heterojunction bivo4/wo3 electrodes for enhanced photoactivity of water oxidation

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Heterojunction BiVO4/WO3 electrodes for enhanced photoactivity of wateroxidation

Suk Joon Hong, Seungok Lee, Jum Suk Jang and Jae Sung Lee*

Received 5th December 2010, Accepted 24th February 2011

DOI: 10.1039/c0ee00743a

Heterojunction electrodes were fabricated by layer-by-layer deposition of WO3 and BiVO4 on

a conducting glass, and investigated for photoelectrochemical water oxidation under simulated solar

light. The electrode with the optimal composition of four layers of WO3 covered by a single layer of

BiVO4 showed enhanced photoactivity by 74% relative to bare WO3 and 730% relative to bare BiVO4.

According to the flat band potential and optical band gap measurements, both semiconductors can

absorb visible light and have band edge positions that allow the transfer of photoelectrons from BiVO4

to WO3. The electrochemical impedance spectroscopy revealed poor charge transfer characteristics of

BiVO4, which accounts for the low photoactivity of bare BiVO4. The measurements of the incident

photon-to-current conversion efficiency spectra showed that the heterojunction electrode utilized

effectively light up to 540 nm covering absorption by both WO3 and BiVO4 layers. Thus, in

heterojunction electrodes, the photogenerated electrons in BiVO4 are transferred to WO3 layers with

good charge transport characteristics and contribute to the high photoactivity. They combine merits of

the two semiconductors, i.e. excellent charge transport characteristics of WO3 and good light

absorption capability of BiVO4 for enhanced photoactivity.

1. Introduction

The photoelectrochemical (PEC) cell is the most advanced

method to produce hydrogen by water splitting using solar light.

Since the first demonstration of this concept in 1972,1 much

attention has been paid to semiconductor materials which could

be used as photoelectrodes.2–4 Although TiO2 is the first and the

most studied anode material employed for the PEC cell,5–7 its

large band gap of 3.2 eV absorbing only UV part of the solar

spectrum is a significant drawback in practical application for

solar hydrogen production. Thus, recent research has focused on

the development of visible light-active materials.8–11

Department of Chemical Engineering and Division of Advanced NuclearEngineering, Pohang University of Science and Technology(POSTECH), San 31, Hyoja-dong, Pohang, Gyeongbuk, 790-784, SouthKorea. E-mail: [email protected]; Fax: +82-54-279-5528; Tel: +82-54-279-2266

Broader context

In heterojunction WO3/BiVO4 electrode, the photogenerated ele

production of photocurrents. Thus by combination of the merits o

teristics of WO3 and good light absorption capability of BiVO4, th

activity relative to bare WO3 and BiVO4 electrodes, respectively, i

light.

This journal is ª The Royal Society of Chemistry 2011

Traditional visible-light photocatalysts are either unstable in

water upon illumination (e.g., CdS and CdSe)12 or have low

activity (e.g., WO3 and Fe2O3).13 Recently, some UV-active

oxides are modified to function as visible-light photocatalysts by

substitutional doping of metals3 or anion doping with N, C, and

S.8,14,15 However, these doped materials, in general, show only

a little absorption in the visible-light region, leading to low

activities.16 New and more efficient visible-light photocatalysts

are needed to meet the requirements of future environmental and

energy technologies driven by solar energy. In contrast to single-

component photocatalysts, composite heterojunction of two

semiconductors has been recognized as an attractive method to

develop a high efficiency material under visible light. It can

combine merits of each component to show synergistic

effects.17,18 In addition, by forming a junction structure, it can

promote efficient electron–hole separation to minimize the

energy-wasteful electron–hole recombination. For example,

Nozik constructed an n-TiO2/p-GaP diode electrode for a PEC

ctrons in BiVO4 are transferred to WO3 layers for efficient

f the two semiconductors, i.e. excellent charge transfer charac-

e heterojunction electrode shows 74% and 730% higher photo-

n photoelectrochemical water oxidation under simulated solar

Energy Environ. Sci., 2011, 4, 1781–1787 | 1781

http://dx.doi.org/10.1039/c0ee00743a






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cell, which was found to be much more active than either n-TiO2

