charge carrier dynamics in hematite photoanodes for solar ...€¦ · for water oxidation to occur...
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Charge Carrier Dynamics in
Hematite Photoanodes for
Solar Water Oxidation
Stephanie R Pendlebury
Department of Chemistry
Imperial College London
Supervisors: Prof J R Durrant and Dr J Tang
Submitted for Degree of Doctor of Philosophy
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Except where specific reference is made, the material contained in this thesis is the result of my
own work. This dissertation has not been submitted in whole or in part of a degree at this or any
other university, and does not exceed 100 000 words in length.
S R Pendlebury
May 2012
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Abstract
Although the field of solar water splitting is now forty years old, in recent years there has been
an upsurge of research in this area, with the aim of using sunlight to produce hydrogen cheaply
and efficiently. Hematite (α-Fe2O3) is of particular interest as a photoanode material for solar water
splitting, due to its optimum band gap (2.0-2.2 eV) and visible light absorption and stability.
Various modifications – including nanostructuring and doping – have been investigated as routes
to improved efficiencies, thought to be limited by long visible light absorption depths, low charge
carrier mobilities and slow hole-transfer kinetics. Additionally, an anodic applied bias is required
for water oxidation to occur on hematite. Improved understanding of the role of applied bias and
the processes limiting the performance of hematite photoanodes will lead to more directed routes
to photoanode architectures with increased efficiencies.
This Thesis describes the results of transient absorption spectroscopy studies, in conjunction
with photoelectrochemical measurements, of hematite photoanodes. Transient absorption
spectroscopy on microsecond-second timescales allows direct monitoring of the recombination,
trapping and reaction of photogenerated holes, both in isolated hematite films, and in photoanodes
in a fully functional photoelectrochemical cell. Transient photocurrent measurements probe
electron extraction from the photoanode on microsecond-millisecond timescales.
The charge carrier dynamics are found to be strongly dependent on the electron density, which
is controlled by applied electrical bias. The photocurrent generated is found to correlate with the
population of long-lived holes, determined by the kinetics of electron-hole recombination.
Generally, effects which lower electron density result in retarded electron-hole recombination
kinetics, increasing the population of long-lived holes and hence increasing the photocurrent.
Following an introduction and review of the literature, the first results chapter reports that the
effect of a positive applied bias is to retard the otherwise dominant electron-hole recombination,
increasing the lifetime of photogenerated holes such that water oxidation can occur. The relative
timescales of recombination, electron extraction and water oxidation as a function of applied bias
are discussed in the following chapter, in conjunction with the results of excitation density studies.
The third results chapter compares the charge carrier dynamics in photoanodes with different
nanomorphologies. The fourth results chapter discusses the effect of an energetic trap state on
charge carrier dynamics, while the effects of surface treatment with cobalt, which is shown to
retard recombination at low applied bias, is reported in the final results chapter. Overall
conclusions are drawn and the implications of these for photoelectrode design are discussed.
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Acknowledgements
First and foremost my thanks must go to Prof James Durrant, for giving me the opportunity to
work in this field, and for providing guidance, support and a ready smile over the duration. Thanks
also to Dr Junwang Tang for his enthusiasm and copious suggestions. I am always grateful to past
and present members of the Durrant, O’Regan, Klug, Haque and de Mello groups for their advice
and help in the lab, general camaraderie and good humour. Tea, cake and crosswords were much
appreciated. Thanks also to Dr Piers Barnes and Dr Steven Dennison for introducing me to the
theory, the literature and for many helpful discussions, and to Dr Xiaoe Li for sharing her extensive
knowledge and equally extensive collection of lab books. Particular thanks are due to Dr Monica
Barroso and Dr Alex Cowan for their help with just about everything, and for our many, many
discussions over scribbly bits of paper - I think we might have fitted together the edge pieces of the
Fe2O3 puzzle.
This project was entirely dependent on those who provided me with samples to measure: Prof
Michael Grätzel, Dr Kevin Sivula and the rest of the EPFL team; Dr Monica Barroso; Dr Steven
Dennison, Chin Kin Ong et al from Chemical Engineering; Dr Junwang Tang and Prof Jinhua Ye –
many thanks to you all.
I am forever grateful to my parents, who have always encouraged me in everything I have
embarked upon.
And to Richard, for everything.
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Table of Contents
Abstract ........................................................................................................................................................ 2
Acknowledgements ..................................................................................................................................... 4
Table of Contents ........................................................................................................................................ 5
List of Figures .............................................................................................................................................. 7
List of Symbols and Abbreviations ......................................................................................................... 13
List of Publications ................................................................................................................................... 14
Chapter I: Introduction ............................................................................................................................. 16
1.1 Background and Introduction to Solar Water Splitting ............................................ 16
1.2 Theory of Photoelectrochemical Water Splitting .................................................... 17
1.3 Literature Review: Hematite Photoanodes for Water Oxidation ............................. 21
1.3.1 Semiconductor properties of Fe2O3 ................................................................ 21
1.3.2 Kinetic Studies of Water Oxidation on Fe2O3 ................................................. 23
1.4 Project objectives .................................................................................................. 29
Chapter II .................................................................................................................................................... 32
Materials & Methods............................................................................................................................ 32
2.1 Materials: Fe2O3 photoanodes .............................................................................. 33
2.1.1 Undoped and Si-doped APCVD hematite ...................................................... 33
2.1.2 Undoped and doped USP hematite ............................................................... 33
2.1.3 Thick solid PLD hematite ............................................................................... 34
2.1.4 Thin solid ALD hematite ................................................................................ 34
2.1.5 Porous microwave heated hematite .............................................................. 35
2.1.6 Thick solid SP Si-doped hematite .................................................................. 35
2.1.7 Colloidal Ti-doped hematite ........................................................................... 35
2.2 Methods ................................................................................................................ 36
2.2.1 PEC............................................................................................................... 36
2.2.2 TAS ............................................................................................................... 38
2.2.3 TPC ............................................................................................................... 40
Chapter III ................................................................................................................................................... 42
Identification of Photogenerated Hole Absorption in Hematite Photoanodes ........................... 42
3.1 Introduction ............................................................................................................ 43
3.2 Experimental .......................................................................................................... 44
3.3 Charge carrier dynamics of isolated hematite films ................................................ 46
3.4 Charge carrier dynamics of hematite under applied bias ....................................... 55
3.5 Discussion ............................................................................................................. 58
3.6 Conclusions ........................................................................................................... 61
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Chapter IV ................................................................................................................................................... 62
Correlation of Photocurrent with Long-Lived Hole Population as a Function of Applied Bias ....................................................................................................................................................... 62
4.1 Introduction ............................................................................................................ 63
4.2 Experimental .......................................................................................................... 64
4.3 Transient absorption studies of photogenerated holes ........................................... 64
4.4 Transient photocurrent studies of photogenerated electrons .................................. 68
4.5 Excitation density studies ....................................................................................... 70
4.6 Discussion ............................................................................................................. 71
4.7 Conclusions ........................................................................................................... 77
Chapter V .................................................................................................................................................... 78
Comparison of Solid and Mesoporous Hematite Photoanodes ................................................... 78
5.1 Introduction ............................................................................................................ 79
5.2 Experimental .......................................................................................................... 80
5.3 Comparison of carrier dynamics in solid and mesoporous hematite ....................... 81
5.4 UV versus visible excitation ................................................................................... 89
5.5 Conclusions ........................................................................................................... 94
Chapter VI ................................................................................................................................................... 96
Influence of Trap States on Charge Carrier Dynamics ................................................................... 96
6.1 Introduction ............................................................................................................ 97
6.2 Experimental .......................................................................................................... 98
6.3 Spectroscopic Study of Trap State ......................................................................... 98
6.4 Discussion ........................................................................................................... 103
6.5 Conclusions ......................................................................................................... 108
Chapter VII ................................................................................................................................................ 110
Effect of Co-Based Catalysts on Hematite Charge Carrier Dynamics: Comparison of Co2+
and Co-Pi ............................................................................................................................................ 110
7.1 Introduction .......................................................................................................... 111
7.2 Experimental ........................................................................................................ 113
7.3 Effect of Co-adsorption on charge carrier dynamics ............................................. 114
7.4 Discussion ........................................................................................................... 119
7.5 Conclusions ......................................................................................................... 122
Chapter VIII: Concluding Remarks ....................................................................................................... 124
IX: References .......................................................................................................................................... 128
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List of Figures
Scheme 1.1 Semiconductor-electrolyte junction: ECB and EVB are the potentials of the conduction and
valence band edge, EF is the Fermi level, Eg is the band gap, ω is the width of the space-charge (depletion)
layer, VOC is open-circuit potential under illumination, EFn and EFp are the electron and hole quasi-Fermi
levels, respectively, under illumination, and uredox is the redox potential of the electrolyte. ............................ 18
Scheme 1.2 Three-electrode photoelectrochemical cell, with nanostructured Fe2O3 photoanode (working
electrode), reference electrode and metal counter electrode. ......................................................................... 19
Scheme 1.3 Crystal structure of hematite (from reference 29). Left: unit cell showing pairs of face-
sharing octahedral aligned along the c-axis. Right: FeO9 dimer. ................................................................... 22
Scheme 1.4 (a) Typical UV-vis spectrum of hematite; (b) schematic of proposed hematite band structure;
(c) updated band structure of hematite, showing strong Fe 3d/O 2p VB hybridisation. .................................. 23
Scheme 1.5 Proposed reaction scheme for water oxidation on hematite photoanodes (adapted from
references 8 and 17). ...................................................................................................................................... 25
Fig 1.1 SEM images of Si-doped (A and B) and undoped (C) APCVD hematite photoanodes. A: side-
view; B and C: top-down view. From reference 13. ........................................................................................ 33
Fig 1.2 Top-down SEM images of undoped (left) and Nb-doped (right) USP hematite photoanodes.
Images courtesy of Monica Barroso. ............................................................................................................... 34
Fig 1.3 Top-down (c) and side-view (d) SEM images of undoped ALD hematite photoanodes. From
reference 14. .................................................................................................................................................... 34
Fig 1.4 Top-down SEM image of undoped MH hematite photoanode. Image courtesy of Junwang Tang.
......................................................................................................................................................................... 35
Fig 1.5 Top-down SEM image of colloidal Ti-doped hematite photoanode. Insert: before encapsulation.
Image from reference 73. ................................................................................................................................ 35
Scheme 2.1 Photoelectrochemical system and three-electrode cell used for current/voltage,
chronoamperometry and IPCE measurements. See text for details. ............................................................. 36
Fig 2.1 Example of current/voltage curve of a hematite photoanode in the dark (grey) and under white
light illumination (blue). The dark current onset potential is ~0.65 VAg/AgCl, while the photocurrent onset
potential is ~0 VAg/AgCl. Nanostructured Si-doped CVD hematite photoanode under EE (“front-side”)
illumination at approximately 1 Sun intensity, in 0.1 M NaOH......................................................................... 37
Scheme 2.2 General schematic of the transient absorption systems employed - see text for details. ..... 39
Fig 3.1 UV-vis spectra of various hematite photoanodes employed in this study: undoped and Si-doped
nanostructured hematite deposited by atmospheric pressure chemical vapour deposition (CVD; blue lines);
mesoporous undoped hematite deposited by ultrasonic spray pyrolysis (USP, red line) and by microwave
heating (MH, green line). No correction has been made for reflection. .......................................................... 45
Fig 3.2 Transient absorption (TA) spectra of undoped CVD hematite in an argon atmosphere and inset: in
a methanol-saturated argon atmosphere, using 337 nm SE excitation (0.20 mJ.cm-2
at Fe2O3 surface).
Spectra shown were measured 5 and 80 μs after the laser pulse. ................................................................. 46
Fig 3.3 (a) Transient absorption decays of three different hematite films in an argon atmosphere; inset:
the same transient decays normalised and shown on log-log axes, exhibiting power-law-like decay kinetics.
TA decays probed at 600nm with 337 nm excitation (~0.2 mJ.cm-2
). (b) IPCE spectra of the same hematite
8
films in a three-electrode cell with 0.1 M NaOH electrolyte (pH ~12.8), at 0.4 VAg/AgCl and under SE
illumination. CVD: dendritic nanostructured Fe2O3 (undoped); USP: mesoporous “platelet” Fe2O3; MH:
mesoporous Fe2O3 deposited by microwave heating. ..................................................................................... 47
Fig 3.4 Comparison of TA decays of undoped (pale blue/orange) and Si-doped (dark blue/brown) CVD
hematite in an argon atmosphere, probed at (a) 600 nm and (b) 900 nm (EE 180 μJ.cm-2
, 337 nm excitation).
Inset: normalised log-log plots. ........................................................................................................................ 49
Fig 3.5 TA spectrum (at 5 μs after the laser pulse) of undoped CVD hematite in an argon atmosphere, at
excitation intensities of 200, 100 and 50 μJ.cm-2
(EE 337 nm excitation). ...................................................... 50
Fig 3.6 Excitation intensity behaviour of undoped CVD hematite in an argon atmosphere, probed at 580
nm (left), 650 and 900 nm (right). Excitation intensities were varied between 27 and 500 μJ.cm-2
(337 nm SE
excitation). The TA amplitude at 1 μs is plotted in (a) probed at 580 nm, and (b) probed at 600 nm (green
triangles) and 900 nm (red rhombuses). The same behaviour is observed at 1-80 μs. The TA decay kinetics
are shown in (c) probed at 580 nm and (d) probed at 650 nm (900nm decays are very similar to those
probed at 650 nm); inset: the same decays normalised. ................................................................................. 51
Fig 3.7 TA decays (probed at 580 nm; SE 337nm, 190 μJ.cm-2
excitation) of undoped CVD hematite in an
argon atmosphere, in 0.1M NaOH, and with hole scavengers including methanol (~0.75 M in 0.1M NaOH)
and iodide (2 mM). Decay dynamics are also essentially identical in the presence of thiocyanate and iso-
propanol. Inset: comparison of SE and EE TA decays of hematite in aqueous KI. ...................................... 52
Fig 3.8 TA decays of an isolated undoped CVD hematite film in water (black) and aqueous AgNO3
solution (2 mM; blue/green), probed at 580 nm (left) and 650 nm (right). Charge carrier dynamics probed at
900 nm are similar to those probed at 650 nm. EE 337nm, 90 μJ.cm-2
excitation. ....................................... 53
Fig 3.9 TA spectrum of an isolated USP Si-doped hematite film in aqueous hydrogen peroxide solution
(~0.38 M). EE 337nm, 0.13 mJ.cm-2
excitation. ............................................................................................ 54
Fig 3.10 Comparison of TA decays of an isolated USP Si-doped hematite film in water (pale blue/orange)
and in aqueous hydrogen peroxide solution (~0.38 M; dark blue/brown) probed at 580 nm and 900nm (EE
337nm, 0.13 mJ.cm-2
excitation). Hydrogen peroxide causes bleaching of the 580 nm signal on timescales
>10 μs, and significantly increases the amplitude of long-lived signals probed at ≥650 nm. .......................... 54
Fig 3.11 Photocurrent/voltage curves for CVD undoped hematite under white light illumination (~1 Sun,
SE) in 0.1M NaOH without (black curve) and with (red curve) ca. 0.2 mM methanol. The dark current is
negligible in the potential region shown........................................................................................................... 56
Fig 3.12 TA decays of CVD undoped hematite in a three-electrode cell under applied bias, probed at (a)
580 nm and (b) 900 nm. In 0.1M NaOH under applied bias of -0.1 VAg/AgCl (blue/orange curve) and +0.4
VAg/AgCl (black curves). SE pulsed (0.33 Hz) 355 nm excitation; excitation densities are matched to those in
Figure 3.7. ........................................................................................................................................................ 56
Fig 3.13 TA decays (probed at 580 nm) of undoped nanostructured hematite in a three-electrode cell
under applied bias. (a) In 0.1M NaOH under applied bias of -0.1 VAg/AgCl (blue curve) and +0.4 VAg/AgCl (black
curve). Upon the addition of methanol in the positive bias condition (red curve), the faster decay indicates the
more facile oxidation of methanol by photogenerated holes. The decay of an isolated hematite film (no bias)
in 0.1M NaOH (green curve) is similar to that under negative applied bias. (b) Comparison of TA decays
with (red) and without (black) methanol at -0.1VAg/AgCl. ................................................................................... 57
Scheme 3.1 Representation of the effect of applied positive electrical bias on the Fermi level of a
nanostructured hematite photoanode. Applied positive bias decreases the background electron density
9
relative to open-circuit conditions, reducing the rate of electron-hole recombination and increasing the
lifetime of photogenerated holes, allowing water oxidation to occur. .............................................................. 59
Fig 4.1 Transient absorption spectra in Si-Fe2O3 CVD at (a) -0.2 VAg/AgCl and (b) +0.4VAg/AgCl at 10 ms,
100 ms, 500 ms and 1 s after the excitation pulse (EE, 355 nm). At early timescales there is a strong bleach
(negative absorption) at wavelengths <625 nm. The spectrum at +0.4VAg/AgCl is essentially the spectrum of
the photogenerated holes. ............................................................................................................................... 65
Fig 4.2 Transient absorption and photocurrent density data for a Si-doped CVD Fe2O3 film as a function
of applied electrical bias. (a) Transient absorption signals (1 μs to 2 s, EE 355 nm excitation, probed at 650
nm) at various applied bias (in 0.1 V increments: pale grey -0.4 VAg/AgCl, brown +0.6 VAg/AgCl). The arrow
indicates increasing number of long-lived holes with increasing positive bias at water-splitting timescales. (b)
Correlation of long-lived photogenerated hole signal amplitude at 100 ms (red diamonds) with photocurrent
(blue line; under 355 nm EE illumination (ca. 550 μW.cm-2
, giving ~54 μA.cm-2
photocurrent at 1.23 VRHE)). 66
Fig 4.3 Median lifetime (t50%)* of the fast decay phase of the transient absorption signal for
photogenerated holes from Figure 4.2a, versus applied bias. ........................................................................ 67
Fig 4.4 TPC decays (measuring extracted electrons) overlaid on corresponding transient absorption
decays (measuring photo-holes) of a Si-doped CVD hematite photoanode, EE excitation at 355 nm, at -0.2,
+0.2, +0.4 and +0.6VAg/AgCl. The TPC axis is shifted upwards and scaled to maximise overlap with the TA
decay. .............................................................................................................................................................. 69
Fig 4.5 Steady-state photocurrent and transient absorption data for a Si-doped CVD Fe2O3 photoanode
as a function of excitation intensity at +0.4 VAg/AgCl. (a) Variation of steady-state photocurrent amplitude
(under 355 nm EE illumination); the red line is the best fit to the data. (b) Variation of transient absorption
photogenerated hole signal (probed at 650 nm, EE 355 nm excitation from 23 μJ.cm-2
(dark green) to 2.21
mJ.cm-2
(brown); the laser intensity used for the majority of the measurements described herein is 200
μJ.cm-2
). (c) normalised slow TA phase at 125 ms - the timescale of water oxidation (2.1 s) is independent
of excitation intensity. Inset: normalised at 10 μs - the fast phase decays more rapidly with increasing
excitation density. (d) Ratio of amplitude of fast and slow decay phases of transient absorption as a function
of excitation intensity; inset: variation of amplitudes with excitation intensity. At the very lowest excitation
intensities (<200 μJ.cm-2
) we approach pseudo-first-order recombination behaviour (i.e. within the small
perturbation regime). ....................................................................................................................................... 70
Table 4.1 Estimated values of potential drop (ΔφSC) across the radius of spherical undoped hematite
nanoparticles of various sizes for two different Debye lengths (LD). ............................................................... 75
Fig 5.1 UV-vis spectra of various types of hematite photoanodes. Spectra taken of the same area of the
photoanodes as used for TAS and PEC measurements. Vertical black lines indicate 355 and 525 nm. ...... 80
Fig 5.2 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination (ca. 1 Sun) intensity), 10
mV.s-1
) from different types of hematite photoanodes: solid Fe2O3 30 nm (pale blue) and 57 nm (dark blue)
thick (ALD); colloidal Ti-Fe2O3 (green); thick (1 μm) solid SP Sn-Fe2O3 (red). ............................................... 81
Fig 5.3 Transient absorption (TA) decays of holes (probed at 700 nm) in 30 nm thick ALD Fe2O3
photoanodes as a function of applied bias, at 0 (blue), +0.3 (green) and +0.6 VAg/AgCl (brown).
Measurements were made using a three electrode cell with 0.1 M NaOH electrolyte; EE 355 nm excitation
(25 μJ.cm-2
, corresponding to an approximate initial photogenerated hole density of 8x1018
holes.cm-3
). ..... 82
Fig 5.4 Correlation of long-lived photo-hole population (as measured by the amplitude of the TA decay at
200 ms) with photocurrent at +0.4VAg/AgCl, both under 355 nm illumination for various hematite photoanodes
10
under EE (front-side) and SE (back-side) illumination. The best-fit straight line has an intercept of 0.001(4)
mΔOD and gradient of 0.19(4). ....................................................................................................................... 82
Fig 5.5 (a) TA decays of holes in 30 nm thick solid ALD (probed at 700 nm; blue) and ~500 nm thick
nanostructured CVD Fe2O3 photoanodes (probed at 600 nm; EE brown, SE orange). (b) The same TA
decays normalised at 3 ms to show the relative timescales of water oxidation. Measurements were made at
potentials were the photocurrent was almost saturated: 0.4 VAg/AgCl for CVD and 0.6 VAg/AgCl for ALD
photoanodes. ALD measurements used EE 355 nm excitation (25 μJ.cm-2
, corresponding to ~2.4x1013
holes.cm-2
); CVD measurements used EE 355 nm excitation (190 μJ.cm-2
, corresponding to ~1.5x1013
holes.cm-2
, assuming a roughness factor of 20), SE excitation densities were matched to this. .................... 84
Fig 5.6 (a) TA decays of holes (probed at 700 nm) in 30 nm thick ALD Fe2O3 photoanodes at +0.6 VAg/AgCl
as a function of excitation density. EE 355 nm excitation at 5 (grey), 25 (black), 50 (blue), 100 (purple) and
250 μJ.cm-2
(pink). (b) The same TA decays normalised at 3 ms, showing that the kinetics of water oxidation
are independent of excitation density. ............................................................................................................. 84
Fig 5.7 TA decays of holes in colloidal Ti-doped (green), thick (~1 μm) solid Si-doped (purple), 57 and 30
nm thick solid ALD (dark and pale blue, respectively) Fe2O3 photoanodes at positive applied bias where
photocurrent is approximately saturated. EE 355 nm excitation; average excitation density ca 2x1018
-3x1019
photogenerated holes.cm-3
. ............................................................................................................................. 85
Fig 5.8 TA decays of holes (probed at 650 nm) in colloidal Ti-doped Fe2O3 photoanodes at 0.25 (just
anodic of the photocurrent onset potential), 0.4 and 0.6 VAg/AgCl. EE 355 nm excitation at 50 μJ.cm-2
. The
fast decay phase (on the microsecond to hundreds of milliseconds timescale) is significantly longer-lived
than in other hematite photoanodes studied. .................................................................................................. 86
Fig 5.9 Transient photocurrent (TPC) from pulsed light (EE 355 nm; the same excitation densities are
employed as for TAS measurements) excitation of colloidal Ti-doped (green), thick solid SP Si-doped
(purple) and thin solid ALD (blue) hematite photoanodes. The photoanodes were held at positive applied
bias where photocurrent is approximately saturated. Photocurrent transients are normalised for ease of
comparison. ..................................................................................................................................................... 88
Fig 5.10 Comparing EE (pale colours) and SE (dark colours) TPC from colloidal Ti-doped (green, left)
and thick solid SP Si-doped (purple, right) hematite photoanodes (355 nm pulsed excitation). The
photoanodes were held at positive applied bias where photocurrent is approximately saturated.
Photocurrent transients are normalised for ease of comparison. .................................................................... 88
Fig 5.11 TPC from solid SP Si-doped hematite photoanodes (~1 μm thick) under 355 nm (purple) and
525 nm (grey) excitation, illuminated SE (left) and EE (right). The photoanodes were held at positive applied
bias (0.5 VAg/AgCl) where photocurrent is approximately saturated; similar decays are observed at potentials
just anodic of the photocurrent onset. The number of photons absorbed was ~3.0x1018
cm-3
in each
measurement. Photocurrent transients are normalised for ease of comparison; inset: data before
normalisation. .................................................................................................................................................. 89
Fig 5.12 TA spectra of nanostructured CVD Si-doped hematite photoanodes at +0.4 VAg/AgCl (i.e. the
spectra of photogenerated holes) under 355 nm and 525 nm SE excitation. Spectra under EE excitation are
very similar, especially at long timescales. ...................................................................................................... 91
Fig 5.13 TA decays of holes (probed at 650 nm) photogenerated by 525 nm excitation (SE, 0.16 mJ.cm-2
)
in nanostructured CVD Si-doped hematite photoanodes as a function of applied bias, from -0.3 VAg/AgCl
(grey) to +0.4 VAg/AgCl (red). Dynamics under EE excitation are similar. ......................................................... 91
11
Fig 5.14 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (red curves) and 355 nm
(blue curves) excitation (SE, number of photons absorbed matched) in nanostructured CVD Si-doped
hematite photoanodes at 0 VAg/AgCl (left), and +0.4 VAg/AgCl (right). ................................................................ 92
Fig 5.15 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (red curves) and 355 nm
(blue curves) excitation (SE, number of photons absorbed matched) in thick solid SP Si-doped hematite
photoanodes at 0 VAg/AgCl (left), and +0.5 VAg/AgCl (right). ............................................................................... 93
Fig 6.1 (a) Current/voltage curves from nanostructured Si-Fe2O3 photoanodes (in 0.1M NaOH, pH ~12.8,
white light illumination, 10 mV.s-1
). (b) Chopped light photocurrent transients from nanostructured Si-Fe2O3
photoanodes at +0.2 VAg/AgCl (355 nm illumination). ........................................................................................ 98
Fig 6.2 TA decay dynamics of Si-Fe2O3 CVD photoanodes under EE 355 nm pulsed excitation (0.20
mJ.cm-2
) probed at 650 nm (positive signal) and 575 nm (negative signal) at +0.1 VAg/AgCl (green) and +0.4
VAg/AgCl (orange). The “fast decay phase” probed at 650 nm and the bleach probed at 575 nm occur on the
same timescale (1 μs to ~20 ms). ................................................................................................................... 99
Fig 6.3 Transient absorption spectra of Si-Fe2O3 photoanodes at (a) -0.7 VAg/AgCl and (b) +0.4 VAg/Agcl at
10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black through blue to grey) after the excitation pulse. EE 355
nm pulsed excitation (0.20 mJ.cm-2
). ............................................................................................................... 99
Fig 6.4 Decay dynamics of Si-Fe2O3 photoanodes probed at 575 nm as a function of applied bias, (a)
from -0.7 to +0.4 VAg/AgCl (black through blue to brown); (b) focusing on -0.7 to -0.3 VAg/AgCl, showing that the
decay dynamics are identical cathodic of -0.4 VAg/AgCl. .................................................................................. 100
Fig 6.5 Overlay of long-lived hole population (given by the amplitude of the transient decay at 100 ms
probed at 650 nm), magnitude of the bleach (probed at 10 μs at 575 nm, inverted and multiplied by 0.1 for
ease of comparison) on the photocurrent density curve (355 nm EE excitation). ......................................... 101
Fig 6.6 Inverted TPC decays (grey, black) overlaid on transient absorption decays (green, orange)
probed at 575 nm under applied bias at +0.1 and +0.4 VAg/AgCl. Si-Fe2O3 CVD photoanodes under 355 nm
EE pulsed excitation. ..................................................................................................................................... 102
Scheme 6.1 Effect of applied bias on trap state and transient absorption bleach (probed at 575 nm). At
negative applied bias, the mid-bandgap state is occupied by electrons, so acts as a hole trap (recombination
centre); a positive transient absorption signal is observed. At positive applied bias, the Fermi level lies below
the trap state, which acts as an electron trap; a negative transient absorption signal (bleach) is observed.
Detrapping of electrons and extraction to the external circuit results in the recovery of the bleach. ............ 105
Scheme 6.2 Effect of applied bias on occupancy of the trap state probed at 575 nm. When the Fermi
level lies above the mid-bandgap state, this state is occupied by electrons (reduced), so acts as a hole trap
(recombination centre). When the Fermi level lies below the trap state is oxidised and acts as an electron
trap. Positive bias increases the width of the space charge layer (lowers the Fermi level in nanoparticulate
films), so more trap states are oxidised. ........................................................................................................ 106
Fig 7.1 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination, 10 mV.s-1
) of Si-Fe2O3
APCVD photoanodes (dark grey), after Co-treatment with Co(NO3)2 (blue), and after repeated Co-treatment
(pale blue) for SE (dashed; “back-side”) and EE (solid; “front-side”) illumination. Inset: expansion of the
photocurrent onset region. ............................................................................................................................. 114
Fig 7.2 Chopped light photocurrent transients from nanostructured Si-Fe2O3 photoanodes before and
after Co-treatment, at +0.2 VAg/AgCl (355nm EE illumination). SE illumination gives similar results but with
lower photocurrent densities. ......................................................................................................................... 115
12
Fig 7.3 Transient absorption decays of isolated Si-Fe2O3 photoanodes before (black) and after (coloured)
Co2+
-adsorption (355nm 0.20 mJ.cm-2
EE excitation, 0.1M NaOH, no applied bias), probed at (a) 575 nm (b)
650 nm and (c) 900 nm. ................................................................................................................................ 115
Fig 7.4 Charge carrier dynamics of photogenerated holes in Si-Fe2O3 photoanodes before (black) and
after (coloured) Co-treatment (355nm 0.20 mJ.cm-2
EE excitation, 0.1M NaOH) probed at 650 nm under
applied bias: (a) -0.1 VAg/AgCl (b) +0.2 VAg/AgCl and (c) +0.4 VAg/AgCl. ............................................................. 116
Fig 7.5 TPC decays probing electron extraction from Si-Fe2O3 photoanodes before (dark colours) and
after Co-treatment (pale colours) at -0.1 VAg/AgCl (green) and +0.4 VAg/AgCl (orange). Pulsed 355nm 0.20
mJ.cm-2
EE excitation, 0.1M NaOH electrolyte. ............................................................................................ 117
Fig 7.6 Transient absorption spectra of Si-Fe2O3 photoanodes before (left) and after Co2+
-adsorption
(right) at 0 VAg/AgCl (top) and +0.4 VAg/AgCl (bottom), at 10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black
through blue to grey) after the excitation pulse. There is a striking similarity between Si-Fe2O3 at +0.4 VAg/AgCl
and Co/Si-Fe2O3 at 0 VAg/AgCl. ........................................................................................................................ 118
Fig 7.7 Transient absorption decays probed at 575 nm under applied bias at (a) 0 VAg/AgCl (just cathodic of
the photocurrent onset potential in the absence of cobalt), and (b) +0.4 VAg/AgCl, (where significant
photocurrent is generated even in the absence of cobalt). Before (black) and after (coloured) Co2+
-
adsorption; cobalt increases the magnitude of the bleach, particularly at low positive applied bias............. 119
Scheme 7.1 Proposed effect of Co2+
-adsorption/Co-Pi deposition on hematite photoanodes................ 121
13
List of Symbols and Abbreviations
a absorption coefficient
A absorbance (of light) (a.u.)
ALD atomic layer deposition
(AP)CVD (atmospheric pressure) chemical vapour deposition
APCE absorbed photon to current conversion efficiency (internal quantum efficiency)
CB conduction band
e- electron
e0 electronic charge (1.602x10-19 C)
ECB position of the conduction band edge (V)
EF Fermi level (V)
EFB flatband potential
Eg bandgap (eV)
EVB position of the valence band edge (V)
ε Dielectric constant (permittivity; C2.J-1.m-1)
ε0 Permittivity of free space (8.854x10-5 C2.J-1.m-1)
EE electrolyte-electrode illumination (i.e. from the front)
FTO fluorine-doped tin oxide
h+ hole
i current density (A.cm-2)
IPCE incident photon to current conversion efficiency (external quantum efficiency)
λ wavelength (nm)
LD Debye length (nm)
MH microwave heated
ND donor density (cm-3)
OER oxygen evolution reaction
ΔOD change in optical density (absorbance) (a.u.)
P power density of illumination (W.cm-2)
PEC Photoelectrochemical
RHE reversible hydrogen electrode
SCLJ semiconductor-liquid (electrolyte) junction
SE substrate-electrode illumination (i.e. from the back)
SP spray pyrolysis
T transmission (of light)
t time (s)
t50% lifetime (s)
TAS transient absorption spectroscopy
TPC transient photocurrent
USP ultrasonic spray pyrolysis
Von photocurrent onset potential
V potential (V)
VB valence band
14
List of Publications
1. “Dynamics of Photogenerated Holes in Nanocrystalline α-Fe2O3 Electrodes for water
Oxidation Probed by Transient Absorption Spectroscopy”
S. R. Pendlebury, M. Barroso, A. J. Cowan, K. Sivula, J. Tang, M. Grätzel, D. Klug,
J. R. Durrant
Chemical Communications 2011, 47, 716-718
2. “Activation Energies for the Rate-Limiting Step in Water Photo-oxidation by
Nanostructured α-Fe2O3 and TiO2”
A. J. Cowan, C. J. Barnett, S. R. Pendlebury, M. Barroso, K. Sivula, M. Grätzel,
J. R. Durrant, D. R. Klug
Journal of the American Chemical Society 2011, 133, 10134-10140
3. “The Role of Cobalt-Phosphate in Enhancing the Photocatalytic Activity of α-Fe2O3
towards Water Oxidation”
M. Barroso, A. J. Cowan, S. R. Pendlebury, M. Grätzel, D. R. Klug, J. R. Durrant
Journal of the American Chemical Society 2011, 133, 14868-14871
4. “Correlating Long-Lived Photogenerated Hole Populations with Photocurrent Densities in
Hematite Water Oxidation Photoanodes”
S. R. Pendlebury, A. J. Cowan, M. Barroso, K. Sivula, J. Ye, M. Grätzel, D. R. Klug,
J. R. Durrant
Energy and Environmental Science 2012, 5, 6304-6312
5. “Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar
water splitting”
M. Barroso, C. Mesa, S. R. Pendlebury, A. J. Cowan, T. Hisatomi, K. Sivula,
M. Grätzel, D. R. Klug, J. R. Durrant
Proceedings of the National Academy of Sciences 2012, in press (Early Edition Article)
15
Chapter I: Introduction 16
Chapter I: Introduction
1.1 Background and Introduction to Solar Water Splitting
With rapidly increasing concentrations of carbon dioxide and other greenhouse gases in
the atmosphere and fluctuating oil prices, it is imperative that affordable renewable, carbon-
free forms of energy are developed and commercialised within the next few years.
