Screened Coulomb Hybrid DFT Study on Electronic Structure and

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Screened Coulomb Hybrid DFT Study on Electronic Structure and Optical Properties of Anionic and Cationic Te-Doped Anatase TiO2 Moussab Harb* KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ABSTRACT: The origin of the enhanced visible-light optical absorption in Tedoped bulk anatase TiO2 is investigated in the framework of DFT and DFPT within HSE06 in order to ensure accurate electronic structure and optical transition predictions. Various oxidation states of Te species are considered based on their structural location in bulk TiO2. In fact, TiO(2−x)Tex (with isolated Te2− species at Te−Te distance of 8.28 Å), TiO2Tex (with isolated TeO2− species at Te−Te distance of 8.28 Å), TiO2Te2x (with two concomitant TeO2− species at Te−Te distance of 4.11 Å), and Ti(1−2x)O2Te2x (with two neighboring Te4+ species at nearest-neighbor Te−Te distance of 3.05 Å) show improved optical absorption responses in the visible range similarly as it is experimentally observed in Te-doped TiO2 powders. The optical absorption edges of TiO(2−x)Tex, TiO2Tex, and TiO2Te2x are found to be red-shifted by 400 nm compared with undoped TiO2 whereas that of Ti(1−2x)O2Te2x is red-shifted by 150 nm. On the basis of calculated valence and conduction band edge positions of Te-doped TiO2, only TiO(2−x)Tex and Ti(1−2x)O2Te2x show suitable potentials for overall water splitting under visible-light irradiation. The electronic structure analysis revealed narrower band gaps of 1.12 and 1.17 eV with respect to undoped TiO2, respectively, resulting from the appearance of new occupied electronic states in the gap of TiO2. A delocalized nature of the gap states is found to be much more pronounced in TiO(2−x)Tex than that with Ti(1−2x)O2Te2x due to the important contribution of numerous O 2p orbitals together with Te 5p orbitals. Using DFT within GGA approximation, J. W. Zheng et al.13 have reported the density of states (DOS) of anionic and cationic Te-doped TiO2. They suggested that the observed red shift of the optical absorption edge in Te-doped TiO2 is due to a rigid valence band shift upon doping. In contrast, no systematic exploration of the different possible anionic and cationic Tedoped configurations was achieved. Moreover, no accurate simulation of the UV−visible optical absorption response was carried out as a function of the different doped configurations for giving more relevant interpretation of the available experimental data. Hence, the theoretical explanation of the nature and structural location of defect responsible for visible-light absorption in Te-doped TiO2 needs to be clarified for a rational improvement of photocatalytic materials. In the present work, we adopt an advanced methodology for accurate electronic structure and optical absorption calculations of undoped and Te-doped anatase TiO2 based on density functional theory (DFT) and density functional perturbation theory (DFPT) within the screened coulomb hybrid (HSE06) exchange-correlation formalism. We systematically explore several possible anionic and cationic Te-doped configurations in bulk anatase TiO2 by considering substitutional Te species for O and Ti sites, interstitial Te species, and mixed substitutional−

1. INTRODUCTION The electronic band gap of a semiconductor is a key parameter characterizing the nature of the absorbed light in the material. TiO2 has a relatively large band gap (3.2 eV for anatase and 3.0 eV for rutile) requiring only UV light (290−400 nm) for any electronic excitation from the valence band to the conduction band, which only corresponds to 3−5% of the solar energy. The band gap engineering of TiO2-based materials is of major importance for solar energy applications such as water splitting for solar hydrogen production.1−6 Doping of TiO2 with nonmetals7−11 seems to be a good way for improving the optical absorption response of TiO2 in the visible range by introducing new occupied electronic levels in the gap (near the top of the valence band). Indeed, the creation of localized impurity states in the gap is a limiting factor in water splitting for the expected mobility of the photogenerated holes to the catalytic surface. This can be improved by the possible creation of strongly delocalized impurity states in the gap, i.e., well-mixed impurity states with the O 2p states governing the valence band of TiO2. V. Stengl et al.12 have synthesized Te-doped TiO2 powders using homogeneous hydrolysis of titanium oxosulfate with urea in aqueous solution in the presence of amorphous tellurium. The doped samples revealed enhanced visible-light optical and photocatalytic properties compared with undoped TiO2. However, the electronic origin of the visible-light responsive Te-doped TiO2 still remains unclear. © 2013 American Chemical Society