or p-GaP single-component electrode for the decomposition of

water.19 The CdS/TiO2 composite materials showed greater

photoactivities than bare CdS or TiO2 because it could absorb

visible light due to the low band gap of CdS and improve the

charge separation of photogenerated electrons and holes by

transferring photoelectrons from CdS to TiO2.20,21

In this work, we studied the WO3/BiVO4 heterojunction elec-

trode for the enhanced photoactivity of water oxidation. WO3 is

an attractive material, which can absorb the blue part of the solar

spectrum up to ca. 500 nm corresponding to the band gap energy

of ca. 2.7 eV.22,23 In addition, WO3 is stable in aqueous solution

under irradiation. Thus, WO3 has been the most popular visible-

light absorbing photoanode for PEC cells, yet its band gap

energy of 2.7 eV limits the theoretical solar-to-hydrogen (STH)

efficiency to ca. 4.5%. We have modified WO3 for utilizing more

sunlight by forming a heterojunction electrode with BiVO4.

BiVO4 is also a well-known photocatalytic material with a band

gap of 2.4 eV. However, its photoactivity is usually low because

of its inferior charge transport properties. If we combine the

merits of these two materials, i.e., WO3 with good charge

transport properties and BiVO4 with good optical absorption

properties, by forming a heterojunction, a synergistic improve-

ment of the photoactivity is expected.

Thus, BiVO4 can be a promising candidate material as the

partner of the heterojunction electrodes for enhancing the

activity of WO3. In a composition-wise similar approach,

Chatchai et al. studied the WO3/BiVO4 composite electrode for

enhancing the activity of BiVO4 electrode by using WO3 layer.25

However, their WO3 was not a photoactive layer itself, but

a buffer layer similar to SnO2.24 In the present work, we studied

WO3/BiVO4 heterojunction electrodes to red-shift the absorption

edge of WO3 and thereby to enhance the photoactivity of WO3

electrode for water oxidation, i.e. the half-reaction of water

splitting under solar light. The electrochemical and photo-

electrochemical properties were studied to understand the origin

of the synergistic effect of the heterojunction electrodes and to

evaluate the potential of the heterojunction electrodes for PEC

cell application.

2. Experimental

The WO3 films were prepared by polymer-assisted direct depo-

sition method as reported previously.26 Briefly, ammonium

metatungstate (AMT) and polyethyleneimine (PEI) were dis-

solved in distilled water. A completely dissolved solution was

used to coat F-doped SnO2 (FTO) glass substrate. After coating,

the films were dried at 80 �C and calcined at 550 �C for 90 min.

The BiVO4 films were prepared as reported elsewhere with some

modification.27 Thus, the Bi(NO3)2 and ammonium vanadate

hydrate were dissolved in distilled water and a separate PEI

solution in distilled water was prepared. After complete disso-

lution, two solutions were mixed to obtain a coating solution.

Coating and calcination procedures were identical to the ones

used to prepare WO3 films. For composite electrodes, each layer

was coated on the top of previous layers after calcinations

between coatings.

The crystalline phase of the products was determined using an

X-ray diffractometer (Mac Science Co., M18XHF) with

1782 | Energy Environ. Sci., 2011, 4, 1781–1787

monochromatic Cu Ka radiation at 40 kV and 200 mA. The

morphology of samples was investigated using a scanning elec-

tron microscope (JEOL JSM-7401F, HR-Field Emission Elec-

tron Microscope) instrument operated at 10 kV. The film

thickness was measured by a profilometer (alpha-step 500 surface

profiler, KLA Pencor).

