Artic le Materials Science

Comparison of the effect of hydrogen incorporation and oxygenvacancies on the properties of anatase TiO2: electronics, opticalabsorption, and interaction with water

Hui Jin • Lianzhou Wang • Debra J. Searles •

Chenghua Sun

Received: 3 September 2013 / Accepted: 18 October 2013 / Published online: 11 March 2014

� Science China Press and Springer-Verlag Berlin Heidelberg 2014

Abstract Hydrogenation has been recently developed as

an approach to improve the visible-light response of titanium

dioxide (TiO2); however, the effect of hydrogenation on the

electronics and optical absorption of anatase TiO2 has been

widely debated. In this work, the electronic structures and

optical properties of hydrogenated TiO2 and its interaction

with water have been studied using the density functional

theory plus Hubbard model. A comparison of the effect of

hydrogenation and introduction of oxygen vacancies (OVs)

to TiO2 is presented. It is found that both hydrogenation and

OVs can promote the absorption of visible light and

strengthen the adsorption of water. Compared with OVs,

hydrogen incorporation can lead to local distortion and even

amorphous structures when it is heavily doped.

Keywords Titanium dioxide � Hydrogenation �Oxygen vacancy � Density functional theory �Photocatalysis

1 Introduction

Fabricating photocatalysts with visible light activity is a

major challenge for solar hydrogen production [1]. Gen-

erally, the band structures of photocatalysts need to be

tuned to satisfy several requirements, including narrowed

band gaps to absorb visible light, and proper positions of

the valence band (VB), and conduction band (CB) [2].

A typical example is titanium dioxide (TiO2), which can

only work under ultraviolet (UV) light as determined by its

wide band gap (e.g. 3.20 eV for anatase TiO2) [3–6]. To

improve its response to visible (Vis) light, extensive efforts

have been made to narrow the band gap, such as doping

with metals or nonmetals [7–14]. Typically, dopants are

introduced to replace Ti or O, and thus change the positions

of the VB and/or CB, leading to smaller band gaps [15].

In recent years, hydrogen-incorporated TiO2, denoted as

H@TiO2, has attracted much attention in the band-gap

engineering of TiO2 [16–26]. A unique feature of H-dopants

is that H can be incorporated at the interstitial sites. For

instance, by high-pressure hydrogenation at 200 oC, Chen

et al. [16] obtained black H@TiO2 and achieved a narrowed

gap of 1.54 eV, in which case most visible light and even a

part of the near infrared (NIR) light can be absorbed. Dis-

ordered layers with a thickness of around 1 nm have been

observed, which was believed to account for the observed

Vis–NIR absorption [16]. Wang et al. [17] prepared black

TiO2 (rutile) nanowires via annealing in H2 steam at

200–550 �C, and their samples are highly crystalline. As

revealed by X-ray photoelectron spectroscopy (XPS),

hydrogenation leads to the formation of oxygen vacancies

(OVs) but has little effect on the location of the VB edge,

and thus, it was concluded that OVs account for the dark

color of H@TiO2 [17]. Under high pressure and high tem-

perature (450 �C), our team also obtained highly crystalline

SPECIAL ISSUE: Advanced Materials for Clean Energy

H. Jin � L. Wang � D. J. Searles � C. Sun (&)

Australian Institute for Bioengineering and Nanotechnology,

The University of Queensland, Brisbane, Qld 4072, Australia

e-mail: chenghua.sun@monash.edu

H. Jin � L. Wang

ARC Centre of Excellence for Functional Nanomaterials, School

of Chemical Engineering, The University of Queensland,

Brisbane, Qld 4072, Australia

D. J. Searles

School of Chemistry and Molecular Biosciences, The University

of Queensland, Brisbane, Qld 4072, Australia

C. Sun

School of Chemistry, Monash University, Melbourne,

VIC 3800, Australia

123

Chin. Sci. Bull. (2014) 59(18):2175–2180 csb.scichina.com

DOI 10.1007/s11434-014-0229-2 www.springer.com/scp

H@TiO2 (anatase), and it is found that the sample colors

strongly depend on the hydrogenation degree, varying from

gray, blue to black [18]. Hoang et al. [19] treated rutile TiO2

arrays at 500 �C under H2/Ar atmosphere, and interestingly,

the TiO2 film remains crystalline and white after the

hydrogenation treatment. Starting from amorphous TiO2,

Naldoni et al. [20] obtained black TiO2 nanoparticles via

heating under H2 stream at 400–500 �C. Both OVs and

disordered structures are presented in their H@TiO2 sam-

ples, and the black color was attributed to the co-existence

of OVs and surface disorder [20].

