Tailoring Band Structure of TiO2 To Enhance Photoelectrochemical

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Tailoring Band Structure of TiO to Enhance Photoelectrochemical Activity by Codoping S and Mg Yanyu Liu, Wei Zhou, Yinghua Liang, Wenquan Cui, and Ping Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00623 • Publication Date (Web): 05 May 2015 Downloaded from http://pubs.acs.org on May 10, 2015

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The Journal of Physical Chemistry

Tailoring Band Structure of TiO2 to Enhance Photoelectrochemical Activity by Codoping S and Mg Yanyu Liua, Wei Zhoua, Yinghua Liangb, Wenquan Cuib, and Ping Wu*a a

Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Faculty of Science, Tianjin University, Tianjin 300072, People’s Republic of China

b

College of Chemical Engineering, Hebei United University, Tangshan, 063009, People’s Republic of China

*To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86 (0)22 27408599 Fax: +86 (0)22 27406852.

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ACS Paragon Plus Environment


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Abstract: Doping with foreign element is an effective method to narrow the band gap of TiO2. Here, the S doping and (S, Mg) codoping effects on the electronic structure and optical properties of the anatase TiO2 were investigated at the HSE06 hybrid functional level. The hybridization of the O 2p, Ti-t2g and S 3p states creates the intermediate band for the doped TiO2, which makes the absorption onset extend into the visible-light region. The introduction of Mg promotes the corporation of S into TiO2. The heavy doping with S further improves the absorption efficiency and eliminates the unoccupied intermediate band. The band alignment confirms that the (S, Mg) codoping with two O atoms substituted by S atoms in TiO2 is desirable for the overall water splitting.

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INTRODUCTION A desirable photocatalyst for water splitting must tightly meet material properties criteria such as appropriate band gap (1.6-2.2 eV), high carrier mobility, and correct band-edge positions that straddle the water redox potential levels.1,2 However, most catalysts are wide-band-gap semiconductors (TiO2-3.2 eV, ZnO-3.2 eV, SnO2-3.6 eV),3-5 which fails them to utilize the abundant visible light. Therefore, band gap engineering has been an important issue for optimizing the performance of these catalysts. The discovery of narrowing the band gap by incorporating foreign elements into TiO2, ZnO, and SnO2 has aroused a great deal of research interest. Among these materials, TiO2 has received much attention as a promising candidate material for photocatalyst applications due to its strong oxidation, a long lifetime of the photoexcited carriers, and high chemical stability.2,3,6-8 It is well known that the conduction band minimum (CBM) and the valence band maximum (VBM) determine the reducing and oxidizing power. The higher the CBM energy to the hydrogen production level, the stronger the reducing power is. While the deeper the VBM energy, the stronger the oxidizing power is.2 Since the CBM of TiO2 is slightly higher than the hydrogen production level, the anatase TiO2 is optimal. It is due to the CBM of anatase phase is about 0.1 eV higher than the rutile TiO2.2 Besides that, the anatase phase also has more catalytic activity and higher electron mobility than the rutile phase.9 Thus, the anatase TiO2 is selected as the subject in this work. For anatase TiO2, two drawbacks must be considered: first, as discussed earlier, its wide-band-gap makes the visible light hardly utilized in photocatalytic reactions. Second, the CBM is much closer to the standard hydrogen electrode potential (SHE). One approach must upshift the VBM of TiO2 and keep the position of the CBM unchanged or raise the CBM. A common method is to incorporate anion into TiO2,

