Anion-Doped NaTaO3 for Visible Light Photocatalysis - The Journal of

Oct 3, 2013 - The results are shown in Figure 6b. Two occupied states are above the valence band of NaTaO3, which mainly consist of C 2p and S 3p stat...
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Anion-Doped NaTaO3 for Visible Light Photocatalysis Baochang Wang,† Pushkar D. Kanhere,‡ Zhong Chen,‡ Jawad Nisar,¶,∥ Biswarup Pathak,§ and Rajeev Ahuja*,†,∥ †

Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden ‡ School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, 639798 Singapore, Singapore ¶ Pakistan Atomic Energy Commission (PAEC), P.O. Box. 2151, Islamabad, Pakistan § Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Khandwa Road, Indore, 452017, India ∥ Condensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, 751 20 Uppsala, Sweden ABSTRACT: In this paper, we have employed DFT and HSE06 methods to study the doping effects on the NaTaO3 photocatalyst. N, S, C, and P monodoping and N−N, C−S, P−P, and N−P codoping have been studied. The redopants’ formation energies have been calculated, and we find S monodoping is energetically more favorable than any other elemental doping. The mechanism of anion doping on the electronic properties of NaTaO3 is discussed. We find the band gap reduces significantly if we dope with anionic elements whose p orbital energy is higher than the O 2p orbitals. N and S can shift the valence band edge upward without losing the ability to split water into H2 and O2. Doublehole-mediated codoping can decrease the band gap significantly. On the basis of our calculations, codoping with N−N, C−S, and P−P could absorb visible light. However, they can only decompose water into H2 when the valence band edge is above the water oxidation level.

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INTRODUCTION Development of visible light photocatalysts for energy and environmental applications has gained significant attention in the recent years. To date, several materials systems have been investigated and modifications of these materials have been carried out to improve their photocatalytic activities.1,2 Among all these, NaTaO3 is one of the most efficient photocatalysts for pure water under ultraviolet (UV) irradiation.3,4 On account of its superior photocatalytic properties, NaTaO3 is considered as a good host material for developing visible light photocatalysts.5−9 Doped NaTaO3 compounds, with the dopants like Bi, Co, Cr, Cu, N, and Fe, have been studied for visible light photocatalysis. These studies show that doping is a promising way to induce visible light absorption in NaTaO3. Although a few doped systems are reported in the literature, efficient photocatalytic system which works under visible light is lacking among the family of NaTaO3 materials. In order to design such a system, the effect of various dopants on the band gap of NaTaO3 should be studied in detail. The literature on the doped photocatalysts shows that doping of anions in wide band gap semiconductors is one of the successful strategies to achieve visible light photocatalystsis.10,11 Photocatalysts such as A-doped TiO2 (where A = C, N, S) and N-doped SrTiO3 have been studied for various photocatalytic applications under visible light. Thus, detailed studies on anion-doped NaTaO3 systems are useful to develop efficient photocatalytic materials © XXXX American Chemical Society

systems. The electronic structure calculations have become an indispensable tool to understand the band structure of a photocatalytic material, which in turn very helpful for designing efficient photocatalysts. Therefore, density functional theory(DFT-)based band structure calculations are extensively used to explore the electronic properties of the photocatalytic materials. Moreover, with the use of hybrid DFT calculations, the accurate description of band structures can be easily predicted. In this article, we report the electronic structure of anion-doped (N-, P-, C-, or S-doped) NaTaO3 using DFT and hybrid DFT calculations. Codoping of C−S, N−P, N−N, P−P have also been studied along with their respective monodoped systems. This is because codoped systems such as C−S and N− N are reported to be highly beneficial for the visible light photocatalysis. Moreover, the effect of dopants on the band gap and band edge positions are discussed in detail. The formation energy of the various dopants under different reaction conditions is also investigated.