Although hydrogenated TiO2 has been prepared and

tested by several groups, the effect of hydrogen incorpo-

ration on the electronics and the optical properties of TiO2

hve not been fully understood yet [27–32]. Currently, it is

widely accepted that hydrogen can pass through the TiO2

surface and diffuse into the lattice, and the diffusion barrier

depends on the surface and the diffusion paths [18, 27–30].

As to the origin of the visible light adsorption, two struc-

tural reasons are proposed. Zhao et al. [32] found that

hydrogenation on oxygen is more favorable than that on

titanium; as a result, Ti3? is generated, and mid-gap states

are introduced, which they determine is the origin of the

Vis light absorption. In contrast, Chen et al. [31] suggested

that the mid-gap states actually origin from the disordered

structures, rather than Ti3?, and amorphous layers can

promote the hydrogen diffusion. Therefore, it is desirable

to further discuss the role of hydrogenation on the prop-

erties of TiO2.

In this work, density functional theory (DFT) calcula-

tions have been carried out to examine the electronics,

optical properties, and interaction of hydrogenated TiO2

with water. Given OV can lead to the formation of Ti3?, all

results are compared with TiO2 containing OVs.

2 Computational methods

Anatase TiO2 is modeled by a periodic (1 9 2) slab of (101)

surface with three Ti–O layers, as shown in Fig. 1. As a

reference, OV is generated by removing one oxygen atom

randomly, and labeled as OV@TiO2, as shown in Fig. 1a.

For H@TiO2, four hydrogen atoms are incorporated through

(101) into the lattice and located at the interstitial sites to

represent the H@TiO2, as shown in Fig. 1a. This above

model is based on two considerations: (1) (101) is the most

stable facet for clean TiO2 crystals [33], and although TiO2

with high percentage of minority surface (001) has been

synthesized in recent years [34–38], most crystals of TiO2

using in reports on H@TiO2 have not used that synthesis and

(2) hydrogen incorporation through the (001) surface is more

difficult (barrier: 1.49 eV) than that through (101) (barrier:

0.88 eV) [18]. Moreover, TiO2(001) containing H-dopants

and OVs has been tested. It is found that the conclusion

regarding the difference between OVs and H-doping

obtained with the (101) slab models and presented in this

work has no change if (001) model is employed. Over the

surface, a vacuum space of 15 A has been introduced to

eliminate the interaction between neighboring slabs.

Spin-polarized DFT has been carried out to optimize the

geometry and calculate the electronic structures within the

generalized-gradient approximation (GGA) [39], together

with the exchange–correlation functional of Perdew-Burke-

Ernzerhof (PBE) [40, 41], as implemented in the Vienna

ab initio simulation package (VASP) [42, 43]. The reci-

procal space has been spanned with a plane-wave basis

with a kinetic energy cutoff of 350 eV. The k-space is

sampled by a Monkhorst–Pack mesh of 7 9 7 9 1. Given

that OV is involved, the on-site electron correlation is

essential, and thus DFT plus Hubbard model (DFT ? U)

[44–48] has been employed in our calculations. In our case

U = 4.0 eV has been selected based on early publications

[49]. From the optimized geometries, the imaginary

dielectric function, labeled as Im, has been obtained by

calculating the frequency-dependent dielectric matrix with

a large number of empty bands, using the formula descri-

bed by Gajdos et al. [50].

3 Results and discussion

3.1 Electronic structures

The effect of H-dopants and OVs on the electronic struc-

tures of TiO2 has been firstly studied, and the spin-polar-

ized band structures are plotted in Fig. 2. For undoped

TiO2, the calculated band-gap (Eg) is 2.75 eV, which

agrees well with published results with U = 4.0 eV [49].