Received: January 25, 2013 Revised: June 8, 2013 Published: June 11, 2013 12942

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Table 1. Doping Models and Stoichiometries (including x, m, p, and k Values) of the Various Anionic and Cationic TeDoped TiO2 Systems with 2 atom % of Te Impurities

interstitial Te species for 2 atom % of Te impurities (atomic concentration close to those obtained experimentally). Moreover, we explore for each doping type several geometrical configurations of Te such as isolated, separated, or gathered in order to determine the most stable cases. Then, we calculate the UV−vis optical response of the various Te-doped TiO2 systems, and we determine the electronic structure of the optimal doped systems as well as the localized or delocalized nature of the new electronic states created upon doping.

doping model (configuration) 1Tes@O (isolated) 2Tes@O (dimerized) 1Tei (isolated)

2. COMPUTATIONAL DETAILS 2.1. Total Energy Calculations. We used spin-polarized density functional theory with periodic boundary conditions, as implemented in VASP 5.214−18 package for total energy calculations within PBE functional19 and projector-augmented plane wave (PAW) approach.18−20 A cutoff energy of 400 eV for wave functions and 605.4 eV for augmentation charge were used. Electron smearing was employed using the tetrahedron method with Bloch corrections with a width of 0.2 eV. The convergence criterion for the electronic self-consistent cycle was fixed at 0.01 meV. The ion coordinates and lattice parameters were fully optimized until all components of the residual forces were less than 0.01 eV/Å. 2.2. Supercell Models. We considered both 2 × 2 × 1 and 2√2 × 2√2 × 1 anatase supercells with 16 TiO2 functional units (Ti16O32) or 32 TiO2 functional units (Ti32O64), respectively, for the simulation of Te-doped TiO2 structures. The Brillouin zone was sampled with 3 × 3 × 3 Monkhorst−Pack k-point grid21 for both supercells. We explored several anionic and cationic Te doping models by paying particular attention to key configurations showing isolated, separated, or dimerized Te species not investigated in the literature. Substitutional Te species at O (labeled by 1 or 2Tes@O) and Ti (labeled by 1 or 2Tes@Ti) sites, interstitial Te species (labeled by 1 or 2Tei), as well as mixed substitutional− interstitial Te species (labeled by 1Tes@O+1Tei) were considered. For a generic supercell of doped anatase with the formula Ti(n−k)O(2n−m)Tep containing initially 3n atoms in total, and modified with p Te atoms, m O vacancies, and k Ti vacancies, the atomic concentrations of Te impurity, and of O and Ti vacancies are defined as p/3n, m/3n, and k/3n, respectively. This supercell model leads to the corresponding stoichiometry Ti(1−kx)O(2−mx)Tepx, with x = 1/n, notation used throughout this manuscript. The various anionic and cationic Te-doped TiO2 systems considered in our study together with their stoichiometries are given in Table 1. Note that the supercell models remain overall neutral while the Te impurity centers induce local charge redistributions. In 1 or 2Tes@O models, Te species are formally Te2−, while in 1 or 2Tes@Ti models, Te4+ species are created. Note that in 1 or 2Tei models, Te species are formally neutral. The construction of the various supercells was performed using the Materials Studio graphical interface.22 2.3. Electronic Structure and Optical Absorption Calculations. Density of states (DOS) calculations of undoped and Te-doped anatase were investigated for the geometries optimized with the PBE functional by employing the screened coulomb hybrid (HSE06) exchange-correlation functional,23 as implemented in VASP 5.2.14−18 The tetrahedron method with Bloch corrections was also employed for the electron smearing with a width of 0.2 eV. More detailed information about this formalism was given in our previous paper.11