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COMPUTATIONAL METHODS In the present study, the total energy and electronic structure calculations were carried out within the framework of the Received: July 16, 2013 Revised: October 1, 2013

A

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density functional theory (DFT) using the projected augmented wave (PAW) method12 as implemented in the VASP package.13−15 The PAW potentials with the valence states 5d and 6s for Ta, 2s and 2p for O, 2s for Na, 3s and 3p for S and P, and 2s and 2p for N and C have been employed. The exchange correlation interaction was treated in the level of the Perdew−Burke−Ernzerhof (GGA-PBE) exchange correlation functional.16 The supercell approach has been employed to study the doping effects on the electronic structure of NaTaO3. A 2 × 2 × 1 supercell has been constructed with 80 atoms in it. Anionic monodoping is simulated by substituting one of the O atoms by an anionic atom that corresponds to the defect concentration of 2%. For the codoping study, we have substituted two O atoms with two anionic dopants. The Heyd−Scuseria−Ernzershof (HSE) screened potential method has been used to calculate the band gap of the pure and doped systems to eliminate the error in the band gap calculations within the standard DFT methods. The Heyd−Scuseria− Ernzershof screened Coulomb potential method is a hybrid density functional method which is based on the screened Coulomb potential for the exchange interactions.17 Using a screened Coulomb potential for Hartree−Fock (HF) exchange enables us to do a more accurate description of the electronic structure calculations. A screened Coulomb potential is the one where the full Coulomb potential part is split into short-range and long-range components.18 In this work, the splitting parameter ω was set to 0.2, conforming to the HSE06 functionals. HSE methods can give the results with significantly smaller errors than pure DFT calculations.19

Figure 2. Total and partial density of states of NaTaO3 given by DFT.

extension with the relatively different amount below and above the Fermi level. The calculated band gap is 2.62 eV, which is much smaller than the experimental value, 4.1 eV, due to the common DFT problems.21 Hybrid functional (HSE06) methods have been employed to calculate the band gap of this system resulting in 3.90 eV, which is very close to experimental value. In order to shift the conduction band downward, the dopant’s d orbital energy should be smaller than that of the Ta 5d; on the other hand, we need the dopant with a higher 2p orbital energy than that of O 2p to shift the valence band upward. The formation energy can be calculated using the expression22

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ΔHD, q = (E D, q − E H) + Σαnαμα + q(Ev + EF )

RESULTS AND DISCUSSION At room temperature, the NaTaO3 is orthorhombic, with space group Pcmn (Figure 1).20 The Ta cation is 6-fold coordinated

where ED,q and EH are the total energies of the doped system and the pristine material, nα is the number of α atoms added (nα > 0) or removed (nα < 0) from host to create a defect, and μα is the chemical potential of the α atom, which can be expressed as μα = μelem α +Δμα. The chemical potential μα is constrained by keeping the host compound stable. The sum of the chemical potential of components Na, Ta, and O must equal to the formation energy of NaTaO3, with 3ΔμO + ΔμNa + ΔμTa = ΔHf(NaTaO3). Thus, the atomic chemical potential can be varied in the range of O rich (ΔμO = 0 and ΔμNa + ΔμTa = ΔHf(NaTaO3)), Na rich with ΔμNa = 0 and 3ΔμO + ΔμTa = ΔHf(NaTaO3), and Ta rich with ΔμTa = 0 and 3ΔμO + ΔμNa = ΔHf(NaTaO3). The other constraints we need to take under consideration is that the elements of Na, Ta, and O do not form compounds as competing phases. Competing phases of NaO and Ta2O5 have been calculated in our study. When doping with N, C, P, and S, the dopants should avoid forming possible competing phases with Ta and Na atoms. For example, because the NaS compound is formed in a natural way, we need to apply the constrain with ΔμS + ΔμNa ≤ ΔHf(NaS). In our study, we have considered the competing phases of N2NaTa, NaS, TaC, and Ta3P for N, S, C, and P doping. Figure 3 shows the allowed chemical domains (dark gray area) for NaTaO3 in the ΔμNa and ΔμO plane. The calculated formation energies of different dopants in NaTaO3 have been shown in Figure 4, and the minimum formation energy is summarized in Table 1. When doping with N, the minimal formation energy is 0.50 eV with ΔμO = 0 eV (O-rich), ΔμN = −5.53 eV, and ΔμNa = −2.89 eV (Figure 4a). When doping with S, the results are quite different. The formation energy is negative, with the value of −0.67 eV. The corresponding chemical potentials are ΔμO = −4.58 eV, ΔμNa = −2.53 eV, and

Figure 1. Crystal structure of NaTaO3. The Na, O, and Ta atoms are denoted as large yellow ball, small red ball, and small brown ball, respectively.