Producing hydrogen by splitting water using sunlight could be one solution. Such “solar-
hydrogen” could also be used as a chemical feedstock when reacted with carbon dioxide or
nitrogen,1 replacing feedstocks currently derived from fossil fuels.
Although hydrogen can be produced by the conventional electrolysis of water, this is an
energy intensive process. Using sunlight to photogenerate charge carriers in a
semiconductor electrode, which then electrochemically dissociate water is a more
environmentally friendly and potentially lower cost alternative. The advantage of solar water
splitting over using photovoltaics to drive conventional electrolysis is that the photon energy
is converted directly in to chemical energy, simplifying the device so potentially reducing
costs and increasing efficiency.
Many semiconductor materials have been investigated for use as photoelectrodes,
including metal oxide semiconductors such as TiO2, WO3, SrTiO3, Fe2O3, and small band-
gap semiconductors such as GaAs, CdSe and CdS. Although small band-gap
semiconductors absorb more of the solar spectrum, and so are potentially more efficient,
their band energies may be unsuitable for the evolution of both O2 and H2. Non-oxide
semiconductors are often severely corroded or photocorroded under water dissociation
conditions. The practicality of large-scale photoelectrochemical cells for hydrogen
production based on III-V semiconductors is also limited by the high cost of these
materials.2-4
Photodissociation of water by a semiconductor was first discovered using TiO2;5
subsequently this material has been extensively studied. However, the band gap for
anatase TiO2 is 3.2 eV (equivalent to 388 nm), so the maximum theoretical photoconversion
efficiency is only 2.2%.6 Because irradiance changes rapidly with wavelength in the UV
region of the solar spectrum, a small decrease in the size of the band gap can lead to a large
increase in the maximum theoretical efficiency. With a band gap of 2.70 eV, equivalent to
459 nm, WO3 has a maximum possible photoconversion efficiency of 4.8%. Both TiO2 and
WO3 are stable to oxygen evolution.
Unlike TiO2 and WO3, iron oxide (Fe2O3) can absorb light in the visible region of the solar
spectrum. The band gap is usually reported as between 2.0 and 2.2 eV, hence Fe2O3 can
absorb sunlight of wavelengths up to ~600nm – approximately 38% of the solar spectrum7 –
17 Chapter II: Materials & Methods
resulting in a maximum possible photoconversion efficiency of 12.9%.6 This band gap also
lies within the 2.0 – 2.25 eV range for maximum photon to chemical conversion efficiency.8
Fe2O3 is one of the smallest-bandgap semiconductors that is stable to oxygen evolution; it is
stable in neutral and basic solutions,9 and has also been reported as somewhat stable at
acidic pH.10, 11 Although Fe2O3 is an intrinsic n-type semiconductor (due to oxygen
vacancies), it can be doped to produce p-type behaviour.1, 10
In addition to its relatively small band gap and stability under water photolysis conditions,
Fe2O3 is non-toxic and formed of highly abundant elements. Photoanodes can be prepared
by a variety of techniques, including – but not limited to – spray pyrolysis,12 chemical vapour
deposition13 and atomic layer deposition.14 However, several factors limit the water photo-
oxidation efficiency, including a somewhat long absorption depth for visible light15 coupled
with a short hole diffusion length.16, 17 Hole transfer kinetics at the semiconductor-electrolyte
junction have also been reported to be relatively slow, potentially limiting water oxidation
efficiency.17-20 An anodic applied bias is necessary for water photo-oxidation to occur.
These factors are discussed in detail in Section 1.3.
1.2 Theory of Photoelectrochemical Water Splitting
The semiconductor-liquid junction (SCLJ) is analogous to the semiconductor-metal
Schottky barrier.2, 21, 22 When a semiconductor surface is brought in contact with an
electrolyte, charge is transferred between the semiconductor and the electrolyte until the
system has reached equilibrium, i.e. the Fermi levels are at the same energy. Because
there is no formal energy of states in the semiconductor band gap (although defect states
may exist there), the change in Fermi level position will be much greater for the
semiconductor than for the solution. The following descriptions are for n-type
semiconductors (such as Fe2O3), in which electrons are the majority charge carrier and the
Fermi level lies just below the conduction band edge. This results in a build-up of negatively
charged ions in solution (Helmholtz and Gouy layers) at the interface and depletion of
electrons from the near-surface region of the semiconductor. This depletion (or space-
charge) layer is part of the electric double layer at the interface. The extent of the space-
charge layer in to the semiconductor bulk depends on the semiconductor donor density (ND),
i.e. the level of doping, as shown in Equation 1.1, where ω is the width of the space-charge
layer, ΔφSC is the potential drop across the space-charge layer; other symbols have their
usual meaning. A high donor (electron) density results in a narrow space-charge layer, while
a lower donor density results in a wider space-charge layer with a weaker electric field
across it.
Chapter I: Introduction 18
Δ
(1.1)
The separation of electrons and holes at the junction causes an electric field, resulting in
band bending across the space-charge layer, so electrons have higher energy at the surface
than in the bulk, as shown in Scheme 1.1. The potential drop in the space-charge layer is
determined by the difference between the Fermi levels of the solution and the semiconductor
when it is free from excess charge (i.e. no band bending). It should be noted that in
nanostructured or particulate semiconductors, no band bending will occur if the particle size
is smaller than the width of the space-charge layer.
A photon with energy greater than that of the semiconductor band-gap energy (Eg), may
excite an electron from the valence band (VB) of the semiconductor in to the conduction
band (CB), leaving a positively charged hole in the valence band. Where band bending is
present, the photogenerated charge carriers migrate (drift) in opposite directions; the
minority charge carriers (holes in an n-type semiconductor) migrate to the surface. Where
there is no band bending, charge carriers migrate by diffusion. Charge separation induces
an electric field which counteracts band bending, and raises the Fermi level. The
thermodynamic upper limit for the energy that can be extracted from the separated
photogenerated charge carriers is the difference between the Fermi levels of the
semiconductor and the solution.
In photoelectrochemical (PEC) water splitting cells, the semiconductor electrode is the
working electrode, while the counter electrode is usually platinum, as shown in Scheme 1.2.
It may be advantageous to employ semiconductor photo-electrodes for both the cathode (p-
Scheme 1.1 Semiconductor-electrolyte junction: ECB and EVB are the potentials of the conduction
and valence band edge, EF is the Fermi level, Eg is the band gap, ω is the width of the space-
charge (depletion) layer, VOC is open-circuit potential under illumination, EFn and EFp are the
electron and hole quasi-Fermi levels, respectively, under illumination, and uredox is the redox
potential of the electrolyte.
n-type
semiconductor electrolyte
Before equilibrium (dark)
ECB
EF
EVB
uredox
EF uredox
ω
ECB
EVB
After equilibrium (dark)
EFn
uredox
ω
EFp
VOC
ECB
EVB
Steady-state illumination
19 Chapter II: Materials & Methods
type) and anode (n-type). However, for research purposes a three-electrode system is
usually employed, where an electrical bias (which controls the semiconductor Fermi level) is
applied between the semiconductor working and reference electrodes, while the current
flows between the working and counter electrodes. Saturated calomel electrodes (SCE) or
Ag/AgCl (SSC) are commonly used. For comparability, potentials can be converted to those
versus the reversible hydrogen electrode (RHE) using the Nernst equation:
(1.2)
where E°ref is the standard potential of the reference electrode (approximately +0.2 VRHE
for both SCE and SSC), and E is the potential applied versus the reference electrode used.
The applied bias changes the position of the semiconductor Fermi level, resulting in a
difference between the positions of the Fermi levels of the semiconductor and the solution,
i.e. the system is no longer in equilibrium.
The flatband potential (VFB) is the position of the Fermi level at zero band bending, in
which situation charge carriers readily recombine, so there is essentially no photocurrent.
Thus the flatband potential is sometimes approximated as the onset potential of the
photocurrent (Von), assuming that VFB is high enough (see below). However, this is not
usually a valid approximation as an overpotential is often required to drive the reaction. The
flatband potentials of the valence band and conduction band are determined by the nature of
the material but also shift with pH (Equation 1.2), due to changes in the extent to which the
Scheme 1.2 Three-electrode photoelectrochemical cell, with nanostructured Fe2O3 photoanode
(working electrode), reference electrode and metal counter electrode.
sunlight
En
erg
y
CB
VB
EF
nanostructured
Fe2O3 anode
e–
h+
metal
cathode
A
electrolyte
H+
H2
H2O
O2
1.23 V
V
reference
electrode
Chapter I: Introduction 20
surface is protonated. For a heavily-doped n-type semiconductor (such as Fe2O3), the Fermi
level lies close to the conduction band edge.
Conduction band electrons at the surface of the semiconductor can reduce redox couples
with a redox potential more positive than the flatband potential, i.e. ECB is a measure of the
reduction potential of photogenerated electrons. Likewise, valence band holes are capable
of oxidising couples with a redox potential more negative than the valence band flatband
potential; EVB is a measure of photogenerated holes’ oxidation potential. The largest
possible photopotential is the difference between the semiconductor Fermi level in the dark
and the flatband potential.
The overall reaction for water splitting is H2O → H2 + ½O2 (1.3)
In basic solution: anode reaction 2h+ + 2OH- → ½O2 + H2O (1.4a)
cathode reaction 2e- + 2H2O → H2 + 2OH- (1.5a)
In acidic solution: anode reaction 2h+ + H2O → ½O2 +2H+ (1.4b)
cathode reaction 2e- + 2H+ → H2 (1.5b)
There are generally considered to be three criteria to be met by a semiconductor
photoelectrode for overall water splitting. 1. The band gap must be larger than 1.229 eV (the
potential corresponding to the Gibbs free energy change for water dissociation: H2O → H+ +
OH-), but small enough to efficiently absorb sunlight. 2. The redox potentials for H+/H2 and
O2/OH- must lie within the band gap. 3. The semiconductor must be stable under
photoelectrolysis conditions. 4. The material should be abundant and cheap. Concerning
point 1, it has been estimated that a bandgap greater than ~2.5 eV is necessary for water
photolysis by an oxide semiconductor without an applied bias, in order to overcome losses.23
Additionally, Equation 1.4 suggests that water oxidation occurs via a concerted 4-hole
transfer process, however, the actual mechanism may proceed along a series of single-hole
transfer steps. The potentials for such single-hole oxidation (reduction) reactions are
significantly more positive (negative) than the equilibrium four-hole oxidation of water
(+1.229 VRHE).24 This also has some significance for the minimum band gap necessary for
water photolysis to occur without an applied bias.
If the Fermi level of the semiconductor is anodic of (lies below) the H+/H2 redox potential,
an external voltage must be applied to raise the Fermi level of the semiconductor such that
photogenerated electrons reduce protons at the counter electrode. The term photoassisted
water splitting should be used when an applied bias is necessary for the reaction to
proceed.25 The term photocatalytic water slitting is often applied to systems in which no
applied bias is necessary. Photocatalytic systems often consist of a suspension of small
semiconductor particles in solution, in which each particle simultaneously evolves hydrogen
and oxygen. The disadvantage of this is the difficulty in separating the evolved gases, which
are potentially explosive.
21 Chapter II: Materials & Methods
The activity of a photoelectrode is usually assessed by measuring the current/voltage (i/V)
curve in the dark and in the light, and by determining the IPCE (incident photon to current
conversion efficiency, i.e. external quantum yield) as a function of wavelength at a particular
potential (see Section 2.2.1). IPCE does not take in to consideration any applied bias.
There are several other methods for calculating the efficiency of water photolysis, which
have been reviewed previously6, 8 and are not considered here. The influence of the choice
of light source on measured photoconversion efficiencies for semiconductor electrodes has
been investigated by Murphy et al.6 Although artificial light sources are more convenient and
practical for measuring photoconversion efficiencies than using solar radiation, their spectra
do not accurately replicate the solar spectrum at the Earth’s surface. Efficiencies calculated
using artificial light sources are often higher than those thermodynamically possible from the
global AM1.5 spectrum (the standard solar reference spectrum). The various light sources
employed in studies reported in the literature means that direct comparison of published
results is not always possible.
Although many different semiconductor materials have been tested for water photolysis
activity, none have yet fulfilled all four of the criteria outlined above for water photolysis
without applied bias.26 An applied bias is almost always necessary for water oxidation to
occur, however this could be overcome using a tandem cell arrangement, with either a
photocathode and photoanode, or a photovoltaic solar cell.10, 27 Since the water oxidation
half-reaction involves four holes for each molecule of O2 produced, this is generally
considered to be more difficult to achieve than the proton reduction half-reaction (two
electrons per H2 molecule). Consequently, much research effort has concentrated on
photoanode materials for water photo-oxidation, i.e. n-type semiconductors such as TiO2,
WO3 and Fe2O3. Metal oxides are favoured due to their stability under water photolysis
conditions. Fe2O3 has a band-gap of ~2 eV, making it a particularly popular choice.
1.3 Literature Review: Hematite Photoanodes for Water Oxidation
1.3.1 Semiconductor properties of Fe2O3
There are several polymorphs of Fe2O3, the structural and magnetic properties of which
have previously been reviewed.28 Hematite (α-Fe2O3) has a corundum-type crystal structure
and is the most thermodynamically stable polymorph. As such, hematite is the Fe2O3
polymorph most commonly employed as a photoanode material. The crystal structure of
hematite is shown in Scheme 1.3.29 Pairs of face-sharing octahedra (Fe2O9 dimers) are
aligned along the c-axis ([001] direction). Maghemite (γ-Fe2O3; inverse spinel structure) is
metastable, transforming to hematite above 400 °C, and has also attracted some interest as
a water photo-oxidation material.
Chapter I: Introduction 22
Hematite is a popular photoanode material due to its stability under water photolysis
conditions, and apparently optimal band-gap of 2.0-2.2 eV, allowing the absorption of
wavelengths up to ~600 nm. A typical hematite UV-vis spectrum is shown in Scheme 1.4a.
It was initially assumed that the bottom of the α-Fe2O3 valence band (VB) had mainly O 2p
character, while the top of the VB was mainly Fe 3d. The absorption peak at ~2.4 eV was
attributed to an Fe 3d→Fe 3d transition, while the peak at ~3.2 eV was attributed to an O
2p→Fe 3d transition.30 While the latter is a charge transfer transition so absorbs strongly,
the d→d transition absorbs more weakly (this forbidden transition is phonon-assisted;
magnetic coupling between adjacent Fe cations and hybridisation of Fe 3d/O 2p orbitals are
also likely to break the octahedral symmetry31). This results in a relatively long absorption
depth for visible light; ~100 nm for 500 nm light.15 More recent soft X-ray spectroscopy and
density functional theory (DFT) studies have indicated that the valence band consists of
strongly hybridised Fe-d and O-p orbitals.31, 32 While spectroscopic studies have suggested
that the top of the VB is strongly hybridised, DFT calculations have suggested that the band
gap is primarily between Fe-d states, with the oxygen density of states being largely >1.5 eV
below the top of the VB (Scheme 1.4).
Unfortunately the water oxidation efficiency of hematite under visible light is severely
limited by this long absorption depth coupled with a short hole diffusion length. The hole
diffusion length has been reported as 2-4 nm and ~20 nm.16, 17 This is an indicator of low
mobility and/or rapid electron-hole recombination (discussed further below). The electron
mobility in hematite is also thought to be low (0.01-0.1 cm2.V-1.s-1).33, 34 Conductivity is highly
anisotropic;35 hematite films oriented with the (001) basal plane perpendicular to the
substrate have been reported to facilitate the collection of photogenerated electrons.13
Although the valence band edge lies below (positive of) the O2/H2O redox potential and as
such is suitable for water oxidation, the conduction band edge lies ~0.4 V positive of the
H2/H+ potential; a positive applied bias is necessary for proton reduction to occur.17, 36 The
Scheme 1.3 Crystal structure of
hematite (from reference 29). Left:
unit cell showing pairs of face-sharing
octahedral aligned along the c-axis.
Right: FeO9 dimer.
23 Chapter II: Materials & Methods
kinetics of water oxidation on hematite are also thought to be sluggish (discussed in the
following section).
Nevertheless, advances in the fabrication of more complex hematite-based photoanodes
have led to significant improvements in water photo-oxidation activity (usually reported as
photocurrent density at 1.23 VRHE under simulated AM 1.5 illumination). Nanostructured
porous hematite12, 37-39 allows the use of thick films to harvest visible light whilst holes are
photogenerated close to the SCLJ. Doping is often found to increase efficiencies; it is often
suggested this is due to improved electron transport properties.13, 40, 41 Heterojunction
photoanodes have also been fabricated, employing materials with good electron transport
properties to aid electron extraction.42, 43 Various surface modifications have been used,
including thin overlayers of Al2O3 and Ga2O3, thought to reduce electron-hole recombination
by relieving lattice strain.44 A thin overlayer of p-type Fe2O3 has also been shown to increase
the water photo-oxidation activity of hematite.45 Additionally, surface modification with
various materials thought to catalyse water oxidation has led to efficiency gains.13, 46, 47
Although in all cases an applied bias was necessary for water oxidation to occur, this could
be overcome using a tandem cell arrangement.10, 27 For a more detailed review of advances
in water oxidation by hematite photoanodes, the reader is referred to a recent publication.48
1.3.2 Kinetic Studies of Water Oxidation on Fe2O3
Significant improvements in water oxidation properties of hematite photoanodes reported
in recent years have been complimented by increasingly detailed understanding of the
mechanisms and kinetics of the various processes occurring, including recombination,
electron collection, water oxidation etc. Several different photoelectrochemical and transient
optical techniques have been employed in these kinetic studies.
The actual detailed mechanism of water photo-oxidation on hematite is currently unknown.
A recent theoretical study investigated the stability of various Fe2O3 surface terminations
Scheme 1.4 (a) Typical UV-vis spectrum of hematite; (b) schematic of proposed hematite band
structure; (c) updated band structure of hematite, showing strong Fe 3d/O 2p VB hybridisation.
400 500 600 700 800 9000.0
0.5
1.0
1.5
2.0a
bs /
a.u
.
wavelength / nm
3 eV
2 eV
(a) Fe 3d
Fe 3d ?
(c)
Fe 3d-O 2p
Fe 3d
Fe 3d
O 2p
CB
VB
2 eV3 eV
(b)
Chapter I: Introduction 24
under photoelectrochemical conditions.49 The ability of different surface terminations to
photo-oxidise water was assessed by calculating the free energies of reaction intermediates
in reaction mechanisms involving a series of one-electron transfer steps. Results indicated
that thermodynamically spontaneous water oxidation is only possible on certain surface
terminations; the driving force for water oxidation by valence band holes is extremely small.
No definite conclusions could be drawn regarding which electron transfer step is the rate-
determining step.
Early photoelectrochemical and impedance studies of hematite photoanodes reported low
faradaic rate constants for water oxidation (0.1-1.0 cm.s-1, cf. 103-104 cm.s-1 for TiO2 and
WO3; it should be noted that rate constants may also have units of s-1).17, 50 It was
suggested that this could be the result of an “energy mismatch” between the Fe2O3 d- and
oxygen p-orbitals (the valence band in TiO2 and WO3 is O 2p in character). Slow charge
transfer kinetics at the hematite surface are likely to result in accumulation of holes at the
Fe2O3 surface. This hole accumulation will affect band-bending and hence electron-hole
recombination.50, 51
Photocurrent transients under chopped light excitation of hematite photoanodes are often
observed.3, 17-19, 40, 43, 45, 50-58 It is generally accepted that these are associated with surface
recombination of conduction band electrons with either of surface-accumulated holes and/or
surface-bound oxidation intermediates (effectively a particular type of surface-bound hole).
The relatively long lifetime of these transients in the absence of hole scavengers (on the
order of 1 s) suggests that the electron-capture cross-section of these surface-bound
holes/intermediates is small, indicating a negatively charged ion, such as OH-, bound to an
Fe ion.51 Efficient hole scavengers, such as H2O2 and [Fe(CN)6]4-, “capture” holes at the
surface faster than H2O, so have been used to investigate the limiting processes in water
oxidation on hematite photoanodes in three-electrode PEC cells.18, 19 Assuming unity hole-
transfer efficiency to the scavenger, this technique has been used to try to separate
recombination and charge transfer limitations. Such studies provide some evidence that
slow hole transfer kinetics may limit the efficiency of water oxidation, but also that bulk
electron-hole recombination and recombination via surface states may be problematic.
Photocurrent measurements and impedance modelling of nanostructured hematite in the
presence and absence of H2O2 have been employed to try to separate the effects of bulk
and surface recombination.19 These revealed a hole injection barrier at the hematite/water
interface, which is not evident is the presence of H2O2. Water oxidation was suggested to be
limited by surface recombination at low applied bias. Surface recombination rapidly
decreased at potentials anodic of the photocurrent onset, attributed to deactivation of
surface traps. Bulk electron-hole recombination was shown to gradually decrease with
increasing positive bias, in proportion with the increasing width of the space-charge layer.
25 Chapter II: Materials & Methods
Smaller hematite particles appeared to result in reduced bulk recombination, i.e. improved
charge separation.
The effect of several fast, one-electron redox shuttles (hole scavengers) and variation in
electrolyte pH on steady-state photocurrent densities were investigated using thin, solid
hematite photoanodes.18 Similarly, this study indicated that hole transfer at the SCLJ is the
rate limiting step of water oxidation, while back electron transfer from the conduction band
and/or surface states to oxidised surface species (i.e. surface recombination) is likely to be
the dominant loss pathway which limits the open circuit photovoltage.
Several other frequency-domain studies of water oxidation on hematite have been
published during the course of the research reported in this Thesis. Techniques include
(photo)electrochemical impedance spectroscopy ((P)EIS),53, 55, 59, 60 intensity-modulated
photocurrent spectroscopy (IMPS),51 and potential-modulated and light-modulated
absorption spectroscopies (PMAS and LMAS, respectively).53 Further evidence for slow
hole transfer kinetics on hematite has been provided by these frequency-domain analyses.
A number of such studies have employed a model whereby both hole transfer to the
electrolyte and surface electron-hole recombination occur via the same surface states, as
shown in Scheme 1.5.51, 59, 60 The sluggish hole transfer kinetics at the semiconductor
surface (reported rate constants range from approximately 0.1 to 100 s-1, depending on
applied bias and light intensity51, 60) were reported to result in hole accumulation at the
surface. It has been suggested that these accumulated holes (likely to be surface-bound
water oxidation intermediates such as M-OHx) cause partial Fermi-level pinning.51, 59, 60
PMAS and LMAS have been used to compare the kinetics and intermediates of the
oxygen evolution reaction (OER) on hematite in the dark (PMAS) and in the light (LMAS).53
Very similar spectra of a surface-hole intermediate were obtained, indicating that the
reaction mechanism proceeds via the same intermediate in electrochemical (dark) and
photoelectrochemical water oxidation. The lifetime of this intermediate (consumed by the
Scheme 1.5 Proposed reaction scheme for water
oxidation on hematite photoanodes (adapted from
references 14 and 51).
hν
e-
h+
hole
transfer
recombination
hole flux tosurface
semiconductor electrolyte
Chapter I: Introduction 26
OER or recombination with electrons) was found to be on the order of 50 ms. Under solar
light intensities, this is thought to result in a surface hole concentration on the order of 1012
cm-2, approximately 1% of a monolayer. It was suggested that the hole transfer kinetics are
at least partially limited by the low mobility of surface-trapped holes, and that the diffusion of
holes across the hematite surface is necessary in order to produce high-valent Fe-species to
drive water oxidation.
Frequency-domain analyses have also been employed to elucidate the effects of surface
modifications to hematite photoanodes. For example, EIS was used to investigate why a
thin Al2O3 overlayer results in increased water oxidation activity.55 This overlayer was shown
to reduce the resistance associated with charge transfer at the SCLJ and increase the
capacitance of the space-charge layer. Together with chopped-light photocurrent
measurements, these results indicate that the Al2O3 layer does not act as a catalyst, but
instead reduces recombination by “passivating” surface states.
IMPS has also been used to clarify the effect of Co2+ surface treatment of hematite.51
Such cobalt treatments are thought to result in the deposition of a cobalt oxide/hydroxide
species, which are known to be electrocatalysts.61 Hence the improved photocurrents
obtained from Co-treated hematite photoanodes are generally assumed to be the result of
an increase in the water oxidation kinetics. Cobalt is thought to act as a “hole reservoir” for
the four oxidising equivalents required for the production of each O2 molecule.13, 62 However,
the IMPS study demonstrated that the effect of Co-treatment is to suppress surface electron-
hole recombination; no evidence of accelerated water oxidation kinetics was found.51 This is
discussed further in Chapter VI.
Impedance-based measurements allow sample characterisation under working conditions,
and fitting of the frequency-domain response to an appropriate electrical model can provide
information about charge transport, trapping and transfer at the SCLJ, in addition to the
flatband potential and donor density (from the Mott-Schottky relation). However, such
techniques rely upon monitoring electrical outputs, and therefore cannot directly monitor
water oxidation by the minority charge carriers (holes). Additionally, impedance-based
measurements rely on fitting empirical data to a model equivalent circuit, with the results
being dependent on the model chosen.
Time-resolved surface photo-voltage measurements have recently been employed to
investigate the timescale of charge separation at the surface of a nanostructured hematite in
air.63 The accumulation of holes at the Fe2O3 surface was observed, however, this
technique is in its infancy for use in studying hematite photoanodes, and as such current
studies are extremely limited. This type of measurement has the potential to provide useful
information about the dynamics of hole accumulation and consumption at the semiconductor
surface, when applied to photoelectrodes under working conditions.
27 Chapter II: Materials & Methods
Transient Absorption Spectroscopy
Transient absorption spectroscopy (TAS) is a pump-probe technique which monitors the
change in optical transmission of a sample due to the absorption of light by photogenerated
charge carriers (electrons and holes). Absorption by charge carriers generated by the pump
beam modulates the transmission of the probe beam. The change in absorption of the
sample is thus a measure of the change in concentration (population) of photogenerated
charge carriers as a function of time after the pump pulse (see Section 2.2). Hence TAS
allows – in theory, at least – measurement of the generation, relaxation, trapping,
recombination, reaction etc of photogenerated charge carriers, depending on the timescale
of the measurement. It is thus a potentially extremely useful tool for studying charge carrier
dynamics in semiconductor photoelectrodes. Although TAS is commonly used to study dye-
sensitised solar cells (usually based on TiO2),64 until the publication of the investigations
described in this Thesis, literature reports of TAS of Fe2O3 were limited. A handful of studies
were published, however the timescales probed were in the femto- to nano-second region
and hence unlikely to be relevant to the timescale of water oxidation. Excitation densities
were typically high (thus not representative of solar irradiance) and the effect of electrical
bias had not been investigated. Nevertheless, these studies provide some information about
the charge carrier dynamics in hematite at timescales prior to those employed in these thesis
studies.
Several time-resolved optical studies of UV-excited charge carrier dynamics in Fe2O3
nanocrystal (on the order of 2-20 nm) suspensions have been reported.65-67 Generally,
decays with picosecond lifetimes were found to be independent of pump or probe
wavelength, hematite/maghemite phase, dopant, pH, or surface adsorbates. It should be
noted that changes to the surface environment are unlikely to affect charge carrier dynamics
on such short timescales. Differences in decay kinetics with excitation density and surface
adsorbates were observed by measurements on longer timescales (up to 3.5 μs) and with
greater excitation densities.67 It is often assumed that thermalised electrons in hematite
absorb in the “red” region of the spectrum while holes absorb in the “blue” region, with little
evidence to support this and somewhat arbitrary divisions between red/blue regions.
However, trapped electrons (attributed to FeII) have been shown to absorb broadly across
500-900 nm in a study of electron injection into the γ-Fe2O3 CB by pulse radiolysis; no
evidence of free CB electrons was observed.68
Transient optical measurements on femto- and pico-second timescales have also been
made of 100 nm thick hematite films on α-Al2O3/α-Cr2O3 substrates, using 407 nm excitation
to avoid absorption by the substrate.69 Differences in initial decay kinetics probed in the blue
(<560 nm) and red regions of the spectrum were attributed to hot hole and electron
relaxation, respectively. Hot CB electron relaxation to the band edge was found to occur on
Chapter I: Introduction 28
timescales of hundreds of femtoseconds. Rapid electron-hole recombination and electron
trapping occurred within 5 ps. The lifetime of trapped electrons, probed at 560 and 680 nm,
was reported to be on the order of hundreds of picoseconds to nanoseconds. These results
are in contrast to those reported for nanocrystals of hematite, where trapped electrons,
probed at similar wavelengths (650-850 nm), were found to recombine on a timescale of
tens of picoseconds with no trapped charges observable beyond 100 ps.65 This extremely
rapid recombination was attributed to non-radiative recombination facilitated by a high
density of intrinsic trap states. The very different results of these two studies could be due to
stronger electron-phonon coupling in the nanocrystals,69 although differences in excitation
density may also be responsible.
Charge carrier dynamics in hematite and in α-Fe2O3/α-Cr2O3 core-shell nanoparticles
have been compared using femtosecond TAS.70 Since the CB and VB in α-Cr2O3 lie above
those in α-Fe2O3, photogenerated holes are expected to accumulate in α-Cr2O3, while
electrons should accumulate in α-Fe2O3. This improved charge separation should result in
reduced charge carrier recombination and thus longer transient lifetimes. However, very
little difference in ps decay kinetics was observed for the two systems. Similarly to the α-
Fe2O3/α-Cr2O3 films,69 electron relaxation occurred within 5 ps, and trapping on a timescale
of ~10 ps. This lack of evidence for charge separation at the interface was attributed to fast
recombination and trapping, resulting in very short free charge-carrier lifetimes.70 However,
if charge transport in these materials occurs via a “trap-detrap” mechanism (as in TiO2
nanoparticles64) rather than by free carriers, it is unlikely that such short-timescale
measurements would be able to probe differences in charge separation.
Since these thesis studies began, two further ultrafast TAS studies of hematite for water
oxidation have been reported. Hematite nanorods with a thin coating of WO3 nanoparticles
have been shown to produce greater photocurrent densities than bare hematite nanorods.43
Transient absorption decay dynamics of the bare and WO3-coated nanorods were very
similar on <5 ps timescales. The decay lifetime on 10-100 ps timescales was slightly faster
for the heterojunction when probed at 580 nm, but identical decay dynamics were observed
when probing at 675 nm. These results were interpreted as evidence that WO3 promotes
the extraction of holes from Fe2O3, although it is unclear why no change in decay dynamics
is observed at 675 nm.
Picosecond charge carrier dynamics in Sn-doped hematite nanowires with different
dopant concentrations and morphologies have been compared.39 Although varying these
factors resulted in significant differences in water oxidation efficiency, transient decay
dynamics were essentially unchanged; decay lifetimes were similar to those reported for
hematite nanoparticles.65, 66 These results indicate that picosecond timescale
29 Chapter II: Materials & Methods
measurements may not provide useful information about charge carrier dynamics pertinent
to water oxidation.
Prior to the studies described in this thesis, time-resolved optical measurements of
hematite were restricted to sub-microsecond timescales. As described above, these studies
have provided information about the timescales of hot electron relaxation to the CB edge
(hundreds of femtoseconds), and rapid electron-hole recombination and trapping (5-10 ps).
Generally charge transfer to adsorbates or heterojunction materials is not observable on
these sub-microsecond timescales.
Whilst no studies on microsecond or longer timescales had been reported for hematite,
transient absorption studies of water oxidation on nanostructured TiO2 had been published.
Charge carrier dynamics in a nanoporous TiO2 film have been investigated by using
chemical hole (methanol) and electron scavengers (Ag+ or Pt).71 In an argon atmosphere,
the transient absorption decay kinetics of photogenerated electrons and holes (probed at
800 and 460 nm, respectively) were found to be identical, indicating the absence of
quenching mechanisms other than electron-hole recombination. At low excitation densities,
the lifetime of the power-law decay decreased with increasing excitation intensity, according
to the trap-detrap model of electron transport64 (discussed further in Chapter III). In an
aqueous environment (neutral pH), the hole signal was found to decay with a half-life of 0.27
s, indicating that water oxidation occurs on a timescale of hundreds of milliseconds at neutral
pH.
1.4 Project objectives
As discussed in the previous section, although hematite has attracted considerable
research interest as a photoanode material for solar water oxidation, prior to these thesis
studies the mechanism and kinetics of water photo-oxidation on hematite were poorly
understood. The purpose of this project was to investigate the physical processes which
limit water photo-oxidation efficiencies of hematite photoanodes. Transient absorption
spectroscopy (TAS) is used to monitor the photogenerated charge-carrier dynamics in
hematite on the microsecond-seconds timescale. The timescale of these measurements is
likely to be pertinent to the timescale of water oxidation, and is significantly longer than those
of time-resolved optical studies previously reported in the literature. In conjunction with
measurements of the photocurrent response, these studies allow the comparison of charge-
carrier dynamics with water oxidation activity. Specifically, the objective of these
investigations was to explain why some types of hematite photoanode exhibit greater water
photo-oxidation activity than others, i.e. to develop structure-function relationships.