The photoelectrochemical measurements were carried out

using a standard three-electrode cell with a Ag/AgCl (3.0 M

NaCl) reference electrode, a platinum foil as a counter electrode

and a potentiostat (Potentiostat/Galvanostat Model 263A

EG&G Princeton Applied Research). In order to measure the

photoactivity, the fabricated film was connected to a copper wire

with silver paste, and then the exposed FTO surface was covered

with epoxy resin. The photocurrent–potential curve was recor-

ded under simulated solar spectrum generated by a solar simu-

lator (Oriel 91160) with AM 1.5G filter. The light intensity of the

solar simulator was calibrated to 1 sun (100 mW cm�2) using

a reference cell certified by the National Renewable Energy

Laboratories (NREL), USA. To measure the performance vs.

light wavelength (IPCE), we used an Hg lamp (450 W, Oriel), and

a monochromator with a bandwidth of 5 nm. The electro-

chemical impedance spectroscopy (EIS) was performed using

a potentiostat and frequency response detector (FRD100, PAR).

The Nyquist plots were measured at 0.7 V (vs. Ag/AgCl) with an

AC amplitude of 20 mV, frequency of 100 kHz–100 mHz under

AM 1.5G illumination (1 sun). The measured spectra were fitted

by using the ZSimpWin program (from Echem software). The

0.5 M Na2SO4 solution was used for the all

electrochemical measurements. For converting the obtained

potential (vs. Ag/AgCl) to RHE (NHE at pH ¼ 0), the following

equation was used.

ERHE ¼ EAgCl + 0.059 pH + EAgCl0,

EAgCl0 (3.0 M NaCl) ¼ 0.209 V at 25 �C.

3. Results and discussion

3.1 Physicochemical properties

To investigate the effect of heterojunction formation between

WO3 and BiVO4, we prepared six samples, i.e., bare WO3,

composites 1–4, and bare BiVO4. The number denoting hetero-

junction electrodes indicates the number of BiVO4 layers in

composite electrodes. All films consist of five layers of WO3 or

BiVO4 with the same thickness. The configuration of samples is

represented in Scheme 1. The upper layer was deposited on top of

the lower layers that had been calcined after deposition. Each

layer had the same thickness and total film thickness was the

same around 3 mm.

Fig. 1 shows X-ray diffraction patterns of bare WO3, BiVO4,

and composites 1–4. The high intensity peaks at 2q of 26.6, 34.2,

37.8, and 51.6� of all samples are due to FTO used as the

substrate. In the bare WO3, the peaks of 23.3, 23.8, and 24.6� are

observed corresponding to (002), (020), and (200) planes for the

monoclinic phase of WO3 (JCPDS 43-1035). The phase of WO3 is

transformed from triclinic to monoclinic at ca. 400 �C and thus,



Scheme 1 The electrodes made of bare WO3, BiVO4 and composites of

different compositions on FTO substrate. The light gray colour indicates

BiVO4 and dark gray WO3.

Fig. 1 X-Ray diffraction patterns of bare WO3, BiVO4 and composite

films (W ¼ monoclinic WO3, B ¼ monoclinic BiVO4, F ¼ FTO).

Fig. 2 UV-Vis absorbance spectra of bare WO3, BiVO4, and composite

films.

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monoclinic phase was observed in our sample that was calcined

up to 550 �C.28 The monoclinic WO3 is known to show the

highest photocatalytic effect than other crystal phases.29,30 In

case of bare BiVO4, the main peaks at 18.5 and 29.4� represent

scheelite-monoclinic structure (JCPDS 14-0688). The tetragonal-

zirconia phase of BiVO4 is transformed to monoclinic at 400–

500 �C.31 Thus, the calcination at 550 �C in this work formed the

monoclinic phase of BiVO4. Because only monoclinic BiVO4

structure has good photocatalytic activity,32 these BiVO4 films

contain proper phase as an active photoanode. In the composite

1, the small peak of BiVO4 starts to appear at 18.5� and increases

with increase of BiVO4 layers. In contrast, the WO3 peaks


decreased from composite 1 to 4 because of the reduced number

of WO3 layers.