with six oxygen anions, forming a tilt octahedra. The Na cations are located in the central position among these octahedra. The calculated lattice constants are a = 5.428 Å, b = 5.510 Å, and c = 7.750 Å, which is in good agreement with experimental data (a = 5.484 Å, b = 5.521 Å, and c = 7.795 Å).20 The calculated bond length of Ta and O is 1.83 Å. The calculated density of states of pure NaTaO3 is shown in Figure 2. The valence band mainly consists of O 2p states, and the conduction band is mainly composed of Ta 5d states. Where the binding characteristics are concerned, there are both covalent and ionic characteristics, as the Ta 5d and O 2p states are hybridized and degenerated over a large part of their B

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Table 1. Minimum Formation Energy and the Corresponding Chemical Potential of O, Na, and Dopantsa N P C S a

formation energy

ΔμO

ΔμNa

ΔμXb

0.50 2.74 3.0 −0.67

0 −4.59 −4.08 −4.58

−2.89 0 0 −2.53

−5.53 0 0 0

The units are in eV. bX is the dopant.

and two neighbor Ta atoms form bonds there. Two of the three 2p electrons of the N atom will be hybridized with Ta 5d electrons and located at the top of the valence band shown in Figure 5a. There are two distinct peaks around the Fermi level. There is one peak on top of the valence band lying in the gap with the Fermi level going through, which corresponds to the unpaired electron. The conduction band edge remains unchanged. The bond length of the N atom and Ta atom is 2.0 Å, which is slightly larger than that of the O and Ta bond. This suggests that the bond between N and Ta is slightly weaker than that of O and Ta atoms. The gap between the impurity level and the conduction band edge is 2.93 eV, which is 0.97 smaller than the pure NaTaO3. The narrower gap would make the adsorption edge of N-doped NaTaO3 move to the UV−visible light range. Sulfur has the same valence electrons as oxygen, but the 3p orbital energy of S is 2.2 eV higher than the O 2p orbitals. When O is substituted with S, the 3p orbitals of S atom will interact with Ta 5d orbitals. The bond length of S and Ta atoms is 2.34 Å, which is much larger than that of O and Ta (1.83 Å) and even larger than the bond of N and Ta atoms, indicating a weaker binding. There are three localized bands appearing above the valence band that come from the

Figure 3. Formation energy of NaTaO3. The dark gray area is the allowed chemical domain for NaTaO3. The formation energy calculations are based on standard DFT.

ΔμS = 0 eV (Figure 4b). The negative formation energy means that doping with S is energetically favorable. For C doping, the minimal ΔHC,0 is at the Na-rich condition with ΔμNa = 0 eV and ΔμO = −4.08 eV. The minimal formation energy is 3.0 eV at the condition of ΔμC ≤ 0 eV, sitting at the upright corner of the formation energy map (Figure 4c). On the other hand, for P doping, the formation energy is slightly smaller than C doping, with a formation energy of 2.74 eV. The chemical potential at the point of ΔμNa and ΔμN equals 0 eV, which is also located at the upright corner of formation energy map (Figure 4d). For nitrogen-doped NaTaO3, the energy of N 2p orbital is 1.9 eV higher than O 2p.23 Additionally, nitrogen has one less electron compared to oxygen, so the substitution of N on the O site in NaTaO3 generates one hole in the system. The N atom

Figure 4. Formation energy of N, S, C, and P-monodoped NaTaO3. The color of the map corresponds to the value of formation energy. The formation energy calculations are based on standard DFT. C

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Figure 5. Calculated density of states N, S, C, and P-monodoped NaTaO3 with HSE06 methods.The DOSs for doped compounds are shifted so that the peaks of the O 2s states (at the farthest site from the dopant) are aligned with each other.

