Layered Perovskite Sr2Ta2O7 for Visible Light Photocatalysis: A First

Feb 21, 2013 - The layered perovskite Sr2Ta2O7 has been investigated for efficient visible light photocatalysis using the first principles study. ...
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Layered Perovskite Sr2Ta2O7 for Visible Light Photocatalysis: A First Principles Study Peng Liu,*,†,‡ Jawad Nisar,‡ Rajeev Ahuja,†,‡ and Biswarup Pathak*,§ †

Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden ‡ Condensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, 751 20 Uppsala, Sweden § Department of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Khandwa Road, Indore, 452017, India ABSTRACT: The layered perovskite Sr2Ta2O7 has been investigated for efficient visible light photocatalysis using the first principles study. The electronic structure of Sr2Ta2O7 is tuned by the anionic (N)/cationic (Mo, W) mono- and codoping. Such doping creates impurity states in the band gap and therefore reduces the band gap significantly. The absolute band edge position of the doped Sr2Ta2O7 with respect to the water oxidation/reduction potential depends a lot on the p/dorbital’s energies of anionic/cationic dopants, respectively. The stability of the co-doped system is governed by the Coulomb interactions and charge compensation effects.

1. INTRODUCTION

The ideal band gap for any visible-light driven photocatalyst is around 2.0 eV for the effective utilization of the solar spectrum. However, the ideal band gap (∼2 eV) of the semiconductor is not the only factor to improve efficiency of the H2 evolution. Along with their band gap, the band positions of the semiconductor with respect to the water oxidation/ reduction potential are equally important.16 The condition is that the conduction band minimum (CBM) of the semiconductor should be higher (more negative) than the hydrogen reduction potential level (H+/H2), while the valence band maximum (VBM) should be lower (more positive) than the water oxidation potential level (O2/H2O).17 Therefore, the controlled band gap engineering is very important for the effective utilizations of solar-to-hydrogen conversion. Recently, Mukherji et al. have shown that nitrogen doped Sr2Ta2O7 increases the visible light absorption and improves the photocatalytic hydrogen production by 87%.18 Kim et al. showed that barium-doped Sr2Ta2O7 exhibits a higher photocatalytic activity19 due to the maximum visible light absorption. Therefore, doping a foreign element into the UV active photocatalyst is a very promising strategy to improve their visible light photocatalytic activity.20−23 However, generally mono-doping creates impurity states in the band gap which acts as a recombination center. Moreover, mono-doping is associated with the spontaneous formation of charge compensating defects which can even destabilize the system.24 Co-doping is a very promising strategy to overcome such

Hydrogen is one of the most promising alternatives to fossil fuel; therefore, hydrogen production is the foremost concern for our future’s energy. Photocatalysis of water is the cleanest and safest way to produce hydrogen.1−4 Therefore, the search for an efficient photocatalyst is the major concern for the hydrogen production. The metal oxides with layered structures, such as K4Nb6O17,5 La2Ti2O7,6 K2La2Ti3O10,7 LiCa2Ta3O10,8 Sr2Ta2O7,9 and Ba5Ta4O15,10 show high photocatalytic activity for water splitting under UV irradiation. These tantalate and niobate based layered perovskite structures are reported to be very promising photocatalyst materials owing to their high photocatalytic activities9 in comparison with the standard known bulk type of materials such as TiO2.11 In the case of photocatalytic water splitting, the H2 and O2 evolution will occur on the photocatalyst surface. Therefore, on a photocatalyst surface, the distances between the H2 and O2 evolution sites are very important because, if they are in very close proximity, then there is a high chance of them reacting back to produce water. However, that is very unlikely in a layered perovskite structure where the H2 and O2 evolution sites are far from each other.12 Besides, the anisotropy generated from such a layered structure increases the photocatalytic ability. Most of the experimetally reported layered perovskite photocatalyst materials9,10,13−15 are tantalate based and very efficient materials for the UV-light driven photocatalysis. Among all of these, Sr2Ta2O7 is reported to be a very promising UV photocatalyst9 with high hydrogen evaluation rate; however, none of these photocatalysts is visible light active. Therefore, the biggest challenge is to make them visible light active. © 2013 American Chemical Society