Chapter I: Introduction 30
This objective can be broken down into a series of aims:
1) Identification of the transient absorption signal associated with
photogenerated holes. The decay kinetics of photogenerated holes are of greatest
interest, since the holes are responsible for water oxidation. In order to determine
the timescale of water oxidation, it is necessary to find an appropriate probe
wavelength to probe holes (preferably one where the hole signal is not overlapped
with the electron signal), which - on the timescale of these measurements - are likely
to be trapped rather than free carriers. This information is not available from the
literature, since previous studies were conducted on significantly shorter timescales,
potentially prior to hole relaxation/trapping.
2) Determination of the timescale of water oxidation on hematite. Previous
electrochemical studies of hematite photoanodes have indicated that the faradaic
rate constant for water oxidation is low. Photogenerated holes can be directly
monitored with TAS, allowing the timescales of hole recombination and transfer to the
electrolyte to be determined. The competition between the oxygen evolution reaction
and electron-hole recombination/electron-oxidation intermediate recombination is
likely to be one of the major factors limiting water oxidation efficiency.
3) Elucidation of the role of positive applied bias. An anodic applied bias is
necessary for water oxidation to occur on hematite, since the CB edge lies below the
H+/H2 redox potential. However, changing the Fermi level is also likely to affect
electron transport and electron-hole recombination. Additionally, changing the
applied bias may change the rate of water oxidation.
4) Comparison of charge carrier dynamics in various hematite photoanodes with
different morphologies, dopants etc. The relative timescales of electron
extraction, recombination and water oxidation will influence the water oxidation
efficiency of a given photoanode. TAS is used in conjunction with
photoelectrochemical measurements to investigate how differences in
nanomorphology and doping affect the relative timescales of these processes.
5) Investigation of the effect of a water oxidation catalyst on the charge carrier
dynamics and kinetics of water oxidation. Deposition of electrochemical water
oxidation catalysts on the semiconductor surface has been shown to markedly
improve the photocurrent/voltage characteristics of hematite photoanodes. However,
the mechanism of this improvement is unclear. Photogenerated hole kinetics are
monitored with TAS in order to determine whether hole transfer to the catalyst
occurs, and/or the timescale of water oxidation is reduced, or whether the catalyst
increases water oxidation efficiency by some other mechanism.
31 Chapter II: Materials & Methods
Although these studies are centred upon hematite photoanodes for water oxidation, the
techniques and methodologies outlined herein are transferrable to other semiconductor
photoelectrodes. The structure-function relationships summarised in Chapter VIII are also
likely to be applicable to many photoelectrode materials other than hematite.
Chapter II: Materials & Methods 32
Chapter II
Materials & Methods
In this section, details of the various hematite photoanodes investigated in the studies
reported in this thesis are given. The main measurement techniques used to examine these
photoanodes – photoelectrochemistry (PEC), transient absorption spectroscopy (TAS), and
transient photocurrent (TPC) – are also described.
33 Chapter II: Materials & Methods
2.1 Materials: Fe2O3 photoanodes
Most hematite photoanodes described in this section were deposited on fluorine-doped tin
oxide (FTO) glass substrates, apart from PLD films, which were deposited on indium-doped
tin oxide (ITO) glass.
2.1.1 Undoped and Si-doped APCVD hematite
Nanostructured hematite photoanodes deposited by atmospheric pressure chemical
vapour deposition (APCVD) were supplied by Michael Grätzel’s research group at EPFL.
The preparation method for the undoped and Si-doped APCVD photoanodes has been
reported in detail in the literature,13 but is described briefly here. Precursor solutions of
Fe(CO)5 and tetraethoxysilane (TEOS) are bubbled with Ar gas to create two vapour
streams, which are mixed with air and directed vertically onto an FTO glass substrate heated
to 450 °C. This forms an approximately circular spot of hematite which is thickest in the
centre (ca. 500 nm). The concentration of Si in the doped films is ca. 1.5%. Undoped
hematite is deposited from Fe(CO)5 alone. This deposition method produces nanoporous,
dendritic “cauliflower-like” nanostructured photoanodes, with a roughness factor of ~20
(Figure 1.1).
2.1.2 Undoped and doped USP hematite
Hematite photoanodes deposited by ultrasonic spray pyrolysis (USP) have a platelet-like
mesoporous structure ca. 200m thick, consisting of “leaflets” aligned perpendicular to the
FTO substrate, 5-10 nm thick and 50-100 nm in length (Figure 1.2). The USP preparation
method has been reported in detail in the literature,12 but is described briefly here. Small
droplets of the Fe(III) acetylacetonate precursor solution are generated by pumping the
solution through an ultrasonic spray nozzle. Compressed air is used to carry the precursor
droplets to a tubular oven, where they are oxidised on the surface of the heated substrate.
Nb-doping is achieved by adding 0.5% niobium ethoxide to the precursor solution. Si-doping
Fig 1.1 SEM images of Si-doped (A and B)
and undoped (C) APCVD hematite
photoanodes. A: side-view; B and C: top-down
view. From reference 13.
Chapter II: Materials & Methods 34
is achieved in a similar way. These photoanodes were made at EPFL in Michael Grätzel’s
lab, and were supplied by Dr Monica Barroso.
2.1.3 Thick solid PLD hematite
Solid (dense) hematite films approximately 600 nm thick were deposited by pulsed laser
deposition (PLD) onto ITO glass substrates heated to 550 °C. A sintered pressed-hematite
disc is used as the target, which is placed in an O2-filled chamber and irradiated with a 355
nm laser (65 mJ.pulse-1, 10 kHz). After deposition, photoanodes are sintered for 30 minutes
by maintaining the substrate temperature.72 These photoanodes were supplied by Prof
Jinhua Ye, NIMS, Japan.
2.1.4 Thin solid ALD hematite
Atomic layer deposition of hematite results in thin solid (dense) films. The thickness of the
films is carefully controlled since this method results in the deposition of a conformal layer of
hematite per ALD cycle (Figure 1.3). Nitrogen carrier gas is used to transport the ozone
and ferrocene precursors to a heated chamber where hematite is deposited on the heated
FTO glass substrate. After deposition, photoanodes are sintered at 500 °C for 30 minutes.14
These photoanodes were supplied by Prof Thomas Hamman and Ben Klahr, Michigan State
University.
Fig 1.2 Top-down SEM images of undoped (left)
and Nb-doped (right) USP hematite photoanodes.
Images courtesy of Monica Barroso.
Fig 1.3 Top-down (c) and side-view (d) SEM
images of undoped ALD hematite photoanodes.
From reference 14.
35 Chapter II: Materials & Methods
2.1.5 Porous microwave heated hematite
These photoanodes are prepared by dissolving iron nitrate and urea precursors in de-
ionised water in an autoclave at room temperature, then heating in a microwave oven at 200
°C for 20 minutes with a piece of clean FTO glass in the solution, resulting in the deposition
of red Fe2O3 powder onto the FTO. Photoanodes are then sintered at
500 °C for 30 minutes. The deposited MH iron oxide film consists of a
porous network of roughly spherical nanoparticles ca. 100-300 nm in
diameter (Figure 1.5). These photoanodes were provided by Dr
Junwang Tang, UCL.
2.1.6 Thick solid SP Si-doped hematite
Relatively dense films of Si-doped hematite approximately 1 μm thick are deposited by
spray pyrolysis from a precursor solution of 50 mM Fe(III) acetylacetonate and 1 mM
tetraethoxysilane (TEOS; Si4+ source). This technique is similar to that described in Section
2.1.2 above, but without the use of an ultrasonic spray nozzle. These photoanodes were
provided by Dr Steven Dennison and Chin Kin Ong, Chemical Engineering Department,
Imperial College.
2.1.7 Colloidal Ti-doped hematite
Porous Ti-doped hematite photoanodes consisting of a nanoporous network of 30-40 nm
particles are produced by doctor blading a colloidal film of hematite nanoparticles with a Ti4+
source (titanium isopropoxide at 5 atom % with respect to iron) onto FTO glass. This is first
annealed at 500 °C to remove organics and sinter the nanoparticles, then coated with a thin
layer of mesoporous silica as a confinement scaffold to encapsulate the nanoparticles, and
annealed briefly at 800 °C to activate the hematite. The confinement scaffold is removed
after annealing.73 These photoanodes were also provided by
Michael Grätzel’s research group at EPFL.
Fig 1.4 Top-down SEM image of undoped MH hematite photoanode.
Image courtesy of Junwang Tang.
Fig 1.5 Top-down SEM image of colloidal Ti-doped hematite
photoanode. Insert: before encapsulation. Image from
reference 73.
Chapter II: Materials & Methods 36
2.2 Methods
Hematite photoanodes were typically heat-treated before PEC, TAS or TPC
measurement, except during Co-treatment studies since heat-treatment of the Co-treated
films caused a significant and irreversible reduction in photocurrent. Photoanodes were
heated at 400 °C for ca 30 minutes in order to remove contaminants on the Fe2O3 surface.
2.2.1 PEC
A three-electrode configuration in a home-made PTFE cell with quartz windows with an
Autolab potentiostat (PGSTAT12) controlled by Nova v1.6 software was used for
current/voltage (i/V) and chronoamperometry (including IPCE) measurements. Electrolyte
solutions of 0.1 M NaOH, typically pH 12.8, were prepared from NaOH (reagent grade, used
as received from Sigma-Aldrich) and Milli-Q-water (Millipore Corp., 18.2 MΩ cm at 25 °C).
Initially, electrolyte solutions were de-aerated for ca 30 minutes prior to measurement using
nitrogen gas (BOC) however no significant differences were observed between
measurements before and after de-aeration. In later studies, the electrolyte was not de-
aerated prior to measurements. A Pt-gauze counter electrode was used, while the
Ag/AgCl/3 M NaCl reference electrode (Bioanalytical Systems Inc.) was protected from
degradation by the alkaline electrolyte by the use of a home-made double junction
configuration with a 0.5 M NaClO4 junction electrolyte (pH ~6) and a 3 Å molecular sieve
porous junction frit. Potentials are primarily reported versus the Ag/AgCl/3 M NaCl (referred
to as “Ag/AgCl”) reference electrode; these were converted to those versus the reversible
hydrogen electrode (RHE) using the Nernst equation: ERHE = E°Ag/AgCl + EAg/AgCl + 0.059pH,
where E°Ag/AgCl is the standard potential of the Ag/AgCl reference (ca. 0.21 VRHE at 25 °C),
and EAg/AgCl is the potential versus Ag/AgCl.
Scheme 2.1 Photoelectrochemical system and three-electrode cell used for current/voltage,
chronoamperometry and IPCE measurements. See text for details.
potentiostat
monochromator
photoelectrode(WE)
RE CE
Xe lamp
37 Chapter II: Materials & Methods
A 75 W ozone-free Xe lamp (Hamamatsu Photonics) was used as the light source, either
monochromated (Optical Building Blocks Corp.) or “open” (0 nm). The lamp output was
adjusted with neutral density filters such that the white light incident on the cell was
equivalent to ~1 Sun in intensity, although there are some differences between the spectral
distributions of the Xe lamp and the solar spectrum.6 The illuminated area of the Fe2O3
working electrodes was typically 0.12-0.25 cm2; approximately the same part of the Fe2O3
film was illuminated as probed during TAS/TPC measurements. For EE (“electrolyte-
electrode”, i.e. front-side illumination) measurements, a piece of blank substrate was used
as a filter to ensure that the same light intensity was incident on the Fe2O3 for both EE and
SE (“substrate-electrode”, i.e. back-side illumination) measurements. Neutral density filters
were used to modify the output of the lamp for light intensity studies. The cell was allowed to
equilibrate for 10-30 minutes before measurements. A diagram of the system used for PEC
measurements is shown in Scheme 2.1, and example i/V curves of a hematite photoanode
in the dark and under illumination are shown in Figure 2.1.
IPCE (incident photon to current conversion efficiencies) were calculated from steady-
state photocurrent measurements under monochromatic light. For each wavelength, the
dark current was allowed to stabilise before the photoanode was illuminated (typically
around 60 s). The stabilised steady-state photocurrent (typically 60-120 s after illumination
began) was used for IPCE calculations. The absolute light intensity of the incident light was
measured using a calibrated photodiode (Thorlabs S120 UV-sensor). IPCE values were
calculated as a function of wavelength using Equation 2.1, where h is Planck’s constant
(6.626x10-34 m2.kg.s-1), c is the speed of light in a vacuum (2.998x108 m.s-1), e is the
Fig 2.1 Example of current/voltage curve of a hematite photoanode in the dark (dark grey) and
under white light illumination (light grey). The dark current onset potential is ~0.65 VAg/AgCl, while
the photocurrent onset potential is ~0 VAg/AgCl. Nanostructured Si-doped CVD hematite
photoanode under EE (“front-side”) illumination at approximately 1 Sun intensity, in 0.1 M NaOH.
-0.2 0.0 0.2 0.4 0.6
0
1
2
3
4
cu
rre
nt
den
sity /
mA
.cm
-2
bias / V vs Ag/AgCl
white light
dark
Chapter II: Materials & Methods 38
electronic charge (1.602x10-19 C), F is the incident photon flux at wavelength λ (nm), I is the
intensity (mW.cm-2) of incident light, and iph is the photocurrent density (mA.cm-2). The
intensity of the incident light was corrected for absorption by the quartz window of the PEC
cell.
(2.1)
2.2.2 TAS
Transient absorption spectroscopy is a pump-probe technique which monitors the change
in optical transmission of a sample due to the absorption of light by photogenerated charge
carriers (electrons and holes). A short, relatively intense pulse of light (the “pump”, typically
from a laser) is used to excite electrons across the band gap of the semiconductor sample.
The transmission of a second, weaker “probe” beam (typically a tungsten or xenon lamp,
often monochromated) through the sample is monitored by a photodiode detector linked via
an oscilloscope and/or data acquisition (DAQ) card to a computer. Absorption by charge
carriers generated by the pump beam modulates the transmission of the probe beam. The
change in absorption (ΔOD) of the sample is thus a measure of the change in concentration
(population) of photogenerated charge carriers as a function of time after the pump pulse.
Hence transient absorption spectroscopy allows – in theory, at least – the generation,
relaxation, trapping, recombination, reaction etc of photogenerated charge carriers to be
monitored, depending on the timescale of the measurement.
The change in optical density (absorption) of a sample is determined by measuring the
current generated by the photodiode as a function of time, which is proportional to the
amount of light transmitted through the sample:
(2.2)
where V is the voltage measured by the photodiode and T is the transmission of the
sample at time t after excitation by the laser pulse. Since T = 10-A (where A is the absorption
of the sample):
(2.3)
For very small value of ΔOD (the difference in absorption between the ground and excited
states), we can approximate this to
(2.4)
39 Chapter II: Materials & Methods
Hence
(2.5)
Two different TAS systems were used to acquire the data discussed in this Thesis. A
general schematic of the TAS systems is shown in Scheme 2.2. The first is a microsecond-
millisecond system employing a nitrogen laser (GL-3300, Photon Technology International
Corp.; 337 nm, 1 ns pulse width) and 100 W tungsten lamp (Bentham 1IL) as the pump and
probe, respectively. The pump illumination was directed to the sample via a liquid light
guide. Wavelength selection was achieved using monochromators before and after the
sample. The photocurrent from the detectors was processed by an AC-coupled pre-amplifier
to extract the transient signals, which were magnified by a home-built amplifier-filter system.
The signals were recorded with a digital oscilloscope (Tektronix TDS220) and transferred to
a computer for analysis. The DC offset of the photocurrent from the detector was subtracted
using the pre-amplifier, thus small absorbance changes (<10-5) could be measured. The
output from the light guide was measured using a Coherent energy meter; the intensity of
the pump illumination on the sample was modified by placing neutral density filters placed
between the light guide and the sample.
The second TAS system was used for studies of hematite photoanodes in a complete
photoelectrochemical (PEC) cell on microsecond-second timescales. Bandgap excitation
was achieved using the third harmonic of a Nd:YAG laser (Surelite I-10, Continuum; 355nm,
6 ns pulse duration) transmitted through a liquid light guide. The output from the light guide
was measured using a Newport power meter; and was varied using an iris before the light
guide and by adjusting the Q-switch timing on the laser. A 75 W Xe lamp (Hamamatsu
Photonics) was employed as the probe beam, with monochromators before and after the
sample. In order to measure on the seconds timescale, a home-built feedback loop was
used to correct the variation in the Xe lamp output at these timescales. A home-built pre-
Scheme 2.2 General schematic of the transient absorption systems employed - see text for details.
probeXe lamp
quartz cuvette/ PEC cell
monochromator monochromator detector
oscilloscope/ DAQ card
computer
1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.00
0.25
0.50
0.75
1.00
1.25
m
OD
time / s
Chapter II: Materials & Methods 40
amplifier was used to process the photocurrent from the silicon PIN photodiode detector
(Hamamatsu Photonics). The microsecond timescale signal was recorded with a digital
oscilloscope (Tektronix TDS220) and the millisecond-second timescale signal was
processed by a DAQ card (National instruments, NI-6221). A three-electrode cell controlled
using a Ministat 251 (Thompson Electrochemical) was used for measurements with bias
applied to the photoanode. A borosilicate glass cell was used with the same reference and
counter electrode configuration as described above; the reference electrode was masked to
prevent damage by UV-irradiation. The i/V curves in the dark and light were checked
immediately before TAS measurements commenced.
Laser intensities were relatively low (typically ca. 200 μJ.cm-2 incident on the cell, except
for excitation intensity studies) in order to be more comparable to solar irradiation. Laser
repetition rates (0.25-2.0 Hz) were chosen such that the transient absorption signal decayed
to zero before the next laser pulse. Data were collected on the timescale of 1 μs to 2s. TA
decays were measured by averaging over 300-1000 laser pulses, allowing signals on the
order of 10-5 ΔOD to be acquired.
2.2.3 TPC
Transient photocurrent (TPC) measurements were made using the same microsecond-
second TAS system and three-electrode cell as described above, but with the probe beam
blocked. An oscilloscope probe (Tektronix TekP6139A) and digital oscilloscope (Tektronix
TDS220) were used to measure the voltage drop (as a function of time after each laser shot)
across an external measurement resistor (typically 47 Ω) in series with the counter electrode.
In order to acquire the full rise and decay of the signal with good time resolution, each TPC
curve was compiled of 2-3 measurements at different timescales. Each timescale was
measured by averaging over 100-750 laser pulses, such that the conditions used were
comparable to those for TAS. The raw data were converted from volts to current using
Ohm’s Law to give photocurrent as a function of time after the laser pulse.
It should be noted that the time constants associated with both the photocurrent rise and
decay (at early timescales) are likely to be limited by the measurements resistor. The time
constant of the photocurrent decay is given by
(2.6)
where CSCL is the capacitance of the space-charge layer, and the bracketed terms give the
total resistance of the system, typically consisting of the resistance of the measurement
resistor, the electrolyte and the semiconductor. The early timescale (e.g. <100 μs)
photocurrent signal may also be limited by the potentiostat response.
TPC measurements of thin hematite photoanodes employing measurement resistors from
100Ω to 1 kΩ are shown in Figure 2.2. It is clear that although the TPC response at
41 Chapter II: Materials & Methods
timescales <30 μs is dependent on the resistance of the measurement resistor, the
photocurrent decays are independent of Rmeasure.
These results suggest that the TPC decay lifetime on timescales greater than a few tens
of microseconds is limited by the resistance(s) associated with the hematite photoanode (i.e.
not RC-limited by Rmeasure). The measurement resistors employed in these thesis studies are
47 to 220 Ω. These resistances are significantly smaller than those reported for hematite
photoanodes from impedance studies. Klahr et al have reported the total resistivities (due to
photoanode and cell) for 60 nm thick ALD hematite photoanodes in pH 13.3 KOH, which
correspond to resistances of approximately 2x105 to 6x103 Ω at applied potentials of 0 to 0.6
VAg/AgCl. These values were obtained under one sun intensity illumination; resistivities
increased with decreasing light intensity.59 Le Formal et al employed impedance
spectroscopy to determine resistances and capacitances associated with the semiconductor
“bulk” and charge transfer at the SCLJ for APCVD Si-doped hematite photoanodes in pH
13.6 NaOH.55 The approximate capacitance and resistance values reported are tabulated
together with the RC time constants calculated from these in Table 2.1. Again, the
resistances associated with the photoanode are significantly larger than those of Rmeasure
employed in these thesis studies.
Potential
/ V vs RHE RSC / Ω CSC / F τSC / s RCT / Ω CCT / F τCT / s
0.8 103 2.5x10-4 0.25 105 2.5x10-4 2.5
1.4 103 1.5x10-4 0.15 105 1.5x10-4 1.5
1E-7 1E-6 1E-5 1E-4 1E-3 0.01
0.0
0.2
0.4
0.6
0.8
1.0
1.2cu
rre
nt
/ m
A
time / s
100
1 k
+0.2 VAg/AgCl
1E-7 1E-6 1E-5 1E-4 1E-3 0.01
0.0
0.5
1.0
1.5
2.0
2.5
cu
rre
nt / m
A
time / s
100
220
1 k
+0.6 VAg/AgCl
Fig 2.2: Transient photocurrent (TPC) decays of a 57 nm thick ALD hematite photoanode, with
various measurement resistors: 100 Ω (black), 220 Ω (blue), and 1 kΩ (red) at 0.2 and 0.6 VAg/AgCl in
0.1 M NaOH (pH ~12.8). Pulsed EE excitation (355 nm, 0.20 mJ.cm-2
, 0.25 Hz).
Table 2.1: Approximate reported resistances (R) and capacitances (C) of the semiconductor (SC
subscript) and charge-transfer at the SCLJ (CT subscript) from impedance measurements of Si-
doped hematite APCVD photoanodes (from reference 55), with calculated time constants (τ).
Chapter III: Identification of Photogenerated Hole Absorption 42
Chapter III
Identification of
Photogenerated Hole Absorption
in Hematite Photoanodes
In which the initial results of transient absorption studies of hematite on the microsecond
to second timescale are discussed. The effect of various chemical scavengers on an
isolated hematite film, and the effect of applied bias on a hematite photoanode in a complete
photoelectrochemical cell are investigated. This allows the identification of the transient
absorption signals due to the long-lived holes that are responsible for water oxidation.
43 Chapter III: Identification of Photogenerated Hole Absorption
3.1 Introduction
As previously discussed in detail in Section 1.3.1, nanocrystalline, mesoporous hematite
(α-Fe2O3) films are considered some of the most promising candidates for the photoanode
material of a photoelectrochemical (PEC) water oxidation device. However, a positive
electrical bias is required for water oxidation to occur on hematite, generally considered
necessary to increase the reduction potential of electrons for proton reduction. A thorough
understanding of charge carrier dynamics and the effect of applied electrical bias are crucial
to devising strategies for enhancing the performance of such photoanode materials.
Photoelectrochemical and impedance methods are typically based on measurements of
electrons extracted from the photoanode to the external circuit (in the case of n-type
semiconductors, which normally have poor hole conductivities). As such, monitoring the
hole dynamics critical for water oxidation presents a significant challenge. Transient
absorption spectroscopy (TAS) is an alternative technique which allows monitoring of – at
least in principle – the recombination, trapping and reaction of both photogenerated
electrons and holes. As such it is an extremely useful tool for probing the charge carrier
dynamics of photoelectrode materials. Until recently TAS was rarely used to study
photoelectrode materials for PEC cells, thus the dynamics of charge separation, transport
and recombination in hematite were poorly characterised prior to these thesis studies.
Transient absorption studies of water oxidation by TiO2 have previously been conducted
by members of our group. The transient absorption spectra of the photogenerated electrons
and holes in TiO2 were identified using Pt and methanol as electron and hole scavengers
respectively.71 Photogenerated hole absorption peaks at 460 nm, while electron absorption
increases with increasing wavelength up to 1000 nm. On the microsecond to second
timescales measured, the photogenerated charge carriers are expected to reside in trap
states. High excitation densities resulted in more rapid electron-hole recombination and
suppressed quantum yield for oxygen production, consistent with the trap-detrap model of
electron transport.64 Recently, TAS has also been used to study the mechanism of water
oxidation on TiO2 in a complete PEC cell for the first time.74 Water oxidation was found to
occur on a timescale of hundreds of milliseconds under positive electrical bias. Comparison
of the electron collection efficiency – from transient photocurrent measurements – and the
quantum efficiency for water oxidation – from dissolved oxygen measurements – showed
that the conversion efficiency of long-lived holes to O2 is approximately 100 %. This implies
that the dominant loss mechanism in water oxidation on TiO2 is charge carrier
recombination.
This chapter describes the first studies of the competition between charge carrier
recombination and water oxidation on Fe2O3, including the first transient absorption
Chapter III: Identification of Photogenerated Hole Absorption 44
measurements of hematite at timescales greater than nanoseconds, and under applied
electrical bias. Transient absorption spectroscopy was employed in conjunction with
photocurrent/voltage measurements of nanostructured undoped hematite photoanodes.
Relatively low excitation density conditions were employed. Initially, isolated hematite films
(i.e. not connected to a photoelectrochemical cell) were studied by TAS on microsecond-
second timescales, likely to correspond to the timescale of water oxidation. The effect of
various chemical scavengers on the charge carrier dynamics was investigated. This was
followed by TAS and photoelectrochemical measurements of hematite photoanodes in a
three-electrode cell. Evidence for two distinct photogenerated species was found, although
only one species was evident at long (seconds) timescales. Charge carrier dynamics were
found to be strongly dependent on applied bias, while water oxidation was observed to occur
on a timescale of seconds. It is suggested that positive applied bias is necessary to reduce
electron-hole recombination by lowering the background electron density, thus increasing
the lifetime of photogenerated holes such that water photo-oxidation occurs.
3.2 Experimental
Most results discussed in this chapter were obtained using undoped α-Fe2O3
photoanodes, which are prepared by atmospheric pressure chemical vapour deposition
(APCVD) and have a dendritic nanoporous structure.37 Other types of photoanodes studied
include Si-doped APCVD, and undoped and Si-doped hematite deposited by ultrasonic
spray pyrolysis (USP), which have a mesoporous “leaflet” structure.12 Undoped hematite
photoanodes prepared by microwave heating were also used as a comparison, which
consist of a porous assembly of roughly spherical particles with diameters on the order of
100 nm. The UV-vis spectra of some of these photoanodes are shown in Figure 3.1 below.
These show the typical hematite absorption spectrum, with strong absorption in the UV
region (O 2p→Fe 3d) but significantly weaker absorption in the visible (>450 nm), due to the
forbidden nature of this d-d transition.30 The absorption edge is around 600 nm; apparent
absorption at longer wavelengths indicates light scattering and/or the presence of mid-
bandgap states. This is particularly notable for the microwave deposited hematite
photoanodes.
Transient absorption measurements on the µs-s time scale were obtained using band-gap
excitation at 337 nm or 355 nm (~0.2 mJ.cm-2 after absorption by cell, 0.33-2.0 Hz), and a
monochromatic probe beam, as described in detail in the Methods section. Dark- and photo-
current/voltage curves were recorded prior to TAS measurements with applied bias, with a
150 W ozone-free Xe lamp light source (approximately equivalent to 1 Sun intensity,
45 Chapter III: Identification of Photogenerated Hole Absorption
although there are some differences between the spectral output of the lamp and that of the
solar spectrum6).
IPCE measurements were made as described in detail in the Methods section. Hematite
photoanodes were illuminated from the SE side (i.e. through the substrate) in a three-
electrode configuration, with 0.1M NaOH (pH ~12.8) electrolyte degassed with nitrogen.
Steady-state photocurrents under monochromatic illumination were measured at +0.4
VAg/AgCl (~1.35 VRHE), which is significantly anodic of the photocurrent onset potential.
Fig 3.1 UV-vis spectra of various hematite photoanodes employed in this study: undoped and Si-
doped nanostructured hematite deposited by atmospheric pressure chemical vapour deposition
(CVD; blue lines); mesoporous undoped hematite deposited by ultrasonic spray pyrolysis (USP,
red line) and by microwave heating (MH, green line). No correction has been made for reflection.
400 500 600 700 800 9000.0
0.5
1.0
1.5
2.0
abs /
a.u
.
wavelength / nm
CVD Fe2O
3
CVD Si-Fe2O
3
USP Fe2O
3
MH Fe2O
3
Chapter III: Identification of Photogenerated Hole Absorption 46
3.3 Charge carrier dynamics of isolated hematite films
The transient absorption (TA) spectrum of undoped nanoporous hematite films measured
in an argon atmosphere is shown in Figure 3.2. The TA spectrum is characterised by a long-
lived (µs-ms) absorption peak at ~575 nm and a broad tail that extends from ~650 nm to the
near IR. Thus the spectrum is clearly divided into two regions either side of 650 nm, which is
indicative of two distinct species. The transient species probed at ~575 nm are also
observed to decay faster than transients probed at wavelengths greater than 625 nm,
providing further evidence for the existence of two distinct species, as discussed further
below. Charge carrier trapping in Fe2O3 has been reported to occur on a picosecond
timescale.65, 69 The spectral features observed here (>1 μs timescales) are likely to
correspond to trapped holes and/or electrons, rather than free carriers. Typical transient
decays in the absence of applied bias (Figures 3.3a) generally show dispersive power-law-
like decay dynamics, as evidenced by the almost linear decays when shown on log-log axes.
Such power law decay kinetics are typical of bimolecular recombination in the presence of
charge trapping, as has been reported previously for nanocrystalline TiO2 films.71, 74 This
behaviour is described by the trap-detrap model, which invokes dispersive electron transport
through a disordered semiconductor.64 In this model, charge carriers become trapped in
localised states, such that charge transport occurs via the thermal excitation of carriers from
these trap states (carriers do not experience long-range forces). Thus the kinetics of charge
transport are dominated by the time constants of release from these localised states. This
Fig 3.2 Transient absorption (TA) spectra of undoped CVD hematite in an argon atmosphere and
inset: in a methanol-saturated argon atmosphere, using 337 nm SE excitation (0.20 mJ.cm-2
at
Fe2O3 surface). Spectra shown were measured 5 and 80 μs after the laser pulse.
600 700 800 900 10000.0
0.2
0.4
0.6
0.8
mO
D
wavelength / nm
5 s Ar
80 s Ar
600 700 800 900 10000.0
0.2
0.4
0.6
0.8
5 s Ar-MeOH
80 s Ar-MeOH
47 Chapter III: Identification of Photogenerated Hole Absorption
has been successfully modelled by a “continuous time random walk” (CTRW), in which
carriers diffuse with a step time taken from a power-law waiting-time distribution. This model
can be applied to the kinetics of scavenging and bimolecular recombination in
semiconductors. This results in the decay of electrons and holes according to Equation 3.1
at the timescales considered herein, where 0 < α < 1.
(3.1)
Fig 3.3 (a) Transient absorption decays of three different hematite films in an argon atmosphere;
inset: the same transient decays normalised and shown on log-log axes, exhibiting power-law-
like decay kinetics. TA decays probed at 600nm with 337 nm excitation (~0.2 mJ.cm-2
). (b)
IPCE spectra of the same hematite films in a three-electrode cell with 0.1 M NaOH electrolyte
(pH ~12.8), at 0.4 VAg/AgCl and under SE illumination. CVD: dendritic nanostructured Fe2O3
(undoped); USP: mesoporous “platelet” Fe2O3; MH: mesoporous Fe2O3 deposited by microwave
heating.
1E-6 1E-5 1E-4 1E-30.0
0.1
0.2
0.3
0.4
0.5
0.6 CVD
USP
MH
mO
D
time / s
(a)
1E-6 1E-5 1E-4 1E-30.01
0.1
1
350 400 450 500 550 600
0
1
2
3
4
5
6
7
CVD
USP
MH
SE
IP
CE
(%
)
wavelength / nm
(b)
Chapter III: Identification of Photogenerated Hole Absorption 48
Fitting a power law function to the TA decays of charge carriers in isolated Fe2O3 films,
such as those in Figure 3.3a, gives α in the range 0.42-0.50. This is consistent with
bimolecular recombination as described by the CTRW model. Although the TA decays are
well described by a power-law function over microsecond timescales, at there is some
divergence after ~500 μs. This suggests that the CTRW model does not fully describe
charge carrier recombination in hematite, particularly at long timescales. The power-law
waiting time distribution is thought to arise from the thermal activation of an exponential
distribution of trap states.64 Some deviation from this model may occur if the distribution of
trap states in hematite is not exponential. For example, several studies have suggested that
a high density of localised trap states occurs in hematite, resulting in partial Fermi level
pinning.12, 59, 75 Additionally, the CTRW model of bimolecular recombination assumes that
one of the recombining species is stationary while the other is fixed, which may not be a
valid assumption in this case.
Comparison of the transient absorption decays and IPCE (external quantum yield) spectra
shown in Figure 3.3 suggests that there is some correlation between the initial amplitude and
lifetime of the TA decays and the water oxidation efficiency. Although all three types of
photoanode are formed of undoped, porous hematite, there are significant differences in
IPCE and TA decay dynamics. The CVD Fe2O3, which has a dendritic nanostructure, has
the greatest IPCE of the three films, and also has TA decays with the highest initial
amplitude and longest lifetime. The MH Fe2O3, in contrast, has very poor activity for water
oxidation, as evidenced by extremely low IPCE values. MH Fe2O3 has TA decays with an
initial amplitude approximately one quarter that of the CVD film, and also significantly shorter
decay lifetimes. USP Fe2O3 produces IPCE values between those of the CVD and MH
Fe2O3, and while the USP TA signal intensity is significantly greater than that of the MH film,
USP photoanodes exhibit the same faster decay kinetics as the MH hematite. Greater TA
signal intensity and slower decay kinetics are both indicative of slower charge carrier
recombination. These results suggest that the greater efficiency of the CVD photoanodes is
due to slower electron-hole recombination. Although the poorer performance of the USP
and MH photoanodes may in part be due to their slightly lower light absorption, it is likely that
their performance is limited by electron-hole recombination. This is particularly true for MH
Fe2O3, which is produced by a low-temperature method so is likely to be less crystalline than
the CVD and USP photoanodes. Recombination at grain boundaries is also likely to be
more significant in the MH Fe2O3, which consists of a porous assembly of roughly spherical
particles with diameters on the order of 100 nm.