overlapping of S 3p orbitals and Ta 5d orbitals. This can be seen from the plot of the density of states of the S-doped system (Figure 5b). The band gap of the sulfur-doped NaTaO3 is 3.1 eV, 0.8 eV less than pure NaTaO3. Both the valence band edge and conduction band edge have been shifted upward, with a value of 1.35 and 0.35 eV, respectively. Thus, the narrowing of the band gap is very helpful for allowing NaTaO3 to absorb near-visible-range light. For the carbon-doped NaTaO3, as C has two less electrons than O and the energy of C 2p orbital is 2.3 eV higher than that of O, the 2p states of C atom overlap with Ta 5d states. There are two C−Ta bonds formed with the bond length 2.09 Å, which is slightly larger than Ta−O bond in pure system and comparable with Ta−N bond. Therefore, C−Ta bond is stronger than that of S−Ta. As can be seen from the density of states plot (Figure 5c), there are three peaks in the gap above the valence band maximum with two of them filled. The band gap was reduced to 1.42 eV with an unoccupied peak above the Fermi level. Moreover, HSE06 also predicts that the conduction band edge is also shifted 0.21 eV upward. The reduced band gap makes it possible to absorb visible light; however, the localized hole states in the band gap always works as a recombination center, which is not good for the efficiency of photocatalysis. Phosphorous also has one less valence electron than oxygen. When doped with phosphorous, some localized states are appearing within band gap, which mainly comes from the overlap of Ta 5d and P 3p states. There are three electrons in P 3p states, though Ta and P atoms form two bonds there. Thus, one extra unpaired electron has a higher energy, above the Fermi level(Figure 5d). The bond strength of P−Ta bond is much weaker because the bond length of it is 2.39 Å, much larger than that of Ta−O (1.83 Å) bond. However, the

conduction band is also shifted upward, which is similar to S and C doping. The band gap given by HSE06 is 1.55 eV, which is also good for visible light absorption. As discussed above, monodoping could introduce holes to the system. For example, N and P bring one hole and C brings two holes to the system. These hole states sit at the top of valence band. Sometimes these hole states work as recombination center, degrading the photocatalysis property (C and P doping). Codoping is one possible way to avoid this deficiency.24−26 In order to eliminate the single-hole state, we have tried N− N codoping. The stable configuration for the N−N codoping is the one with two nitrogen atoms substituting the two nearest oxygen, respectively. After substitution, the distance between two nitrogen atoms becomes 1.41 Å (Table 2), much smaller Table 2. Bond Length and Binding Energy of Codoping with Two Different Anion Atoms NaTaO3

bond length Å

binding energy (eV)

O−O N−N C−S N−P P−P

2.83 1.41 1.79 1.73 2.09

1.02 0.94 1.39 0.91

than the original distance of two oxygen atoms (2.83 Å) in pure NaTaO3, indicating very strong coupling between the two nitrogen atoms. As each N atom has an unpaired electron, when two N atoms move close to each other, the two unpaired p electrons will be hybridized with each other, forming a bonding state and an antibonding state with a large energy separation. The bonding state sits inside of the valence band and the antibonding state sits at the conduction band. The D

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Figure 6. Calculated density of states of codoping with N−N (a), C−S (b), N−P (c), and P−P (d) in NaTaO3 with HSE06 methods. The black curves are the total density of states of pure NaTaO3, and the red curves are the total density of states of doped NaTaO3. The DOSs for doped compounds are shifted so that the peaks of the O 2s states (at the farthest site from the dopant) are aligned with each other.

mainly consist of C 2p and S 3p states. The band gap is 1.70 eV, which is also good for visible light absorption. The shifting of the conduction band edge is also observed in our calculations. N and P codoping is also investigated in our study. As both N and P have one less valence electron than O, codoping with them will eliminate the hole state localized at the Fermi level. The double-hole-mediated coupling is also observed in this system, which mainly results from the overlapping of P 3p and O 2p states (Figure 6c). Additionally, the N and P atoms form a strong bond with a bond length of 1.73 Å (Table 2). The band gap is largely reduced due to this effect (1.34 eV). When P and P codoping is considered, there is one hole state lying in the band gap. There is a weak coupling between the two P atoms with the bond length of 2.09 Å (Table 2). When the two P 3p orbitals interact with each other, the unpaired P 3p electrons will overlap, forming a bonding and an antibonding state. The hole state originates from the antibonding state. As it is just above the Fermi level, there is a very small gap for P−P-doped NaTaO3 (Figure 6d). Three well-separated states are appearing just below the Fermi level, which are states of overlapped of P 3p and Ta 5d states. The band alignment is shown in Figure 7; the position of the valence band edge of pure NaTaO3 is adopted from experiment.27 Water can be decomposed into H2 and O2 for pure NaTaO3, as both water oxidation and reduction energy levels are within the band gap. As shown in the band alignment, it is still possible for the N-doped NaTaO3 photocatalyst to produce H2 and O2 with its valence band edge moving 0.97 eV upward. It is also possible for the S-doped NaTaO 3 photocatalyst to produce H2 and O2. For C-doped NaTaO3, the valence band edge is above the water oxidation level, which is not possible for O2 production. The impurity band, 0.6 eV