Received: November 5, 2012 Revised: February 21, 2013 Published: February 21, 2013 5043

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issues.25,26 The intermediate states appearing in the band gap can be tuned by choosing different combinations and concentrations of p−n dopants. In addition, in the cationic/ anionic co-doped systems, the stabilities can be improved due to the presence of Coulomb coupling. As the valence band position of Sr2Ta2O7 is deep enough (Figure 1b) with respect to the water oxidation (H2O/O2)

functional and 25% Hartree−Fock (HF) (exact) exchange. The resulting exchange-correlation energy formula is as follows: PBEO E XC =

1 HF 3 E X + E XPBE + ECPBE 4 4

PBE Here, EHF and EPBE are the X is the HF exchange energy. EX C PBE exchange and correlation energies, respectively. In hybrid functionals, only the local part of the exact exchange energy is treated within HF theory, whereas the remaining part is treated by DFT. A k-points mesh of 3 × 1 × 5 was found to be sufficient to reach convergence for GGA-PBE calculations and 2 × 1 × 3 for the PBE0 calculations. In all calculations, the tolerance for energy convergence was set to 10−5eV. A planewave cutoff energy of 500 eV was used for the GGA-PBE and PBE0 method. The PAW potentials with the valence states 4s, 4p, and 5s for Sr, 4s and 3d for Ti, 2s and 2p for O and N, and 6s and 5d for Ta have been used. For the doping cases, a 1 × 2 × 1 supercell with 88 atoms was used, where O and Ta were substituted by dopant complexes such as anions and cations, respectively. The absorption curves can be obtained from the imaginary part of the dielectric constant using Kramers− Kroning dispersion relations.33

3. RESULTS AND DISCUSSION 3.1. Pristine Sr2Ta2O7. Sr2Ta2O7 possesses layered perovskite structure with an orthorhombic space group of Cmcm. The calculated (GGA-PBE) lattice parameters (a = 3.983 Å, b = 27.599 Å, and c = 5.746 Å) of Sr2Ta2O7 are in very good agreement with the experimental values of a = 3.937 Å, b = 27.198 Å, and c = 5.692 Å.34 The optimized crystal structure of Sr2Ta2O7 is presented in Figure 1a. The calculated band gap of pristine Sr2Ta2O7 is 2.93 eV, which is much smaller than the experimental value of 4.6 eV.9 It is a well-known fact that the GGA-PBE method underestimates the band gap of metal oxides.35 Therefore, we have used the hybrid density functional method (PBE0) for a more accurate description of their electronic structures.20,36,37 Our calculated band gap of pristine Sr2Ta2O7 using the hybrid functional method (PBE0) is 4.63 eV, which is in excellent agreement with the experimental band gap of 4.6 eV.9 Thus, for our further study, we have used the GGA-PBE method for the structural and energetic calculations, whereas the PBE0 functional is used for their electronic structures and absorption spectra calculations. The band edge alignment of pristine Sr2Ta2O7 is presented in Figure 1b, which indicates that the CBM is 1.66 eV more negative than the reduction potential of H+/H2 and VBM is 1.71 eV more positive than the oxidation level of O2/H2O.9 The total and partial densities of states (PDOS) of Sr2Ta2O7 are presented in Figure 2. It is found that the valence band maximum is mainly composed of O 2p and Sr 4p orbitals, whereas the conduction band minimum has been predominantly composed of Ta 5d orbitals. Therefore, our PDOS analysis shows that the valence band edge mainly consists of an anionic O atom, whereas the conduction band edge is mainly composed of a cationic Ta atom. Hence, the shift of the valence band edge and conduction band edge positions can be tuned with the anionic and cationic doping, respectively. 3.2. Mono-Doping in Sr2Ta2O7. We have studied the mono-anionic (N) and cationic (Mo and W) doping of Sr2Ta2O7 to understand the effect of an extra hole or electron in the electronic structure of pure Sr2Ta2O7. For the N anionic mono-doping, we have substituted one of the oxygen atoms in the 88 atoms (Sr:Ta:O = 16:16:56) supercell of Sr2Ta2O7