Although various ultrafast TAS studies have previously indicated that recombination
occurring on sub-nanosecond timescales limits the efficiency of Fe2O3 for water oxidation,39,
65, 69, 70 the results discussed above demonstrate that recombination on microsecond-
49 Chapter III: Identification of Photogenerated Hole Absorption
millisecond timescales is also important. Figure 3.4 compares the TA decays of isolated
undoped and Si-doped CVD films. Si-doping is widely reported to improve the efficiency of
hematite photoanodes; this is often attributed to improved electron transport to the back
contact.37 It is evident from Figure 3.4a that, in isolated hematite with no external circuit for
electron extraction, Si-doping does not result in significantly more intense transient
absorption signals. This indicates that approximately the same number of photogenerated
charge carriers remain at 1 μs without and with Si-doping. Indeed, Figure 3.4b shows that
Si-doping actually increases the rate of decay (but only at wavelengths >625 nm), i.e. Si-
doping causes faster charge carrier recombination in isolated hematite. This can be
explained by considering that Si acts as an electron donor, by substituting Fe3+ with Si4+.76
Fig 3.4 Comparison of TA decays of undoped (blue/red) and Si-doped (dark grey) CVD hematite
in an argon atmosphere, probed at (a) 600 nm and (b) 900 nm (EE 180 μJ.cm-2
, 337 nm
excitation). Inset: normalised log-log plots.
1E-6 1E-5 1E-4 1E-3 0.010.0
0.2
0.4
0.6
0.8
m
OD
time / s
Si-Fe2O3 600nm
Fe2O3 600nm
(a)
1E-6 1E-5 1E-4 1E-3
0.1
1
1E-6 1E-5 1E-4 1E-30.00
0.05
0.10
mO
D
time / s
Si-Fe2O3 900nm
Fe2O3 900nm
(b)
1E-6 1E-5 1E-4 1E-3
0.1
1
Chapter III: Identification of Photogenerated Hole Absorption 50
Increasing the background electron density is likely to increase the rate of electron-hole
recombination, thus increasing the decay rate of photoinduced transients. However, in a
complete photoelectrochemical cell, where electrons are extracted to an external circuit, Si-
doping results in significantly larger transient absorption signals, shown in Chapter IV.
There are several indications that two distinct species are probed by transient absorption
spectroscopy in the 550-1000 nm region of the spectrum. The microsecond-millisecond TA
spectrum has a large, sharp peak at ~575 nm, but a broad, less intense absorption
extending from ~625 nm to the NIR. Transients probed at ~575 nm exhibit different
behaviour to those probed at wavelengths greater than 625 nm. In addition to the difference
in effect of Si-doping on decay kinetics, transients probed at ~575 nm generally decay faster
than those probed at long wavelengths. There are also differences in excitation density
behaviour and in the effect of hydrogen peroxide as a chemical scavenger, as discussed
below.
The excitation density dependence of charge carrier dynamics in isolated undoped CVD
photoanodes is shown in Figures 3.5 and 3.6. Again, there is a clear difference in behaviour
between the ~575 nm peak and the broad absorption at wavelengths greater than 625 nm.
In the intensity range studied (27-500 μJ.cm-2), a linear excitation dependency is observed
when probing at 580 nm (Figure 3.6a). However, the number of photogenerated charge
carriers remaining tends towards saturation when probed at 650 and 900 nm (Figure 3.6b).
Additionally, while identical decay kinetics are observed over this excitation intensity range
when probing at 580 nm, higher laser intensities result in marginally faster decay kinetics
when probing at 650 nm (inset Figure 3.6c and d, respectively). A similar, although much
Fig 3.5 TA spectrum (at 5 μs after the laser pulse) of undoped CVD hematite in an argon
atmosphere, at excitation intensities of 200, 100 and 50 μJ.cm-2
(EE 337 nm excitation).
600 700 800 900 10000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
m
OD
wavelength / nm
200 J.cm-2
100 J.cm-2
50 J.cm-2
51 Chapter III: Identification of Photogenerated Hole Absorption
stronger, effect has been reported for nanocrystalline TiO2, which was attributed to faster
electron-hole recombination at higher laser intensities in accordance with the trap-detrap
model.71
The excitation density dependence of the charge carrier dynamics can be partially
interpreted by considering the relative numbers of states available for the two
photogenerated species to occupy. The concentration of the charge carrier (change in
absorption) probed at 580 nm is linearly dependent on the excitation density (laser intensity).
This suggests that there are many states available for this species (photogenerated hole or
electron) to occupy, so the signal does not saturate. Saturation behaviour is, however,
observed when probing at 650-900 nm, suggesting that there are a limited number of states
available for this species to occupy, such that at sufficiently high laser intensities all the
states are occupied.
Fig 3.6 Excitation intensity behaviour of undoped CVD hematite in an argon atmosphere, probed at
580 nm (left), 650 and 900 nm (right). Excitation intensities were varied between 27 and 500 μJ.cm-2
(337 nm SE excitation). The TA amplitude at 1 μs is plotted in (a) probed at 580 nm, and (b) probed
at 600 nm (green triangles) and 900 nm (red rhombuses). The same behaviour is observed at 1-80
μs. The TA decay kinetics are shown in (c) probed at 580 nm and (d) probed at 650 nm (900nm
decays are very similar to those probed at 650 nm); inset: the same decays normalised.
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0
580 nm (1 s)
linear fit
mO
D
laser intensity / mJ.cm-2
(a)
0.0 0.1 0.2 0.3 0.4 0.50.00
0.05
0.10
0.15 650 nm (1 s)
900 nm (1 s)
mO
D
laser intensity / mJ cm-2
(b)
1E-6 1E-5 1E-4 1E-3 0.010.0
0.2
0.4
0.6
0.8
1.0
1E-6 1E-5 1E-4 1E-3 0.010.0
0.5
1.0
m
OD
time / s
500 J.cm-2
330 J.cm-2
160 J.cm-2
100 J.cm-2
50 J.cm-2
27 J.cm-2
(c)
1E-6 1E-5 1E-4 1E-3 0.010.00
0.05
0.10
0.15
1E-6 1E-5 1E-4 1E-3 0.011E-3
0.01
0.1
m
OD
time / s
(d)
Chapter III: Identification of Photogenerated Hole Absorption 52
Chemical scavengers, such as methanol (a hole scavenger) or Ag+ (an electron
scavenger) are commonly employed in order to identify transient absorption signals due to
photogenerated electrons and holes.71 However, the TA decay kinetics of hematite
immersed in argon, 0.1 M NaOH electrolyte, and various hole scavengers – including
methanol and iodide (Figure 3.7), thiocyanate and iso-propanol (not shown) – are almost
identical. The TA spectrum is unchanged in the presence of methanol, as shown in Figure
3.2. The yield and decay dynamics of photogenerated charge carriers monitored by the μs-
ms transient absorption signals at 580 nm and 900nm are essentially insensitive to chemical
scavengers. This behaviour is in stark contrast to that of TiO2, which can photo-oxidise
methanol on a timescale of nanoseconds.71 The differences in decay dynamics between
Fe2O3 and TiO2 is considered in detail in the Discussion section.
A comparison of SE (“back-side”) versus EE (“front-side”) excitation is shown for hematite
in the presence of iodide in the inset of Figure 3.7. Similar decay dynamics are observed in
the presence of other hole scavengers, including 0.1M NaOH, and 0.1M NaOH with added
methanol. In the presence of hole scavengers, the SE and EE decay dynamics are almost
identical, however, at millisecond timescales the EE curve exhibits marginally faster decay
dynamics. This is consistent with the interpretation that under EE illumination charge
carriers are generated closer to the semiconductor-liquid junction than under SE illumination
(the absorption depth of 337 nm light is ~30 nm 15), and thus have a shorter distance to
diffuse to reach the SCLJ. The small degree of scavenging observed on the millisecond
timescale suggests that the TA signal observed at this timescale may be attributed to
Fig 3.7 TA decays (probed at 580 nm; SE 337nm, 190 μJ.cm-2
excitation) of undoped CVD hematite
in an argon atmosphere, in 0.1M NaOH, and with hole scavengers including methanol (~0.75 M in
0.1M NaOH) and iodide (2 mM). Decay dynamics are also essentially identical in the presence of
thiocyanate and iso-propanol. Inset: comparison of SE and EE TA decays of hematite in aqueous KI.
10-6
10-5
10-4
10-3
10-2
0.00
0.05
0.10
0.15
0.20
0.25
m
OD
time / s
Ar(g)
NaOH(aq)
NaOH/CH3OH(aq)
KI(aq)
1E-6 1E-5 1E-4 1E-3 0.01
0.01
0.1
SE KI
EE KI
53 Chapter III: Identification of Photogenerated Hole Absorption
photogenerated holes. Although reproducible, this scavenging effect is almost negligible,
indicating that only an essentially insignificant fraction of photogenerated holes are
scavenged, even under EE excitation (in the absence of applied electrical bias). In an inert
Ar atmosphere, SE and EE decay dynamics are identical (not shown), since under these
conditions no scavenging occurs.
It appears that, although holes at the top of the Fe2O3 valence band (ca. +2 V versus
RHE) are thermodynamically able to oxidise iodide (E0 = +1.35 VRHE),77 thiocyanate (+1.64
VRHE)77 and methanol (+0.02 VRHE),78 in the absence of applied bias, the photogenerated
holes observed do not have a long enough lifetime to oxidise these species. When
evaluating the lack of methanol and water oxidation on the µs-ms timescales by Fe2O3 holes
it is also important to consider the thermodynamics of the one-electron intermediates in
these oxidation reactions which would lead to a considerably lower thermodynamic driving
force.24
The charge carrier dynamics in hematite in the presence of an electron scavenger were
investigated by employing silver ions under weak UV excitation (to minimise deposits of
silver metal onto the hematite surface). No deposition of silver onto the hematite surface
was observed during TA measurements of hematite in the presence of Ag+, in contrast to the
behaviour of TiO2, which turns black due to Ag-deposition after only a few tens of laser
pulses in the presence of silver ions.71 The charge carrier dynamics of the species probed at
580 nm are unchanged in the presence of silver ions, as shown in Figure 3.8. However,
when probing at wavelengths ≥650 nm, the addition of silver ions causes an increase in the
TA decay lifetime and initial signal intensity. These results indicate that photogenerated
holes are most probably probed at ~650-1000 nm, confirmed by studies under positive
applied bias (discussed below).
Fig 3.8 TA decays of an isolated undoped CVD hematite film in water (black) and aqueous AgNO3
solution (2 mM; blue/green), probed at 580 nm (left) and 650 nm (right). Charge carrier dynamics
probed at 900 nm are similar to those probed at 650 nm. EE 337nm, 90 μJ.cm-2
excitation.
1E-6 1E-5 1E-4 1E-3 0.010.00
0.05
0.10
0.15
1E-6 1E-5 1E-4 1E-3m
OD
time / s
Ag+
(aq)
H2O
650 nm
1E-6 1E-5 1E-4 1E-30.0
0.1
0.2
1E-6 1E-5 1E-4 1E-3
m
OD
time / s
Ag+
(aq)
H2O
580 nm
Chapter III: Identification of Photogenerated Hole Absorption 54
Using hydrogen peroxide as a scavenger results in substantial changes to the transient
absorption decay dynamics across the 550-1000 nm spectral range, as shown in Figures 3.9
and 3.10. The spectrum in the presence of H2O2 has several similarities to the spectrum
under positive applied bias (see Chapter IV), including a bleach around 575 nm and
increased long-lived signal at ~625-900 nm. The decay kinetics of signals probed ≥650 nm
are significantly retarded in the presence on H2O2, similar behaviour to decay kinetics in the
presence of Ag+. When probing around 575 nm, decay kinetics are initially identical in water
and H2O2, but at ~10 μs H2O2 causes an abrupt increase in the decay kinetics, followed by
bleaching of the signal.
Fig 3.9 TA spectrum of an isolated USP Si-doped hematite film in aqueous hydrogen peroxide
solution (~0.38 M). EE 337nm, 0.13 mJ.cm-2
excitation.
Fig 3.10 Comparison of TA decays of an isolated USP Si-doped hematite film in water (black) and in
aqueous hydrogen peroxide solution (~0.38 M; blue/red) probed at 580 nm and 900nm (EE 337nm,
0.13 mJ.cm-2
excitation). Hydrogen peroxide causes bleaching of the 580 nm signal on timescales
>10 μs, and significantly increases the amplitude of long-lived signals probed at ≥650 nm.
550 600 650 700 750 800 850 900-0.05
0.00
0.05
0.10
0.15
0.20
mO
D
wavelength / nm
1 s
10 s
100 s
800 s
1E-6 1E-5 1E-4 1E-3 0.01-0.05
0.00
0.05
0.10
0.15
0.20
m
OD
time / s
H2O
2
H2O
580 nm
1E-6 1E-5 1E-4 1E-3 0.01
1E-3
0.01
0.1
1
1E-6 1E-5 1E-4 1E-3 0.01
0.00
0.02
0.04
0.06
0.08
0.10
m
OD
time / s
H2O
2
H2O
900 nm
1E-6 1E-5 1E-4 1E-3 0.010.01
0.1
1
55 Chapter III: Identification of Photogenerated Hole Absorption
Hydrogen peroxide is likely to act as an electron-scavenger via the photo-Fenton reaction,
which has been shown to occur under alkaline conditions.79, 80 The photo-Fenton reaction
occurs when photo-generated Fe(II) species react with hydrogen peroxide to form hydroxide
radicals: Fe(II) + H2O2 → ∙OH + Fe(III). In this case, Fe(II) is equivalent to a photogenerated
electron in the hematite photoanode, likely to be trapped in states below the conduction
band edge. The hydroxyl radical can also be formed by the photolysis of H2O2. The
hydroxyl radical is a strong oxidising agent and as such is also likely to act as an electron
scavenger. However, hydrogen peroxide has previously been employed as an efficient hole
scavenger for hematite photoanodes under applied bias.19 The Fe2O3/H2O2 system is
complex and it is possible that electron scavenging may occur initially, followed by hole
scavenging at long timescales, for example. Different scavenging behaviour may also occur
in isolated hematite films and in photoanodes under applied bias.
Together with the similarity of the TA spectra of hematite in the presence of H2O2 and at
positive applied bias, the change in decay dynamics in the presence of H2O2 at 625-1000 nm
on microsecond-second timescales indicate that these transient measurements are probing
photogenerated holes. It is apparent that the photo-oxidation of H2O2 by these holes occurs
significantly faster than that of other hole scavengers investigated (Figure 3.7), as evidenced
by the significant change in initial signal intensity in the presence of H2O2 (Figure 3.10).
The behaviour probed at ~575 nm is more complex. If this signal were associated with
photogenerated electrons, the transient lifetime and possibly also the initial amplitude would
be expected to decrease in the presence of H2O2, concomitant with the increase in signal
intensity and lifetime observed at longer wavelengths. However, this is not the case. The
initial signal intensity and decay kinetics are essentially unchanged by H2O2, but at >10 μs
the decay kinetics abruptly accelerate and a bleach is observed. This bleach is similar to
that observed at ~575 nm under positive applied bias. The effect of applied bias on the
transient absorption dynamics of hematite photoanodes is now addressed.
3.4 Charge carrier dynamics of hematite under applied bias
In order to investigate the hole kinetics of hematite in a fully-functional PEC cell, a three-
electrode cell with Ag/AgCl/saturated KCl reference electrode, Pt gauze counter electrode
and de-aerated 0.1M NaOH electrolyte with/without ca. 0.2 mM methanol was used. Typical
current/voltage data obtained in the dark and under white light irradiation are shown in
Figure 3.11. As expected, significant photocurrent is only observed with the application of
positive potential to the hematite photoanode, assigned to water photo-oxidation. The
presence of methanol results in a shift in the photocurrent onset potential and significant
enhancement of photocurrent, assigned to methanol photo-oxidation.
Chapter III: Identification of Photogenerated Hole Absorption 56
Figure 3.12 shows the transient absorption decays probed at 580 and 900 nm, collected
using a complete PEC cell under 355 nm pulsed light irradiation, measured under applied
biases versus Ag/AgCl of -0.1 V (corresponding to approximately open circuit) and +0.4 V
(corresponding to significant photocurrent generation). At -0.1 V, the transient absorption
exhibits microsecond decay dynamics, very similar to those observed for isolated films (as
shown in Figure 3.7). An applied positive bias of +0.4 V results in the appearance of a much
longer lived decay phase (Figure 3.12) at both 580 and 900 nm. At early (ca 1 μs-1 ms)
timescales, a bleach (negative signal) is observed under positive applied bias at 580 nm.
This bleach is similar to that observed in the presence of H2O2, and is effectively the
inversion of the strong positive peak observed in the TA spectrum in the absence of applied
bias. This behaviour is assigned herein to the photo-oxidation and reduction of a trap state
located just below the conduction band edge, and is investigated in detail in Chapter VI.
However, at long (1 ms-1 s) timescales, TA decays probed at 580 and 900 nm exhibit very
Fig 3.11 Photocurrent/voltage curves for
CVD undoped hematite under white light
illumination (~1 Sun, SE) in 0.1M NaOH
without (black curve) and with (red curve)
ca. 0.2 mM methanol. The dark current is
negligible in the potential region shown.
Fig 3.12 TA decays of CVD undoped hematite in a three-electrode cell under applied bias,
probed at (a) 580 nm and (b) 900 nm. In 0.1M NaOH under applied bias of -0.1 VAg/AgCl
(blue/orange curve) and +0.4 VAg/AgCl (black curves). SE pulsed (0.33 Hz) 355 nm excitation;
excitation densities are matched to those in Figure 3.7.
-0.2 0.0 0.2 0.4 0.60.00
0.05
0.10
0.15
0.20
with methanol
without methanol
cu
rre
nt d
en
sity / m
A c
m-2
potential / V vs Ag/AgCl
0.0 0.5 1.0 1.5 2.0
0.000
0.005
0.010
0.015
0.020
m
OD
time / s
+0.4 VAg/AgCl
-0.1 VAg/AgCl
(b) 900 nm
1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.02
0.04
0.06
0.08
0.0 0.5 1.0 1.5 2.0
0.00
0.01
0.02
0.03
0.04
0.05
m
OD
time / s
+0.4 VAg/AgCl
-0.1 VAg/AgCl
(a) 580 nm
1E-5 1E-4 1E-3 0.01 0.1 1-0.05
0.00
0.05
0.10
0.15
57 Chapter III: Identification of Photogenerated Hole Absorption
similar behaviour under applied bias, indicating that in fact the same species is probed at
these timescales across the 550-1000 nm region, in contrast to the behaviour observed at
microsecond timescales. The observed increase in lifetime of these signals under applied
positive bias – effectively an electron scavenger – indicates that these long-timescale (>1
ms) TA signals are probing photogenerated holes.
Addition of methanol (a hole scavenger) to the electrolyte solution cathodically shifts the
onset potential and increases the photocurrent density (Figure 3.11). The effect of methanol
on the TA decay probed at 580 nm is shown in Figure 3.13. Under positive applied bias,
addition of methanol to the electrolyte results in a significant reduction in the lifetime of the
TA decay. The slow decay phase probed at 580 nm under +0.4VAg/AgCl can be fitted with a
stretched exponential function with a lifetime of 3±1 s, decreasing to 400±100 ms in
presence of methanol.
The appearance of a long lived transient signal under positive applied bias is strongly
indicative of the formation of long-lived holes which have avoided rapid recombination. The
faster decay of this long-lived signal in the presence of methanol indicates that the transient
lifetime is determined by surface oxidation kinetics, confirming the assignment of this signal
to surface active holes. In this context, the faster kinetics in the presence of methanol are
consistent with the relatively facile oxidation of methanol compared to water. Additional
evidence for this interpretation is given by comparing the TA decays in the presence and
absence of methanol at -0.1 VAg/AgCl (Figure 3.13b), where essentially no photocurrent is
observed. Although addition of methanol results in increased photocurrent at this potential,
the photocurrent density is extremely small (ca. 10 μA.cm-2) even in the presence of the hole
Fig 3.13 TA decays (probed at 580 nm) of undoped nanostructured hematite in a three-electrode
cell under applied bias. (a) In 0.1M NaOH under applied bias of -0.1 VAg/AgCl (blue curve) and +0.4
VAg/AgCl (black curve). Upon the addition of methanol in the positive bias condition (red curve), the
faster decay indicates the more facile oxidation of methanol by photogenerated holes. The decay of
an isolated hematite film (no bias) in 0.1M NaOH (green curve) is similar to that under negative
applied bias. (b) Comparison of TA decays with (red) and without (black) methanol at -0.1VAg/AgCl.
0.0 0.5 1.0 1.5 2.0
0.00
0.01
0.02
0.03
0.04m
OD
time / s
+0.4V
+0.4V with methanol
-0.1V
OC
(a)
0.00 0.25 0.50 0.75 1.00
0.00
0.01
0.02
0.03
m
OD
time / s
NaOH/methanol
NaOH
(b)
Chapter III: Identification of Photogenerated Hole Absorption 58
scavenger. The TA decays at this potential are essentially identical, with almost no long-
lived signal remaining on the seconds timescale.
It is noted that the amplitude of this long lived hole signal is only ~10% of the initial
photoinduced hole signal at 1 μs (Figure 3.7), indicating that even under positive applied
bias, the majority of photogenerated holes still undergo rapid electron-hole recombination on
the microsecond timescale. The application of a positive bias is clearly necessary even for
the oxidation of methanol, since TA decay dynamics are essentially unchanged by the
addition of this hole-scavenger in the absence of applied bias (Figures 3.2 and 3.7). The
chemical identity of these long-lived holes is not currently known, but could be for example
Fe ions bound to surface hydroxyl radicals.
3.5 Discussion
The results reported above indicate the presence of two distinct photogenerated species,
one with a narrow but intense absorption centred around 575 nm, and one that has a broad
absorption from ~625 nm to the near-IR. The bleaching behaviour of the former in the
presence of hydrogen peroxide and at positive potentials suggests that this feature is
associated with the photo-oxidation and -reduction of a trap state located just below the
conduction band edge, and is investigated in detail in Chapter VI. However, at long (1 ms-1
s) timescales under applied bias, decays probed at 580 and 900 nm exhibit very similar
behaviour, indicating that the same species is probed at these timescales across the 550-
1000 nm region. The observed increase in lifetime of these signals under applied positive
bias, and reduction in lifetime on the addition of methanol, indicate that these long-timescale
(>1 ms) signals are probing photogenerated holes. The existence of long-lived (hundreds of
milliseconds to seconds lifetime) holes is apparently necessary not only for water oxidation
to occur, but also for the oxidation of more easily oxidised hole scavengers such as
methanol. In the absence of a positive applied bias, photogenerated holes do not have a
long enough lifetime for oxidation to occur.
These results could be interpreted in terms of rapid bulk recombination, or slow hole
transfer kinetics at the semiconductor-electrolyte junction resulting in recombination of
surface-trapped holes. It is often considered that the low intrinsic faradaic rate constant for
water oxidation on Fe2O3 limits the performance of this material.17 On the other hand, fast
bulk recombination (indicated by the short hole diffusion length16, 17) is thought to prevent
holes generated in the semiconductor bulk from reaching the surface. The observation that a
positive bias is necessary to generate long lived holes capable of driving surface oxidation
reactions is consistent with the perception that rapid recombination may be the key loss
process in hematite photoelectrodes. According to Gerischer’s electric double-layer theory of
59 Chapter III: Identification of Photogenerated Hole Absorption
the semiconductor-electrolyte junction,2 a positive bias will increase the width of the
depletion layer and facilitate removal of electrons. However, it has been suggested that this
band bending model does not apply to the nanoporous structures investigated here.37 In the
latter case, the effect of positive bias can be interpreted as a decrease of the background
electron density throughout the film (Scheme 3.1). Under either interpretation, the overall
effect will be a decrease in electron-hole recombination, increasing the yield of longer-lived
holes, as observed in this work. This provides an additional explanation for the requirement
of applied positive bias for water photo-oxidation by iron oxide, specifically that decreased
electron density is necessary to reduce recombination and allow long-lived photoholes to
diffuse to the surface and oxidise water.13 Scheme 3.1 illustrates a system where
recombination is assumed to be mediated by free conduction band electrons, but the results
observed are also consistent with trapped-electron mediated recombination.
Electron-hole recombination has been shown to be the key loss process limiting water
photolysis by TiO2 photoelectrodes.74 Titania (specifically anatase) appears to differ from the
hematite photoanodes studied herein in that scavenging of photogenerated charge carriers –
e.g. by methanol or Ag+ – can be competitive with recombination, even in the absence of
applied bias.71 It is clear that oxidation of scavengers by photogenerated holes in hematite
is not competitive with charge carrier recombination in the absence of applied electrical bias.
Indeed, significant and rapid electron-hole recombination occurs prior to the timescale of
water oxidation, even under positive applied bias. The hole diffusion length is an indicator of
Scheme 3.1 Representation of the effect of applied positive electrical bias on the Fermi level of a
nanostructured hematite photoanode. Applied positive bias decreases the background electron
density relative to open-circuit conditions, reducing the rate of electron-hole recombination and
increasing the lifetime of photogenerated holes, allowing water oxidation to occur.
Chapter III: Identification of Photogenerated Hole Absorption 60
the relative significance of electron-hole recombination: more dominant recombination
results in a shorter diffusion length. The hole diffusion length in hematite is notoriously short
(reported as 2-4 nm or 20 nm,16, 17) but is significantly longer in titania.81 Recombination may
be more dominant in hematite due to some or all of the following factors. (i) Greater electron
(donor) density in hematite, resulting in faster electron-hole recombination kinetics. The
donor density in undoped hematite is typically 1017-1018 cm-3,36, 37 while anatase TiO2 varies
between 1016 to 1019 cm-3.82 Values for the dielectric constant of hematite are varied,83 but
are the same order of magnitude as those for TiO2 (anatase or rutile), so the degree of
electronic shielding is likely to be similar. (ii) A greater distance for photogenerated holes to
travel to reach the semiconductor surface, due to a longer absorption depth for incident light
in hematite. At the UV excitation wavelengths used in this study (337 and 355 nm), the
absorption coefficient (α, the reciprocal of which gives the light penetration depth) is similar
in hematite and titania.15, 83-85 (iii) Lower charge carrier mobilities in hematite. The hole
mobility in titania has been reported as 16 cm2.V-1.s-1 (at room temperature),86 while the hole
mobility in hematite is thought to be <0.1 cm2.V-1.s-1.87 Electron mobilities in hematite and
titania have been reported as 0.01-0.1 cm2.V-1.s-1 and 0.4 cm2.V-1.s-1, respectively.33, 34, 86
Theoretical calculations have also indicated that hematite hole mobilities are lower than
electron mobilities,35 in contrast to TiO2.86 However, these theoretically determined hematite
electron mobilities are several orders of magnitude smaller than those determined
experimentally.33-35 (iv) Additional relaxation pathways in hematite resulting in faster
recombination, as suggested by ultrafast TAS studies.39, 65 (v) Lower driving force for
oxidation of scavengers on hematite (since the hematite valence band is less positive than
that of anatase or rutile TiO2), or greater activation energy barrier to oxidation. (vi)
Differences in particle size: TAS studies of TiO2 (anatase) on comparable timescales to
those described herein employed nanoporous titania photoanodes with an average particle
diameter of 15 nm.71, 74 However, the hematite photoanodes employed in the studies
described herein generally have significantly larger particle sizes, on the order of hundreds
of nanometres. Significantly, the CVD hematite photoanodes have a dendritic
nanomorphology in which the smallest particles have a diameter of ca 10-20 nm. These
photoanodes give the highest efficiencies and largest transient absorption signals of the
various hematite photoanodes studied. Points (i) and (ii) are unlikely to contribute
significantly to the difference in charge carrier dynamics observed between hematite and
titania. There is little evidence in the literature for point and (iv), although this is still a valid
hypothesis. Point (v) is discussed in more detail in the following chapter. This suggests that
developing nanostructured photoanodes consisting of particles with diameters <20 nm is key
to efficient water photo-oxidation on hematite.
61 Chapter III: Identification of Photogenerated Hole Absorption
3.6 Conclusions
Transient absorption studies of hematite indicate that photogenerated charge carriers are
relatively long-lived, with lifetimes on the microsecond timescale, even in the absence of
applied bias or chemical scavengers. Trapped photogenerated holes absorb broadly in the
visible-NIR region; a sharp peak in the transient absorption spectrum is observed at ~580
nm. Unlike TiO2, the charge carrier dynamics of Fe2O3 are unchanged in the presence of
hole scavengers, suggesting that charge carrier dynamics are dominated by electron-hole
recombination. However under applied positive electrical bias, at which significant
photocurrent is observed, the lifetime of photogenerated holes is significantly increased,
allowing the oxidation of methanol and water to occur. Water oxidation on undoped
nanostructured hematite occurs on a timescale of seconds; very long-lived holes are
necessary for water oxidation. These results indicate that the role of a positive applied bias
is more complex than previously thought. Positive electrical bias not only increases the
reduction potential of electrons at the cathode, but also reduces the background electron
density in hematite. This results in decreased electron-hole recombination and thus
increased hole lifetime, such that photogenerated holes can diffuse to the semiconductor-
electrolyte junction and oxidise water.
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 62
Chapter IV
Correlation of Photocurrent with
Long-Lived Hole Population
as a Function of Applied Bias
In which the charge carrier dynamics in Si-doped nanostructured hematite photoanodes
as a function of applied electrical bias are presented. Transient absorption spectroscopy is
used to probe the photogenerated hole dynamics, while transient photocurrent
measurements follow the dynamics of electron extraction. The effect of excitation intensity
on the kinetics of recombination and water oxidation are also discussed.
63 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
4.1 Introduction
In the previous chapter, it was demonstrated that application of a positive (anodic)
electrical bias to a hematite photoanode retards electron-hole recombination. This increases
the lifetime of photogenerated holes by several orders of magnitude, allowing water
oxidation to occur (on a timescale of seconds). This result qualitatively suggests that
photocurrent generation is dependent on the generation of long-lived holes. It is widely
recognised that electron-hole recombination is a major limitation of metal oxide – including
Fe2O3, TiO2 and WO3 – photoanodes for water photo-oxidation. Direct measurements of
such recombination losses and correlation of these losses with the photocurrent as a
function of voltage have been limited to date.
Recently, members of this group used microsecond-second TAS as a function of
temperature to determine the activation barrier to water oxidation on nanostructured
hematite and mesoporous titania photoanodes.20 There was no observable change in hole
decay dynamics in TiO2, in the temperature range 26-54 °C, indicating that any activation
barrier to water oxidation on titania is too small to be measured by this method.
Photogenerated hole dynamics in hematite, however, clearly exhibited a strong temperature
dependence. The activation energy to water photo-oxidation on hematite was determined to
be 0.45 eV, independent of the applied bias at potentials anodic of the photocurrent onset
potential. This work demonstrated that the slower water oxidation kinetics on hematite are
associated with a thermal barrier to water oxidation.
As described in section 1.3, hematite has several advantageous qualities as a photoanode
material for solar water-splitting photoelectrochemical (PEC) cells.48 However, several
factors limit the water photo-oxidation efficiency, including a somewhat long absorption
depth for visible light (ca. 100 nm for λ = 500 nm 15) coupled with a short hole diffusion
length (2-20 nm 16, 17). Hole transfer kinetics at this junction have also been reported to be
relatively slow, potentially limiting water oxidation efficiency.17-20
The highest performing hematite photoanodes reported to date (Si-doped APCVD-
deposited films with IrO2 surface-treatment) achieve incident photon to current efficiencies
(IPCEs) of ~20% at 500 nm and 50% at 300 nm.48 However, such high photo-oxidative
quantum efficiencies are typically only achieved under strong anodic bias conditions,
typically at least 1.23 V versus the reversible hydrogen electrode (RHE), which is equivalent
to the equilibrium dark water-oxidation potential. A key challenge for enhancing the
thermodynamic efficiency of water photo-oxidation by such photoelectrodes is to reduce this
requirement for anodic bias.
This chapter addresses the requirement for an anodic bias via a quantitative analysis of
the correlation between applied bias and the charge carrier dynamics within hematite
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 64
photoanodes. Transient absorption spectroscopy is used to follow the dynamics of
photogenerated holes as a function of applied electrical bias. These optical measurements
are complimented by transient photocurrent studies of electron collection by the external
circuit. This allows direct monitoring of the potential dependence of electron-hole
recombination in addition to the dynamics of water oxidation. These transient studies enable
a quantitative consideration of parameters influencing the efficiency of water oxidation by
hematite photoelectrodes. The relationship between the photogenerated hole decay
dynamics and the photocurrent density in a variety of hematite photoanodes is investigated.
Elucidation of the relative timescales of electron extraction and electron-hole recombination
lead to a greater understanding of the processes determining the efficiency of water photo-
oxidation by hematite photoelectrodes, and their dependence upon applied electrical bias.
Additionally, excitation density studies provide some insight in to the mechanism of water
oxidation on hematite.