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whose p orbital energy higher than that of the oxygen, such as C, N, and S.1 This method successfully leads to noticeable redshift for the optical absorption edge.7,10-14 However, an acceptor level induced by C or N doping, just above the Fermi level, would act as the recombination center. Such unoccupied energy levels are detrimental for the photoinduced current.2 Although the introduction of S in TiO2 would not create such states because it belongs to the same group VI with O, the doping of S in TiO2 will be very difficult owing to the large difference in size between S and O. The codoping on Ti and O sites was proposed to enhance the solubility of alloying elements, which has shown better optical absorption than monodoping in the visible-light region.15-19 Obviously, the codoping is one of viable ways for the efficient absorption of sunlight. In addition, to guarantee the H2 evolution, the noble metals were usually adopted as cocatalysts or codoping elements in previous reports.3,20-22 However, the noble metals were scarce which is not suitable for the practical applications. Invoked by this concept and based on our earlier work,23 Mg can be chosen as the codoping element with S because of the Mg doping does not affect the position of the CBM. Furthermore, the introduction of Mg also can help the absorption edge shift into the long-wavelength region in TiO2. COMPUTATIONAL DETAILS The most of early ﬁrst-principles calculations were based on density functional theory with the local density or generalized gradient approximations (DFT-LDA or GGA). However, they fail to estimate the band gap observed in optical experimental value due to a residual self-interaction present within the O 2p shell in LDA or GGA.2,16 A new hybrid functional (the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional) in DFT corrects the problem and provides greatly improved descriptions

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of electronic structure. In the present work we use the HSE06 hybrid functional to study the structural, electronic and optical properties of anatase TiO2.24 ,25 The first-principles DFT calculation were carried out using projector augmented wave (PAW) pseudopotentials, as implemented in the Vienna ab initio Simulation Package (VASP).26,27 In the HSE06 hybrid functional, the exchange-correlation functional uses a mixing parameter (α = 0.25) to incorporate the Perdew-BurkeErnzerhof (PBE) functional and HF exact exchange functional. The electron wave function was expanded in plane waves up to a cutoff energy of 400 eV. MonkhorstPack k-point sets of 5 × 5 × 2, and 4 ×4 × 4 were used for 12-, 48-atom supercells and 2 × 3 × 1 for anatase (101) surface. For the surface TiO2, the supercells are separated from each other in the z direction with 15 Å thick vacuum layers. The cell volume and atomic positions were relaxed until the residual forces were below 0.01 eV/Å. The obtained lattice constants are listed in Table I. And the band gap of TiO2 can be enlarged from 1.2 eV of PBE to 3.1 eV of HSE06 which is much close to the experimental results (3.2 eV). Table I Lattice constants for the anatase TiO2 determined by HSE06 compared with theoretical and experimental values. Theory (LDA+U)28

Experiment29

This work

a/Å

3.84

3.78

3.80

c/Å

9.59

9.50

9.47

deq/Å

1.96

1.93

1.94

dap/Å

2.00

1.97

1.98

For investigating the optical properties of a material, it is needed to calculate one dielectric tensor component. The complex dielectric function ε(ω) is given as

ε (ω ) = ε1 (ω ) + iε 2 (ω )

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(1)


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which is closely relationship with the electronic response. The imaginary term ε2(ω) of the dielectric constant is very important for any materials, which can be derived from the momentum matrix elements between the occupied and unoccupied wave functions, and the real part ε1(ω) is deduced from imaginary part ε2(ω) by the wellknown Kramer–Kronig transformation.

 4π 2 e 2  i MJ 2 2 ∑∫  m ω  i, j

ε 2 (ω ) = 

ε1 ( ω ) = 1 +

2

π

P∫

∞

0

2

f i (1 − fj ) δ ( E f − Ei − ω ) d 3 k

ω 'ε 2 (ω ' ) d ω '

(ω

'2

(2)

(3)

−ω2 )

Then, the corresponding absorption coefficient α (ω) can then be the expressed byε1 (ω) and ε2 (ω)

α (ω ) = 2ω  ε12 (ω ) + ε 22 (ω ) − ε1 (ω )  

1

2



(4)

In addition, the impurity concentration can affect the optical properties. Under thermodynatic equilibrium conditions, the concentration of the impurity is given by

c = N sites exp − E

f

k BT

(5)

where E f is the formation energy, N sites is the number of sites can be incorporated on,

k B is the Boltzmann constant, and T is the temperature. Obviously, the doping concentration is determined by the formation energy: E f = Ed − E p + ∑ ni µ i

(6)

where Ed and Ep are the total energies of the supercell with and without the impurity,

ni is the number of the corresponding species is added or removed and µi is the chemical potential of a species in the supercell, which represents the speciﬁc 6


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equilibrium growth condition. A correction term of the difference between the electrostatic potentials of the defected and un-defected systems is used to align the band structure. And a jellium background was used for charged system as charge correction.