Figure 1. (a) Crystal structure of bulk pristine Sr2Ta2O7 with TaO6 octahedra where Sr, Ta, and O atoms are represented by green, blue, and red spheres, respectively. (b) Band edge alignment of pure Sr2Ta2O7 with respect to the water redox potentials.

potential, we chose to dope with an anionic element (nitrogen) which has a higher p-orbital energy than that of the oxygen. However, nitrogen doping will add a hole to the system. Therefore, we considered doping with a cationic element which has one more valence electron than the Ta atom. Hence, we chose to dope with Mo and W in place of Ta, which will add one extra electron to the system. Then, we have studied the codoping (Mo−N and W−N) in order to keep the total number of electrons constant as in pure Sr2Ta2O7. Hence, we have investigated the effects of mono- and co-doping on the electronic structure of Sr2Ta2O7 and their band positions are aligned with respect to the water redox potential. The formation energies for the mono- and co-doping systems are calculated in order to find out the possibilities of such doping. The relative stabilities of the co-doped systems are compared with their mono-doped system on the basis of their relative binding energies.

2. COMPUTATION DETAILS In our work, the first-principles calculations were performed using the projected augmented wave (PAW) method,27 as implemented in the Vienna ab initio simulation package (VASP).28−30 The exchange-correlation interaction was treated at the level of the GGA using Perdew−Burke−Ernzerhof (GGA-PBE).31 The Brillion zone was integrated using Monkhorst−Pack generated sets of k-points.32 The PBE0 method is defined as the mixing of the 75% GGA-PBE 5044

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Figure 2. The calculated total and partial density of states of pristine Sr2Ta2O7 using the PBE0 functional. The vertical dashed line represents the Fermi level.

which corresponds to a N doping concentration of 1.78%, whereas, for the cationic doping, we have substituted one of the Ta atoms by a W or Mo atom which corresponds to a doping concentration of 6.25%. The spin polarized calculations are used for all the doped systems in order to get their proper ground state structure. The N-doped Sr2Ta2O7 gives a total magnetic moment of +1.0 μB, out of which the N atom alone contributes a magnetic moment of +0.67 μB. This is because the nitrogen atom has one less valence electron than the oxygen atom, and therefore it adds a hole to the system. The total and partial density of states (DOS) of the N-doped system are presented in Figure 3a. It shows that N 2p orbitals split into occupied and unoccupied states, which are mixing well with the O 2p orbitals and appearing at the valence band edge as well as in the band gap. Long et al.38 found a similar trend for the N-doped TiO2 in their spin polarized calculations. We also have plotted the spin charge density of mono-anionic and cationic doping of Sr2Ta2O7 in Figure 4. From Figure 4a, we can see that the N atom not only shows a spin polarized effect on itself but also induces some magnetization to the neighboring O atoms. The absolute band position of N-doped Sr2Ta2O7 is aligned (Figure 5) with respect to the water oxidation/reduction potential. The valence band maximum (VBM) of N-doped Sr2Ta2O7 is pushed upward by 0.52 eV which is mainly contributed by mixing of N and O 2p orbitals. Moreover, some unoccupied impurity states appear in the band gap; therefore, the effective band gap of N-doped Sr2Ta2O7 is reduced to 2.57 eV (Figure 3a). Hence, the VBM can be easily tuned on the basis of the dopant’s (N) 2p orbital’s energy39 which is in good agreement with the experimental findings.18 However, the unoccupied acceptor states appearing above the Fermi level could act as an electron−hole recombination center. Therefore, it is very important to remove such unwanted impurity states in the band gap so that the visible light activity of the photocatalyst can be further improved. The cationic doping in Sr2Ta2O7 is considered for tuning the conduction band edge. We have substituted one Ta of Sr2Ta2O7 by a Mo atom because in the periodic table the Mo atom places at the next group to the Ta atom and their atomic sizes are almost equal. We have used the spin polarized