When an OV is introduced, four local states (LS), with two

occupied (dotted lines) and two unoccupied states (thick

Fig. 1 (Color online) Slab models for a OV@TiO2 and b H@TiO2.

H atoms have been labeled directly, and O is denoted as a sphere with

a circle, with the rest as Ti atoms. OV is indicated by the bigger

sphere in (a)

2176 Chin. Sci. Bull. (2014) 59(18):2175–2180

123

black lines), are generated, as shown in Fig. 2a, but there is

almost no change in the band gap. With insertion of four

H-atoms at interstitial sites, H-atoms directly bond with

oxygen, leading to the breakage of Ti–O bonds, as shown

in Fig. 1b. Electronically, four local occupied states

between Fermi energy (dashed thin line) and VB are gen-

erated, and these are shown as dotted green lines in Fig. 2b.

This is not surprising since each hydrogen can contribute

one electron to the system. In addition, the Eg has increased

from 2.75 to 2.97 eV, indicating that such hydrogen

incorporation does not narrow but enlarge the Eg.

Similarly, it was recently reported that interstitial boron

also has no effect on the Eg narrowing of TiO2 [51]. For

sunlight harvest, both OV@TiO2 and H@TiO2 may offer

the capacity to adsorb visible light due to the existence of

local states, which will be further explored below.

3.2 Optical adsorption

For hydrogenated TiO2, it has been widely observed that

the color can change from white to gray, blue, and even

black, depending on the hydrogenation conditions [18].

–2

–1

0

FQΓ Z Γ

En

erg

y (e

V)

(a)

Spin up

Eg

–2

–1

0

En

erg

y (e

V)

Spin down

FQΓ Z Γ

–3

–2

–1

0

En

erg

y (e

V)

(b)

Spin up

Eg

FQΓ Z Γ–3

–2

–1

0

En

erg

y (e

V)

Spin down

FQΓ Z Γ

Fig. 2 Calculated band structures for a OV@TiO2 and b H@TiO2. Fermi energy, occupied, and unoccupied local states are indicated by red,

green, and blue dashed lines, respectively

0 1 2 3 4 5 60

1

2

3

4

5<010>

Im <001>

<100>

Energy (eV)

(a)

0 1 2 3 4 5 60

1

2

3

4

5

<010>

Im <001>

<100>

Energy (eV)

(b)

Fig. 3 Calculated imaginary part of the dielectric function for a OV@TiO2 and b OV @TiO2

Chin. Sci. Bull. (2014) 59(18):2175–2180 2177

123

To understand the effect of hydrogenation on optical

adsorption, the imaginary part of the dielectric functions of

OV@TiO2 and H@TiO2 (labeled as Im) has been calcu-

lated. To depict the anisotropy, the value of Im along the

\100[,\010[, and\001[directions are shown in Fig. 3.

For both OV@TiO2 and H@TiO2, two adsorption peaks

are observed, located at 1.0–2.0 eV and 3.0–5.0 eV, which

can be assigned to the LS ? CB and VB ? CB excita-

tions based on the calculated band structures. In principle,

the excitations from occupied LS to unoccupied LS can

also contribute to the adsorption in the range of 1.0–2.0 eV,

depending on the mobility of electrons in these states.

The effect of adding more H-atoms to H@TiO2 can be

considered, and it is found that heavy hydrogenation may

lead to significant distortion, resulting in the formation of

defects and even amorphous layers. Chen et al. [16]

reported that long-term hydrogenation can lead to the for-

mation of amorphous layers and TiO2 samples become

black. Combining the calculated band structures and opti-

cal profiles, it is speculated that heavy hydrogenation

introduces a large number of local states, which are

involved in the optical adsorption, and thus, result in the

color change of the TiO2 crystals. In terms of optical

adsorption, OV@TiO2 and H@TiO2 show similar features,

except that the first adsorption peak of H@TiO2 is dis-

tributed over a wider range, which may be related to the

local distortion associated with the hydrogenation. Exper-

imentally, blue TiO2 containing -1.0 wt% hydrogen

becomes gray after the dehydrogenation, supporting the

above conclusion that H-dopants can effectively extend the

distribution of local states and strengthen the adsorption

of visible lights. Therefore, with respect to OV@TiO2,

H@TiO2 offers a more flexible approach to adjust the

optical adsorption of TiO2 photocatalysts.