RESULTS AND DISCUSSIONS 1. S monodoping in Bulk TiO2 To obtain the influence of S monodoping on the photochemical activity of anatase TiO2, a detailed analysis of the structural and electronic structure of S-doped anatase TiO2 was performed. Substituting 3.125% O with S atoms in anatase TiO2, marked by S@O, leads to remarkably structural changes due to the large size difference between the both atoms. The S-Tieq and S-Tiap bond lengthes, 2.20 Å and 2.36 Å, are longer than that Ti-O ones [Table I]. The structural modification will affect the electronic structure. The total and partial density of states (DOS) were calculated for the pristine anatase TiO2 and S-doped TiO2 [Fig. 1(a) and (b)]. Obviously, the valence band mainly consists of the O 2p states while the conduction band is most composed of the Ti 3d states for the pristine TiO2, which is similar with previous reports.2,3 The band gap obtained is 3.1 eV, which slightly underestimates experiment by 0.1 eV. In addition, the VBM is very deep relative to the water oxidization potential, which is beneficial for the oxygen production.

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Figure 1. Left: Total and projected DOS and right: The optical properties of pure, S@O, 2S@2O bulk TiO2 successively. As depicted in the Fig. 1(b), the S dopant mildly perturbs the band edge of the valence and conduction band. And the O 2p states and the Ti 3d states are also primary in the valence band and the conduction band, respectively. Clearly, the S doping scarcely affect the valence and conduction bands of the TiO2. Notwithstanding, a new delocalized intermediate bands (IB) forms just below the Fermi level for 3.125% doping concentration with respect to the undoped material. The IB arises from the O 2p, Ti-t2g and S 3p states above the VBM [Fig. 1(b)]. The presence of the IB can effectively narrow the optical band gap and improve the visible light response. It is obvious that the S doping in anatase TiO2 is superior to C-/N- doped anatase TiO2, in which an unoccupied and localized states within the band gap are present. The localized states would form the photoinduced carrier recombination centers, which reduces the photocatalytic efficiency.2 As stated earlier, the CBM of anatase TiO2 is slightly above the SHE, thus the CBM should keep unchanged or better upshfit to guarantee the hydrogen production during tuning the band gap. Therefore, the DOS for different structural models was aligned with the core energy level of O-1s orbital. From the Fig. 1(a) and (b), it can be found that the coincorporation of S atom barely

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changes the electronic structure and the position of the conduction band. Therefore, the S element is an optimal dopant to narrow the band gap of TiO2 to realize watersplitting under visible radiation. In what follows, the S doping effect on the optical-response properties will be discussed in detail. Owing to the optical anisotropy of the crystal, the absorption spectra should be considered in different directions. According to the Eq. (4), the optical absorption spectra were calculated for the pure and S-doped TiO2 in Fig. 1(d) and (e). It is found that pure TiO2 just respond to the UV light, while an absorption onset extends into the visible-light region for S-doped system. The notable redshift effect should be due to the IB below the Fermi Level. As shown in the Fig. 1(d) and (e), the S monodoping not only can effectively reduce the band gap to absorb the rich visible light, but can also simultaneously strengthen the absorption for overall the UV light. In addition, an optical anisotropy was observed in absorption spectra, which indicates the optimal incident light’s polarization direction is direction for the bulk TiO2. Although this reduction for the band gap makes the TiO2 able to absorb the visible light, the absorption efficiency is relatively low. Therefore, we need further reduce the band-gap or improve the absorption efficiency in the visible light region. As we know, increasing the impurities concentration can further decrease the band gap and enhance the absorption of sunlight. The concentration of S mixed with TiO2 is varied from 1.23% to 10% in experimental reports.30-32 Therefore, the effect of the heavy doping with S (6.25%) on the electronic and optical properties has been investigated, which are shown in Fig. 1(c) and (f). The heavy doping with 6.25% O replaced by S atoms, is labeled 2S@2O. According to the total and partial DOS, the heavy doping also hardly influences the electronic structure of the valence and conduction bands. But the area of IB is substantially enlarged owing to the stronger