4.2 Experimental
All results presented in this chapter were obtained using Si-doped hematite photoanodes
deposited by atmospheric pressure chemical vapour deposition (APCVD - referred to as
“CVD” herein), as described in the literature.13 For a typical UV-vis spectrum of these
photoanodes, see Figure 3.1. These photoanodes produce significantly larger photocurrent
densities and transient signals than the undoped hematite CVD photoanodes discussed in
the previous chapter. Measurements were made close to the centre of the film, at a point
where the hematite is ca. 400 nm thick.
Transient absorption spectroscopy with applied bias (on the microsecond to seconds
timescale), transient photocurrent and photoelectrochemical measurements were made as
described in the Methods section. TAS (on the µs-s time scale) and TPC measurements
were obtained using pulsed band-gap excitation at 355 nm (typically ~0.2 mJ.cm-2 after
absorption by cell, 0.25-33 Hz).
4.3 Transient absorption studies of photogenerated holes
Photogenerated holes in hematite photoelectrodes exhibit a broad photoinduced
absorption in the visible/near infrared (see Figure 4.1).20, 88 Transient signals for probe
wavelengths below 625 nm are complicated at early times by the presence of narrow,
intense optical signals from charge carriers trapped in localised intraband states, discussed
further in Chapter VI. To simplify the data reported herein, the transient absorption observed
at 650 nm is employed as a probe of photogenerated hole dynamics in such films. Similar
dynamics were however observed for all probe wavelengths between 625 and 900 nm.
65 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
The variation of the photo-hole transient absorption signal with applied electrical bias in
CVD Si-doped hematite, probed at 650 nm, is shown in Figure 4.2a. We observe
qualitatively similar behaviour for all the photoanodes that we have examined, including
doped, undoped, nanostructured and solid hematite. On the timescales of these
measurements (microseconds-seconds), the transient absorption decays exhibit two phases.
A fast phase is observed between 1 μs and ca. 20 ms, with a bias-dependent median
lifetime (t50%), assigned to electron-hole recombination, supported by evidence discussed
below. This fast phase is followed by a slower phase with a lifetime on the hundreds of
milliseconds to seconds timescale that exhibits a bias-dependent amplitude. (It should be
noted that there is some variation in the timescale of the slow transient absorption decay
phase between individual photoanodes of the same type.) We have previously shown that
the decay time of this slow phase is accelerated in the presence of methanol (consistent with
the relatively facile oxidation of methanol compared to water) indicating that this slow phase
results from a surface oxidation reaction, specifically (for aqueous electrolytes in the
absence of chemical hole scavengers) to water photo-oxidation.
Here the yield of long-lived holes as a function of applied bias is considered, before
returning to examine the fast phase decay, assigned to electron-hole recombination, in more
detail. The increasing amplitude of the slow phase of the transient absorption decay with
applied bias is indicated by the arrow in Figure 4.2a. At low applied bias (-0.4 to -0.2
VAg/AgCl) there are very few photo-holes remaining on the 100 ms timescale. As an
increasingly positive bias is applied, the amplitude of this photogenerated hole signal
increases until it saturates at ~0.4 VAg/AgCl, after which it becomes approximately constant
with bias, as illustrated in Figure 4.2b (red diamonds, measured at 100 ms). Also shown in
Fig 4.1 Transient absorption spectra in Si-Fe2O3 CVD at (a) -0.2 VAg/AgCl and (b) +0.4VAg/AgCl at
10 ms, 100 ms, 500 ms and 1 s after the excitation pulse (EE, 355 nm). At early timescales there
is a strong bleach (negative absorption) at wavelengths <625 nm. The spectrum at +0.4VAg/AgCl is
essentially the spectrum of the photogenerated holes.
600 700 800 900
-0.2
-0.1
0.0
0.1
10 ms
100 ms
500 ms
m
OD
wavelength / nm
(a) -0.2VAg/AgCl
600 700 800 900
-0.2
-0.1
0.0
0.1
10 ms
100 ms
500 ms
1 s
m
OD
wavelength / nm
(b) +0.4 VAg/AgCl
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 66
Figure 4.2b is the photocurrent density/voltage curve measured for the same photoelectrode.
It is apparent there is an excellent, quantitative correlation between the amplitude of the
long-lived hole signal (measured at 100 ms) and the photocurrent, assigned to water
oxidation. It should be noted that the small increase in photocurrent between -0.3 and 0
VAg/AgCl is scan speed dependent, and most probably does not correspond to water photo-
oxidation but rather to accumulation of holes at the surface/oxidation of trap states.55 This is
examined further in the Discussion section.
Fig 4.2 Transient absorption and photocurrent density data for a Si-doped CVD Fe2O3 film as a
function of applied electrical bias. (a) Transient absorption signals (1 μs to 2 s, EE 355 nm
excitation, probed at 650 nm) at various applied bias (in 0.1 V increments: pale grey -0.4 VAg/AgCl,
brown +0.6 VAg/AgCl). The arrow indicates increasing number of long-lived holes with increasing
positive bias at water-splitting timescales. (b) Correlation of long-lived photogenerated hole signal
amplitude at 100 ms (red diamonds) with photocurrent (blue line; under 355 nm EE illumination
(ca. 550 μW.cm-2
, giving ~54 μA.cm-2
photocurrent at 1.23 VRHE)).
-0.4 -0.2 0.0 0.2 0.4 0.6
0.000
0.025
0.050
0.075
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.6 0.8 1.0 1.2 1.4 1.6
ph
oto
cu
rre
nt
de
nsity /
mA
/cm
2
bias / V vs Ag/AgCl
h+ a
mp
litu
de
at
10
0 m
s /
mO
D bias / V vs RHE
(b)
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
0.25
m
OD
time / s
-0.4 - +0.6 VAg/AgCl
(a)
67 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
The quantitative correlation between the amplitude of the long-lived hole signal at various
applied bias and the photocurrent/voltage curve was observed for all the semiconductor
photoanodes examined, including Si-doped and undoped nanostructured CVD films, Nb-
doped mesoporous films fabricated by ultrasonic spray pyrolysis and dense undoped films
deposited by pulsed laser deposition, shown in Chapter V. This correlation provides further
evidence for the importance of long-lived holes in driving water photo-oxidation, and
indicates that this correlation is generic to a broad range of hematite photoelectrodes.
The processes associated with the faster decay phase observed in the transient
absorption data (1 μs-20 ms, Figure 4.2a) are now considered. The median lifetime (t50%)* 89
of this fast phase is plotted versus applied bias in Figure 4.3. It is apparent that this decay
half-time increases by three orders of magnitude as the bias voltage is increased anodically,
reaching a plateau at a lifetime of ~3ms. This behaviour is analogous to that reported
previously for the bias dependence of TiO2 electron/dye-cation recombination in dye
sensitised titania films.90 As for these dye-sensitised titania films, the increase in
recombination time with increasing positive bias is due to a reduction in electron density in
the film induced by the positive bias. Similarly the plateau observed under positive bias is
assigned to the regime where the density of electrons photogenerated by the laser pulse
exceeds those present in the dark under positive bias conditions, such that further
reductions in dark electron density by more positive bias do not result in further retardation of
* The value of t50% was approximated as the time t at which the ratio [ΔOD(t, V)/(ΔOD(1 ms, V)-
ΔOD(30 ms, V))] equals 0.5.
Fig 4.3 Median lifetime (t50%)* of the fast decay phase of the transient absorption signal for
photogenerated holes from Figure 4.2a, versus applied bias.
-0.4 -0.2 0.0 0.2 0.4 0.61E-3
0.01
0.1
1
10
t 50% (
from
1 u
s)
/ m
s
bias / V vs Ag/AgCl
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 68
the observed recombination dynamics. Further support for these assignments comes from
the excitation density and photocurrent transient data reported below.
The time resolution of these TA measurements does not allow us to monitor
recombination dynamics faster than ~1 μs. There have been several reports of faster
(picosecond-nanosecond) recombination dynamics in hematite photoelectrodes,39, 65, 67, 69
although such studies were typically undertaken with much higher peak laser powers (and
therefore charge carrier densities) than those employed herein. Indeed it is already
apparent from Figure 4.2a that, at negative potentials, a significant fraction of charge
recombination occurs prior to 1 μs. It is likely that picosecond timescale measurements
probe, at least in part, geminate electron-hole recombination, whereas at the longer
timescales (>1 μs) reported herein only non-geminate recombination occurs.
4.4 Transient photocurrent studies of electron extraction
The transient absorption data reported above are assigned to the absorption of
photogenerated holes within hematite photoanodes. A transient absorption signal in the
550-950 nm region clearly assignable to photogenerated electrons has not been observed.
Instead, transient photocurrent (TPC) measurements are employed to probe the timescale of
electron extraction from hematite photoanodes, measured under the same transient
excitation conditions used for transient absorption measurements. Photocurrents measured
through the back FTO contact primarily correspond to extracted electrons, although there
may be a small cathodic contribution from back-reaction of electrons with surface trapped
holes.91 The early timescale TPC rise is likely to be limited by the measurement resistor;
only the TPC decay at longer timescales (ca. >10 μs) is considered here as this does not
appear to be limited by the measurement resistor (see section 2.2.3).
Figure 4.4 shows transient absorption decays (measuring photogenerated holes) at -0.2,
+0.2, +0.4 and +0.6 VAg/AgCl overlaid with the corresponding TPC decays (measuring
extracted electrons). The TPC signals are strikingly similar to the “fast phase” of the
transient absorption decay curves for all bias conditions, although the TPC decay tracks the
fast phase of the transient absorption decay more closely at low applied bias. Very similar
results are achieved with SE excitation. It is of particular note that at +0.4 VAg/AgCl, bias
conditions which result in the generation of the slow transient absorption decay phase
assigned to the long lived holes driving water oxidation, the TPC curve decays to zero by ca
20 ms. It is thus apparent that under bias conditions where water oxidation is observed,
electron extraction occurs on a timescale much faster than the kinetics of water oxidation.
69 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
These transient photocurrent decays can be most easily understood as monitoring the
recovery of electron density towards dark equilibrium following the pulsed excitation. This
recovery in electron density will result from both electron-hole recombination and electron
extraction by the external circuit. Electron-hole recombination will also be dependent upon
this excess electron density. The strong similarity between the TPC and TA decay kinetics
at low bias can thus be explained as follows. The “fast phase” TA signal (monitoring holes)
decays as recombination decreases the number of holes in the film. At potentials cathodic
of significant photocurrent, recombination dominates over electron extraction, so
recombination is also the main process by which the electron population decreases. Since
both the TPC and TA decays are dominated by the same process, their decay kinetics will
be similar. However, at potentials with significant photocurrent, electron extraction and
water oxidation become significant in comparison to charge recombination, resulting in a
small but notable difference between the TPC and TA decay kinetics. An additional factor
likely to impact upon the kinetics of the TPC decays may be the time taken for electrons to
move through the hematite film to the back contact, although very little difference in TPC
decay kinetics is observed between SE and EE excitation. It is noted that the TPC decay
kinetics more closely resemble the recovery of the TA bleach at ~575 nm, associated with
the photo-reduction of a particular trap state. This is discussed in detail in Chapter VI.
Fig 4.4 TPC decays (measuring extracted electrons) overlaid on corresponding transient
absorption decays (measuring photo-holes) of a Si-doped CVD hematite photoanode, EE
excitation at 355 nm, at -0.2, +0.2, +0.4 and +0.6VAg/AgCl. The TPC axis is shifted upwards and
scaled to maximise overlap with the TA decay.
-0.2 VAg/AgCl
1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
m
OD
time/s
0.0
0.1
0.2
0.3
TP
C / m
A
1E-5 1E-4 1E-3 0.01 0.1 10.00
0.05
0.10
0.15
0.20
0.25
m
OD
time/s
0.0
0.5
1.0
1.5
TP
C /
mA
+0.2 VAg/AgCl
1E-5 1E-4 1E-3 0.01 0.1 10.00
0.05
0.10
0.15
0.20
m
OD
time/s
-1
0
1
2
TP
C / m
A
+0.4 VAg/AgCl
1E-5 1E-4 1E-3 0.01 0.1 10.00
0.05
0.10
0.15
0.20
m
OD
time/s
-2
-1
0
1
2
3
TP
C / m
A
+0.6 VAg/AgCl
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 70
4.5 Excitation density studies
Steady state photocurrent densities measured at 0.4 VAg/AgCl anodic bias for the same
photoanode as a function of steady state light intensity are shown in Figure 4.5a. It should
be noted that the excitation density range for the transient (laser) measurements is much
greater than for the steady-state (continuous wave) measurements; only the very lowest
laser intensities are likely to correspond to the charge density regime of the steady-state
photocurrent measurements. Figure 4.5b shows transient absorption decays measured
under the same bias as a function of excitation density from 23 μJ.cm-2 to 2.21 mJ.cm-2
Fig 4.5 Steady-state photocurrent and transient absorption data for a Si-doped CVD Fe2O3
photoanode as a function of excitation intensity at +0.4 VAg/AgCl. (a) Variation of steady-state
photocurrent amplitude (under 355 nm EE illumination); the red line is the best fit to the data. (b)
Variation of transient absorption photogenerated hole signal (probed at 650 nm, EE 355 nm
excitation from 23 μJ.cm-2
(dark green) to 2.21 mJ.cm-2
(brown); the laser intensity used for the
majority of the measurements described herein is 200 μJ.cm-2
). (c) normalised slow TA phase at
125 ms - the timescale of water oxidation (2.1 s) is independent of excitation intensity. Inset:
normalised at 10 μs - the fast phase decays more rapidly with increasing excitation density. (d)
Ratio of amplitude of fast and slow decay phases of transient absorption as a function of
excitation intensity; inset: variation of amplitudes with excitation intensity. At the very lowest
excitation intensities (<200 μJ.cm-2
) we approach pseudo-first-order recombination behaviour (i.e.
within the small perturbation regime).
0.01 0.1 10.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
m
OD
time / s
(c)
1E-5 1E-4 1E-3 0.01 0.1 10.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 20000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
ratio fast:slow phase amplitudes
ratio
fa
st:slo
w p
ha
se
am
plit
ud
es
excitation intensity (355 nm) / J.cm-2
(d)0 1000 2000
0.00
0.25
0.50
0.75
1.00
1.25
slow decay
phase amplitude m
OD
fast decay phase amplitude
0 50 100 150 2000.00
0.25
0.50
0.75
1.00
cu
rre
nt d
en
sity / m
A.c
m-2
Light Intensity (%)
(a)
1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.00
0.25
0.50
0.75
1.00
1.25
1.50
m
OD
time / s
(b)
71 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
(other data in this chapter being collected at 200 μJ.cm-2). It is apparent that as the
excitation density is increased, the fast transient absorption decay phase becomes faster
and increasingly dominant, consistent with our assignment of this decay phase primarily to
bimolecular electron-hole recombination. Of particular interest is the observation that the half
time of the slow decay phase, assigned to water oxidation by photogenerated holes, is
independent of excitation (and therefore hole) density, as shown in Figure 4.5c. This
observation is discussed further below. The linear dependence of the steady-state
photocurrent density on light intensity under these anodic bias conditions (Figure 4.5a) is
also discussed below. This again has important implications for the processes limiting
photocurrent generation under these bias conditions.
4.6 Discussion
Before discussing in detail the results of these studies, it is important to appreciate the
advantages and limitations of using transient absorption spectroscopy to monitor the yields
and dynamics of photogenerated holes. The core advantage of this technique is that it
allows the presence of photogenerated holes to be directly monitored by the observation of
their optical absorption. This differs from alternative electrochemical based techniques (such
as impendence spectroscopies and photocurrent measurements)12, 13, 17-19, 36, 37, 46, 55, 56, 58, 72,
75, 92, 93 which rely upon monitoring electrical outputs, and therefore cannot directly monitor
hole transfer at the SCLJ. This ability to directly monitor photogenerated holes is particularly
important for photoanodes as it is the photogenerated holes which drive the key electrode
function of water oxidation. However, transient absorption spectroscopy has a significant
disadvantage in that it has a limited sensitivity and therefore requires the use of relatively
intense excitation pulses to generate sufficiently high hole density to be detectable. The
transient absorption system employed for the studies described here has a particularly high
sensitivity, and the excitation densities significantly below those in other transient absorption
studies of hematite photoelectrodes. Nevertheless, our 200 μJ.cm-2 excitation pulses
correspond to absorbed photon densities of ~1019 cm-3. This charge density, while
equivalent to the background electron density of undoped (and below that of Si-doped)
hematite,37 is most probably greater than the charge carrier density generated under
continuous solar irradiation. It is important to appreciate this limitation during the following
discussion.
Using transient absorption spectroscopy to monitor photogenerated holes, and transient
photocurrent to monitor the extraction of photogenerated electrons, the charge carrier
dynamics in hematite photoanodes were observed to be strongly dependent upon applied
potential. The decay of photogenerated holes exhibits two phases, on timescales of ca 1 μs
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 72
to 20 ms and 20 ms to >2 s. The fast decay phase is assigned to electron-hole
recombination. This electron-hole recombination becomes slower as the steady state
electron density in the film is reduced under positive electrical bias. This dependence is very
strong, with a 400 mV anodic shift in applied bias resulting in a 1000-fold retardation of the
electron-hole recombination rate (Figure 4.3). The lifetime (t50%) of this fast decay phase
reaches a plateau value of ~3 ms at approximately the same potential as the photocurrent
onset occurs. Although this is likely to correspond to the regime where the density of
electrons photogenerated by the laser pulse exceeds those present in the dark under
positive bias conditions, it should be noted that this lifetime is calculated from the observed
ΔOD at 1 μs. However, recombination is also likely to occur on the nanosecond timescale,
(i.e. prior to the timescale of these transient measurements), which will also partially
determine the population of long-lived holes remaining after ~20 ms. A more accurate
assessment of t50% could be achieved by considering the initial hole signal on the
nanosecond timescale, however these data are not currently available. This may also
contribute to the plateau in t50% under the same bias conditions where the population of long-
lived holes is still increasing. Additionally, there may be some effect of the early-timescale
bleach observed at ~575 nm on the TA decays probed at 650 nm.
The transient photocurrent measurements indicate that electron collection by the external
circuit is also complete by ca. 20 ms, with kinetics similar to those of the fast transient
absorption decay phase. The slow transient absorption decay phase is assigned to residual
photogenerated holes which have not recombined with electrons. There is a quantitative
correlation between the amplitude of this slow decay phase (i.e. the population of long-lived
holes) and the electrode photocurrent monitored as a function of applied bias. The onset of
photocurrent generation at positive bias is attributed to the pronounced retardation in
electron-hole recombination with applied positive potential, allowing electron extraction to the
external circuit and leading to the generation of long-lived holes. These long-lived holes
then go on to drive water oxidation on a timescale of hundreds of milliseconds to seconds.
This analysis is consistent with a recent electrochemical impedance analysis of analogous
silicon doped hematite photoanodes,55 wherein a two-phase response was also reported; the
low frequency element was assigned to charge transfer processes at the semiconductor-
liquid junction, and the high frequency element to processes occurring within the
semiconductor, including charge transport.
The activation energies and rate constant for water oxidation on nanostructured undoped
hematite have previously been studied by this group.20 This rate constant and activation
energy for water oxidation were observed to be independent of applied bias. A similar
conclusion regarding the rate constant is also apparent for the silicon doped films employed
herein from the bias independence of the slow decay phase lifetime apparent in Figure 4.2a.
73 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
These observations indicate that the reactivity of photogenerated long-lived holes is
independent of applied bias. This conclusion is consistent with the above analysis, i.e. the
photocurrent is dependent only upon the yield of long lived holes, with this yield being
determined by kinetics of electron-hole recombination on faster timescales.
The long-lived photogenerated hole signal observed for hematite photoelectrodes was
previously assigned to holes localised at or near the Fe2O3 surface, prior to charge transfer
to surface-bound or electrolyte species.20 This charge due to holes accumulated at/near the
Fe2O3 surface after electron extraction is most probably balanced by negative ions in the
electrolyte, forming the Helmholtz layer. Under approximately 1 Sun illumination conditions,
the photocurrent density at 0.4 VAg/AgCl is ~2.1 mA.cm-2 (for these nanostructured, Si-doped
hematite photoanodes). Taking the hole lifetime (water oxidation timescale) to be 1 s, and
assuming a surface roughness of 20,13 this results in a surface hole density on the order of
1015 holes.cm-2. This interpretation is consistent with recent reports of photo-hole
accumulation at/near the Fe2O3 surface under water-photolysis conditions.18, 19, 53
It should be noted that the best correlation between the population of long-lived holes (as
determined from the amplitude of the TA signal at 100 ms) and photocurrent density is best
with an i/V scan rate of 20 mV.s-1, i.e. not the steady-state photocurrent. As the scan rate is
reduced, the amplitude of the current cathodic of the photocurrent onset potential
(associated with recombination via surface states) decreases, and the photocurrent rise
becomes more abrupt. These results suggest that the amplitude of the long-lived hole signal
actually measures the population of holes that are initially transferred to water/surface-bound
water states, and does not take account of the back-reaction of electrons with this oxidation
intermediate (evidenced by photocurrent transients in chopped-light measurements55). It is
likely that this back-reaction occurs on timescales longer than those probed by the TPC
measurements reported herein, i.e. >100 ms.
A quantitative correlation between long-lived photo-hole population and photocurrent
amplitude appears to be a common feature of several metal oxide photoanodes for water
oxidation. A similar association between long-lived holes and water oxidation has also been
observed for nanoporous TiO2 photoanodes, both in the presence of a chemical electron
scavenger71 and under anodic bias,74 and for a nanoporous WO3 film.94 The results reported
herein for different hematite photoanodes indicate that there is a general requirement for
long-lived holes in order for water oxidation to occur. This can be attributed to the slow
timescale for the reaction of these holes with water. The reaction timescale of these holes is
accelerated when oxidising chemical hole-scavengers such as methanol.88 The slow
timescale of water oxidation on hematite can be explained, at least in part, by the
observation of a significant (45 kJ.mol-1) activation barrier for this reaction.20 The activation
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 74
barrier to methanol oxidation is considerably lower, confirming that the water oxidation
timescale is limited by slow kinetics at the semiconductor-liquid junction.
It should be noted that the actual hole lifetime required for water oxidation depends on the
timescale of the oxidation reaction on a given material. For instance, the hole lifetime
required for water oxidation on nanoporous TiO2 is approximately two orders of magnitude
shorter than for nanostructured undoped α-Fe2O3 under similar conditions.74, 88 However, it
is clear from the transient measurements reported herein that the timescale of water
oxidation on nanostructured Si-doped hematite is approximately two orders of magnitude
slower than the timescale of electron extraction to the external circuit, and that significant
electron-hole recombination occurs on even faster timescales. The requirement for an
enhancement of the water oxidation rate by two orders of magnitude is a daunting task.
Hence strategies to increase the yield of long-lived holes and therefore to optimise the
performance of hematite photoanodes should focus upon decreasing the level of electron-
hole recombination, either through material design to directly retard the kinetics of electron-
hole recombination, or by accelerating the kinetics of electron extraction by the external
circuit. Indeed, electron mobility in hematite is known to be low,50 and the short hole
diffusion length16, 17 is indicative of rapid electron-hole recombination.
It is striking that, after recovery of the electron density in the film to its equilibrium value
(corresponding to when the photocurrent transient has decayed to zero at ~20 ms), the
lifetime of the remaining long-lived holes is independent of the applied potential. Given that
this applied potential is likely to modulate the average electron density in the film, this
observation strongly suggests that these long-lived holes are localised in a depletion region
(space-charge layer) generated by band bending at the semiconductor-electrolyte interface.
Although several electrochemical impedance studies of hematite in the literature have
provided information about the flatband potential and donor density of various types of
hematite photoanodes,17, 36, 37, 50, 55, 93 the quantitative extent of the depletion region through a
nanostructured photoanode is at present unclear. In semiconductor particles with one
dimension greater than the width of the space-charge layer, some degree of band bending is
likely to exist. The CVD hematite photoanodes studied herein have a dendritic
nanostructure in which the smallest feature size is ca 10-20 nm but which consist of larger
features closer to the substrate, on the order of hundreds of nanometres.13 Following the
methodology outlined by O’Regan et al,95 Equation 4.1 is used to estimate the voltage
difference between the centre of a particle and the semiconductor-liquid junction for an
undoped hematite photoanode.
(4.1)
75 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
where
is the Debye length. The radius of the particle (which is
approximated as spherical) is r, while the voltage drop across the radius of the particle is
ΔφSC. Undoped hematite has a donor (electron) density, ND ~1017-1018 cm-3 and dielectric
constant, ε ~30-60.36, 57, 83 This gives a Debye length of 4.6-21 nm. Nanostructured
undoped hematite photoanodes deposited by CVD have feature sizes in the range ca. 20-
250 nm.13 The estimates of ΔφSC for different feature sizes are shown in Table 4.1 below.
The voltage difference between the centre of a particle and the semiconductor-liquid junction
is in the region of ca. 1-20 mV for a 10 nm radius particle, or ca. 25-500 mV for a 100 nm
radius particle. These values indicate that some degree of band-bending is likely to be
present in undoped CVD hematite photoanodes; Si-doping increases the donor density to
1020 cm-3,37 thus decreasing the width of the space-charge layer (although the dielectric
constant will also change).2 Hence it is very likely that a degree of band-bending occurs in
both undoped and Si-doped hematite nanoporous CVD photoanodes.
particle radius / nm ΔφSC for LD = 4.6 nm / mV ΔφSC for LD = 21 nm / mV
10 20 1
50 500 24
125 3.2 x 104 150
The effect of applied bias on these photoanodes is most likely a combination of the classic
band-bending model of the semiconductor-electrode interface2 and that of a nanoparticulate
film with no band-bending. In particles large enough for band-bending to occur (nearer the
substrate), increasing positive applied bias increases the width of the space-charge layer so
sweeps more photogenerated holes to the SCLJ, reducing the rate of recombination and
increasing the population of long-lived holes. In smaller particles, charge carrier transport is
by diffusion only, so positive bias lowers the Fermi level, resulting in diminished background
electron density, leading to decreased recombination and longer hole lifetimes, allowing
more holes to reach the SCLJ. The UV excitation light employed here is absorbed within
~30 nm of solid hematite.15 Hence we are likely measuring a combination of the band-
bending and nanoparticulate regimes in nanoporous photoanodes, particularly with EE
(“front-side”) illumination.
As increasing positive bias is applied, the width of the space-charge layer increases, so
more holes are photogenerated within the space charge layer. The flat-band potential (i.e.
the potential at which no band-bending occurs) of hematite is reported to be ca. 0.4 VRHE,37
equivalent to ca. -0.6 VAg/AgCl under the conditions used in this study. Hence the lowest
Table 4.1 Estimated values of potential drop (ΔφSC) across the radius of spherical undoped
hematite nanoparticles of various sizes for two different Debye lengths (LD).
Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 76
potential used in this study (-0.4 VAg/AgCl) is close to flat-band, where very few holes are
photogenerated in the space charge layer. At this potential, the transient absorption signal
of photogenerated holes decays rapidly; almost no photogenerated holes remain by ~200 μs
and most recombination occurs prior to the timescale of our transient absorption
measurement (Figure 4.2a). This suggests that the photogenerated holes measured on the
timescale of our transient absorption studies (microseconds-seconds) are mainly those
within the space charge layer and nanoparticulate regions of the photoanode.
Finally, the light intensity dependence of the transient and steady-state measurements is
considered. In general, water photo-oxidation by hematite photoelectrodes depends upon
two processes which might be expected to behave non-linearly with charge carrier densities,
and therefore light intensity: electron-hole recombination and water oxidation. The multi-
electron nature of water oxidation is of particular interest, with four holes being required to
drive the oxidation of two molecules of water, releasing one molecule of oxygen. There are
currently extensive efforts to develop “multi-hole” catalysts for this reaction, motivated in part
by recent advances in our understanding of the structure and function of the manganese-
based water oxidation centre of Photosystem II in higher plants.96 In this regard, it is
particularly striking that the photo-oxidation current observed under positive (anodic) bias for
these films scales linearly with light intensity. There are currently few studies reported in the
literature of the excitation intensity dependence of photocurrent densities for hematite.16, 50, 75,
97
Electron-hole recombination is expected to be a bimolecular process dependent upon
both electron and hole densities. This is supported by both the bias and excitation density
dependence of the fast transient absorption decay phase shown herein, where the rate of
recombination increases with increasing electron density. However, we note that under
steady state conditions, the electron density in the film is likely to be dominated by the dark
or resting electron density. † In this limit, the electron density in the film becomes
independent of light intensity, and electron-hole recombination can be expected to be
linearly dependent on light intensity (effectively corresponding to pseudo-first order
recombination behaviour). We note that the fast phase decay time and relative amplitude
(compared to the slow decay phase amplitude) of our transient absorption data are almost
identical for the two lowest excitation conditions, shown in Figure 4.5. This behaviour
suggests that we are approaching the pseudo-first-order limit in our transient measurements
at the lowest excitation intensities (≤200 μJ.cm-2).
Of equal, or greater, interest is the independence of the half-time of the slow decay phase
upon excitation density and therefore photogenerated hole density (estimated as ca. 1015
† This is particularly true for the steady-state (continuous wave) study reported herein where
excitation density range is likely much lower than for the transient (laser) measurements
77 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population
holes.cm-2 under solar illumination – see above). The population of long-lived holes is
observed to increase almost ten-fold over the range of excitation densities employed, whilst
the decay time of these long-lived holes remains constant. This behaviour strongly suggests
that the rate determining step in the reaction of these surface trapped holes with
water/surface-bound water species is first order in hole density, and is therefore a single-
hole oxidation. This is consistent with our previous report on the thermal activation
measurements for this reaction (and also single laser shot analysis of water oxidation on
TiO2), which are indicative of water oxidation proceeding via the single-hole oxidation of, for
example, surface-bound hydroxyls (OH-).20, 24 The potentials for such single-hole oxidation
reactions are significantly more positive than the equilibrium four-hole oxidation of water
(+1.23 V versus RHE). However, it appears that the highly oxidising nature of
photogenerated holes in hematite is sufficient to drive water oxidation via a series of single-
hole oxidation steps, without a significant requirement for the accumulation of multiple holes
to drive a concerted multi-hole oxidation reaction, in agreement with a recent theoretical
study of water oxidation on Fe2O3.49
4.7 Conclusions
Transient absorption and transient photocurrent measurements have been employed to
monitor photogenerated holes and electrons, respectively, in hematite photoanodes as a
function of applied bias. A quantitative correlation between the yield of long-lived
photogenerated holes (on the order of hundreds of milliseconds) and photocurrent density
was demonstrated. The rate of water oxidation by these long-lived holes is independent of
hole density, indicating that water oxidation proceeds via a rate-determining single-hole
transfer step. The bias dependence of the yield of long-lived holes is caused by the strong
bias dependence of electron-hole recombination on the micro- to milli-second timescale.
This rapid recombination phase competes with electron collection by the external circuit.
The key factor limiting the efficiency of water photo-oxidation by these hematite
photoelectrodes, and leading to the requirement of thermodynamically undesirable anodic
electrical biases to enable this photo-oxidation, is not the (albeit slow) timescale of water
oxidation by photogenerated holes but rather the rapid electron-hole recombination which
can prevent efficient electron collection. This suggests that strategies to optimise the
performance of such photoelectrodes should focus not so much upon the acceleration of
water oxidation by the addition of co-catalysts, but rather upon either retarding this electron-
hole recombination or accelerating the kinetics of electron extraction by the external circuit.
Chapter V: Comparison of Sold and Mesoporous Hematite 78
Chapter V
Comparison of
Solid and Mesoporous
Hematite Photoanodes
In which the dynamics of photogenerated holes, probed using transient absorption
spectroscopy, and electrons, probed using transient photocurrent measurements, in solid
and nanostructured hematite photoanodes are compared. The differences in efficiency
under UV and visible excitation are also considered.
79 Chapter V: Comparison of Sold and Mesoporous Hematite
5.1 Introduction
Although one of hematite’s most advantageous properties for solar energy conversion is
its strong absorption of visible light, the internal quantum efficiency (APCE) is significantly
lower under visible excitation than UV.14, 37, 40 This is generally accepted to result from the
combination of the relatively long absorption depth for visible light in hematite (ca. 100 nm
absorption depth for 500 nm light15) together with the short hole diffusion length (2-4 nm;16
20 nm 17). Hematite also has a relatively narrow space-charge layer due to a high donor
density. Consequently, under visible excitation holes are photogenerated several times
further from the semiconductor-liquid junction (SCLJ) than their diffusion length. Under SE
(“back-side”) visible excitation, electrons are generated further from the back contact, so are
also more likely to recombine than under UV excitation. However, there is some evidence
that charge carriers generated by visible light are inherently less active for water oxidation; it
has been suggested that higher APCE values under UV excitation are a consequence of
more energetic holes having larger mobilities.14
The main method of mitigating these losses is to employ porous, nanostructured
photoanodes. This type of morphology allows the use of thick films which absorb a large
fraction of incident light, but in which holes have only a short distance to diffuse to the SCLJ.