Figure 3. Calculated (using PBE0) DOS and PDOS of Sr2Ta2O7 with (a) N, (b) Mo, and (c) W mono-doping. The vertical dashed line indicates the Fermi level.

calculation for the Mo doped system, and the calculated magnetic moment of Mo doped Sr2Nb2O7 is −1.0 μB, where the largest (−0.85 μB) contributions come from the doped Mo atom. As the Mo atom has one more electron than the tantalum atom, it adds one extra electron to the system, thus showing a total moment of −1.0 μB. The total and partial density of states (DOS) of the Mo doped system are plotted in Figure 3b. In the Mo doped Sr2Nb2O7 system, the Fermi level shifts toward the conduction band edge which is in resemblance to an n-type semiconductor. Moreover, the Mo impurity states are appearing in the band gap just below the Fermi level (Figure 3b), but the occupied electronic states are very deep in the band gap. Hence, we can assume that the resultant band gap is the gap between the filled impurity states and the CBM which is 2.01 eV. Such impurity states (acceptor or donor) will appear in the band gap whenever we dope with an element which has higher or lower d-orbital energy.6,9,24 If we ignore the impurity states appearing in the band gap, then the band gap would have been 4.21 eV which is close to that of Sr2Ta2O7. For Mo-doped Sr2Ta2O7, the Mo atom induces the spin charge density on its neighboring Ta atoms and itself to some extent (Figure 4b). Therefore, our spin charge density studies (Figure 4b) show that the unpaired electron is mainly localized on the doped Mo atom and partially on the neighboring Ta atoms. Interestingly, our PDOS (Figure 3b) studies on the Mo-doped Sr2Nb2O7 also give a similar picture where it shows the unpaired electron is contributed maximum by the Mo 4d and Ta 3d orbitals. We 5045

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5). However, the reduction potential level is found to be at a good position with respect to the CBM level; therefore, it can be a promising material for hydrogen production. Meanwhile, we have also considered the W-doped Sr2Ta2O7 system to evaluate the doping effect on the electronic structure of Sr2Ta2O7. The tungsten substitution at the tantalum site acts as a single donor. We found a local magnetic moment of −1.0 μB for the W-doped system. This spin polarization occurs on the d-states of the W atom which gives a magnetic moment of −0.69 μB, while the neighboring atoms are polarized with a small magnetic moment. The electronic structure of the doped system is elaborated by plotting the total and partial density of states (Figure 3c). Some occupied states, which are mainly consisting of W 5d and Ta 5d states are appearing in the band gap but just below the CBM. Thus, after W doping, the effective band gap between the filled impurity state and the CBM is 0.96 eV (Table 1), which is much lower than the ideal Table 1. The Relative Binding Energies (Eb) of Co-Doping and Band Gaps (Eg) of Different Doped Sr2Ta2O7a GGA-PBE dopant pristine N Mo W N−Mo (near) N−Mo (far) N−W (near) N−W (far)

PBEO

Eb (eV)

Eg (eV)

CB shift (eV)

VB shift (eV)

2.31 1.19 2.44 1.69

4.61 2.57 2.01 0.96 3.89 2.32 4.36 2.96

−1.52 −0.51 −0.03 −0.51 −1.25 0.18 −0.67

0.52 2.09 3.64 0.21 1.04 0.43 0.98

a

The respective values are calculated using two different functionals (PBE and PBE0), and the shifting of VBM and CBM are tabulated for mono- and co-doping systems.