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hybridization between the O 2p, S 3p and Ti-t2g states and its width is about 0.8 eV. This not only yields the absorption edge to further extend to the long-wavelength region, but also results in the higher absorption efficiency in the visible and UV light (see Fig. 1(f)). Apparently, further narrowing the band gap is successfully achieved by the heavy doping with S. Besides those, a more distinct optical anisotropy near the absorption edge was observed. In view of the absorption efficiency of the solar one should choose the incident light polarized along direction. However, the heavy doping with S is very difficult to realize in experiment in virtue of the largely different size between S and O. Motivated by the codoping to increase the alloy concentrations, a better approach to facilitate the corporation of S into TiO2 was put forward, which is codoping with Mg element. Based on the Bader charge analysis, MgTi has the charge state of +1.62 which acts a cation dopant. 2. S and Mg coincorporated into TiO2 There are two questions that should be addressed for the coincorporation of (S, Mg) in TiO2. The first one is how to increase the S doping concentration in TiO2, the second one is how to passivate the unoccupied defect bands induced by the Mg doping,20 whose thermodynamic transition level ( ε (0 / - 1) ) is 1.0 eV (the method for thermodynamic transition level calculation was stated in the reference 33). Obviously, the transition barrier is sufficiently large to firmly trap the photo-generated carriers, which prevents the trapped photo-generated carriers participating in the photocatalytic reduction reaction to produce H2. Therefore, the unoccupied defect bands should be passivated. As Eq. (6) and (7) indicate, the formation energy of a dopant, which is closely associated with the atomic chemical potentials of both the host elements and the dopant, determines the solubility of impurities. In order to evaluate the trend of the doping concentration, one just needs to calculate the formation energy

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for various structural models. On basis of the Eq. (7), the formation energies were calculated for S-monodoping and S-Mg codoping in bulk TiO2. And the formation energy of S-monodoping is about 1 eV higher than that of S-Mg coincorporated into TiO2, which implies that the implantation of Mg favors the S doping into the TiO2 lattice. Therefore, the heavy doping with S in TiO2 can be realized by codoping with Mg. And this result is different from the case for improving the solubility of dopants with the conventional acceptor-donor codoping strategy. Similar with the Mg doped TiO2 system, the charge loss of Mg atom in the Mg-S codoped TiO2 system is just 0.06 e more than that of Mg doped TiO2 system on basis of the Bader charge analysis. Therefore, the Mg atom still act as a cation dopant in the Mg-S codoped TiO2 system. The electronic structure of (S, Mg) codoping into TiO2 lattice was calculated and shown in Fig. 2. From the total and partial DOS it can be seen that the localized acceptor induced by the Mg doping is passivated, because it would not act as a carrier recombination center. Comparing with the S monodoping, it deserves noting that the width of the valence band and the conduction band is broadened. These changes, induced by the (S, Mg) codoping, can improve the mobility of photon-generated carriers which is benefit for carriers moving to the surface of catalytic reaction. The high mobility of the photogenerated hole is vital for their access to the catalytic surface with minimizing the electron-hole recombinations. And the band edges still retain the initial positions. Obviously, it can be concluded that the (S, Mg) coincorporation has highly merit on the photocatalytic reaction.

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Figure 2. Total and projected DOS of TiO2 codoped with S and Mg and the inset is the partial electron density distribution of IB states. The blue, brown and yellow spheres stand for Ti, O, Mg, and S atoms, respectively. It is well known that there are three processes in the water-splitting reaction on the photocatalytic surface. Firstly, the band gap of the semiconductor must be smaller than the absorption photon energy. Secondly, the photoinduced electron-hole pairs separate and move to the catalytic surface separately. At last, the band edges of TiO2 should be match with the redox potentials of water. In the second step, the photoexcited electrons transferring from the valence band to the conduction band by the IB relies on its localized or delocalized nature in the (S, Mg)- codoped TiO2. Therefore, the partial electron density distribution of IB states was calculated and shown in the inset of Fig. 2. Obviously, the IB is mainly composed of the S and O p orbitals with slight Ti d orbitals, which is in concordance with the calculated partial DOS results. As the springboard to bridge the VBM and the CBM, the delocalized nature of the IB is of significance. This nature is very in favor of the migration of the photoinduced electrons to transfer from the VBM to the CBM under visible light irradiation. This can be verified by the optical properties of the S monodoping and heavy doping with S. The more delocalized the IB, the higher absorption efficiency.