This is thought to be why Si-doped APCVD hematite photoanodes, which have a
“cauliflower-like” dendritic nanostructure, generate relatively high photocurrents under one
Sun illumination.37 Similarly, arrays of narrow hematite nanorods or nanotubes aligned
perpendicularly to the conducting substrate surface would provide a direct, grain-boundary-
free path for electron transport to the back contact whilst minimising the distance holes must
diffuse to the SCLJ. However, development of such photoanodes has not so far resulted in
higher photocurrents than those generated by APCVD hematite.38, 97, 98 This is likely to be
due to the width of the nanorods/tubes (on the order of 10-25 nm or greater), which could be
significantly larger than the hole diffusion length. Bulk and/or surface defects may also limit
the photocurrent.99 An alternative suggestion for overcoming the long light absorption depth
and short hole diffusion length is to stack several ultra-thin solid photoanodes, in which
photogenerated holes are swept to the semiconductor surface by the electric field (band
bending).100
The two previous chapters in this Thesis focussed on the charge carrier dynamics of
nanostructured hematite photoanodes as a function of applied bias. The population of long-
lived holes responsible for water oxidation was shown to be determined by electron-hole
recombination on sub-millisecond timescales. It was demonstrated that increasing positive
applied bias reduces recombination, likely due to a greater proportion of holes being
generated in the increasingly wide space-charge layer. In this chapter, these analyses are
Chapter V: Comparison of Sold and Mesoporous Hematite 80
extended to various hematite photoanodes with different nanomorphologies and
thicknesses. Photogenerated electron and hole dynamics in thick and thin solid
photoanodes, and in nanostructured and dendritic photoanodes are compared. Additionally,
charge carrier dynamics under visible and UV excitation in hematite photoanodes on
microsecond to second timescales are examined. Transient absorption (TA) measurements
are used to probe photogenerated holes, while transient photocurrent (TPC) measurements
probe the dynamics of electron extraction. Current/voltage and steady-state photocurrent
measurements are also employed. TPC measurements clearly show greater efficiencies
under UV excitation. TA measurements are employed to elucidate the loss mechanisms
under visible excitation, however, the limited sensitivity of this optical technique combined
with the low efficiencies of the solid photoanodes employed in these studies limit the
conclusions that can be drawn. Although the timescale of water oxidation is shown to be the
same on solid and nanostructured films, the timescale of electron extraction to the external
circuit is found to be highly dependent on the morphology of the photoanode. Since more
rapid electron extraction reduces losses by electron-hole recombination, these results are
useful for developing structure-function relationships of photoanodes for water oxidation.
5.2 Experimental
Several different types of hematite photoanode were investigated and are described
briefly here; more details are given in Section 2.1 and the references indicated. Very thin (57
and 30 nm thick) undoped solid (non-porous) hematite deposited by atomic layer deposition
(ALD),14 and undoped solid hematite deposited by pulsed laser deposition (PLD),
approximately 600 nm thick.72 Thick (~1 μm) Si-doped hematite photoanodes deposited by
spray pyrolysis, which are relatively non-porous. Colloidal, porous Ti-doped hematite
Fig 5.1 UV-vis spectra of
various types of hematite
photoanodes. Spectra
taken of the same area of
the photoanodes as used
for TAS and PEC
measurements. Vertical
black lines indicate 355
and 525 nm.
400 500 600 700 800 9000
1
2
3
abs /
a.u
.
wavelength / nm
PLD Fe2O
3
SP Si-Fe2O
3
colloidal Ti-Fe2O
3
USP Nb-Fe2O
3
ALD 57 nm thick Fe2O
3
ALD 30 nm thick Fe2O
3
81 Chapter V: Comparison of Sold and Mesoporous Hematite
photoanodes are produced using a scaffold to encapsulate nanoparticles, which is removed
after annealing, and consist of a nanoporous network of 30-40 nm particles.73 Nb-doped
hematite approximately 200 nm thick with mesoporous “leaflet” nanostructure, deposited by
ultrasonic spray pyrolysis (USP) using a method similar to that already described in the
literature,12 with 0.5% Nb precursor. The UV-vis absorption spectra of these photoanodes
are shown in Figure 5.1. Undoped and Si-doped CVD photoanodes, employed in studies
discussed in previous chapters, were also used (UV-vis spectra shown in Figure 3.1).
Transient absorption spectroscopy with applied bias (on the microsecond to seconds
timescale), transient photocurrent and photoelectrochemical measurements were made as
described in the Methods section. TAS and TPC measurements were obtained using pulsed
band-gap excitation at 355 nm (typically ~0.2 mJ.cm-2 after absorption by cell, 0.25-33 Hz).
Photocurrent/voltage curves for colloidal Ti-Fe2O3, SP Si-Fe2O3 and ALD Fe2O3 (30 and 57
nm thick) are shown in Figure 5.2.
5.3 Comparison of carrier dynamics in solid and mesoporous hematite
The transient absorption (TA) signal of photogenerated holes (probed at 700 nm) in thin
solid hematite is shown as a function of applied bias in Figure 5.3. The decay of the long-
lived hole signal over a timescale of tens of milliseconds to seconds is attributed to water
oxidation (Chapter IV). These very thin (~30 nm thick) hematite photoanodes produce
significantly smaller TA signals compared to those for thicker CVD hematite discussed
previously, due to the greater transparency of these thin films. As previously observed for
nanostructured hematite photoanodes, the amplitude of the TA signal, probed on
Fig 5.2 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination (ca. 1 Sun)
intensity), 10 mV.s-1
) from different types of hematite photoanodes: solid Fe2O3 30 nm (pale blue)
and 57 nm (dark blue) thick (ALD); colloidal Ti-Fe2O3 (green); thick (1 μm) solid SP Sn-Fe2O3 (red).
0.0 0.2 0.4 0.6
0.0
0.2
0.4
0.6
0.8
1.0
curr
ent
density /
mA
.cm
-2
bias / V vs Ag/AgCl
colloidal Ti-Fe2O
3
57 nm thick ALD Fe2O
3
30 nm thick ALS Fe2O
3
SP Si-Fe2O
3
Chapter V: Comparison of Sold and Mesoporous Hematite 82
milliseconds to seconds timescales, increases with increasing positive applied bias. This is
attributed to greater long-lived hole populations as increasing positive bias decreases the
electron density and thus reduces electron-hole recombination.
As demonstrated in the previous chapter, there is a quantitative correlation between the
amplitude of the TA hole signal on the hundreds of milliseconds timescale (i.e. population of
long-lived holes responsible for water oxidation) and the photocurrent density as a function
Fig 5.3 Transient absorption (TA) decays of holes (probed at 700 nm) in 30 nm thick ALD
Fe2O3 photoanodes as a function of applied bias, at 0 (blue), +0.3 (green) and +0.6 VAg/AgCl
(brown). Measurements were made using a three electrode cell with 0.1 M NaOH electrolyte;
EE 355 nm excitation (25 μJ.cm-2
, corresponding to an approximate initial photogenerated hole
density of 8x1018
holes.cm-3
).
Fig 5.4 Correlation of long-lived photo-hole population (as measured by the amplitude of the TA
decay at 200 ms) with photocurrent at +0.4VAg/AgCl, both under 355 nm illumination for various
hematite photoanodes under EE (front-side) and SE (back-side) illumination. The best-fit straight
line has an intercept of 0.001(4) mΔOD and gradient of 0.19(4).
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.02
0.04
m
OD
time / s
0 VAg/AgCl
+0.3 VAg/AgCl
+0.6 VAg/AgCl
0.00 0.05 0.10 0.15 0.200.00
0.01
0.02
0.03
0.04
Nb-Fe2O3 USP EE
Fe2O3 CVD SE
Si-Fe2O3 CVD EE
Nb-Fe2O3 USP SE
Fe2O3 PLD EE
Fe2O3 PLD SE
TA
am
plit
ud
e a
t 0
.2s /
mO
D
photocurrent at 0.4V / mA.cm-2
83 Chapter V: Comparison of Sold and Mesoporous Hematite
of applied bias. This quantitative correlation is observed for all hematite photoanodes
examined, including Si-doped and undoped nanostructured CVD films, Nb-doped
mesoporous films fabricated by ultrasonic spray pyrolysis and dense undoped films
deposited by pulsed laser deposition. Figure 5.4 plots the amplitude of this long-lived hole
signal (monitored at 200 ms and 575 nm) against the photocurrent measured at +0.4 VAg/AgCl
for these different Fe2O3 photoanodes. There is a clear common correlation between the
amplitude of the long-lived photogenerated hole signal and the photocurrent density. This
correlation provides further evidence for the importance of long-lived holes in driving water
photo-oxidation, and indicates that this correlation is generic to a broad range of hematite
photoelectrodes.
Comparison of the long-lived hole decay signals of thin (30 nm) solid and thicker
nanostructured hematite photoanodes (Figure 5.5) also shows a strong similarity in the water
oxidation kinetics. However, there is a small difference in the kinetics of water oxidation
(decay of the long-lived hole signal on the milliseconds-seconds timescale). Water oxidation
appears to be marginally faster on the solid ALD photoanodes than on the nanostructured
CVD photoanodes. This may be due to a greater surface hole density on the solid ALD
photoanodes, or due to differences in surface structure on the atomic scale that lead to a
more catalytic surface. This first explanation is unlikely, since previous excitation density
studies on CVD photoanodes (see Chapter IV) showed that the timescale of this decay is
independent of excitation density. Additionally, the surface density of photogenerated holes
is approximately the same for nanostructured CVD and solid PLD photoanodes, assuming
that the same proportion of photogenerated holes reaches the surface. The latter point
depends on the relative rates of electron-hole recombination in the different photoanodes,
which are unlikely to be identical.
An excitation density study of the long-lived hole dynamics in solid PLD photoanodes was
conducted in order to aid the interpretation of these results (Figure 5.6). As the excitation
density is increased, the amplitude of the long-lived hole signal initially increases, then
saturates. In the previous excitation density study of CVD photoanodes, the long-lived hole
signal was also shown to saturate (Figure 4.6b). This saturation behaviour may occur due to
increasingly rapid electron-hole recombination which negates any gain in initial number of
photogenerated holes. It may also be due to the saturation (filling) of the states available
for this long-lived hole to occupy. The nature of the long-lived hole probed by these TA
studies is currently unknown. It is plausible, however, that this hole is a surface-bound
species, such as a high-valent Fe ion bound to a hydroxide radical. Given the long lifetime
of this hole (ca. 300 ms on ALD hematite; ca. 2 s on Si-doped CVD photoanodes), it is
possible that these states become blocked with unreacted holes, resulting in saturation of
the signal.
Chapter V: Comparison of Sold and Mesoporous Hematite 84
The timescale of water oxidation on these solid ALD photoanodes (~300 ms) is clearly
independent of the excitation (and therefore hole) density. This result is very similar to that
obtained using nanostructured photoanodes, as discussed in Chapter IV. This provides
further evidence that the rate determining step in the reaction of these surface trapped holes
with water/surface-bound water species is first order in hole density, and is therefore a
single-hole oxidation.
Fig 5.5 (a) TA decays of holes in 30 nm thick solid ALD (probed at 700 nm; blue) and ~500 nm
thick nanostructured CVD Fe2O3 photoanodes (probed at 600 nm; EE brown, SE orange). (b) The
same TA decays normalised at 3 ms to show the relative timescales of water oxidation.
Measurements were made at potentials were the photocurrent was almost saturated: 0.4 VAg/AgCl
for CVD and 0.6 VAg/AgCl for ALD photoanodes. ALD measurements used EE 355 nm excitation
(25 μJ.cm-2
, corresponding to ~2.4x1013
holes.cm-2
); CVD measurements used EE 355 nm
excitation (190 μJ.cm-2
, corresponding to ~1.5x1013
holes.cm-2
, assuming a roughness factor of
20), SE excitation densities were matched to this.
Fig 5.6 (a) TA decays of holes (probed at 700 nm) in 30 nm thick ALD Fe2O3 photoanodes at
+0.6 VAg/AgCl as a function of excitation density. EE 355 nm excitation at 5 (grey), 25 (black), 50
(blue), 100 (purple) and 250 μJ.cm-2
(pink). (b) The same TA decays normalised at 3 ms,
showing that the kinetics of water oxidation are independent of excitation density.
1E-4 1E-3 0.01 0.1 1
0.00
0.02
0.04
0.06m
OD
time / s
CVD Fe2O
3 SE
ALD Fe2O
3 EE
CVD Fe2O
3 EE
(a)
1E-4 1E-3 0.01 0.1 1
0
1
2
O
D
time / s
CVD Fe2O
3 EE
CVD Fe2O
3 SE
ALD Fe2O
3 EE
(b)
1E-4 1E-3 0.01 0.1 1
0.0
0.5
1.0
1.5 (b)
m
OD
time / s
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1-0.01
0.00
0.01
0.02
0.03
0.04
0.05
0.06
m
OD
time / s
(a)5-250 μJ.cm-2
85 Chapter V: Comparison of Sold and Mesoporous Hematite
It is unclear why the water oxidation kinetics on the 30 nm thick solid ALD photoanode are
approximately one order of magnitude faster than on nanostructured photoanodes. It is
possible that the conformal and relatively smooth surface of the solid ALD hematite is less
disordered or more appropriately structured for binding water or water oxidation
intermediates in such a way as to catalyse the single-hole oxidation reaction probed by
these TA measurements. However, it is notable that the water oxidation kinetics on slightly
thicker (57 nm thick) ALD hematite are the same as on nanostructured hematite, as shown
in Figure 5.7. This suggests that the faster water oxidation kinetics observed for the very
thin ALD films are related to the thickness of hematite, rather than the surface properties
(which are likely to be very similar for different thicknesses of hematite deposited by ALD).
It is also apparent from Figure 5.7 that the TA decays probing photogenerated holes in
these solid photoanodes do not have the same biphasic decay observed for nanostructured
photoanodes (Chapter IV). The “fast decay phase”, associated with electron-hole
recombination, is observed on the microsecond-millisecond timescale in nanostructured
CVD photoanodes. However, this phase is not observed in thin (30 or 57 nm thick) ALD
photoanodes. Although observable in thicker solid photoanodes, the fast decay phase has a
significantly shorter lifetime. Also shown in Figure 5.7 is the TA decay of photogenerated
holes in colloidal Ti-doped hematite photoanodes. These colloidal photoanodes consist of a
nanoporous network of 30-40 nm particles. This small particle size mans that more holes
are generated close to the semiconductor liquid junction compared to photoanodes
consisting of larger particles, resulting in increased peak photocurrents. However, a serious
Fig 5.7 TA decays of holes in colloidal Ti-doped (green), thick (~1 μm) solid Si-doped (purple),
57 and 30 nm thick solid ALD (dark and pale blue, respectively) Fe2O3 photoanodes at positive
applied bias where photocurrent is approximately saturated. EE 355 nm excitation; average
excitation density ca 2x1018
-3x1019
photogenerated holes.cm-3
.
1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0
0.1
0.2
0.3
m
OD
time / s
colloidal Ti-Fe2O
3
SP 1 m thick Sn-Fe2O
3
ALD 30 nm thick Fe2O
3
ALD 57 nm thick Fe2O
3
Chapter V: Comparison of Sold and Mesoporous Hematite 86
disadvantage of these photoanodes is that the photocurrent onset potential is anodically
shifted compared to CVD hematite photoanodes. This has been attributed to recombination
via surface states at grain boundaries between the colloidal particles, which limits the
efficiency of electron transport to the back contact.73 It has also been suggested that
multiple connections between semiconductor particles in a three-dimensional network result
in slower electron transport to the back contact compared to morphologies with fewer
pathways available to electrons.101
It can be seen from Figures 5.7 and 5.8 that the fast TA decay phase of the colloidal
photoanodes extends over a significantly longer timescale than for nanostructured CVD or
solid photoanodes. While the fast decay phase is complete by ~20 ms in CVD photoanodes
at +0.4 VAg/AgC, the fast decay phase extends to ~400 ms for the colloidal photoanodes at the
same bias (Figure 5.8). The timescale of this fast decay phase was previously found to be
related to the timescale of the transient photocurrent (TPC) decay, and is associated with
electron-hole recombination (Chapter IV). Indeed, the lifetime of this decay phase is
reduced at higher excitation densities (not shown), due to faster electron-hole recombination.
The relative amplitude of the fast decay phase compared to the amplitude of the slow phase
is also much greater for colloidal photoanodes than for CVD photoanodes. These TA results
suggest that electron-hole recombination on micro- to milli-second timescales is a more
significant loss process in colloidal photoanodes than for CVD photoanodes.
Since no transient absorption signal in the 550-950 nm region is clearly assignable to
photogenerated electrons, TPC measurements are employed to qualitatively probe electron
Fig 5.8 TA decays of holes (probed at 650 nm) in colloidal Ti-doped Fe2O3 photoanodes at
0.25 (just anodic of the photocurrent onset potential), 0.4 and 0.6 VAg/AgCl. EE 355 nm excitation
at 50 μJ.cm-2
. The fast decay phase (on the microsecond to hundreds of milliseconds
timescale) is significantly longer-lived than in other hematite photoanodes studied.
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.0
0.1
0.2
0.3
mO
D
time / s
+0.6 VAg/AgCl
+0.4 VAg/AgCl
+0.25 VAg/AgCl
87 Chapter V: Comparison of Sold and Mesoporous Hematite
extraction, as demonstrated in Chapter IV. Although the early timescale TPC response is
likely to be limited by the measurement resistor, the TPC from ca. 10 μs shown in the figures
below does not appear to be limited by the measurement resistor (see Section 2.2.3) and
hence provides information about the timescale of electron transport through the
semiconductor. These measurements are made under the same pulsed excitation
conditions used for transient absorption measurements. The TPC signal monitors the
recovery of electron density towards dark equilibrium following the pulsed excitation. This
recovery in electron density will result from electron-hole recombination and electron
extraction by the external circuit, both of which are dependent on electron transport through
the hematite photoanode. Faster electron extraction to the external circuit is expected to
result in reduced electron-hole recombination and thus higher photocurrent densities and/or
a more cathodic photocurrent onset potential.
The TPC signals from colloidal hematite photoanodes are compared with those from thick
(~1 μm) and thin (57 nm) solid hematite photoanodes in Figure 5.9 at positive applied bias
where the photocurrent is approximately saturated. A double peak in the photocurrent
transients is often observed. The first peak could be attributed to charge reorganisation after
laser pulse excitation;102 the charge extracted during this initial peak is only a small fraction
of the total. The second current peak is qualitatively interpreted as the collection of
photogenerated electrons that have diffused to the back contact; given the relatively late
timescales at which this peak occurs (50 μs-10 ms), this is not thought to be limited by the
measurement resistor. The time at which the (second) photocurrent peak occurs is related
to the distance the photogenerated charge must travel to reach the back contact, known as
“time of flight”.
It is clear from Figure 5.9 that electron extraction occurs significantly faster from the thin
solid photoanode (blue curve) than from the thick solid and colloidal photoanodes. Rapid
electron extraction from the thin film is expected since photogenerated electrons only have a
very short distance to diffuse and/or drift (depending on extent of band bending) to reach the
back contact. With this in mind, it might be expected that electron extraction would be
significantly faster from the colloidal photoanode, than from the solid SP photoanode (~1 μm
thick). However, the TPC from the SP photoanode has an initial peak at ca. 10 μs followed
by a second peak at 3 ms, while colloidal TPC only peaks at 5 ms. Similar behaviour is
observed at potentials just anodic of the photocurrent onset.
Together with the behaviour of the fast TA decay phase (Figure 5.8), these results indicate
that electron transport through colloidal hematite photoanodes is significantly slower than
through solid (or nanostructured CVD) hematite. It could be assumed electron transport is
aided in the solid hematite by band-bending, and that this does not occur in the colloidal
hematite photoanodes. However, it is likely that band bending does occur to some extent in
Chapter V: Comparison of Sold and Mesoporous Hematite 88
the colloidal photoanodes (as discussed Section 4.6), since the hematite particles are 30-40
nm in diameter. Additionally, the effect of band-bending (thought to occur on a length scale
of at most tens of nanometres) on electron transport in the ~1 μm thick solid photoanodes is
unlikely to be significant compared to the thickness of the hematite film. Instead of diffusing
directly to the back contact as in solid photoanodes, electrons in colloidal photoanodes must
“hop” between particles, as in a dye-sensitised solar cell. As discussed above, this is likely
to be a slower process than diffusion through a solid semiconductor.101 Recombination via
surface states at grain boundaries73 is also likely to be a more important loss pathway in
these high-surface area photoanodes than in solid or less porous photoanodes.
Fig 5.9 Transient photocurrent (TPC) from pulsed light (EE 355 nm; the same excitation
densities are employed as for TAS measurements) excitation of colloidal Ti-doped (green), thick
solid SP Si-doped (purple) and thin solid ALD (blue) hematite photoanodes. The photoanodes
were held at positive applied bias where photocurrent is approximately saturated. Photocurrent
transients are normalised for ease of comparison.
Fig 5.10 Comparing EE (green/purple) and SE (black) TPC from colloidal Ti-doped (left) and
thick solid SP Si-doped (right) hematite photoanodes (355 nm pulsed excitation). The
photoanodes were held at positive applied bias where photocurrent is approximately saturated.
Photocurrent transients are normalised for ease of comparison.
1E-5 1E-4 1E-3 0.01 0.1 10.0
0.2
0.4
0.6
0.8
1.0
no
rma
lise
d c
urr
en
t /
A
time / s
SP Si-Fe2O
3
colloidal Ti-Fe2O
3
57 nm thick ALD Fe2O
3
1E-5 1E-4 1E-3 0.01 0.1 10.0
0.2
0.4
0.6
0.8
1.0
no
rma
lise
d c
urr
en
t /
A
time / s
SE
EE
1E-5 1E-4 1E-3 0.01 0.1 1
0.0
0.2
0.4
0.6
0.8
1.0
no
rma
lise
d c
urr
en
t /
A
time / s
SE
EE
89 Chapter V: Comparison of Sold and Mesoporous Hematite
Transient photocurrents from colloidal and thick solid hematite photoanodes illuminated
SE (“back side”) and EE (“front side”) are shown in Figure 5.10. Electron extraction is
significantly faster under SE illumination, since electrons are generated close to the back
contact (the absorption depth (α-1) is ca. 30 nm for 355 nm light15). Consequently, the SE
TPC decay kinetics are very similar for colloidal and thick solid photoanodes, despite the
differences under EE illumination.
5.4 UV versus visible excitation
By changing the wavelength of the excitation light pulse, the absorption depth of the light
is also changed. One of the advantages of hematite for water photo-oxidation is its strong
absorption of visible light (Figure 5.1). However, it is widely known that the efficiency of
water photo-oxidation on hematite is much lower under visible light than UV.14, 37, 40 This has
been linked to the relatively long absorption depth for visible light in hematite (ca 100 nm
absorption depth for 500 nm light15) together with the short hole diffusion length (2-4 nm;16
20 nm 17). Consequently, under visible excitation holes are photogenerated several times
further from the semiconductor-liquid junction than their diffusion length.
This difference in light absorption depth is evident in the TPC from solid SP hematite
photoanodes, shown in Figure 5.11. For these measurements, the excitation (laser pulse)
energies were carefully controlled such that the excitation densities were matched EE/SE
and visible/UV, i.e. the same number of photons were absorbed. Under SE illumination,
electrons generated from UV (355 nm) excitation are extracted from the photoanode faster
Fig 5.11 TPC from solid SP Si-doped hematite photoanodes (~1 μm thick) under 355 nm
(purple) and 525 nm (grey) excitation, illuminated SE (left) and EE (right). The photoanodes
were held at positive applied bias (0.5 VAg/AgCl) where photocurrent is approximately saturated;
similar decays are observed at potentials just anodic of the photocurrent onset. The number of
photons absorbed was ~3.0x1018
cm-3
in each measurement. Photocurrent transients are
normalised for ease of comparison; inset: data before normalisation.
1E-5 1E-4 1E-3 0.01 0.1 1
0.0
0.2
0.4
0.6
0.8
1.0
no
rma
lise
d c
urr
en
t /
A
time / s
SE 525 nm
SE 355 nm
1E-5 1E-4 1E-3 0.01 0.1 1
0.0
0.2
0.4
0.6
0.8
1.0
no
rma
lise
d c
urr
en
t /
A
time / s
EE 525 nm
EE 355 nm
Chapter V: Comparison of Sold and Mesoporous Hematite 90
than those generated by visible (525 nm) excitation, due to the generation of charges closer
to the back contact under UV excitation. Accordingly, under EE illumination the main
photocurrent peak occurs at slightly earlier timescales for visible excitation (800 μs and 3 ms
for visible and UV, respectively, at +0.5 VAg/AgCl). It is also evident from the non-normalised
data (inset in Figure 5.11) that a greater number of charges are extracted under UV
excitation than under visible excitation. Since these measurements were conducted under
the same applied potential, and with equal numbers of photons absorbed, a greater number
of electrons extracted is equivalent to larger APCE (absorbed photon to current conversion
efficiency). These results indicate that the lower IPCE (incident photon to current efficiency)
values typically reported for hematite in the visible region, compared to UV (Figure 3.3), are
not only due to the smaller number of photons absorbed.
It is generally accepted that the long absorption depth of visible light in hematite15 is
responsible for the poor efficiency values in this region of the spectrum. Under both SE and
EE visible illumination, carriers are generated far from the SCLJ, so holes are likely to
recombine before reaching the surface. However, there is an alternative argument: UV
excitation excites electrons from the O 2p orbital (charge transfer transition, ~3 eV), while
visible excitation excites electrons from the Fe 3d orbital (d-d transition, ~2 eV). It has been
suggested that in this way two distinct types of holes are initially generated, but holes
generated in the O 2p band would relax to the Fe 3d band, which is thought to have very low
Faradaic efficiencies for water oxidation.17
Using transient absorption spectroscopy to probe holes in hematite photoanodes
generated by UV (355 nm; 3.5 eV) and by visible (525 nm; 2.4 eV) light, it should be possible
to demonstrate that visible light generates a lower yield of the long-lived holes responsible
for water oxidation, compared to UV excitation. Since ultrafast TAS studies have indicated
that electron relaxation and trapping occur on a picosecond timescale,65, 69 photogenerated
holes are almost certainly trapped (e.g. at the top of the valence band, and/or in mid-
bandgap states) on the timescale of the TA measurements reported herein (1 μs-2 s). As
such it is unlikely that microsecond-second TA measurements would differentiate between
holes generated in the O 2p band and those generated in the Fe 3d valence band. Indeed,
as Figure 5.12 shows, the TA spectra of hematite photoanodes at positive applied bias (i.e.
the spectrum of photogenerated holes) are almost identical under UV and visible excitation.
These results indicate that the same type of hole is probed under UV and visible excitation
on microsecond-second timescales. The main difference is in the intensity of the early
timescale bleach at ~575 nm, which is associated with a particular trap state that lies just
below the conduction band edge, and is discussed in detail in the following chapter.
Consequently, the same probe wavelength can be use to probe holes generated by visible
excitation as by UV excitation (usually 650 nm).
91 Chapter V: Comparison of Sold and Mesoporous Hematite
The visible light photogenerated hole dynamics as a function of applied bias are also
found to be similar to the dynamics under UV excitation (Chapter IV), as shown in Figure
5.13. As for UV excitation, under visible excitation the decay time (t50%) of the fast decay
phase (which occurs on microsecond-millisecond timescales) increases with increasing
positive applied bias, indicating that electron-hole recombination is reduced as electron
density is lowered. The amplitude of the TA signal on ~10 ms-2s timescales, associated
with the long-lived holes responsible for water oxidation, also increases with increasing
positive applied bias. These long-lived holes, which have avoided recombination, drive
water oxidation on a timescale of hundreds of milliseconds to seconds.
Fig 5.12 TA spectra of nanostructured CVD Si-doped hematite photoanodes at +0.4 VAg/AgCl
(i.e. the spectra of photogenerated holes) under 355 nm and 525 nm SE excitation. Spectra
under EE excitation are very similar, especially at long timescales.
Fig 5.13 TA decays of holes (probed at 650 nm) photogenerated by 525 nm excitation (SE,
0.16 mJ.cm-2
) in nanostructured CVD Si-doped hematite photoanodes as a function of applied
bias, from -0.3 VAg/AgCl (grey) to +0.4 VAg/AgCl (red). Dynamics under EE excitation are similar.
600 700 800 900
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
m
OD
wavelength / nm
355nm 10 s
355nm 100 s
355nm 1 ms
355nm 10 ms
355nm 100 ms
355nm 1 s
600 700 800 900
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
m
OD
wavelength / nm
525 nm 10 s
525 nm 100 s
525 nm 1 ms
525 nm 10 ms
525 nm 100 ms
525 nm 1 s
1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
mO
D
time / s
+0.4 V
+0.3 V
+0.2 V
+0.1 V
0 V
-0.1 V
-0.2 V
-0.3 V
Chapter V: Comparison of Sold and Mesoporous Hematite 92
The TA studies of photogenerated hole dynamics discussed so far have shown that holes
generated by visible excitation behave in a qualitatively similar manner to those generated
by UV excitation. Considering literature reports and the IPCE and TPC studies discussed
above, a quantitative comparison would be expected to demonstrate that the population of
long-lived holes (i.e. the amplitude of the TA signal on tens of milliseconds to seconds
timescales) is reduced under visible excitation, compared to UV excitation. However, this is
not the case. Figure 5.14 shows a quantitative comparison of the decay dynamics of holes
(probed at 650 nm) generated by visible and UV excitation in nanostructured hematite
photoanodes. The excitation densities were carefully controlled such that the number of
photons absorbed (i.e. the number of charge carriers photogenerated) was the same at each
wavelength (3.0x1014 holes.cm-2 (geometric)). This is to avoid complications caused by
different rates of electron-hole recombination at different excitation densities, as discussed in
Section 4.5. Although there are some differences in the fast decay phase, associated with
electron-hole recombination, it is clear that the long-lived hole signal, associated with water
oxidation, is almost identical in initial amplitude and lifetime. TA decays are shown for
biases just anodic of the onset potential and at positive applied bias where the photocurrent
is almost saturated (0 and +0.4 VAg/AgCl respectively); the same behaviour is observed at
every potential studied. This result apparently indicates that the same number of long-lived
holes is generated under UV and visible excitation, and that the timescale of water oxidation
is also identical. The latter is to be expected, according to the results discussed above
demonstrating that charge carriers generated by UV and visible excitation relax to form the
same species at timescales faster than those employed in these studies. However, the
former is unexpected, as this suggests that UV and visible illumination are equally efficient at
generating long-lived holes that oxidise water.
Fig 5.14 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (blue curves) and
355 nm (black curves) excitation (SE, number of photons absorbed matched) in nanostructured
CVD Si-doped hematite photoanodes at 0 VAg/AgCl (left), and +0.4 VAg/AgCl (right).
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
0.25
m
OD
time / s
355 nm excitation
525 nm excitation
0 VAg/AgCl
1E-5 1E-4 1E-3 0.01 0.1 10.00
0.05
0.10
0.15
0.20
0.25
0.30
m
OD
time / s
355 nm excitation
525 nm excitation
+0.4 VAg/AgCl
93 Chapter V: Comparison of Sold and Mesoporous Hematite
As discussed above, the absorption depth of light in hematite is strongly dependent on the
wavelength. The CVD photoanodes examined above (Figures 5.12-5.14) have a dendritic
“cauliflower-like” nanostructure, in which the smallest feature size is ca 10-20 nm but which
consist of larger features closer to the substrate, on the order of hundreds of nanometres.13
Consequently, changing the wavelength of the excitation pulse not only changes the
absorption depth but also the semiconductor morphology under investigation. This
complicates the interpretation of such results, as the morphology is likely to affect charge
separation efficiencies and recombination rates. In order to avoid such issues, it is
necessary to employ a photoanode in which the nanomorphology is consistent throughout
the hematite thickness. Although the colloidal Ti-doped hematite photoanodes discussed
above fulfil this requirement, the unusually long lifetime of the TA fast decay phase partially
obscures the long-lived hole signal associated with water oxidation. Solid hematite
photoanodes, however, have a more clearly identifiable long-lived hole signal. The ALD
photoanodes discussed above are too thin to give TA signals with sufficient signal-to-noise
ratio under visible excitation. Instead, the thick SP Si-doped photoanodes were employed.
The TA signals of holes (probed at 650 nm) generated by UV and visible excitation in
these solid photoanodes are shown in Figure 5.15. As before, excitation densities were
carefully controlled such that the number of photons absorbed (i.e. the number of
photogenerated charge carriers) was the same at each wavelength. Despite this, and the
homogeneous morphology of these solid photoanodes, as before there is no difference in
hole decay dynamics between UV and visible generation. The amplitude and lifetime of the
long-lived hole signals are almost identical whether UV or visible excitation was employed.
Fig 5.15 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (blue curves) and
355 nm (black curves) excitation (SE, number of photons absorbed matched) in thick solid SP
Si-doped hematite photoanodes at 0 VAg/AgCl (left), and +0.5 VAg/AgCl (right).
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
m
OD
time / s
525 nm excitation
355 nm excitation
0 VAg/AgCl
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
+0.5 VAg/AgCl
m
OD
time / s
525 nm excitation
355 nm excitation
Chapter V: Comparison of Sold and Mesoporous Hematite 94
These results from transient absorption studies (measuring photogenerated holes) are in
contrast to those from corresponding transient photocurrent studies (measuring electrons
extracted to the external circuit) discussed above (Figure 5.11). Although the TPC and TAS
measurements were made under near-identical conditions (pulsed excitation with the same
excitation densities, repetition rate etc), the TPC studies clearly show that photocurrent
generation (and hence oxygen evolution) is more efficient under UV than visible excitation.
This is also found for nanostructured CVD hematite.
As discussed in the previous chapter, although the TAS system employed for these
studies has particularly high sensitivity, such optical measurements are significantly less
sensitive than corresponding electrical measurements such as TPC. This difference in
sensitivity is evident from the TPC and TA decays shown above: although TPC traces were
averaged over only 100 laser shots (compared to 300-1000 shots for TAS), they generally
have a noticeably cleaner signals than the TA decays. It may be that the TA measurements
employed for this study are not sensitive enough to differentiate between the populations of
long-lived holes generated by UV and visible excitation. The thick solid photoanodes
employed produce only very low photocurrent densities, even under UV illumination.
Consequently, the observation of small differences in the amplitude of the TA long-lived hole
signal (i.e. the population of holes that go on to oxidise water) is very difficult. Additionally,
although the total number of absorbed photons was constant under UV and visible
excitation, the number of photons absorbed within the space charge layer was not.