band gap of any photocatalyst. Figure 4c shows magnetization density induced by W atom and its effects on the doping system. It indicates that the W atom and its neighboring Ta atoms contribute the magnetization density, which are the same atoms corresponding to the gap states below the Fermi level in Figure 3c. 3.3. Co-Doping in Sr2Ta2O7. Mono-anionic (N) or cationic (Mo and W) doping creates impurity states in the band gap (Figure 3) which will act as a recombination center. Moreover, mono-doping is associated with the spontaneous formation of charge compensating defects in the system which can destabilize the system. To avoid such recombination centers and defects, controlled band gap engineering is important and this can be done by the anion−cation codoping.24,40 In this work, the (N, Mo) and (N, W) co-doping in Sr2Ta2O7 have been studied with doping concentrations of 1.78 and 6.25% for N and Mo/W, respectively. For a descriptive study, we have assumed two kinds of configurations; “near” and “far” to simulate the co-doped systems. In the cases of “near” and “far” configurations of the (N, Mo/W) co-doping system, the distances between the dopants are 1.73 and 17.61 Å for the nar and far configurations, respectively. We have investigated the electronic structure by plotting the density of states for the near and far configurations of the (N, Mo)-doped Sr2Ta2O7 system (Figure 6). The effective band gap of (N, Mo) is reduced to 3.89 and 2.32 eV for the “near” and “far” configurations, respectively. In the case of the “near” configuration, VBM is slightly shifted upward by 0.21 eV due to

Figure 4. Spin charge density of mono-doping in Sr2Ta2O7: (a) Ndoped; (b) Mo-doped; (c) W-doped. The green, red, gray, and pink spheres represent Sr, O, Ta, and doped atoms, respectively. The yellow-green color represents the positive (isosurface = 0.003/Å3), and the blue-green color shows the negative (isosurface = 0.004/Å3).

Figure 5. The electronic band edge positions with respect to the water reduction and oxidation potential levels for the pure and doped Sr2Ta2O7 systems.

have aligned the band edge position of Mo doped Sr2Ta2O7, and we have found the water oxidation potential level is more positive than the VBM, which means that water oxidation reaction (H2O/O2) is not thermodynamically favorable (Figure 5046

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Figure 7. The calculated (using the PBE0 functional) DOS and PDOS of (N, W) co-doped Sr2Ta2O7 with (a) near and (b) far configuration. The vertical dashed line indicates the Fermi level.

Figure 6. The calculated (using the PBE0 functional) DOS and PDOS of (N, Mo) co-doped Sr2Ta2O7 with (a) near and (b) far configuration. The vertical dashed line indicates the Fermi level.

the mixing of N 2p and O 2p, and CBM moves downward by 0.51 eV, which is due to the hybridization of Mo 4d and Ta 5d orbitals. However, in the case of the “far” configuration, the respective VBM moves upward by 1.04 eV and CBM moves downward by 1.25 eV. Therefore, the effective band gap is reduced to 2.32 eV which is in good range for the efficient solar-energy conversion. It is observed from their band edge alignment diagram that water oxidation and reduction reactions are thermodynamically feasible for the (N, Mo-far) co-doped system (Figure 5). Therefore, the co-doping of Mo and N neutralizes the electrons and holes in the system, and clean band gaps are obtained which not only avoid the recombination centers but also remove the charge compensating defects in the system for the efficient photocatalyst material. To understand the effect of electron neutralization, we have investigated the electronic structures of the (N, W) co-doped system by plotting their total and partial density of states (Figure 7). It shows that W 5d and Ta 5d are hybridized at the CBM and N 2p and O 2p orbitals contribute to the VBM. The band gaps for the “near” and “far” configurations are 4.36 and 2.96 eV, respectively. In the case of the “near” configuration, the CBM is unaffected, whereas the VBM moves slightly upward by 0.43 eV. For the “far” configuration, the VBM shifts upward by 0.67 eV and CBM moves downward by 0.98 eV; thus, the band gap is reduced to 2.96 eV. For both the “near” and “far” configurations, band gap narrowing is achieved. However, the (N, Mo) co-doped system shows a narrower band gap compared to the (N, W) co-doped system. This is because the Mo 4d orbital’s energy is lower than the W 5d orbital’s energy. Now this is explained in the revised manuscript. Both redox reactions are thermodynamically possible with good reaction rates for both of the configurations, as the VBM is deep enough with respect to the water oxidation potential and the CBM is also at a good position with respect to the water reduction potential level. 3.4. Defect Formation Energy for Doped Sr2Ta2O7. To search for the proper growth conditions, we have calculated the