The optical behavior of the films was investigated by UV-Vis

absorbance spectra as shown in Fig. 2. The light absorption of

bare WO3 started at around 475 nm in correspondence with its

band gap energy. For the bare BiVO4 film, the onset of light

absorption is around 520 nm, again corresponding to its band

gap energy. In the composite films, the band absorption edges

shifted to longer wavelengths with increased number of BiVO4

layers. Thus, the absorption range of the WO3 electrode was

enhanced by coupling of BiVO4 that has a smaller band gap

energy.

Fig. 3 presents scanning electron microscopy (SEM) images of

bare WO3, BiVO4 and composite 1 electrodes. As shown in

Fig. 3a, the bare WO3 film shows a porous morphology with the

small grain sizes below 100 nm. The relatively small grain size

and the well-developed porosity are advantageous for high

photoactivity because of larger number of reaction sites on the

surface and the smaller distance that the holes have to travel to

reach the electrolyte/electrode interfaces to react with water.22,23

The composite 1 film, in which a layer of BiVO4 was deposited on

top of four WO3 layers, also shows high porosity with particle

sizes of �100 nm. However, the bare BiVO4 shows larger grain

sizes than other films. The space between grains is developed by

burning polyethyleneimine (PEI) introduced as a binder to make

films. Thus, the morphology and porosity are determined during

the calcination step when the polymer is burnt away and crys-

tallization of semiconductors takes place. Thus, the final

morphology is dependent on the crystallization materials and

substrates. Note that the BiVO4 layer for bare films was depos-

ited on bare FTO, yet BiVO4 for composite films was grown on

the WO3 layer. The Fig. 3d is the energy dispersive X-ray spec-

trum (EDS) for the cross-section of composite-2 electrode. The

Sn layers were observed in the bottom layers due to the FTO

substrate. The V atoms originated from BiVO4 were observed at

the top layers and W atoms from WO3 between BiVO4 layers and

the FTO substrate. Thus, we can confirm the layer-by-layer

structure of composite electrode as shown in Scheme 1. However,



Fig. 3 Scanning electron microscopy images of (a) bare WO3, (b)

composite 1, and (c) bare BiVO4 films. (d) EDS data for cross-section of

composite 2.

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the interface between different layers is not sharply defined

showing significant interpenetrating tails.

3.2 Photocurrent measurements

The photoactivity of each film was determined by measuring the

photocurrent density generated during water oxidation as AM

1.5G simulated solar light (light intensity: 1 sun or 100 mW cm�2)

was irradiated on the front (semiconductor) side of the film

immersed in 0.5 M Na2SO4 (at pH ¼ 6.6). The WO3 electrodes

are stable in aqueous solution with a wide pH value (pH ¼ 0–7),

while the BiVO4 electrode is weak in acidic media. Thus, the

photocurrent measurements were performed in the neutral elec-

trolyte. Fig. 4 indicates the photocurrent density curves of bare

WO3, BiVO4 and composite 1–4 films. The photocurrents

increased with increasing applied anodic potential representing

a typical n-type semiconductor behavior, because both WO3 and

BiVO4 are n-type semiconductors. The inset of Fig. 4 is

Fig. 4 Photocurrent densities of bare WO3, BiVO4 and composite films

measured under AM 1.5G simulating solar light (1 sun) in 0.5 M Na2SO4

solution. The inset is a magnified view of photocurrent curves near onset

potential. Dark current is for the composite 1 film.


a magnified view in 0–0.5 V (vs. Ag/AgCl) of applied potentials to

determine photocurrent onset potential. The photocurrent onset

of WO3 was around 0.25 V (vs. Ag/AgCl) and the onset poten-

tials of composite films were negatively shifted to ca. 0.05 V

(vs. Ag/AgCl) by introducing the BiVO4 on WO3 films. The onset

potential of photocurrent normally indicates the flat band

potential of the electrode; however, interfacial charge transport

limitation induces errors that lead us to underestimate the

value.33 Thus, we measured the photocurrent–potential curves in

the electrolyte with 0.05 M methanol solution, which acted as an

efficient hole acceptor (Fig. 5). The methanol can reduce the

kinetic barrier for charge transport by capturing the photo-

generated hole efficiently.