5.5 Conclusions
The charge carrier dynamics in solid and nanostructured hematite photoanodes were
investigated using transient absorption spectroscopy and transient photocurrent
measurements to follow photogenerated holes and electrons, respectively. Water oxidation
kinetics were shown to be very similar on solid and nanostructured hematite, occurring on a
timescale of hundreds of milliseconds to seconds. Although water oxidation appears to be
slightly faster on thin (30 nm thick) solid ALD photoanodes than on nanostructured or thicker
(57 nm thick) solid hematite, excitation density studies demonstrated that water oxidation
kinetics are independent of excitation (hole) densities. Transient photocurrent
measurements demonstrate that the timescale of electron collection (i.e. electron transport)
is highly dependent on the nanomorphology of the photoanode. Both transient photocurrent
and transient absorption measurements indicate that electron transport is significantly slower
in colloidal nanoparticulate photoanodes than in solid or nanostructured CVD photoanodes.
Photoanodes in which the morphology is consistent throughout the thickness of the
hematite layer, such as the colloidal and solid hematite photoanodes, allow direct
95 Chapter V: Comparison of Sold and Mesoporous Hematite
comparison of the dynamics of charge carriers generated by UV and visible excitation to be
made. Both steady-state and pulsed excitation transient photocurrent measurements clearly
demonstrated higher photoconversion efficiencies under UV (355 nm) than visible (525 nm)
excitation in thick solid hematite photoanodes. However, there was no observable difference
in the transient absorption decay dynamics. This is most likely because such optical
measurements are significantly less sensitive than corresponding electrical (TPC)
measurements. Observation of larger long-lived hole populations under UV excitation,
compared to those under visible excitation, may be achievable with photoanodes that
produce significantly larger photocurrent densities than those employed in this study.
Chapter VI: Influence of Trap State 96
Chapter VI
Influence of Trap States
on Charge Carrier Dynamics
In which evidence for the existence of a particular hematite trap state a few hundred
millivolts below the conduction band edge is presented. This trap state manifests as a
strong transient absorption signal observed at ~575 nm, which is positive in undoped
hematite in the absence of anodic bias, but negative (bleached) under anodic bias and in
doped hematite.
97 Chapter VI: Influence of Trap State
6.1 Introduction
In Chapter III, evidence was presented of a photogenerated species probed at ~575 nm
which exhibits distinctly different behaviour to that of photogenerated holes (probed at 650-
900 nm). Although a strong positive transient absorption signal is observed at this
wavelength for undoped hematite in the absence of applied bias, when a positive potential is
applied a bleach is observed on microsecond to millisecond timescales. In the presence of
hydrogen peroxide or Ag+ ions, the decay kinetics of the TA signal probed at 575 nm are
accelerated, suggesting that electrons are probed at this wavelength. However, the
bleaching behaviour observed at positive bias and with H2O2 indicates that this interpretation
is over-simplistic. Evidence that these TA signals are associated with a particular energetic
(“trap”) state, situated a few hundred millivolts below the conduction band edge, is presented
in this chapter.
As discussed in previous chapters, charge carrier dynamics in hematite are dominated
by electron-hole recombination. Positive bias lowers the electron density, which reduces
electron-hole recombination, allowing sufficient hole lifetimes for water oxidation to occur
(with a timescale of hundreds of milliseconds to seconds). Ultrafast transient optical studies
have also indicated that rapid, efficient electron hole recombination occurs on pico- to nano-
second timescales, thought to be mediated by a high density of intra-bandgap states.39, 65
Several studies employing Mott-Schottky analysis have provided evidence for intra-bandgap
trap states positioned close to the CB edge and approximately 0.5-0.7 eV below the
conduction band edge.36, 50, 52, 103 Additionally, photocurrent transients under chopped light
excitation of hematite photoanodes are often observed.3, 17-19, 40, 43, 45, 50-58 These are
associated with surface recombination of conduction band electrons with either surface-
accumulated holes and/or surface-bound oxidation intermediates, as discussed in Section
1.3. The magnitude of these photocurrent transients is generally diminished in the presence
of chemical hole scavengers, which prevent the accumulation of holes at the hematite
surface. Surface treatments with thin layers of Al2O3 and Ga2O3 (which are isomorphic with
hematite) have been shown to reduce the amplitude of these transients. It has been
suggested that such overlayers relax lattice strain at the hematite surface, thus reducing the
density of trap states through which surface recombination is thought to occur (“passivation”
of surface states).44 A number of frequency-domain studies have employed a model
whereby both hole transfer to the electrolyte and surface electron-hole recombination occur
via the same surface states.51, 59, 60 However, it is not clear that these numerous different
studies are probing the same type of trap state.
Herein evidence is provided for a particular energetic state, situated a few hundred
millivolts below the conduction band edge. Transient absorption spectroscopy is used in
Chapter VI: Influence of Trap State 98
conjunction with transient photocurrent measurements to elucidate the timescales of electron
trapping, electron extraction to the external circuit and electron-hole recombination involving
this trap state.
6.2 Experimental
Si-doped nanostructured hematite films deposited by APCVD were used in this study.13
Transient absorption spectroscopy with applied bias (on the microsecond to seconds
timescale), transient photocurrent and photoelectrochemical measurements were made as
outlined in the Methods section. Current/voltage measurements were made at a scan rate of
10 mV.s-1. For chronoamperometry measurements, the dark current was allowed to reach a
constant value before the photoanode was illuminated. TAS and TPC measurements both
employed 355 nm, 0.20 mJ.cm-2, 0.25-0.33 Hz EE (“front-side”) pulsed excitation.
6.3 Spectroscopic Study of Trap State
Typical chopped light photocurrent transients from hematite photoanodes are shown in
Figure 6.1b. As discussed above, such photocurrent transient spikes are attributed to
electron recombination with surface-accumulated holes/surface-bound water oxidation
intermediates.55 These transient decays are approximately exponential, with time constants
of ~300 ms. This recombination timescale is significantly longer than that obtained from the
lifetime of the TA fast decay phase (see Chapter IV), which is ~3 ms under the same applied
bias. Given the different conditions employed for these two types of measurements (6 ns
pulse width illumination for TAS, and illumination for tens of seconds for chopped light
photocurrent), the two recombination timescales are not entirely comparable. However, the
large difference in decay lifetimes suggests that different recombination processes are
Fig 6.1 (a) Current/voltage curves from nanostructured Si-Fe2O3 photoanodes (in 0.1M NaOH, pH
~12.8, white light illumination, 10 mV.s-1
). (b) Chopped light photocurrent transients from
nanostructured Si-Fe2O3 photoanodes at +0.2 VAg/AgCl (355 nm illumination).
0.0 0.2 0.4 0.6
0
1
2
3
4(a)
cu
rre
nt d
en
sity /
mA
.cm
-2
bias (V vs Ag/AgCl)
white EE
white SE
dark
80 85 90 95
0.00
0.02
0.04
0.06
0.08
0.10
0.12
cu
rre
nt
de
nsity /
mA
.cm
-2
time / s
+0.2 VAg/AgCl
EE
+0.2 VAg/AgCl
SE
(b)
99 Chapter VI: Influence of Trap State
probed with TAS and chopped light photocurrent transients.
TA decays of the photogenerated hole signal (probed at 650 nm) in nanostructured, Si-
doped hematite under positive applied bias are shown in Figure 6.2. As discussed in
Chapter IV, the positive hole signal consists of two decay phases: the fast decay phase (1
μs to ~20 ms) is associated with electron-hole recombination, while the slow decay phase
(>20 ms) is attributed to water oxidation. Also shown in Figure 6.2 are the decays probed at
575 nm, which exhibit an intense bleach (negative absorption) on microsecond to
millisecond timescales under these positive bias conditions. It is evident that the fast decay
phase (probed at 650 nm) and the bleach (probed at 575 nm) have essentially the same
lifetime. This is discussed further below.
Fig 6.3 Transient absorption spectra of Si-Fe2O3 photoanodes at (a) -0.7 VAg/AgCl and (b) +0.4 VAg/Agcl
at 10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black through blue to grey) after the excitation pulse.
EE 355 nm pulsed excitation (0.20 mJ.cm-2
).
Fig 6.2 TA decay dynamics of Si-Fe2O3 CVD photoanodes under EE 355 nm pulsed excitation
(0.20 mJ.cm-2
) probed at 650 nm (positive signal) and 575 nm (negative signal) at +0.1 VAg/AgCl
(green) and +0.4 VAg/AgCl (orange). The “fast decay phase” probed at 650 nm and the bleach
probed at 575 nm occur on the same timescale (1 μs to ~20 ms).
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
-4
-3
-2
-1
0
1
h+ (650 nm) +0.4 V
h+ (650 nm) +0.1 V
575 nm +0.1 V
575 nm +0.4 V
m
OD
time / s
600 700 800 900-0.02
0.00
0.02
0.04
0.06 10 s
1 ms
10 ms
100 ms
1 s
m
OD
wavelength / nm
(a) -0.7 VAg/AgCl
500 600 700 800 900
-3
-2
-1
0
1
10 s
100 s
1 ms
10 ms
100 ms
1 s
m
OD
wavelength / nm
(b) +0.4 VAg/AgCl
Chapter VI: Influence of Trap State 100
TA spectra of a hematite photoanode at strong cathodic and anodic applied bias are
shown in Figure 6.3 (similar spectra are obtained with EE and SE excitation). This bleach
feature at ca. 525-625 nm is an inversion of the strong positive feature observed in the
spectrum of undoped hematite in the absence of applied positive bias (Figure 3.2). A similar
positive transient absorption feature is observed for Si-doped hematite at strongly negative
applied bias (Figure 6.3a). Although this feature has a positive absorption in undoped
hematite without applied bias, this feature generally has a negative absorption (bleach) in
doped hematite even in the absence of anodic bias. It is generally observed that hematite
photoanodes with greater activity for water oxidation (nanostructured, doped etc) produce a
more intense bleach, i.e. cathodically shift the onset of the bleach. Surface treatment with
cobalt is also observed to shift the onset of this bleach, discussed in the following chapter.
Fig 6.4 Decay dynamics of Si-Fe2O3 photoanodes probed at 575 nm as a function of applied
bias, (a) from -0.7 to +0.4 VAg/AgCl (black through blue to brown); (b) focusing on -0.7 to -0.3
VAg/AgCl, showing that the decay dynamics are identical cathodic of -0.4 VAg/AgCl.
1E-5 1E-4 1E-3 0.01 0.1-0.10
-0.05
0.00
0.05
0.10
(b)
mO
D
time / s
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0 -0.7 V
-0.6 V
-0.5 V
-0.4 V
-0.3 V
-0.2 V
-0.1 V
0 V
+0.1 V
+0.2 V
+0.3 V
+0.4 V
mO
D
time / s
(a)
101 Chapter VI: Influence of Trap State
Given the relatively narrow wavelength region occupied by this spectral feature, and its
inversion with positive/negative applied bias, it is attributed to an intra-bandgap energetic
state that can be either oxidised or reduced (see below).
The TA bleach dynamics as a function of applied bias, probed at 575 nm, are shown in
Figure 6.4. The depth of the bleach clearly increases with increasing positive bias. The
bleach recovers by ~20 ms; at longer timescales the amplitude of the long-lived signal
increases with increasing positive bias, bearing a strong resemblance to the slow decay
phase of photogenerated holes probed at 650 nm (see Chapter IV). These results signify
that the slow part of the decay phase (>20 ms) of the 575 nm signal probes the same kind of
charge carrier as that probed at longer wavelengths, i.e. long-lived holes that are responsible
for water oxidation on the hundreds of milliseconds to seconds timescale, as previously
indicated in Chapter III.
As more negative bias is applied, the depth of the bleach is decreased until, at -0.4 VAg/AgCl
(approximately equivalent to 0.56 VRHE), a positive decay signal is observed. The initial
(positive) amplitude of this signal increases slightly with more negative applied bias,
however, the decay dynamics at -0.5 to -0.7 VAg/AgCl are identical (Figure 6.4b). One possible
explanation for this behaviour is that at potentials cathodic of the flatband potential (0.4 VRHE
for these Si-doped photoanodes;37 -0.56 VAg/AgCl under these conditions), where no band-
bending occurs, the rate of electron-hole recombination is no longer modulated by changing
the applied bias.
The 575 nm bleach signal first appears at potentials ~200 mV cathodic of the
photocurrent onset potential (Figure 6.5). The increasing magnitude of the (inverted) bleach
Fig 6.5 Overlay of long-lived hole population (given by the amplitude of the transient decay at 100
ms probed at 650 nm), magnitude of the bleach (probed at 10 μs at 575 nm, inverted and multiplied
by 0.1 for ease of comparison) on the photocurrent density curve (355 nm EE excitation).
-0.4 -0.2 0.0 0.2 0.4 0.6
0.000
0.025
0.050
0.075
0.100
0.125
0.00
0.02
0.04
0.06
0.08
0.10
0.12
photo
curr
ent
density /
mA
/cm
2
bias / V vs Ag/AgCl
575 nm x -1/10 (10 us)
650 nm (100 ms)
mO
D
Chapter VI: Influence of Trap State 102
with increasing positive bias follows the shape of the photocurrent/voltage curve, but shifted
cathodically. Although both the photocurrent density and the depth of the bleach increase
with increasing positive bias, there is no quantitative correlation between the magnitude of
the bleach and the photocurrent. This is in contrast to the behaviour of the long-lived hole
signal, as probed at 650 nm on the hundreds of milliseconds timescale (see Chapter IV).
Additionally, the bleach recovers on a timescale shorter than that of water oxidation. This
behaviour suggests the bleach does not probe the species directly responsible for water
oxidation, but follows a species which becomes oxidised at potentials cathodic of the onset
of water oxidation. These results are discussed further in the following section.
Finally, the timescale of electron extraction to the external circuit, as measured by TPC,
and the decay kinetics of the transient absorption bleach are compared in Figure 6.6, where
the inverted TPC decay is overlaid on the transient absorption bleach (probed at 575 nm).
As previously, only the TPC decay (>10 μs) is considered. It was shown in Chapter IV that
the TPC decay and the fast transient absorption decay phase (probed at 650 nm) have very
similar decay kinetics, particularly at low applied bias (for nanostructured, Si-doped
hematite). This was attributed to the domination of both TPC and TAS decays by rapid
electron-hole recombination on the micro- to milli-second timescale. At more positive
applied bias, anodic of the photocurrent onset potential, there were significant differences
between the TPC and TA fast phase decay kinetics. However, it is clear from Figure 6.6 that
even at +0.4VAg/AgCl, ~400 mV anodic of the photocurrent onset potential, the TA bleach and
TPC decay kinetics are extremely similar. Indeed, the lifetimes (from fitting with single
Fig 6.6 Inverted TPC decays (grey, black) overlaid on transient absorption decays (green,
orange) probed at 575 nm under applied bias at +0.1 and +0.4 VAg/AgCl. Si-Fe2O3 CVD
photoanodes under 355 nm EE pulsed excitation.
1E-4 1E-3 0.01 0.1 1
-4
-3
-2
-1
0
mO
D
time / s
TAS, 575 nm, +0.1 V
TAS, 575 nm, +0.4 V
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
inverted TPC, +0.1 V
inverted TPC, +0.4 V
TP
C /
mA
103 Chapter VI: Influence of Trap State
exponential decays) of the TPC decay and the recovery of the TA bleach are almost
identical (0.014 s and ~0.013 s at +0.4VAg/AgCl, respectively). These results suggest that the
recovery of the bleach and the TPC decay are probing the same process, i.e. electron
detrapping and extraction to the external circuit. This is discussed in detail below.
6.4 Discussion
The proximity of the strong absorption/bleach signal at 525-625 nm to the conduction
band edge initially suggests that it may be associated with Moss-Burstein type behaviour of
the hematite band edge.104 This occurs when the lower levels of the conduction band
become populated with electrons, resulting in a blue-shift of the absorption edge. However,
the Moss-Burstein phenomenon is relatively rare, only occurring in semiconductor materials
in which the conduction band has a well-defined edge and a small effective density of states,
such that the Fermi level may shift into the conduction band. This is not the case in
hematite, in which the Fermi level is thought to be ~0.3 eV below the conduction band
edge.36 Additionally, it is likely that Fe2O3 has an exponential distribution of states at the
bottom of the CB, similar to that in TiO2 and other disordered, nanocrystalline metal oxides.64
Moreover, the narrowness of the TA feature at ~575 nm and its inversion with positive
applied bias (Figures 3.1 and 6.3) strongly suggest that it is associated with a intra-bandgap
“trap” state with narrow energetic distribution.
Trap states are generally thought to mediate electron-hole recombination. If the intensity
(depth) of the bleach signal at ~575 nm (on the microseconds timescale) is directly
correlated with the degree of electron-hole recombination, we would expect more intense
bleaching to be associated with lower photocurrent yields. However, the opposite is true;
there is a general correspondence between the intensity of the bleach and the activity of a
given photoanode. For example, both Si-doping and Co2+-adsorption increase the
photocurrent at a given anodic bias,13 and both of these treatments result in a deeper bleach
at a given bias (see the following chapter), i.e. cathodic shift of the bleach intensity. Yet the
presence/depth of the bleach does not directly correlate with the photocurrent onset/density
(Figure 6.5). Instead, the bleach occurs at potentials cathodic of the photocurrent onset
potential and the appearance of the long-lived hole signal. Thus the increasing magnitude of
the (inverted) bleach with increasing positive bias roughly tracks the shape of the
photocurrent/voltage curve, but shifted cathodically by ~200 mV. The bleach signal is
observed to recover (the TA signal becomes positive) by ~50 ms, i.e. faster than the TA slow
decay phase assigned to water oxidation, which has a lifetime on the order of 1 s. These
results suggest that although the 575 nm bleach signal is an indicator of water oxidation
Chapter VI: Influence of Trap State 104
activity, the processes associated with this signal are not directly responsible for water
oxidation.
The recovery of the bleach occurs with an almost identical lifetime to that for the TPC
decay lifetime (Figure 6.6), i.e. the timescale of extraction of electrons from the photoanode
(as discussed in Chapters IV and V; TPC decays at such long timescales are apparently not
limited by the measurement resistor, see section 2.2.3). This strongly indicates that the
recovery of the bleach results from electron de-trapping and extraction to the external circuit.
A similar relationship between the TPC decay and bleach recovery was observed for several
other types of hematite photoanode, suggesting that it is general. Although previous
comparisons of the TPC and TAS (probed at 650 nm) decays showed a similarity between
the TPC and the fast decay phase of the photogenerated hole, there were notable
differences with increasing anodic bias (Chapter IV). The similarity between the timescales
of the TPC and fast phase 650 nm TAS decays was attributed to domination of both electron
and hole decay processes by electron-hole recombination. Figure 6.6 clearly shows that
both the transient absorption fast decay phase (as probed at 650 nm) and the recovery of
the bleach (as probed at 575 nm) occur on similar timescales.‡ As such, it is unsurprising
that there is also a similarity between the timescales of the TPC decay and the bleach
recovery.
The interpretation of these results is outlined in Scheme 6.1. At negative applied bias, the
Fermi level lies close to the conduction band edge and the trap state is occupied by
electrons. There is no ground state absorption from the valence band to the trap state, and
no transient absorption bleach is observed. When a positive bias is applied, width of the
space-charge layer increases (in a solid photoanode, the Fermi level is lowered in a film
consisting of nanoparticles). When the Fermi level lies below the trap state, the trap state is
not occupied by electrons, allowing a ground state absorption from the valence band. This
ground state absorption is lost when electrons that are excited across the band gap by the
laser pulse then relax into the trap state, resulting in a transient bleach (negative absorption).
The bleach recovers (the ground state absorption is regained) as electrons detrap from the
trap state and are extracted to the external circuit. Finally, a positive long-lived transient
signal is regained, which has similar characteristics to the long-lived hole signal (probed at
≥650 nm) that is associated with water oxidation. The relative timescales of electron
detrapping (oxidation of the trap state and bleach recovery; 10-4-10-2 s) and water oxidation
(0.1->2 s) could indicate that the trap state must be oxidised before water oxidation can
occur. This has been suggested by previous studies, which concluded that recombination
‡ This is observed for Si-doped nanostructured hematite photoanodes, but not for solid hematite
photoanodes, in which the fast decay phase (probed at 650nm) has a shorter lifetime than that of the bleach (probed at 575 nm) - see Chapter V. However, solid hematite photoanodes also produce TPC decays with essentially identical kinetics to those of the corresponding TA bleach recovery.
105 Chapter VI: Influence of Trap State
via intra-bandgap trap states in the space-charge layer prevents holes from oxidising water;
water oxidation occurs once the trap state is oxidised.103 Although electron-hole
recombination via the 575 nm trap state is likely to affect the long-lived hole population, it is
probably unsafe to assume that the trap state must be oxidised before water oxidation can
occur.
The intra-bandgap state associated with the transient absorption feature at ~575 nm can
evidently act as either an electron or a hole trap, depending on whether the state is pre-
oxidised (unoccupied by an electron) or pre-reduced, respectively. The occupancy of the
trap is determined by the position of the Fermi level, i.e. by the applied bias. The narrow
spectral region involved (~100 nm wide) implies a narrow energetic distribution of the trap
state. The appearance of the bleach at -0.3 VAg/AgCl, ca. 0.3 V anodic of the flatband
potential, indicates that the trap state is positioned a few hundred millivolts below the
conduction band edge.
This trap state is unlikely to occur only at the hematite surface on the atomic scale, since
the bleach is not observed to saturate, even at strong anodic bias (where the photocurrent is
essentially saturated) on solid hematite photoanodes, which have much lower surface area
Scheme 6.1 Effect of applied bias on trap state and transient absorption bleach (probed at 575
nm). At negative applied bias, the mid-bandgap state is occupied by electrons, so acts as a hole
trap (recombination centre); a positive transient absorption signal is observed. At positive applied
bias, the Fermi level lies below the trap state, which acts as an electron trap; a negative transient
absorption signal (bleach) is observed. Detrapping of electrons and extraction to the external
circuit results in the recovery of the bleach.
positive bias
EF
ECB
EVB
trap
recombination
hν(355 nm)
e-
e-
h+
EF
ECB
EVB
loss ofground state absorption(575 nm)
X
e-
EF
ECB
EVB
hν(355 nm)
e-
h+ e-
ground state absorption(575 nm)
550 600 650 700-1.0
-0.5
0.0
0.5
1.0
mO
D
wavelength / nm
550 600 650 700
-3
-2
-1
0
1
mO
D
wavelength / nm
ECB
EVB
hν(575 nm)
e-
EF
Chapter VI: Influence of Trap State 106
than nanostructured photoanodes. This indicates that a large number of these trap states
are available, and are not fully occupied (saturated) even under significant band-bending.
These results suggest that the trap state is not localised at the surface, but extends
throughout the depth of the hematite film, as shown in Scheme 6.2. As discussed in Chapter
IV, due to the high donor (electron) density of hematite, it is likely that some band bending
occurs even in nanostructured photoanodes. As such, the increasing bleach intensity with
increasing positive bias can be understood by considering the variation in the width of the
space-charge layer/position of the Fermi level. As the width of the space-charge layer
increases/Fermi level moves down with positive bias, further trap states are oxidised (no
longer occupied by electrons), so there is a greater loss of ground state absorption resulting
in a more intense transient bleach. As discussed in previous chapters, as the width of the
space-charge layer increases, the rate of electron-hole recombination decreases resulting in
a larger population of long-lived holes and greater photocurrent density. Consequently, the
intensity of the bleach signal provides an indirect indicator of water oxidation activity. Doped
hematite photoanodes are observed to produce more intense bleach signals than undoped
hematite at a given bias. Doped hematite has a higher electron density, hence a narrower
space-charge layer/higher Fermi level (at a given potential), and thus would be expected to
produce a less intense bleach. However, doping is likely to increase the density of intra-
bandgap trap states, which may explain the more intense bleach signals observed in doped
photoanodes.
The model invoked in Scheme 6.2 is similar to that described by Horowitz, where
impedance and photocurrent measurements provided evidence for a deep donor level 0.7-
0.9 eV below and parallel to the CB edge, and a surface state 0.55 eV below the CB
Scheme 6.2 Effect of applied bias on occupancy of the trap state probed at 575 nm. When the
Fermi level lies above the mid-bandgap state, this state is occupied by electrons (reduced), so
acts as a hole trap (recombination centre). When the Fermi level lies below the trap state is
oxidised and acts as an electron trap. Positive bias increases the width of the space charge layer
(lowers the Fermi level in nanoparticulate films), so more trap states are oxidised.
positive bias
EF
ω
ECB
EVB
+
EF
ω
ECB
EVB
++
++
107 Chapter VI: Influence of Trap State
edge.103 Other studies have also reported deep donor levels 0.5-0.7 eV below the CB edge,
in addition to shallow levels closer to the CB edge.36, 50, 52 It is unclear what the chemical
nature of these trap states may be, although Fe2+ states have been postulated as deep
donors. The shallow levels close to the CB edge may be the same trap state that gives rise
to the 575 nm TA bleach. It seems unlikely that the trap state probed at 575 nm in the time-
resolved studies reported herein is equivalent to the deep donor levels 0.5-0.7 eV below the
CB edge, since the 575 nm state is apparently positioned significantly closer to the CB edge.
Several frequency-domain studies of hematite photoanodes have been published
recently.51, 53, 55, 59, 60 One study into the effect of Al2O3 surface layers on recombination
concluded that such overlayers reduce surface recombination by decreasing the number of
surface states through which recombination occurs.55 However, other studies have
employed a model whereby both hole transfer to the electrolyte and surface electron-hole
recombination occur via the same surface states (Scheme 1.3), which could be
intermediates in the water oxidation mechanism.51, 59, 60 Charging of these states is reported
to cause Fermi-level pinning, where a change in applied bias results mainly in a change in
the voltage drop across the Helmholtz layer. Generally, studies based on the type of model
shown in Scheme 1.3 suggest that the photocurrent onset is coincident with hole
accumulation in these surface states. As discussed in Chapter IV, the TA studies reported
herein indicate that the photocurrent onset correlates with the appearance of a signal
associated with long-lived holes responsible for water oxidation. This suggests that TA
measurements monitoring photogenerated holes (probed at 650-1000 nm) on ca. 10 ms to 2
s timescales may be probing the same species as the surface-state accumulated holes
monitored by frequency domain measurements. The TA hole decay kinetics (i.e. rate of
water oxidation) are independent of applied potential and excitation density (Figures 4.2a
and 4.5c). EIS and IMPS studies have indicated that the hole transfer rate constant
increases with increasing anodic bias and increasing light intensity, varying from
approximately 0 to1 s-1 or 1-10 s-1 at potentials cathodic of the photocurrent onset, to ca. 1-4
s-1 or 10-50 s-1 anodic of Von.51, 60 Additionally, a different EIS study reported surface-trapped
hole lifetimes of 100-10 ms at 0.7-0.8 VSCE respectively (in the dark).17 Consequently, it is
unclear whether TA and frequency domain studies are probing the same hole transfer
process. However, the results of TA studies reported herein indicate that water oxidation
kinetics on hematite are extremely slow, which is likely to result in surface hole
accumulation, in agreement with several frequency domain investigations and studies
employing rapid hole scavengers.18, 19, 51, 53, 59, 60
Optical transmission spectra of these surface trapped holes obtained from potential- and
light-modulated (frequency domain) absorption spectroscopies of hematite photoanodes
have recently been reported.53 Under anodic bias conditions, the PMAS and LMAS spectra
Chapter VI: Influence of Trap State 108
exhibit strong absorption around 500 nm, which decreases into the near IR. These spectra
appear similar to the TA spectra of undoped hematite in the absence of applied bias
reported herein (Figure 3.2). However, under anodic bias the TA hematite spectrum has a
strong negative absorption (bleach) around 575 nm. Clearly further experiments are
required in order to rationalise the different behaviour observed using time-resolved and
frequency-resolved absorption spectroscopies.
Frequency domain studies have reported recombination rate constants that typically
decrease with anodic applied bias, in a similar manner to the increase in charge carrier
lifetimes with anodic bias indicated by TA studies reported herein. However, the rate
constants for surface-state recombination obtained from EIS and IMPS studies are typically
on the order of 1-100 s-1,51, 60 i.e. equivalent to lifetimes of 0.01-1 s. This is significantly
slower than those obtained from TA studies, where the fast decay phase (on timescales of
~1 μs-10 ms) was associated with electron-hole recombination. Instead, recombination rate
constants from frequency domain studies, which may be associated with back electron
transfer to surface-bound water oxidation intermediates, suggest that this recombination
process occurs on timescales equivalent to the beginning of the slow TA decay phase, which
is associated with water oxidation. It is likely that the recombination process probed by TAS
on <10 ms timescales is a different recombination process, i.e. not back-reaction of water
oxidation intermediates.
TAS and TPC studies reported herein have provided evidence for a particular trap state,
positioned a few hundred millivolts below the CB edge. Water oxidation would not be
thermodynamically possible from this trap state, as it lies above the H2O/O2 redox potential.
Hence it is unlikely that this is the surface state probed by the EIS and IMPS studies
discussed above.
6.5 Conclusions
Transient absorption studies of hematite photoanodes have provided evidence of a
particular energetic trap state, positioned a few hundred millivolts below the conduction band
edge. The spectral feature associated with this trap state is a narrow but intense absorption
centred around 575 nm in the absence of anodic bias, but inverts under anodic bias
conditions to form an intense bleach. When the Fermi level lies above the trap state, this
state is occupied by an electron, so acts as a recombination centre (hole trap), resulting in a
positive TA signal. When positive bias shifts the Fermi level below the trap state,
photogenerated electrons can relax into the trap state from the conduction band (electron
trapping). This results in a TA bleach, which recovers as the electron is detrapped and
extracted to the external circuit. Electron trapping occurs on a timescale of <10 μs, electron
109 Chapter VI: Influence of Trap State
detrapping and extraction to the external circuit occur on the milliseconds timescale,
significantly faster than the timescale of water oxidation (hundreds of milliseconds to
seconds). The increasing magnitude of the (inverted) bleach with increasing positive bias
roughly tracks the shape of the photocurrent/voltage curve, but shifted cathodically.
However, no evidence is found to link this trap state to the mechanism of water oxidation.
Chapter VII: Effect of Co-Based Catalysts 110
Chapter VII
Effect of Co-Based Catalysts
on Hematite Charge Carrier Dynamics:
Comparison of Co2+ and Co-Pi
In which the effect on charge carrier dynamics of the addition of cobalt-oxide “catalysts” to
the surface of hematite photoanodes is investigated. Photogenerated hole dynamics are
studied in isolated hematite and in photoanodes in a photoelectrochemical cell before and
after the adsorption of Co2+ to the hematite surface. The results of these studies are
compared to those from studies of hematite/Co-Pi composite photoanodes. Elucidation of
the effect of the cobalt-oxide “catalyst” on the charge carrier dynamics is aided by
understanding of the trap state discussed in the previous chapter.
111 Chapter VII: Effect of Co-Based Catalysts
7.1 Introduction
Despite hematite’s many advantageous properties as a photoanode material for photo-
assisted water splitting, the water oxidation efficiency is limited by rapid electron-hole
recombination.88, 105 The rate-determining step of water oxidation on hematite occurs on a
timescale of hundreds of milliseconds to seconds, while electron extraction to the external
circuit occurs on a timescale of milliseconds. As discussed in previous chapters, this implies
that hole transfer to water or surface-bound water species occurs on a timescale at least two
orders of magnitude slower than charge carrier recombination.105 Several studies have
provided evidence of sluggish charge-transfer kinetics at the hematite-electrolyte interface.17-
20 As demonstrated in Chapter V, changing the morphology of the photoanode affects the
electron transport through the semiconductor film, but does not significantly change the
timescale of water oxidation. However, modifying the hematite surface with water-oxidation
catalysts is expected to increase the rate of water oxidation. Significant increases in
efficiency would be expected for a catalyst that could accelerate water oxidation such that
the timescale of hole transfer from hematite becomes commensurate with electron-hole
recombination. The requirement of an applied electrical bias for water oxidation on hematite
is also a severe limitation of such photoanodes. As such, modifications that reduce this
requirement for applied bias are the subject of intense research effort.
Several different electro-catalysts have been investigated for use with hematite
photoanodes, including RuO2, IrO2 and cobalt-oxide based catalysts. Modifying the
photoanode surface with RuO2 has lead to mixed results, and has not been investigated
extensively.106, 107 Deposition of IrO2 nanoparticles on the surface of nanostructured Si-
doped hematite photoanodes results in shifting the onset potential cathodically by 200 mV
and achieving a photocurrent of >3 mA.cm-2 at 1.23 VRHE under simulated sunlight.46
However, the IrO2 nanoparticles detach from the hematite surface after relatively short
periods of time.
Much interest has focussed on cobalt-oxide based electro-catalysts for water oxidation.
The absorption of a monolayer of cobalt ions from Co2+ solution on to hematite photoanodes
results in ~100 mV cathodic shift in photocurrent potential and increased IPCE.13 The Co-
treatment was achieved simply by soaking the photoanode in a 10 mM aqueous solution of
Co(NO3)2 for a few minutes, which is then rinsed off. This is thought to result in the
deposition of approximately one monolayer of cobalt ions on the hematite surface. The
increased efficiency was attributed to accumulation of photogenerated holes at cobalt
centres with oxo/hydroxo-type ligands, allowing a catalytic cycle for water oxidation involving
Co II/III and II/IV couples similar to that which operates in Photosystem II.