2Tei (dimerized) 1Tes@O + 1Tei (dimerized or separated) 1Tes@Ti (isolated) 2Tes@Ti (separated)

Ti(1−kx)O(2−mx)Tepx stoichiometry

Ti(n−k)O(2n−m)Tep supercell

TiO(2−x)Tex k = 0; m = p=1 TiO(2−2x)Te2x k = 0; m =p = 2 TiO2Tex k = 0; m = 0, p=1 TiO2Te2x k = 0; m = 0, p=2 TiO(2−x)Te2x k = 0; m = 1, p = 2 Ti(1−x)O2Tex m = 0; k = p=1 Ti(1−2x)O2Te2x m = 0; k =p=2

n = 16 n = 32 n = 16 n = 32 n = 32 n = 16 n = 32

UV−vis optical absorption calculations of undoped and Tedoped anatase were performed in the framework of the spinpolarized density functional perturbation theory (DFPT), as implemented in VASP 5.214−18 by employing the HSE06 functional.23 We also used the geometries obtained with the PBE functional. The optical properties were calculated through the frequency-dependent dielectric function following a methodology described in ref 24. Electron smearing using the tetrahedron method was also employed with a width of 0.2 eV. The optical absorption curves were obtained by plotting the averaged imaginary part of the frequency-dependent dielectric function over the three (x, y, and z) polarization vectors which represents the real electronic transitions between the occupied and unoccupied electronic states.25,26

3. RESULTS AND DISCUSSION 3.1. Optimized Structures and Relative Energies. In this section, we explore the relative stability of the various possible anionic and cationic Te-doped TiO2 structural configurations considered in our work. We show in Figure 1 the lowest-energy structures obtained for 2 atom % of Te impurities. Moreover, we report in Table 2 their relative energies together with the corresponding local distances. Substitutional Te species for O sites are simulated by replacing single or double O atoms by single or double Te dopants in the 48-atom or 96-atom supercells, respectively. This leads to TiO(2−x)Tex or TiO(2−2x)Te2x, respectively. The lowest-energy structure reveals the formation of a dimerized Te−Te species with a Te−Te bond length of 2.66 Å (Figure 1b and Table 2). A closed-shell singlet state is found to be the most stable spin configuration, associated formally to a diamagnetic (Te2)4− defect substituted for two O2−. The configuration with two Te species separated by a longer Te−Te distance of 8.28 Å is found at 0.87 (or 0.49) eV/Te higher in energy with PBE (or HSE06) (Figure 1a and Table 2). Interstitial Te species are also modeled by adding single or double Te dopants added to the 48-atom or 96-atom supercells, respectively. This leads to TiO2Tex or TiO2Te2x, respectively. The lowest-energy structure shows the formation of two TeO species with Te−O bond lengths of 2.13 and 2.16 Å (Figure 1d and Table 2). A closed-shell singlet state is found to be the most stable spin configuration, associated formally to diamagnetic TeO2− species substituted for O2−. The configuration having two concomitant TeO species with two shorter Te−O bond lengths 12943

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Figure 1. Lowest-energy (b, d, f, h) and metastable (a, c, e, g) structures of the various anionic and cationic Te-doped TiO2 systems for 2 atom % of Te impurities: (a) TiO(2−x)Tex; (b) TiO(2−2x)Te2x; (c) TiO2Te2x; (d) TiO2Tex; (e, f) TiO(2−x)Te2x; (g) Ti(1−x)O2Tex, and (h) Ti(1−2x)O2Te2x. Ti are in gray, O in red, and Te in dark brown.