bond between the two nitrogen atoms is purely covalent, as the distance between them is comparable to that of N2H2 gas (1.45 Å). The binding energy of the codoped system can be calculated by Eb = EDA + EDB − EDA+B − EH, where EDA (EDB) is the total energy of A- (or B-) monodoped NaTaO3, EDA+B is the total energy of A and B-codoped NaTaO3, and EH is the total energy of pure NaTaO3 compound. The calculated binding energy (Table 2) for N−N-, C−S-, N−P-, and P−P-codoped systems are 1.02, 0.94, 1.39, and 0.91 eV, respectively. The positive binding energies for the codoped system support their stability over their corresponding monodoped systems. The N−P has the largest binding energy. The density of states of the N−N-codoped system are shown in Figure 6a. The valence band edge of the doped system has been shifted upward with respect to pure NaTaO3. Moreover, two nitrogen 2p orbitals are interacting with the Ta 5d orbitals that are located on top of the valence band. The bond length of N and Ta atoms becomes 2.19 Å larger than that of single N doping, indicating a weaker binding. The band gap is 2.1 eV, much smaller than that of the pure one, which is suitable for visible light absorption. Like the C-, S-, and P-doped cases, the conduction band edge is shifted to a higher energy range, about 0.40 eV higher than the undoped case (Figure 6a). We also studied the carbon and sulfur codoping in NaTaO3, which could reduce the band gap significantly as the doublehole-mediated coupling observed in other photocatalysts.24−26 After optimization, the distance between C and S atoms becomes 1.79 Å, much smaller than their distance (2.83 Å) before substitution (Table 2). Unlike the N−N bond, the bond between C and S is a mixture of covalent and ionic characteristics. The results are shown in Figure 6b. Two occupied states are above the valence band of NaTaO3, which E

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scholarship council for support. SNIC and UPPMAX are acknowledged for providing computing time.

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(1) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (2) Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 2008, 20, 35−54. (3) Lee, Y. C.; Teng, H.; C., H. C.; Hu, S. Y. Temperature-dependent Photoluminescence in NaTaO3 with Different Crystalline Structures. Electrochem. Solid-State Lett. 2008, 11, P1−P4. (4) Kato, H.; Asakura, K.; Kudo, A. Highly Efficient Water Splitting into H2 and O2 over Lanthanum-doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure. J. Am. Chem. Soc. 2003, 125, 3082−3089. (5) Kanhere, P.; Zheng, J.; Chen, Z. Visible Light Driven Photocatalytic Hydrogen Evolution and Photophysical Properties of Bi3+ Doped NaTaO3. Int. J. Hydrogen Energy 2012, 37, 4889−4896. (6) Kang, H. W.; Lim, S. N.; Park, S. B. Photocatalytic H2 evolution under visible light from aqueous methanol solution on NaBixTa1−xO3 prepared by spray pyrolysis. Int. J. Hydrogen Energy 2012, 37, 4026− 4035. (7) Yang, M.; Huang, X.; Yan, S.; Li, Z.; Yu, T.; Zou, Z. Improved Hydrogen Evolution Activities under Visible Light Irradiation over NaTaO3 Codoped with Lanthanum and Chromium. Mater. Chem. Phys. 2010, 121, 506−510. (8) Yi, Z. G.; Ye, J. H. Band Gap Tuning of Na1−xLaxTa1−xCoxO3 Solid Solutions for Visible Light Photocatalysis. Appl. Phys. Lett. 2007, 91, 254108. (9) Wang, X.; Liu, G.; Chen, Z. G.; Li, F.; Lu, G. Q.; Cheng, H. M. Synthesis and Photoelectrochemical Behavior of Nitrogen-doped NaTaO3. Chem. Lett. 2009, 38, 214−215. (10) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visiblelight Photocatalysis in Nitrogen-doped Titanium Oxides. Science 2001, 293, 269−271. (11) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science 2002, 297, 2243−2245. (12) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B: Condens. Matter Mater. Phys 1994, 50, 17953−17979. (13) Kresse, G.; Hafner, J. Ab initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys 1993, 47, 558−561. (14) Kresse, G.; Hafner, J. Ab initio Molecular-dynamics Simulation of the Liquid-metal-amorphous-semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. Phys 1994, 49, 14251− 14269. (15) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys 1999, 59, 1758−1775. (16) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B: Condens. Matter Mater. Phys 1992, 46, 6671−6687. (17) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207. (18) Heyd, J.; Scuseria1, G. E. Efficient Hybrid Density Functional Calculations in Solids: Assessment of the Heyd-Scuseria-Ernzerhof Screened Coulomb Hybrid Functional. J. Chem. Phys. 2004, 121, 1187. (19) Marsman, M.; Paier, J.; Stroppa, A.; Kresse, G. Hybrid functionals Applied to Extended Systems. J. Phys.: Condens. Matter 2008, 20, 064201. (20) Ahtee, M.; Darlington, C. N. W. Structures of NaTaO3 by Neutron Powder Diffraction. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1980, 36, 1007−1014. (21) Godby, R. W.; Schlüter, M.; Sham, L. J. Accurate ExchangeCorrelation Potential for Silicon and Its Discontinuity on Addition of an Electron. Phys. Rev. Lett. 1986, 56, 2415−2418.