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3. Surface Effect on the Electronic Structure Since many properties of semiconductors are affected by surface processes to a large degree, analysis of doping traits at surface is prominently relevant. The substitutional S dopants in the surface and subsurface layers were modeled. The total energies are calculated, -768.92 eV for the doping at surface, -765.92 eV for the subsurface. The results indicate that the thermodynamically favorable structure is the doping at surface, because of the strain induced by the S atom easily releases in the surface. Therefore, we just need to analyze the electronic and optical properties for the most stable model. In order to obtain a systematic understanding of S monodoping and (S, Mg) corporation into the TiO2 surface, the electronic and optical properties were calculated. The total and partial DOS of pure, S- and (S-Mg)- doping with one or two O atoms replaced by S element at the surface were shown in parts (a), (b), (c) and (d) of Fig. 3, respectively. The corresponding structures are denoted by pure, S@O, S+Mg@O+Ti, and 2S+Mg@2O+Ti, respectively. It is clear that the band gap enlarges by 0.3 eV in comparison with the bulk model for the pristine TiO2. In addition, the electronic structures for the three doping models have significant difference. For the S@O case, one occupied intermediate impurity states separate oneself from the IB. The rest of IB extends into the top of the valence band, broadening the valence band by 0.6 eV. For the case of S+Mg@O+Ti, there are two unoccupied IBs, locating at 1.64 and 2.42 eV, respectively, which possibly act as recombination centers. In order to further analyze the nature of the unoccupied IBs, the thermodynamic transition level ( ε (0 / - 1) ) was calculated.33 The results indicate that the unoccupied IBs have large ε (0 / - 1) of about 1.8 eV. The transition barrier is adequately large to firmly trap the photo-generated carriers, which result in the trapped photo-generated carriers hardly participating in the photo-catalytic reduction

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reaction. When two O atoms are substituted by S, the unoccupied IBs in the S+Mg@O+Ti system were passivated by the heavy doping with S in the 2S+Mg@2O+Ti case, two occupied IBs appear just below the Fermi level. It implies that the incorporation with more delocalized S-3p orbital than O-2p orbital can be utilized to eliminate the unoccupied impurity band which usually acts as recombination centre. As mentioned above the occupied IB is benefit for the photocatalysis. That is to say the 2S+Mg@2O+Ti system will have better photocatalysis efficiency. Although the electronic structures are significantly different, the positions of the VBM and CBM are almost unperturbed.

Figure 3. Total and projected DOS for (a) pure, (b) S@O, (c) S+Mg@O+Ti, and (d) 2S+Mg@2O+Ti surface TiO2. The corresponding optical spectra were drawn in Fig. 4(a), (b), (c) and (d). As shown in the optical spectra, the introduction of the impurity in the TiO2 results in a remarkable redshift for the absorption edge. The IBs can serve as the stepping stones of the photogenerated carrier transition, thus decrease the absorption energy. The absorption onset extends to 2.2 eV, 1.6 eV, 2.2 eV, for S@O, S+Mg@O+Ti and 2S+Mg@2O+Ti case, respectively. According to the optical spectra, the absorption rate, which determines the initial number of photoinduced carriers, should be in order of S@O ≅ S+Mg@O+Ti > 2S+Mg@2O+Ti. In addition, the optical anisotropy has 14


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some change where the optimal incidence direction changes from to directions.

Figure 4. Optical properties of (a) pure, (b) S@O, (c) S+Mg@O+Ti, and (d) 2S+Mg@2O+Ti surface TiO2. 4. Band alignments of Undoped and Doped TiO2 In general, the band edge positions of a semiconductor concerning the redox potentials of the adsorbed species on its surface restrict the photogenerated electron transfer between the semiconductor and the adorbates. When the valence band edge is below the potential of the donor level and the CBM is above the potential of the acceptor level, the water-splitting is able to proceed to produce oxygen and hydrogen simultaneously. To evaluate the doping effect on the photocatalytic activity of TiO2, the position alignment of the band edges was performed (including the reduction and oxidation potentials) and shown in Figure 5. The VBM and CBM positions of the pure TiO2 with regard to the SHE are taken from the experimental values.34 Based on the relative positions as compared with that of pure anatase TiO2, the CBM positions