Table 2. Relative Energies per Te Atom (eV/Te) Obtained with PBE and HSE06 (in Brackets) for the Various Anionic and Cationic Te-Doped TiO2 Systems Reported in Figure 1 Together with the Local Distances (Å) stoichiometry TiO(2−x)Tex TiO(2−2x)Te2x TiO2Te2x TiO2Tex TiO(2−x)Te2x Ti(1−x)O2Tex Ti(1−2x)O2Te2x a

configuration isolated(1a) dimerized(1b) dimerized(1c) isolated(1d) separated(1e) dimerized(1f) isolated(1g) separated(1h)

ΔE/nTe 0.87 (0.49) 0.00 (0.00) 0.10 (0.63) 0.00 (0.00) 1.64 (1.21) 0.00 (0.00) 0.36 (0.27) 0.00 (0.00)

Te−Te

Te−O

a

8.28 2.66 4.11 8.28a 4.07 2.62 8.28a 3.05

2.03/2.14 2.13/2.16 2.01/2.31

Te−Ti 2.44/2.53 2.53/2.65 2.52/2.67 2.58/2.63 2.56/2.82 2.64/2.70

2.04/2.13 1.96/2.09

Indicates the averaged distance to neighboring periodic image cells.

Figure 2. Calculated UV−vis optical absorption spectra with HSE06 for the various anionic Te-doped TiO2 systems reported in Figure 1: (a) TiO(2−x)Tex, TiO2Tex, and TiO2Te2x; (b) TiO(2−2x)Te2x and TiO(2−x)Te2x. Reference undoped TiO2 in dark solid line.

of 2.03 Å forming two additional weaker Te−O bonds of 2.14 Å is found at 0.10 (or 0.63) eV/Te higher in energy with PBE (or HSE06) (Figure 1c and Table 2). Mixed interstitial−substitutional Te species for O sites are simulated by replacing single O atom with single Te dopant and by adding one additional Te dopant to the 96-atom supercell.

This leads to TiO(2−x)Te2x. The lowest-energy structure reveals the formation of a dimerized Te−Te species with a Te−Te bond length of 2.62 Å (Figure 1f and Table 2). The most stable spin configuration is found to be closed-shell spin state, formally associated to diamagnetic Te22− species substituted for O2−. The configuration with two Te species separated by a longer Te−Te 12944