3.3 Interaction with water

As well as sunlight absorption, the charge transfer between

TiO2 and water is another critical factor for photocatalytic

water-splitting, which is determined by the interaction

between water and TiO2. Over a clean (101) surface, it is

widely accepted that water is adsorbed molecularly, and

the calculated adsorption energy is 0.77 eV by standard

DFT (using PBE functional) [52, 53]. In this work, it is

×DO

S (

a.u

.)

Energy (eV)

(b)

H2O

Total

×

100

10

–12 –9 –6 –3 0 3

–9 –6 –3 0 3

DO

S (

a.u

.)

Energy (eV)

(d)

(a)

(c)

H2O

Total

×10

Fig. 4 (Color online) Adsorption of single water (as circled). a Optimized geometries and b DOS for water on OV@TiO2, and c optimized

geometries and d DOS for water on H@TiO2

2178 Chin. Sci. Bull. (2014) 59(18):2175–2180

123

found that the energy is 0.64 eV for single water adsorp-

tion, with a deviation of 0.13 eV due to the Hubbard cor-

rection. With OV in the sub-layer, as shown in Fig. 4a, the

adsorption energy is slightly increased to 0.91 eV. From

the calculated density of states (DOS) profile shown in

Fig. 4b, it is observed that a small fraction of the local DOS

of water has the same energy levels as the occupied LSs

introduced by OVs (indicated by the blue dashed lines).

Those occupied LSs mainly locate at unsaturated Ti-atoms,

and the water-TiO2 interaction can promote the electron

transfer through the Ti–O bonding.

As to water adsorption, several starting geometries are

tested with single water being introduced on the surfaces of

H@TiO2 and OV@TiO2. Figure 4c shows the most pre-

ferred geometry on H@TiO2, and the corresponding DOS

profile is present in Fig. 4d. With respect to water on

OV@TiO2, the adsorption energy is much higher, being up

to 1.52 eV, which is close to that by typical chemical

adsorption. From the optimized geometry, however, there

is no water dissociation, and such high adsorption energy is

still obtainable when the second water is introduced

(averaged Eads = 1.37 eV). If four and eight water mole-

cules are introduced on the surface, the averaged Eads will

decrease to 0.4–0.8 eV, corresponding to typical physical

adsorption. From the DOS profile, the local DOS of water

has almost no overlap with the LSs associated with

H-dopants, which is another difference from water on the

surface of OV@TiO2. Interestingly, a new distribution of

the local DOS of water appears in the VB range, as labeled

by the arrow in Fig. 4d. Given the VB is dominated by O2p

states, the above result may indicate stronger interaction

between water and surface oxygen, which may explain the

increase of adsorption energy. Another possibility is that

water can stabilize the surface layers of H@TiO2: as

revealed in Sect. 3.1, heavy hydrogenation may lead to

strong local distortion and strain energy, while the Ti–Ow

and H–O2c (two-coordinated oxygen on TiO2 surface)

bonding can effectively stabilize those surface atoms and

release local strain energy.

4 Conclusion

Using the DFT ? U approach, the geometries, electronic

structures, optical absorption, and water adsorption have

been discussed and compared for OV@TiO2 and H@TiO2.

Both OV and H-dopants can introduce LS between VB and

CB, which promote the adsorption of visible light, and such

promotion is achieved through the LS ? CB excitation,

rather than narrowing the band-gap. In addition, stronger

water-TiO2 interactions can be achieved through the

introduction of an OV and hydrogenation because both OV

and H-dopants can lead to more lowly saturated surface

atoms, which can actively interact with water. To clarify

whether these mid-gap states can promote water-splitting

or just serve as a recombination center of the electron–hole

pairs, the excited states of water-TiO2 system, and the

charge transfer should be further examined.

Acknowledgments This work was supported by Australian

Research Council through Discovery Project and Future Fellowship

(CHS). The authors also appreciate the generous grants of CPU time

from both the University of Queensland and the Australian National

Computational Infrastructure Facility.

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