Figure 7. Band alignment of doped NaTaO3. The position of the valence band edge of pure NaTaO3 is adopted from experiment.

above valence band edge, works as the recombination center, which is not good for the efficiency of the photocatalysis. Pdoped NaTaO3 can only work as electrode for water reduction rather than oxidation, as is similar for N−N-, C−S-, and N−Pcodoped cases. However, the band gaps for the four doped systems are suitable for visible light absorption. Although it is feasible for P−P-codoped NaTaO3 to generate H2 and O2, the localized hole state in the gap will work as a recombination center, which is a drawback for P−P codoping.

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CONCLUSIONS In this paper, we have conducted PBE and HSE06 methods to calculate the anion doping in NaTaO3 for visible light photocatalysis. The results suggest that the minimum formation energy of N, P, C, and S are 0.49, 2.74, 3.0, and −0.67 eV, respectively. Doping of S is more favorable over the other elements. P monodoping could significantly reduce the band gap to 1.55 eV, followed by C doping with the band gap value of 2.02 eV. N doping changes least to the band gap. Doublehole mediated coupling could reduce the band gap significantly. N−N, C−S, and N−P codoping lead to band gap reductions, with the value of 2.19, 1.70, and 1.34 eV, respectively, which are suitable for visible light photocatalysis. However, the P−P codoping gives a gap energy of 2.50 eV, which is 1.5 eV smaller than the pure one and slightly large for visible light absorption. Although doping changes the band edges, it leads to the position change of the band alignment with respect to the water reduction and oxidation level. According to our calculations, monodoping with N and S could reduce the band gap and water can be decomposed into both H2 and O2. Codoping with N−N, C−S and N−P could result in significant band gap reduction with loss of the ability to oxidize water to O2.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*R. Ahuja. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the Swedish Research Council (VR) for financial support. B.C.W. would like to thank China F

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(22) Persson, C.; Zhao, Y.-J.; Lany, S.; Zunger, A. n-type Doping of CuInSe2 and CuGaSe2. Phys. Rev. B: Condens. Matter Mater. Phys 2005, 72, 035211. (23) Wei, S.-H.; Krakauer, H.; Weinert, M. Linearized Augmentedplane-wave Calculation of the Electronic Structure and Total Energy of Tungsten. Phys. Rev. B: Condens. Matter Mater. Phys 1985, 32, 7792− 7797. (24) Wang, B. C.; Nisar, J.; Pathak, B.; Kang, T. W.; Ahuja, R. Band Gap Engineering in BiNbO4 for Visible-light Photocatalysis. Appl. Phys. Lett. 2012, 100, 182102. (25) Yin, W.-J.; Wei, S.-H.; Al-Jassim, M. M.; Yan, Y. Double-HoleMediated Coupling of Dopants and Its Impact on Band Gap Engineering in TiO2. Phys. Rev. Lett. 2011, 106, 066801. (26) Nisar, J.; Pathak, B.; Wang, B.; Won Kang, T.; Ahuja, R. Hole Mediated Coupling in Sr2Nb2O7 for Visible Light Photocatalysis. Phys. Chem. Chem. Phys. 2012, 14, 4891−4897. (27) Kato, H.; Kudo, A. Water Splitting into H2 and O2 on Alkali Tantalate Photocatalysts ATaO3 (A = Li, Na, and K). J. Phys. Chem. B 2001, 105, 4285−4292.

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