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distance of 4.07 Å forming one substitutional Te species and two TeO species with Te−O bond lengths of 2.01 and 2.31 Å is found to be at 1.64 (or 1.21) eV/Te less stable with PBE (or HSE06) (Figure 1e and Table 2). Substitutional Te species for Ti sites are modeled by replacing single or double Ti atoms with single or double Te dopants in the 48-atom or 96-atom supercells, respectively. This leads to Ti(1−x)O2Tex or Ti(1−2x)O2Te2x, respectively. The lowest-energy structure corresponds to the formation of two Te species separated by a Te−Te distance of 3.05 Å (Figure 1h and Table 2). Each Te species is coordinated with 5 O with 2 short Te−O bond lengths of 1.96 Å and a second shell of 3 O with 3 longer Te−O bond lengths of 2.09 Å (Figure 1h and Table 2). The other O previously bonded to the missing Ti get closer to the other Ti. A closed-shell singlet state is found to be the most stable spin configuration, associated formally to diamagnetic Te4+ defects substituted for Ti4+. The configuration with two Te species separated by a longer distance of 8.28 Å and imposing longer Te−O bonds of 2.04 and 2.13 Å is found at 0.36 (or 0.27) eV/Te higher in energy with PBE (or HSE06) (Figure 1g and Table 2). 3.2. Effect of Te Dopants on the UV−Vis Optical Absorption of TiO2. For undoped TiO2, we have shown in our previous paper11 a calculated band gap of 3.30 eV with HSE06 (Figure 4a), revealing a significant improvement with a slight overestimation of only 0.1 eV compared with experimental data. Note that other hybrid functionals like PBE0 or B3LYP significantly overestimate the band gap by 0.9 eV. The calculated band gap of 3.30 eV for anatase with HSE06 leads to the UV−vis optical absorption response shown in Figure 2 with an absorption onset at 376 nm. For anionic Te-doped systems, TiO2Tex (with separated TeO2− species), TiO2Te2x (with two concomitant TeO2− species), and TiO(2−x)Tex (with isolated Te2− species) reveal roughly similar optical spectra with a significant improvement in the visible region represented by red-shifted absorption edges of about 400 nm with respect to that of TiO2 (Figure 2a). In contrast, TiO(2−x)Te2x (with dimerized Te22− species) and TiO(2−2x)Te2x (with dimerized Te2− species) show different spectral behavior with very broad absorption bands up to high wavelengths (Figure 2b). These doping structures are not hence optimal for an enhanced optical absorption in Te-doped TiO2 under visible-light irradiation and should be avoided during the synthesis steps. Although TiO(2−x)Tex shows a significantly improved optical response in the visible range, it was energetically found at 0.87 eV/Te less stable than TiO(2−2x)Te2x. However, this system cannot be completely excluded because the shape of the optical spectrum matches well the observed experimental data.12 For cationic Te-doped systems, Ti(1−x)O2Tex (with isolated Te4+ species) and Ti(1−2x)O2Te2x (with two neighboring Te4+ species) show two different optical absorption spectra (Figure 3). For Ti(1−x)O2Tex, an optical resonance appeared at 430 nm with a blue-shifted absorption edge in the UV range compared with that of undoped TiO2. In contrast, an improvement in the visible region is obtained for Ti(1−2x)O2Te2x with a red-shifted absorption edge of about 150 nm with respect to that of TiO2 (Figure 3). This result shows that the optical response is strongly dependent on the O environment surrounding the formed Te4+ species. As a consequence, four Te-doped systems show improved optical absorption responses in the visible range similarly as it is experimentally observed in Te-doped TiO2 powders:12 three

Figure 3. Calculated UV−vis optical absorption spectra with HSE06 for the two cationic Te-doped TiO2 systems reported in Figure 1: Ti(1−x)O2Tex and Ti(1−2x)O2Te2x. Reference undoped TiO2 in dark solid line.

anionic systems, TiO(2−x)Tex (with isolated Te2− species), TiO2Tex (with separated TeO2− species), and TiO2Te2x (with two concomitant TeO2− species) and one cationic system Ti(1−2x)O2Te2x (with two neighboring Te4+ species). The optical absorption edges of TiO(2−x)Tex, TiO2Tex, and TiO2Te2x are found to be red-shifted by 400 nm compared with undoped TiO2 whereas that of Ti(1−2x)O2Te2x is red-shifted by 150 nm. Moreover, the shape of these absorption edges also shows a good agreement with the experimental results published in ref 12. This suggests that these four doping species could be at the origin of the enhanced visible-light absorption in Te-doped TiO2. The electronic structure analysis will be now presented to give a deeper understanding of the optical spectra. 3.3. Effect of Te Dopants on the Electronic Structure of TiO2. In this section, we investigate the electronic structure for the various optimal anionic and cationic Te-doped TiO2 structural configurations discussed in Section 3.2 by calculating the density of states (DOS) with HSE06. By correlating DOS and optical absorption results, we aim at determining the electronic origin of the enhanced visible-light optical response obtained in Te-doped TiO2. Let us first consider the three optimal anionic Te-doped systems presented in Figure 2a. The DOS obtained for TiO(2−x)Tex (with isolated Te2− species) shows a narrower band gap of 1.12 eV compared with that of TiO2 resulting from the creation of three new occupied electronic states in the gap of TiO2: two states at the top of the valence band and one at 1.0 eV above the valence band of TiO2 (Figures 4a, b). The new electronic states are mainly composed of O 2p orbitals with a contribution of Ti 3d orbitals and a small contribution of Te 5p orbitals. For TiO2Tex (with separated TeO2− species) and TiO2Te2x (with two concomitant TeO2− species), the DOS reveal narrower optical band gaps of 1.76 and 2.11 eV compared with that of TiO2, respectively, resulting from the creation of new occupied electronic states in the middle of the gap of TiO2 (Figures 4a, c, d). The contribution of O 2p orbitals in the new electronic states is much less important than that in TiO(2−x)Tex. The excitations from the new electronic states to the conduction band of TiO2 are at the origin of the new absorption bands appearing in the visible range, as presented in Figure 2a. If we consider now the optimal cationic Te-doped system Ti(1−2x)O2Te2x (with two neighboring Te4+ species) presented in 12945