In Fig. 5, the onset potential values were observed at �0.05 V

for WO3, �0.65 V for BiVO4, and �0.15 V for composite 1

electrode. Namely, the flat band potential of WO3 was shifted to

negative potential by coupling with BiVO4, whose flat band

potential is more negative. The negative shift indicates larger

accumulation of electrons in the WO3/BiVO4 composite elec-

trode and reflects decreased charge recombination.34,35 This

negative shift is also of practical importance for the photo-

electrochemical water splitting for hydrogen production.

The photocurrents of WO3 were 1.0 mA cm�2 at 0.7 V

(vs. Ag/AgCl) and 1.65 mA cm�2 at 1.2 V (vs. Ag/AgCl) as shown

Fig. 4. The composite 1 exhibited the highest photocurrents of

1.74 mA cm�2 at 0.7 V and 2.78 mA cm�2 at 1.2 V by forming

a composite with the BiVO4 layer. However, the photocurrent of

Fig. 5 The photocurrent–potential curves measured under chopped

light (AM 1.5G) in 0.5 M Na2SO4 + 0.05 M methanol.



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composite films decreased with further increase of BiVO4 layers

from composite 1 to composite 4. The lowest photocurrent was

observed at bare BiVO4 electrode with 0.21 mA cm�2 at 0.7 V and

0.63 mA cm�2 at 1.2 V. Thus, the significant enhancement of

photocurrent was observed by coupling BiVO4 on WO3 forming

a heterojunction in composite 1 electrode, which has an opti-

mized composition (BiVO4/WO3 layers) of 1/4. The photo-

activity of the composite 1 electrode represents enhancement of

74% and 730% relative to bare WO3 and BiVO4 electrodes,

respectively.

3.3 The origin of photoactivity enhancement in heterojunction

composite films

Thus, the photocurrent of WO3 was increased by coupling with

BiVO4 forming heterojunction composite electrodes of proper

compositions. In order to understand the origin of photoactivity

enhancement in the heterojunction system, it is useful to deter-

mine the potential energy diagram of the composite film. To

determine the relative band positions of each material in this

WO3/BiVO4 composite film, we measured flat band potentials

and optical band gaps of WO3 and BiVO4. In Fig. 5, the flat band

potential was measured by the onset potential for the photo-

current. Here, the flat band potential of electrodes was deter-

mined by the Mott–Schottky relation:36

1/C2 ¼ (2/e330Nd)[Va � Vfb � kT/e]

where C ¼ space charge layers capacitance, e ¼ electron charge,

3 ¼ dielectric constant, 30 ¼ permittivity of vacuum, Nd ¼ elec-

tron donor density, Va ¼ applied potential, and Vfb ¼ flat band

potential. The flat band potential (Vfb) was determined by taking

the x intercept of a linear fit to the Mott–Schottky plot, 1/C2, as

a function of applied potential (Va).

As shown in Fig. 6a, the flat band potentials of WO3 and

BiVO4 were �0.19 V and �0.58 V (vs. Ag/AgCl), respectively.

These potentials matched the obtained values in Fig. 5.

Assuming the gap between flat band potential and bottom edge

Fig. 6 (a) Mott–Schottky plot and (b) Tauc plot of bare WO3 and

BiVO4. The Mott–Schottky plots were measured in 0.5 M Na2SO4

solution.


of conduction band is negligible for n-type semiconductors,37 the

conduction band position of WO3 and BiVO4 could be estimated

as indicated in Fig. 7. To calculate valence band position, the

optical band gap was determined by the following Tauc

equation:38

(ahv)n ¼ A(hv � Eg)

where A ¼ constant, hv ¼ light energy, Eg ¼ optical band gap

energy, a ¼ measured absorption coefficient, n ¼ 0.5 for indirect

band gap, and n ¼ 2 for direct band gap materials. Because WO3

has an indirect band gap and BiVO4 a direct band gap, the y axis

of the Tauc plot is (ahv)0.5 for WO3 and (ahv)2 for BiVO4. In the

Fig. 6b, the extrapolation of the Tauc plot on x intercepts gives

the optical band gaps of 2.77 eV and 2.51 eV for WO3 and

BiVO4, respectively.