Chapter VII: Effect of Co-Based Catalysts 112
A self-healing cobalt phosphate (“Co-Pi”) amorphous electro-catalyst has recently been
developed.108 This catalyst is thought to have a Co-oxo/hydroxo structure with molecular
dimensions, consisting of edge-sharing CoO6 octahedra.61 At potentials at which oxygen is
evolved, the average valency of the cobalt ions is ≥3. The mechanism of oxygen evolution
on such Co-Pi catalysts is thought to involve a CoIII to CoIV proton-coupled electron-transfer
step prior to the turnover-limiting process.109 Initial studies investigating the effect of Co-Pi
on hematite photoanodes electrodeposited thick (~200 nm) catalyst layers, by applying a
positive bias to a hematite photoanode submerged in a buffer solution of 0.1 M potassium
phosphate (pH 7) containing 0.5 mM Co(NO3)2 for one hour.110 This Co-Pi treatment
cathodically shifted the photocurrent onset by 350 mV. However, the efficiency of these
composite photoanodes was restricted by non-productive light absorption by the catalyst
(which absorbs in the 350-600 nm region, overlapping with the hematite absorption
spectrum), and kinetically limited due to poor mobility of the proton-accepting electrolyte
through the catalyst layer.62 Thin Co-Pi films photo-electrodeposited (under AM 1.5
simulated irradiation, with anodic bias applied for only 200-500 s) on hematite overcome
these limitations, resulting in greater improvements in photocurrent densities and onset
potentials than electrodeposited Co-Pi or absorbed Co2+ catalyst layers.58 It is thought that
photo-assisted electrodeposition results in deposition only where visible light generates
oxidising equivalents, providing a more uniform distribution of Co-Pi onto the semiconductor
surface than obtained by electrodeposition.
Co-Pi and absorbed-Co2+ catalyst layers are deposited on hematite by very different
methods ((photo)-electrodeposition under positive bias, and adsorption from aqueous
solution, respectively).13, 58 However, these result in similar improvements to the PEC
characteristics of hematite photoanodes, namely a cathodic shift in photocurrent onset
potential by typically 100-200 mV and increased photocurrent densities. Such improvements
are often attributed to “catalysed” water oxidation kinetics, with little experimental evidence
to support faster hole transfer kinetics. Due to the high number of redox states available to
Co, it is thought that clusters of Co ions may act as “hole reservoirs” for the four “oxidising
equivalents” (holes) required for the evolution of each O2 molecule, thus catalysing hole
transfer to water. Although several studies characterising the structure of Co-Pi and
possible electro-catalytic mechanisms for water oxidation have been published recently, as
outlined above, there has been almost no attempt to characterise the chemical nature or
structure of the cobalt-based species deposited by Co2+ treatment.
Despite progress in understanding the mechanism of water oxidation on Co-Pi electro-
catalysts in the dark,109 little is known about the charge carrier dynamics of hematite
photoanodes with such cobalt-oxide type surface catalysts. Surface catalysts are generally
assumed to accelerate water oxidation kinetics. Efficient interfacial hole transfer between
113 Chapter VII: Effect of Co-Based Catalysts
hematite and the cobalt-oxide catalyst could increase the electron gradient within the
underlying hematite, aiding charge separation and thus reducing electron-hole
recombination. It has also been tentatively suggested that Fe2O3 surface states, thought to
mediate recombination, may be “passivated” (although the physical meaning of this is rarely
defined) by Co-Pi type catalysts.51, 110 Transient absorption spectroscopy allows the
dynamics of photogenerated holes in hematite photoanodes to be probed as a function of
applied bias.20, 105 Thus the effects of cobalt-oxide type catalysts on charge carrier dynamics
– potentially including transfer of photogenerated holes from Fe2O3 to cobalt ions – in
hematite photoanodes in a working PEC cell can be investigated. An improved
understanding of how these catalysts work could lead to new design rules for more efficient
composite hematite/catalyst photoanodes.
For the studies reported herein, the Co2+ absorption method was employed together with
nanostructured, Si-doped hematite photoanodes to allow direct comparison to literature
studies;13 these results are compared to those obtained in a parallel study of Co-Pi/Fe2O3
composite photoanodes recently published by this group.111 The dynamics of
photogenerated holes in hematite are probed in isolated films and as a function of applied
bias in a complete PEC cell using transient absorption spectroscopy. Transient photocurrent
measurements are used to investigate the dynamics of electrons extracted to the external
circuit. These techniques allow some understanding of the mechanisms by which cobalt-
oxide type materials improve the efficiency of hematite photoanodes for water oxidation.
Elucidation of the effect of the cobalt-oxide “catalyst” on the charge carrier dynamics is aided
by understanding the processes contributing to the transient absorption bleach observed at
~575 nm under positive applied bias, associated with a particular trap state as discussed in
the previous chapter. It is demonstrated that charge carrier dynamics in hematite
photoanodes that have been treated with Co2+ or Co-Pi are almost indistinguishable, despite
the very different methods employed to deposit these two types of cobalt-oxo/hydroxo
layers. No evidence is found for hole transfer from hematite to cobalt, nor for increased
water oxidation kinetics at low to moderate anodic bias. Instead, the increased water
oxidation activity is attributable to a reduction in electron-hole recombination.
7.2 Experimental
Si-doped nanostructured hematite films deposited by APCVD were used in this study. Co-
treatment of the photoanodes was achieved by soaking the photoanode in an aqueous
solution of ~2mM Co(NO3)2 for approximately 30 minutes, then rinsing briefly with de-ionised
water and drying in a nitrogen stream.13 Where photoanodes were treated with Co a second
time, the treatment was repeated as previously. The photoanodes were not heat-treated
Chapter VII: Effect of Co-Based Catalysts 114
before measurements as this is known to destroy the activity of Co-treated hematite.
Transient absorption spectroscopy with applied bias (on the microsecond to seconds
timescale), transient photocurrent and photoelectrochemical measurements were made as
outlined in the Methods section. For chronoamperometry (chopped light photocurrent)
measurements, the dark current was allowed to reach a constant value before the
photoanode was illuminated. TAS and TPC measurements both employed 355 nm, 0.20
mJ.cm-2, 0.25-0.33 Hz EE (“front-side”) pulsed excitation. The electrolyte was 0.1 M NaOH
(pH ~12.8), not degassed.
7.3 Effect of Co-adsorption on charge carrier dynamics
Current/voltage measurements of nanostructured Si-Fe2O3 photoanodes before and after
Co2+-adsorption are shown in Figure 7.1. The photocurrent density is increased and the
photocurrent and dark current onset cathodically shifted with each application of Co2+. After
two applications, the photocurrent onset is shifted cathodically by approximately 100 mV
(from ca +0.06 to -0.04 VAg/AgCl), and the photocurrent density increased from 1.2 mA.cm-2 to
2.0 mA.cm-2 at +0.27 VAg/AgCl (equivalent to ~1.23 VRHE). These results are comparable to
those already reported in the literature for Co2+ treatment,13 and also very similar to the
cathodic shift in photocurrent onset potential and increase in photocurrent density observed
with Co-Pi deposition onto hematite photoanodes.58
Although initial studies tentatively suggested that the Co-oxide based treatments may
“passivate” hematite surface states,110 Figure 7.2 shows that chronoamperometry transient
photocurrent spikes increase in magnitude after Co2+-adsorption. Such spikes are attributed
Fig 7.1 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination, 10 mV.s-1
) of
Si-Fe2O3 APCVD photoanodes (dark grey), after Co-treatment with Co(NO3)2 (blue), and after
repeated Co-treatment (pale blue) for SE (dashed; “back-side”) and EE (solid; “front-side”)
illumination. Inset: expansion of the photocurrent onset region.
-0.2 0.0 0.2 0.4 0.6
0
1
2
3
4
curr
ent
density /
mA
.cm
-2
bias (V vs Ag/AgCl)
0.0 0.1 0.2
0.0
0.5
1.0
1.5
Co
115 Chapter VII: Effect of Co-Based Catalysts
to recombination via surface states or back-reaction of surface intermediates55 (although
some of the initial photocurrent may be attributed to cobalt oxidation, particularly at low
anodic bias62) on relatively slow (hundreds of millisecond to second) timescales; see
Chapter VI for further discussion of these transients. Consequently, these results suggest
that the Co-treatment does not passivate surface states responsible for this slow
recombination. Similar photocurrent transients have been reported for both Co-Pi and Co2+-
treated hematite photoanodes.58 Although some studies have reported a decrease in the
amplitude of such photocurrent spikes after Co-treatment, inspection of those results
suggests that the transient response may be shifted cathodically, rather than reduced across
the whole potential range.47, 58, 62
In order to understand how the Co2+-adsorption increases the photocurrent and shifts the
onset, TAS and TPC (microsecond to millisecond timescales) were used to monitor the
dynamics of photogenerated holes and electron extraction, respectively. Transient
absorption decays of isolated hematite films in 0.1M NaOH (i.e. with no applied bias) before
Fig 7.2 Chopped light photocurrent transients from nanostructured Si-Fe2O3 photoanodes
before and after Co-treatment, at +0.2 VAg/AgCl (355nm EE illumination). SE illumination gives
similar results but with lower photocurrent densities.
Fig 7.3 Transient absorption decays of isolated Si-Fe2O3 photoanodes before (black) and after
(coloured) Co2+
-adsorption (355nm 0.20 mJ.cm-2
EE excitation, 0.1M NaOH, no applied bias),
probed at (a) 575 nm (b) 650 nm and (c) 900 nm.
0 20 40 60 80 100
0.0
0.1
0.2
0.3
0.4
0.5
curr
ent
/ m
A.c
m-2
time / s
extra Co/Si-Fe2O
3
Co/Si-Fe2O
3
Si-Fe2O
3
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.0
0.1
0.2
0.3
0.4
0.5
m
OD
time / s
(b)
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.00
0.05
0.10
0.15
0.20
m
OD
time / s
(c)
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
-1.00
-0.75
-0.50
-0.25
0.00
m
OD
time / s
(a)
Chapter VII: Effect of Co-Based Catalysts 116
and after a single cobalt treatment are shown in Figure 7.3. It is clear that the lifetime of the
charge carriers probed at these wavelengths is increased by several orders of magnitude in
the presence of cobalt, even in the absence of applied bias. As discussed in Chapters III
and IV, long-lived holes, with lifetimes of hundreds of microseconds to seconds, are
necessary for water oxidation to occur on hematite.
The dynamics of photogenerated holes in hematite photoanodes in a complete
photoelectrochemical cell (i.e. under applied bias) were investigated by probing their
transient absorption at 650 nm (Figure 7.4). In the absence of cobalt, the fast phase of this
decay (ca. 1 μs - 20 ms) is associated with electron-hole recombination, while the slow
decay phase (>20 ms) is attributed to water oxidation (see Chapter IV). Although the effect
of Co2+-adsorption is less dramatic than in the absence of applied bias, it results in
significant differences in the decay dynamics. At low applied bias (<0.4 VAg/AgCl,
approximately equivalent to <1.36 VRHE), the population of long-lived holes, as evidenced by
the amplitude of the TA signal at >20 ms, is greater after Co2+-adsorption. The kinetics of
the fast decay phase, associated with charge carrier recombination, are retarded in the
presence of cobalt, particularly at -0.1 V (Figure 7.4a, close to the photocurrent onset
Fig 7.4 Charge carrier dynamics of
photogenerated holes in Si-Fe2O3
photoanodes before (black) and after
(coloured) Co-treatment (355nm 0.20
mJ.cm-2
EE excitation, 0.1M NaOH)
probed at 650 nm under applied bias:
(a) -0.1 VAg/AgCl (b) +0.2 VAg/AgCl and
(c) +0.4 VAg/AgCl. Insets: decays
shown on linear time scales.
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
m
OD
time / s
Co/Si-Fe2O
3
Si-Fe2O
3
(a) -0.1 VAg/AgCl
0.0 0.5 1.0 1.5 2.0
0.0
0.1
0.2
1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
mO
D
time / s
Co/Si-Fe2O
3
Si-Fe2O
3
(c) +0.4 VAg/AgCl
0.0 0.5 1.0 1.5 2.00.0
0.1
0.2
0.3
0.4
0.5
1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
m
OD
time / s
Co/Si-Fe2O
3
Si-Fe2O
3
(b) +0.2 VAg/AgCl
0.0 0.5 1.0 1.5 2.0
0.0
0.1
0.2
0.3
0.4
117 Chapter VII: Effect of Co-Based Catalysts
potential after Co-treatment). Faster hole transfer (to either the catalyst or to water/surface-
bound water species) would manifest as more rapid decay of the TA hole signal to zero,
which is not observed in this potential range. There is no evidence for accelerated water
oxidation kinetics, nor is there evidence for hole transfer from hematite to the “catalyst” - in
other words, there is no evidence that cobalt ions act as “hole reservoirs”. These results are
consistent with water oxidation occurring via a series of single hole transfer steps, as
discussed in Chapter IV. These results are essentially identical to those obtained from
mesoporous Nb-doped hematite photoanodes with a thin photo-deposited layer of Co-Pi
(CoOx).111
At more positive applied bias (+0.4 VAg/AgCl, Figure 7.4c) where significant photocurrent is
generated even without cobalt, the initial population of long-lived holes is unchanged after
Co2+-adsorption. However, the decay kinetics of the long-lived holes – attributed to water
oxidation – are slightly faster after Co2+-adsorption. Although this provides very tentative
evidence for increased water-oxidation kinetics in the presence of a Co-oxide type material,
it is emphasised that this only occurs under very high anodic bias conditions, equivalent to
ca. +1.36 VRHE.
Transient photocurrent measurements are employed to qualitatively probe electron
extraction, as demonstrated in Chapter IV. Although the early timescale TPC response is
likely to be limited by the measurement resistor, the TPC from ca. 10 μs shown in the figures
below does not appear to be limited by the measurement resistor (see Section 2.2.3) and
hence provides provide information about the timescale of electron extraction to the external
circuit. TPC decay dynamics of hematite photoanodes before and after Co2+-adsorption
Fig 7.5 TPC decays probing electron extraction from Si-Fe2O3 photoanodes before (black/grey)
and after Co-treatment (pale colours) at -0.1 VAg/AgCl (green) and +0.4 VAg/AgCl (orange). Pulsed
355nm 0.20 mJ.cm-2
EE excitation, 0.1M NaOH electrolyte.
1E-4 1E-3 0.01 0.1
0.00
0.25
0.50
0.75
1.00
cu
rre
nt
/ m
A
time / s
Co/Si-Fe2O
3 +0.4 V
Ag/AgCl
Si-Fe2O
3 +0.4 V
Ag/AgCl
Co/Si-Fe2O
3 +0.1 V
Ag/AgCl
Si-Fe2O
3 +0.1 V
Ag/AgCl
Chapter VII: Effect of Co-Based Catalysts 118
were investigated at -0.1 and +0.4 VAg/AgCl (Figure 7.5). The first potential corresponds to the
photocurrent onset potential after Co-treatment, but at which there is very little photocurrent
in the absence of cobalt. At +0.4 VAg/AgCl, significant photocurrent is generated even in the
absence of cobalt. The TPC rise is not considered as this is likely to be RC-limited;112
instead, only the decays (from ~100 μs) are discussed. Although there is a small increase in
the total charge extracted from the hematite photoanodes after Co2+-adsorption (to be
expected given the increased photocurrent densities observed in Figures 7.1 and 7.2), it is
evident that the decay kinetics are identical before and after Co2+-adsorption. These results
indicate that Co2+-adsorption does not affect the electron transport properties of hematite,
which is unsurprising since this treatment is likely only to modify the hematite surface and
not the bulk.
The effect of Co2+-adsorption on hematite photoanodes under applied bias is further
investigated by considering the transient absorption spectra, shown in Figure 7.6. Spectra of
Si-Fe2O3 at 0 and +0.4 VAg/AgCl before and after Co2+-adsorption are shown. Generally,
application of positive bias or cobalt treatment increases the depth of the intense early
Fig 7.6 Transient absorption spectra of Si-Fe2O3 photoanodes before (left) and after Co2+
-adsorption (right)
at 0 VAg/AgCl (top) and +0.4 VAg/AgCl (bottom), at 10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black through
blue to grey) after the excitation pulse. There is a striking similarity between Si-Fe2O3 at +0.4 VAg/AgCl and
Co/Si-Fe2O3 at 0 VAg/AgCl.
500 600 700 800 900-4
-3
-2
-1
0
1
m
OD
wavelength / nm
Si-Fe2O
3 0 V
Ag/AgCl
500 600 700 800 900-4
-3
-2
-1
0
1
Co/Si-Fe2O
3 0 V
Ag/AgCl
m
OD
wavelength / nm
500 600 700 800 900-4
-3
-2
-1
0
1
Co/Si-Fe2O
3 +0.4 V
Ag/AgCl
10 us
100 us
1 ms
10 ms
100 ms
1 s
m
OD
wavelength / nm
500 600 700 800 900-4
-3
-2
-1
0
1
Si-Fe2O
3 +0.4 V
Ag/AgCl
m
OD
wavelength / nm
119 Chapter VII: Effect of Co-Based Catalysts
timescale bleach at ~575 nm (discussed below), and increases the population of long-lived
holes (indicated by the pale blue and grey colours). The similarity between the spectra of
the Co2+/Fe2O3 photoanode at 0 VAg/AgCl and the bare photoanode at +0.4VAg/AgCl is especially
striking, particularly in the 500-650 nm region at 10-100 ms (mid-pale blue) where a weak
negative absorption feature is observed at +0.4 VAg/AgCl without the catalyst, but at 0 VAg/AgCl
with the catalyst. This suggests that the result of Co2+-adsorption is equivalent to an
application of more positive applied electrical bias. Additionally, the similarities between the
spectra before and after Co2+-adsorption indicate that there is no hole transfer from hematite
to the catalyst. These results are also essentially identical to those obtained from Co-
Pi/Fe2O3 composite photoanodes.111
As discussed in Chapter VI, the early timescale bleach at ~575 nm is associated with
photo-induced electron trapping by a particular trap state positioned a few hundred millivolts
below the CB edge. The intensity of this bleach signal is an indicator of the degree of
electron depletion of the hematite. Increased bleach intensity is indicative of increased
space-charge layer width or lowering of the Fermi level (depending on the semiconductor
particle size), as would occur with application of a positive bias. However, Co2+-adsorption
also increases the intensity of this bleach, as shown in the TA spectra (Figure 7.6) and the
dynamics of the TA decays probed at 575 nm, shown in Figure 7.7. This effect is greatest at
more modest potentials, comparable to the difference in long-lived hole dynamics before and
after Co2+-adsorption when probed at 650 nm (Figure 7.4). Both the increased intensity of
the bleach and greater long-lived hole population after Co2+-adsorption are equivalent to the
change in charge carrier dynamics observed upon application of approximately 200 mV
anodic bias. This is comparable to the cathodic shift in onset potential observed after Co2+-
adsorption (Figure 7.1).
Fig 7.7 Transient absorption decays probed at 575 nm under applied bias at (a) 0 VAg/AgCl (just
cathodic of the photocurrent onset potential in the absence of cobalt), and (b) +0.4 VAg/AgCl,
(where significant photocurrent is generated even in the absence of cobalt). Before (black) and
after (coloured) Co2+
-adsorption; cobalt increases the magnitude of the bleach, particularly at low
positive applied bias.
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1-2.0
-1.5
-1.0
-0.5
0.0
Co/Si-Fe2O
3
m
OD
time / s
(a)
Si-Fe2O
3
1E-6 1E-5 1E-4 1E-3 0.01 0.1 1
-4
-3
-2
-1
0
Co/Si-Fe2O
3
m
OD
time / s
(b)
Si-Fe2O
3
Chapter VII: Effect of Co-Based Catalysts 120
7.4 Discussion
As demonstrated in previous chapters, long-lived holes are necessary for water oxidation
on hematite, which occurs on the timescale of hundreds of milliseconds to seconds. It is
clear that Co2+-adsorption causes the long-lived hole population to increase, particularly at
low applied bias and in isolated hematite films. The increased long-lived hole signal (probed
at 650 nm, >10 ms) is accompanied by increased bleaching (probed at 575 nm). The
transient absorption spectra of a hematite photoanode without cobalt-oxide at +0.4 VAg/AgCl
and with cobalt-oxide at 0 VAg/AgCl are strikingly similar. Co2+-adsorption is also observed to
reduce the rate of electron-hole recombination, as evidenced by the longer lifetime of the
fast transient absorption decay phase (<10 ms, probed at 650 nm). The effects of Co2+-
adsorption on hematite charge carrier dynamics are almost identical to those observed on
the application of positive bias, suggesting that Co2+-adsorption results in lowering the
hematite Fermi level and/or increasing the width of the space-charge layer. This
interpretation is consistent with the cathodic shift of the photocurrent onset potential
observed after Co2+-adsorption. As discussed in previous chapters, the application of a
positive applied bias to hematite photoanodes reduces electron-hole recombination and
increases the population of long-lived holes.
Except at high positive applied bias, there is no evidence from the TA studies reported
herein that Co2+-adsorption increases the rate of water oxidation on hematite. At +0.4
VAg/AgCl (~1.36 VRHE), the decay of the long-lived hole signal, attributed to water oxidation, is
slightly faster after Co2+-adsorption, as shown in Figure 7.4c. This provides some tentative
evidence for faster water oxidation, but only at potentials significantly above the
thermodynamic water oxidation potential (1.23 VRHE). Additionally, the similarities in the TA
spectra and the kinetics of long-lived holes also indicate that hole transfer from hematite to
adsorbed cobalt does not occur.
The effect of Co2+-adsorption on the charge carrier dynamics in hematite photoanodes is
nearly identical to that of Co-Pi, studied in parallel within this group.111 The large increase in
charge carrier lifetime in isolated hematite after Co2+-adsorption (Figure 7.3) is almost
indistinguishable from that observed for Co-Pi/Fe2O3 composite photoanodes. The effect of
Co2+ and Co-Pi on charge carrier dynamics in hematite photoanodes under applied bias is
also strikingly similar. Both Co2+-adsorption and Co-Pi deposition have essentially the same
effect as applying a more positive electrical bias: increased amplitude of the long-lived hole
signal, reduced electron-hole recombination rate, and increased 575 nm bleach depth. The
results obtained from transient optical measurements reported herein strongly suggest that
Co2+-adsorption and Co-Pi deposition on hematite result in the formation of the same cobalt-
based material, i.e. a cobalt-oxo/hydroxo structure, as reported for Co-Pi (in which an X-ray
121 Chapter VII: Effect of Co-Based Catalysts
spectroscopy study found little evidence for Co-P bonding).61 This is somewhat surprising,
given the very different deposition techniques employed for each: Co-Pi is deposited under
anodic bias conditions, while no bias is applied for Co2+-adsorption. Structural
characterisation of the Co-oxide type material deposited by Co2+-adsorption, similar to
studies already reported in the literature for Co-Pi, could confirm this hypothesis; such
studies are beyond the scope of this work. However, a recent comparison of the effect of
Co2+-adsorption, electrodeposited Co-Pi, and photo-electrodeposited Co-Pi on hematite
photoanodes also reported strikingly similar effects on the current/voltage characteristics of
the photoanodes.58
The results of these transient spectroscopy studies, namely that Co2+-adsorption and Co-
Pi deposition are not observed to enhance water oxidation kinetics, nor is hole transfer to
cobalt observed, contradict previous assumptions that cobalt-oxide type materials “catalyse”
water oxidation on hematite. Instead, these results indicate that deposition of cobalt-oxide
acts in the same manner as a few hundred millivolts’ anodic bias. This lowers the Fermi
level and/or increases band-bending at the hematite/cobalt-oxide interface. As such, the
cathodically shifted photocurrent onset and increased photocurrent densities are attributed
entirely to decreased electron-hole recombination, leading to more long-lived holes, at least
at potentials cathodic of 1.36 VRHE. This interpretation is summarised in Scheme 7.1. The
results reported herein are also in broad agreement with the conclusions of a recent
photoelectrochemical impedance spectroscopy study, in which Co2+-adsorption was found to
almost completely suppress surface recombination.60 It is possible that a Schottky-type
Co2+ treatment
EF
ω
Fe2O3
h+
e-
hν
recombination
ECB
EVB
H2O
O2
EF
ω
Fe2O3 CoOx
O2
H2O
h+
e-
Scheme 7.1 Proposed effect of Co2+
-adsorption/Co-Pi deposition on hematite photoanodes. It
is also possible that the CoOx layer removes (“passivates”) Fe2O3 surface trap states through
which recombination occurs, however the increase in magnitude of chopped light transient
photocurrent spikes after Co2+
-treatment is not consistent with this explanation.
Chapter VII: Effect of Co-Based Catalysts 122
inorganic heterojunction is formed between the hematite and cobalt-oxide, resulting in
increased band-bending at the hematite surface. A similar junction has been reported for n-
BiVO4/p-Co3O4 composite photoanodes, resulting in reduced electron-hole recombination
due to enhanced charge separation.113 A recent investigation into the nucleation and growth
of Co-Pi has suggested that this material is formed of an array of CoO(OH) clusters,
consisting of edge-sharing CoO6 octahedra.114 It is possible that Co2+/Co-Pi deposition
forms a cobalt oxide structure similar to p-Co3O4. Hematite photoanodes with a thin surface
overlayer of p-type (Mg-doped) hematite have recently been reported to exhibit enhanced
photoelectrochemical properties.45 According to the models of hematite charge carrier
dynamics developed herein, it is possible that such p/n junctions could reduce electron-hole
recombination, and hence enhance charge separation, at the hematite surface. It has been
suggested that cobalt ions may act as “hole reservoirs”, since several oxidation states are
available to cobalt. However, it is possible that a cobalt-oxo/hydroxo layer acts as an
electron reservoir, depleting electrons from the hematite surface and thus reducing the rate
of electron-hole recombination. This effect could be investigated by comparison of the
flatband potential before and after cobalt oxide deposition, however this lies beyond the
scope of these thesis studies.
7.5 Conclusions
The charge carrier dynamics in hematite photoanodes before and after Co2+-adsorption
were studied as a function of applied bias, and the results compared to those obtained from
Co-Pi/Fe2O3 composite photoanodes. The effect of Co2+-adsorption on the charge carrier
dynamics in hematite photoanodes is nearly identical to that of Co-Pi. This indicates that
Co2+-adsorption and Co-Pi deposition on hematite result in the formation of the same cobalt-
based material, i.e. a cobalt-oxo/hydroxo structure. Contrary to previous assumptions, no
evidence was found for hole transfer to the cobalt-oxide, nor for increased water oxidation
kinetics at moderate applied bias. Both Co2+-adsorption and Co-Pi deposition have
essentially the same effect as applying a more positive electrical bias: reduced electron-hole
recombination rate, more intense 575 nm bleach, and increased population of the long-lived
holes responsible for water oxidation. These results are interpreted as evidence that
Co2+/Co-Pi induce increased band-bending at the hematite surface. This increased band-
bending reduces electron-hole recombination, resulting in a greater yield of long-lived holes
– particularly at low applied bias – and hence a cathodic shift in photocurrent onset and
greater photocurrent densities.
123 Chapter VII: Effect of Co-Based Catalysts
Chapter VIII: Concluding Remarks 124
Chapter VIII: Concluding Remarks
The primary focus of the PhD studies reported herein was to investigate the physical
processes which limit water photo-oxidation efficiencies of hematite photoanodes.
Specifically, the objective of these investigations was to explain why some types of hematite
photoanode exhibit greater water photo-oxidation activity than others. Transient absorption
spectroscopy (TAS) was used to monitor the photogenerated charge-carrier dynamics,
primarily of holes, in hematite photoanodes on the microsecond-seconds timescale.
Transient photocurrent (TPC) measurements on timescales of microseconds to tens of
milliseconds were employed to probe the kinetics of electron extraction to the external
circuit. TAS and TPC allowed the timescales of electron-hole recombination, electron
trapping/detrapping, water oxidation and electron extraction to be determined. These
techniques were used in conjunction with measurements of photocurrent response as a
function of voltage, allowing comparison of charge-carrier dynamics with water oxidation
activity.
It was shown in Chapters III and IV that charge carrier dynamics in hematite are
dominated by electron-hole recombination on microsecond to millisecond timescales.
Transient absorption studies of hematite photoanodes in a working PEC cell indicated that
the role of a positive applied bias is more complex than previously thought. Positive
electrical bias not only increases the reduction potential of photogenerated electrons, but
also reduces the background electron density in hematite. This was found to result in
decreased electron-hole recombination and thus increased hole lifetime, such that
photogenerated holes can diffuse/drift to the semiconductor-electrolyte junction and oxidise
water. The timescale of water oxidation was found to be on the order of hundreds of
milliseconds to seconds, hence extremely long-lived holes are required for water oxidation to
occur. A quantitative correlation between the yield of long-lived photogenerated holes and
photocurrent density was demonstrated. The rate constant of water oxidation by these long-
lived holes was independent of hole density, indicating that water oxidation proceeds via a
rate-determining single-hole transfer step. These results support proposed water oxidation
mechanisms consisting of multiple sing-hole transfer steps.
The charge carrier dynamics in hematite photoanodes with various different morphologies,
including thick and thin solid (non-porous), nanoparticulate colloidal, and nanocrystalline
dendritic nanostructured hematite films were compared in Chapter V. Water oxidation
kinetics were shown to be very similar on solid and nanostructured hematite. Transient
photocurrent measurements demonstrate that the timescale of electron collection (i.e.
electron transport) and electron-hole recombination losses are highly dependent on the
125 Chapter VIII: Concluding Remarks
nanomorphology of the photoanode. Since hole transfer (water oxidation) is a slow process,
more rapid electron extraction is likely to reduce losses by electron-hole recombination.
Thus hematite photoanodes should be engineered to maximise rapid electron transport to
the back contact. This may be achieved by employing heterostructures with a material with
high electron mobility between hematite and the back contact, or by using deposition
techniques which minimise grain boundaries.
In Chapter III, results of transient absorption studies with chemical scavengers and
applied bias suggested two distinct photogenerated species: trapped holes and a second
species, with a narrow but intense absorption (in undoped hematite with no applied bias)
centred around 575 nm. The bleaching behaviour of this signal at positive potentials
suggests that this feature is associated with the photo-oxidation and -reduction of a trap
state located just below the conduction band edge; further evidence for this interpretation
was discussed in Chapter VI. TAS and TPC were employed to probe the electron
trapping/detrapping and recombination associated with this trap state. Although trap state
oxidation was found to occur prior to hole transfer to water/surface-bound water species, no
direct evidence was found to suggest that this trap state is directly involved in the water
oxidation mechanism. Instead, the magnitude of the TA bleach was modelled as an
indicator of the degree of electron depletion of the hematite film.
Finally, the effect of Co2+-adsorption on the charge carrier dynamics in hematite
photoanodes was investigated and compared with that of Co-Pi in Chapter VII. These two
cobalt surface treatments, deposited under very different conditions, were found to result in
essentially identical charge carrier dynamics. This indicated that Co2+-adsorption and Co-Pi
deposition on hematite result in the formation of the same cobalt-based material, i.e. a
cobalt-oxo/hydroxo structure. No evidence was found for hole transfer to the cobalt-oxide,
nor for increased water oxidation kinetics at moderate anodic bias. Cobalt treatments were
demonstrated to have essentially the same effect as applying a more positive electrical bias.
These results were interpreted as evidence that Co2+/Co-Pi induce increased band-bending
at the hematite surface. This increased band-bending reduces electron-hole recombination,
resulting in a greater yield of long-lived holes – particularly at low applied bias – and hence a
cathodic shift in photocurrent onset and greater photocurrent densities. The results of these
investigations imply that materials that are thought to be electrocatalysts for water oxidation
may not actually catalyse water photo-oxidation on semiconductor materials. The term
“catalyst” should be used with caution.
These studies have elucidated the relative timescales of electron-hole recombination,
electron trapping/detrapping, water oxidation and electron extraction to the external circuit in
hematite photoanodes with a variety of different nanomorphologies. Generally, effects which
Chapter VIII: Concluding Remarks 126
lower electron density result in retarded electron-hole recombination kinetics, increasing the
population of long-lived holes and hence increasing the photocurrent.
Future Work
Although the primary objectives of this project have been achieved, further studies could
advance our understanding of the physical processes occurring in hematite photoanodes,
and test the models of charge carrier dynamics constructed during these thesis studies.
Some potential areas of investigation are outlined below.
Ultrafast TAS: The time resolution of the TA measurements reported herein does not
allow charge carrier dynamics faster than ~1 μs to be monitored. It is evident that, even
under anodic applied bias, significant electron-hole recombination occurs on sub-
microsecond timescales. Significant differences in recombination dynamics on microsecond
were observed between solid and nanostructured hematite; does the “fast decay phase”
occur on sub-microsecond timescales in solid hematite? Additionally, what processes
associated with the 575 nm trap state occur on <1 μs timescales?
IR-probe TAS: No transient absorption signal clearly assignable to photogenerated
electrons in hematite has yet been observed. It is possible that photogenerated electrons in
hematite absorb at wavelengths greater than 1000 nm. Direct optical measurement of
electrons would allow testing of the models of electron-hole recombination and electron
trapping/detrapping and extraction developed during these PhD studies.
TAS and TPC under white light bias: The TAS and TPC studies reported in this thesis
were conducted in the “dark”, i.e. with no white light incident on the photoanode.
Consequently, these measurements were not made under working conditions typical for
photoelectrochemical cells. Future studies should compare charge carrier dynamics under
white light bias to those reported herein.
Heterojunction photoanodes: As discussed above, heterojunctions of hematite with a
layer of high-electron-mobility material may reduce electron-hole recombination losses by
rapidly extracting electrons to the external circuit. TAS and TPC studies of such
heterojunction photoanodes could be employed to determine whether more rapid electron
extraction from the hematite does occur, and may also elucidate the effects of potential
electron-hole recombination at the junction.
According to the models of hematite charge carrier dynamics developed during these
thesis studies, it is possible that thin surface overlayers of p-type materials could reduce
electron-hole recombination by enhancing band-bending at the hematite surface. Transient
absorption studies of hematite photoanodes with thin overlayers materials such as Co3O4 or
Mg-doped Fe2O3 could aid the understanding of how such p/n junctions enhance the
photoelectrochemical properties of hematite photoanodes.
127 Chapter VIII: Concluding Remarks
Chapter IX: References 128
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