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Figure 4. Calculated density of states (DOS) with HSE06 for the optimal anionic and cationic Te-doped TiO2 systems reported in Figures 2 and 3: (a) TiO2, (b) TiO(2−x)Tex, (c) TiO2Tex, (d) TiO2Te2x, and (e) Ti(1−2x)O2Te2x. Color legend: total DOS in black, DOS projected on Ti in blue, on O in green, and on Te in red. Fermi level is represented by the horizontal line.

NHE potential were taken from the experimental values.27 For Te-doped TiO2, the conduction band edge positions were obtained from the DOS plots according to the relative positions as compared with that of pure TiO2, and the valence band edge positions were deduced from the absolute band gap values. As shown in Figure 5, TiO2 presents an ability to reduce H+ ions as its relative conduction band edge position is 0.4 eV more negative than the H+/H2 potential. The valence band edge position is much more positive than the O2/H2O potential. On the basis of the above standard of an active photocatalyst for water splitting and the degradation of organic molecules, we discuss and evaluate the photocatalytic activity of the various anionic and cationic Te-doped TiO2 systems. For TiO(2−x)Tex (with isolated Te2− species), the conduction band edge position is shifted upward by 0.3 eV over TiO2, and so, the reduction ability of H+ ions is maintained. The valence band edge position is moved upward by 1.32 eV over TiO2 (i.e., 0.25 eV more positive than the O2/H2O potential), and so, the water oxidation ability is greatly improved. Similar result was obtained for Ti(1−2x)O2Te2x (with two neighboring Te4+ species). In contrast, for TiO2Tex (with separated TeO2− species) and TiO2Te2x (with two concomitant TeO2− species), the conduction band edge positions are both shifted upward by 0.5 eV over TiO2, and so, their reduction ability remains possible. Nevertheless, their valence band edge positions are moved upward by 2.13 and 2.48 eV over TiO2, respectively (i.e., 0.59 or 0.94 eV more negative than the O2/H2O potentials, respectively), leading to unsuitable positions for water oxidation reaction. 3.5. Electron Density Based Impurity States Analysis. After excitation, the generated electrons and holes should be separated in the bulk. Then, they should migrate to the surface of the catalyst in order to oxidize water and reduce H+ ions. Indeed, in our case the holes are released from the impurity levels created in the band gap of TiO2. Therefore, it is important to discuss their localized or delocalized nature. We show in Figure 6 the total electron densities corresponding to the new electronic states created upon doping in the gaps of the two optimal systems presented in Figure 5. For TiO(2−x)Tex (with isolated Te2− species), the electron density analysis shows an important contribution of p orbitals on the Te species and p orbitals on the majority of the O species in