Based on these values of flat band potentials and optical band

gap energies, we constructed the potential energy diagram for the

composite film in Fig. 7. The conduction band and valence band

of BiVO4 are more negative than the corresponding bands of

WO3. This thermodynamic condition favours the facile injection

of photogenerated electrons from the conduction band of BiVO4

to that of host WO3. Thus, when this composite electrode is

illuminated with simulated solar light, excited electrons are

generated in the conduction band of both WO3 and BiVO4. The

photogenerated electrons in BiVO4 move to the conduction band

of WO3 easily due to the potential difference, and then the

excited and migrated electrons in WO3 are collected onto FTO.

This facile electron transfer would reduce the chance of recom-

bination with holes formed in the valence bands of the two

semiconductors. The holes migrate to the semiconductor/elec-

trolyte interface either directly or after transfer from WO3 to

BiVO4. The reduced recombination would naturally induce

photoactivity enhancement.

To evaluate the kinetics of charge transfer in this composite

electrode system, the electrochemical impedance spectroscopy

(EIS) measurements were performed. The EIS is a powerful tool

to study electrochemical behaviour, especially charge transfer

phenomena.39,40 Fig. 8 shows EIS spectra measured under

Fig. 7 Schematics of the potential energy diagram for the WO3/BiVO4

heterojunction composite electrode.



Fig. 8 Electrochemical impedance spectra of bare WO3, BiVO4 and

composite 1 and 4 electrodes. The solid line was fitted by ZSimpWin

software using the proposed equivalent circuit model. The EIS was

measured at 0.7 V (vs. Ag/AgCl) under simulated solar illumination in

0.5 M Na2SO4 solution. The inset shows an equivalent circuit for the

photoanodes.

Fig. 9 Incident photon to current conversion efficiency (IPCE) of bare

WO3, BiVO4 and composite 1 and 4 electrodes. The IPCE was measured

at 0.7 V (vs. Ag/AgCl) under 0.5 M Na2SO4 solution.

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simulated solar light illumination and presented in Nyquist

diagram in the frequency range of 100 kHz–100 mHz for bare

WO3, BiVO4, and composite 1 and 4 electrodes. The Nyquist plot

can be interpreted in terms of the equivalent circuit as displayed

in the inset. In the plot, symbols indicate the experimental results

and the lines represent fitting results by using an equivalent

circuit. Here, the EIS data were measured using a three electrode

cell system, thus the arc in Nyquist plot indicates the charge

transfer kinetics on the working electrode.

In the equivalent Randle circuit, Rs is the solution resistance,

Q1 is the constant phase element (CPE) for the electrolyte/elec-

trode interface, and Rct is the charge transfer resistance across the

interface of electrode/electrolyte. Thus, the arcs in the Nyquist

plot are related to charge transfer at interface of the photo-

electrode/electrolyte. The fitted values of Rct were 780.4, 803.6,

1390, 8803 U for bare WO3, composite 1, composite 4, and bare

BiVO4 electrodes, respectively. The efficient charge transfer at

the interface between photoelectrode/electrolyte hinders the

charge recombination and induces the facile charge transport of

electrons through the films.35 Thus, the bare WO3 has a very

good efficiency of charge transfer showing the lowest Rct. The

largest Rct for BiVO4 indicates that the charge transfer charac-

teristics of BiVO4 are poor. The Rct values of composites 1 and 4

lie between those of BiVO4 and WO3. The Rct of composite 1, the

most efficient heterojunction electrode, was only slightly larger

than bare WO3 electrode. Thus, the charge transfer rate in the

bare WO3 and composite 1 are similar. Thus, we can conclude

that the low activity of bare BiVO4 is due to its poor charge

transfer characteristics and by forming the heterojunction with

WO3, the charge transfer rate is improved to become as good as

that of WO3.