defect formation energy of the doped Sr2Ta2O7 system. The defect formation energy ΔHf(D) of a defect of α atom can be defined as a function of the chemical potentials and the number of host and impurity atoms1,41 ΔΗ f (D) = E T(D) − E T(H) + q(Ε F + E V ) −

∑ nαμαelem α

(1)

Here the first two terms ET(D) and ET(H) are the total energies of the supercells with and without defects, respectively. Ev represents the energy of the VBM of the defect free system, whereas Ef is the Fermi energy. Here, we have considered that all the defect formation is in a neutral charge state; therefore, q = 0. nα is the number of atoms that have been added or removed. Here nα = 1 or −1 if an atom is added or removed, respectively. μelem is the chemical potential of the atom added α or removed. The chemical potential of the elements Sr, Ta, and bulk gas O cannot exceed those of bulk μbulk Sr and μTα and gas O μO . elem bulk elem bulk elem gas Therefore, μSr ≤ μSr , μTα ≤ μTα , and μO ≤ μO . When Sr2Ta2O7 is in equilibrium with reservoirs of Sr, Ta, and O, the sum of the chemical potentials of Sr, Ta, and O atoms must be equal to that of the bulk Sr2Ta2O7, i.e., 2 μSr + 2 μTa + 7 μO = μSr2Ta2O7(bulk). The heat of formation of Sr2Ta2O7 is Δ = μSr2Ta2O7(bulk) − 2 μSr(bulk) − 2 μTa(bulk) − 7 μO(gas). As the Δ value is negative for stable materials, ΔμSr, ΔμTa, and ΔμO satisfy Δ + 2μsr(bulk) ≤ 2μSr ≤ 2μsr(bulk)

(2a)

Δ + 2μTa(bulk) ≤ 2μTa ≤ 2μTa(bulk)

(2b)

Δ + 7μO(gas) ≤ 7μO ≤ 7μO(gas)

(2c)

For the host elements, μSr and μTa are calculated with bulk pure metals of Sr and Ta, while μO(gas) = 1/2 μO2, which is calculated with one O2 molecule centered in a 20 × 20 × 20 Å3 cubic box. For dopants, we suppose that the nitrogen gas and the bulk Mo and W act as N, Mo, and W reservoirs, respectively. 5047

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Figure 8. The calculated defect formation energy (eV) of Sr2Ta2O7 is plotted as a function of chemical potentials of Ta (ΔμTa) and O (ΔμO). The defect formation energy is plotted for (a) Mo and W mono-doped, (b) N mono-doped, (c) (N−Mo-near) co-doped, (d) (N−Mo-far) co-doped, (e) (N−W-near) co-doped, and (f) (N−W-far) co-doped. The different colored lines (c−f) show how the co-doped defect formation energy changes as the chemical potentials of Ta (ΔμTa) and O (ΔμO) change. The defect structure cannot be formed in the large white region because the chemical potential of Sr (ΔμSr) is exceeded (eq 2a).

could be due to the strong Coulomb coupling in the anion− cation pairs. The relative binding energy of co-doping systems is calculated for determining the relative stabilities of the system with respect to their mono-doped system, which is as follows.42