Figure 3, the DOS indicates a narrower optical band gap of 1.17 eV compared with that of TiO2 resulting from the creation of one new occupied electronic state at 1.0 eV above the valence band of TiO2 (Figures 4a, e). This new electronic state is composed of an important contribution of O 2p orbitals with a small contribution of Te 5p orbitals and Ti 3d orbitals. The excitation from this new electronic state to the conduction band of TiO2 is responsible for the new absorption band appearing in the visible region, as shown in Figure 3. 3.4. Evaluation of the Photocatalytic Activity. The ability of a semiconductor to undergo photoinduced electron transfer to adsorbed species on its surface is governed by the band energy positions of the semiconductor with respect to the redox potential of the adsorbate. Thermodynamically, the relevant potential level of the acceptor species is required to be below (more positive than) the conduction band potential of the semiconductor. The potential level of the donor species needs to be above (more negative than) the valence band potential of the semiconductor to donate an electron to the vacant hole.2 To evaluate the influence of Te-doping on the photocatalytic activity of TiO2, the valence and conduction band edge positions of the various anionic and cationic Te-doped TiO2 systems discussed in Section 3.3 as compared with those of undoped TiO2 are depicted in Figure 5. For undoped TiO2, the values of the valence and conduction band edge positions with respect to

Figure 5. Calculated valence and conduction band edge positions of the various anionic and cationic Te-doped TiO2 systems reported in Figure 4 as compared with those of the of pure TiO2. These values are given with respect to vacuum level (in eV) and also with respect to the NHE potential (in V). 12946

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Figure 6. Calculated total electron density with HSE06 for the gap states created upon doping in (a) TiO(2−x)Tex and (b) Ti(1−2x)O2Te2x. Isovalue is 0.003 au.

the cell (Figure 6a). For Ti(1−2x)O2Te2x (with two neighboring Te4+ species), the electron density analysis reveals a weak contribution of p orbitals on the Te species and p orbitals on the O species only coordinated with Te (Figure 6b). It is important to mention that the contribution of p orbitals on O in Ti(1−2x)O2Te2x is much less important than that obtained with TiO(2−x)Tex. Therefore, the delocalized nature of the gap states seems to be more pronounced in TiO(2−x)Tex than that obtained with Ti(1−2x)O2Te2x.

Notes

4. CONCLUSION We have investigated the origin of the enhanced visible-light optical absorption in Te-doped bulk anatase TiO2 in the framework of DFT (including the perturbation theory approach DFPT) within the screened coulomb hybrid (HSE06) exchangecorrelation formalism. The use of this robust and advanced methodology led to an accurate description of the electronic structure and optical absorption properties of this material. TiO(2−x)Tex (with isolated Te2− species at Te−Te distance of 8.28 Å), TiO2Tex (with isolated TeO2− species at Te−Te distance of 8.28 Å), TiO2Te2x (with two concomitant TeO2− species at Te−Te distance of 4.11 Å), and Ti(1−2x)O2Te2x (with two neighboring Te4+ species at nearest-neighbor Te−Te distance of 3.05 Å) show improved optical absorption responses in the visible range similarly as it is experimentally observed in Te-doped TiO2 powders. The optical absorption edges of TiO(2−x)Tex, TiO2Tex, and TiO2Te2x were found to be redshifted by 400 nm compared with undoped TiO2 whereas that of Ti(1−2x)O2Te2x was red-shifted by 150 nm. On the basis of calculated valence and conduction band edge positions of Tedoped TiO2, only TiO(2−x)Tex and Ti(1−2x)O2Te2x showed suitable potentials for overall water splitting under visible light irradiation. The electronic structure analysis of these two systems revealed narrower band gaps of 1.12 and 1.17 eV with respect to undoped TiO2, respectively, resulting from the appearance of new occupied electronic states in the gap of TiO2. The gap states were found to be much more delocalized in TiO(2−x)Tex than those appearing in Ti(1−2x)O2Te2x due to the important contribution of numerous O 2p orbitals together with Te 5p orbitals.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks King Abdullah University of Science and Technology (KAUST) for the available supercomputing facilities.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +966.2 808.07.88. Fax: +966.2.802.12.72. E-mail: [email protected]. 12947

REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp400880b | J. Phys. Chem. C 2013, 117, 12942−12948