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of doped TiO2 are obtained from the band structure and the VBM positions are calculated from the absolute band gap values.3

Figure 5. Band edge alignment of (a) pure, (b) S@O, (c) S+Mg@O+Ti, and (d) 2S+Mg@2O+Ti surface TiO2 with respect to the water reduction and oxidation potentials, which are marked horizontal lines with labels (H+/H2) and (H2O/O2 ). The Fermi level is denoted as a horizontal dotted line for various structures. Obviously, all samples have a strong redox potential, which is ascribed to the H+/H2 potential and the O2/H2O potential within the band edges, except for the S+Mg@O+Ti case. Although only UV light is utilizable for the pure TiO2, the abundant visible light can be absorbed for S@O and 2S+Mg@2O+Ti cases due to the presence of IBs. Especially for the 2S+Mg@2O+Ti case, there is relatively higher driving force for H2 production, which sufficiently guarantees the H2 production under visible radiation. For the S+Mg@O+Ti case, the unoccupied IBs lie with the the H+/H2 potential and the O2/H2O potential, which is detrimental for H2 production. Therefore, the IBs lead to low photoelectrochemical efficiency. According to the results of the band alignment, we advise that 2S+Mg@2O+Ti codoing in TiO2 would be a strong candidate for water splitting under visible radiation because the CBM is unchanged and the Fermi level is lower than the O2 production level.

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CONCLUSIONS In summary, we investigate the S monodoping and (S, Mg) codoping effects on the electronic structure and optical properties of the anatase TiO2 by employing the density functional theory with the HSE06 hybrid functional. The new intermediate band is the origin of the absorption in the visible light region for S monodoped and (S, Mg) codoped in the anatase TiO2. The intermediate band is induced by the hybridization of the O and S p orbitals with the Ti d orbitals. The delocalized nature of the intermediate band is of significance for photogenerated electron transferring from the top of the valence band to the bottom of the conduction band due to the reduction of the absorption energy. The heavy doping with S yields the absorption onset to shift the longer-wavelength region and the corporation of Mg indeed facilitates the S doping in the anatase bulk TiO2. In addition, the doping at surface also makes the absorption onset extend to the visible light owing to the IBs. But based on the results of the band alignment, only the S@O and 2S+Mg@2O+Ti cases satisfy the hydrogen and oxygen production requirement under visible radiation. Due to the unoccupied IBs, the S+Mg@O+Ti is not suitable for the hydrogen production. In addition, the 2S+Mg@2O+Ti codoing in TiO2 would be a strong candidate for water splitting under visible radiation because the CBM is unchanged and the Fermi level is close to the O2 production level.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86 (0)22 27408599 Fax: +86 (0)22 27406852

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ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51074112) and (11247224). The supercomputing resources were supported by High Performance Computing Center of Tianjin University, China.

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Enhanced

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Table of Contents

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Fig. 1 Left: Total and projected DOS and right: The optical properties of pure, S@O, 2S@2O bulk TiO2 successively. 69x48mm (600 x 600 DPI)


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Fig. 2 Total and projected DOS of TiO2 codoped with S and Mg and the inset is the partial electron density distribution of IB states. The blue, brown and yellow spheres stand for Ti, O, Mg, and S atoms, respectively. 69x48mm (600 x 600 DPI)



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Fig. 3 Total and projected DOS for (a) pure, (b) S@O, (c) S+Mg@O+Ti, and (d) 2S+Mg@2O+Ti surface TiO2. 69x48mm (600 x 600 DPI)


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Fig. 4 Optical properties of (a) pure, (b) S@O, (c) S+Mg@O+Ti, and (d) 2S+Mg@2O+Ti surface TiO2. 69x48mm (600 x 600 DPI)



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Fig. 5 Band edge alignment of (a) pure, (b) S@O, (c) S+Mg@O+Ti, and (d) 2S+Mg@2O+Ti surface TiO2 with respect to the water reduction and oxidation potentials, which are marked horizontal lines with labels (H+/H2) and (H2O/O2 ). The Fermi level is denoted as a horizontal dotted line for various structures. 69x48mm (600 x 600 DPI)


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Tailoring Band Structure of TiO2 To Enhance Photoelectrochemical

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