Although BiVO4 absorbs a larger fraction of solar light than

WO3 due to its narrower band gap as shown in Fig. 2, electrons

and holes formed in BiVO4 are not fully utilized for water

decomposition reaction because its poor transfer characteristics

lead to the recombination. However, when a composite


heterojunction electrode is formed, the relative band positions of

WO3 and BiVO4 shown in Fig. 7 provide a thermodynamic

driving force of charge transfer in the composite electrode. Thus,

photoelectrons formed in BiVO4 would move to WO3 avoiding

charge recombination. These photoelectrons could be efficiently

used in WO3 for current generation due to its good transfer

characteristics. This could be confirmed by measuring the inci-

dent photon-to-current conversion efficiency (IPCE) spectra.

Fig. 9 shows IPCE spectra of bare WO3, BiVO4, and

composite 1, 4 electrodes measured at 0.7 V (vs. Ag/AgCl) in

0.5 M Na2SO4. The IPCE is defined by following equation,

IPCE (at l) ¼ [J � 1240]/[Pmonol]

where J ¼ photocurrent density (mA cm�2), Pmono ¼ light power

density (mW cm�2) at l, and l ¼ wavelength of incident light

(nm). The bare WO3 shows rising IPCE from 480 nm and BiVO4

from 540 nm in agreement of their band gap energies. Despite the

range of light absorption of BiVO4 is larger than WO3, the

photoactivity was much less because of poor charge transfer

characteristics as evidenced by the EIS study discussed above.

The composite 1 also showed the onset of IPCE at 540 nm like

BiVO4. Because WO3 cannot absorb the light with the wave-

length between 500 and 540 nm, the IPCE in this range for

composite 1 is originated from the absorption by the BiVO4

layers. But, unlike bare BiVO4, the composite electrode can

utilize this light of 500–540 nm for water oxidation, because the

good charge transfer characteristic at the interface between

composite 1 electrode/electrolyte induced rapid transfer of

photoelectrons formed in BiVO4 to WO3. The thermodynamic

energy band structure as shown in Fig. 7 helped the generated

photoelectron to migrate from the conduction band of BiVO4 to

that of WO3. Thus, the composite 1 heterojunction electrode

combines the advantages of two semiconductors, i.e. excellent

charge transport characteristics of WO3 and good light absorp-

tion capability of BiVO4. This seems to be the origin of the

synergy in enhanced photoactivity by forming composite elec-

trodes of two semiconductors. The composite 4 behaves more



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like bare BiVO4 because of improper composition of the

composite.

Conclusions

We investigated the heterojunction composite anode of

WO3/BiVO4 to enhance the photoactivity of WO3 by coupling

with BiVO4. The composite electrode with optimized composi-

tion of one top BiVO4 layer and four WO3 layers on FTO showed

the highest photocurrent with 74% (at 0.7 V) increase relative to

bare WO3, and 730% relative to bare BiVO4. The EIS study

demonstrated poor charge transport characteristics of BiVO4,

which were responsible for the low photoactivity of the bare

BiVO4 film. But when it is coupled with WO3 forming hetero-

junction composite anode, the photogenerated electrons in

BiVO4 are transferred to WO3 layers with good charge transport

characteristics and contribute to the high photoactivity. Thus,

the composite electrode combines merits of the two semi-

conductors, i.e. excellent charge transfer characteristics of WO3

and good light absorption capability of BiVO4. This seems to be

the origin of the synergy in enhanced photoactivity by forming

heterojunction electrodes of the two semiconductors. Thus

fabrication of heterojunction WO3/BiVO4 is an efficient method

to enhance the performance of WO3 electrode for water splitting

in PEC cells.

Acknowledgements

This work was supported by the Hydrogen Energy R&D Centre

and Korean Centre for Artificial Photosynthesis funded by the

Ministry of Education, Science and Technology of Korea and the

Brain Korea 21 program.

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heterojunction bivo4/wo3 electrodes for enhanced photoactivity of water oxidation

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