The calculated formation energies for doped systems are displayed in Figure 8. The formation energy increases with the chemical potentials of the host atoms because the vacancies are difficult to form under the host-rich conditions. Anionic N mono-doping is energetically more favorable (Figure 8b) than the cationic Mo/W mono-doping (Figure 8a) under the O- and Ta-poor conditions, respectively. At the Ta-poor conditions, the W doping is more energetically favorable than the Mo doping which might be due to their similar atomic sizes. For the co-doping system, their formation energies are more favorable than those for their respective mono-doping system. It is energetically favorable to form (N−Mo) co-doped (Figure 8c,d) and (N−W) co-doped (Figure 8e,f) systems under Tapoor and O-rich conditions. The reduction of the formation energy suggests that anionic (N) and cationic (Mo, W) doping can be greatly promoted by anionic−cationic co-doping. This

ΔE b = E(A) + E(B) − E(A + B) − E(pure)

(2)

Here E(A), E(B), and E(A + B) are the total energies of the anionic mono-doped, cationic mono-doped, and co-doping systems, respectively. E(pure) is the total energy of the pristine layered perovskite Sr2Ta2O7. The relative binding energies of the (N, Mo) co-doped system for “near” and “far” configurations are 2.31 and 1.19 eV, respectively. The positive value indicates that co-doping is more favorable in comparison with their respective mono-doping system, which is due to the neutralization of acceptor and donor states. The relative 5048

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binding energies of the (N, W) co-doped system for “near” and “far” configurations are 2.44 and 1.69 eV, respectively, which indicates that the N atom and W atom are coupled in the near configuration. 3.5. Absorption Curves. We have studied the optical absorption spectra for mono-doping of N, and Mo and codoping of (N−Mo) and (N−W) with the “far” configuration using the PBE0 method and compared with the pristine Sr2Ta2O7 (Figure 9). It shows that N mono-doping and (N−

experimentalist to investigate the cationic−anionic co-doping study on Sr2Ta2O7 for the visible-light photocatalysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.L.); [email protected] (B.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the Swedish Research Council (VR and FORMAS) and Stiffelsen J. Gust Richerts Minne (SWECO) for financial support. J.N. is thankful to the Higher Education Commission (HEC) of Pakistan, P.L. also would like to thank Chinese Scholarship Council (CSC), B.P. and R.A. would like to acknowledge Wenner-gren Foundation for financial support. SNIC and UPPMAX are acknowledged for providing computing time.

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REFERENCES

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Figure 9. The calculated optical absorption curves for pure, N and Mo mono-doped, and N−Mo (far) and N−W (far) co-doped perovskite Sr2Ta2O7.

W) and (N−Mo) co-doping with the “far” configuration can harvest a longer wavelength of the visible light spectrum as compared to the pristine Sr2Ta2O7 for the efficient photocatalysis.

4. CONCLUSIONS We have performed hybrid functional calculations for the mono- and co-doping of Sr2Ta2O7 by the controlled band gap engineering for efficient solar-driven photocatalyst. The occupied or unoccupied states of dopants appear in the band gap by anion (N) or cation (Mo/W) doping which reduces the band gap significantly; however, such states act as electron− hole recombination centers which reduce the efficiency of the photocatalysts. To remove such unwanted impurity states, (anionic−cationic) co-doping has been introduced in Sr2Ta2O7. It is found that (N, Mo) and (N, W) co-doped systems with the “far” configuration reduce the band gap significantly without generating any impurity states in the band gap. Our band edge alignment diagram shows that the water oxidation and reduction reactions are thermodynamically favarable for both [(N, Mo) and (N, W)] of the co-doped Sr2Ta2O7 systems, therefore promising material for visible light photocatalysis. Moreover, our defect formation energy calculations show that such anionic−cationic co-doping is energetically more favorable compared to their mono-doping systems. Therefore, our study not only explains the experimentally reported improved photocatalysis activity of N-doped Sr2Ta2O7, but we show that the photocatalytic activity of the N-doped system can be further improved if the impurity states appearing in the band gap can be removed via co-doping. Interestingly, such kind of co-doping43−45 is already reported to be good for visible light photocatalysis. Therefore, we believe our results will guide the 5049

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