Electronic Structure, Optical Properties, and Photocatalytic Activities of

Article pubs.acs.org/JPCC

Electronic Structure, Optical Properties, and Photocatalytic Activities of LaFeO3−NaTaO3 Solid Solution Pushkar Kanhere,† Jawad Nisar,‡ Yuxin Tang,† Biswarup Pathak,‡ Rajeev Ahuja,‡,§ Jianwei Zheng,∥ and Zhong Chen*,† †

School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798 ‡ Condense Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, 751 20 Uppsala, Sweden § Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden ∥ Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore ABSTRACT: A solid solution photocatalyst, Na1−xLaxFe1−xTaxO3 (x up to 0.06), was prepared by the conventional solid-state method. The photophysical properties of the samples were studied by various experimental techniques and the electronic structures were investigated by using screened hybrid density functional (HSE06) calculations. The solid solution photocatalyst showed absorption of visible light extending up to 450 nm. Upon loading of platinum nanoparticles cocatalyst, the photocatalytic hydrogen evolution of 0.81 μ·mol·h−1·g−1 was obtained for 2% doping of LaFeO3 in NaTaO3, under visible radiation (λ > 390 nm; 20% methanol solution). The photocatalytic properties of the solid solution were found to be better than Fe doped NaTaO3 compounds on account of the suitable band structure. The electronic structure analysis revealed that, in the case of Fe doping at the Ta site, unoccupied electronic states in between the band gap appear that are responsible for the visible-light absorption. However, in the case of La and Fe codoping (passivated doping) the mid-gap electronic states are completely filled, which makes the band structure suitable for the visible-light photocatalysis. The present solid solution of perovskites (LaFeO3 and NaTaO3) sheds light on the interesting photophysical properties and photocatalytic activities which could be beneficial for the photocatalysts derived from these compounds. reaction under UV radiation.15,16 Specifically, the La doped NaTaO3 system showed surface nanosteps and significant increment in the efficiency of the water-splitting reaction.17,18 The valence band of NaTaO3 is mainly formed by the Ta 5d orbital and thus doping at the Ta site would significantly alter the band structure, while codoping at the Na site would help maintain the ionic charge balance. Several modifications of NaTaO3, such as doping of N19 and Bi20,21 and codoping of La−Cr22 and La−Co23 have been carried out. These studies demonstrate that the visible-light driven photocatalytic activity can be achieved by the strategy of band engineering. Orthorhombic NaTaO3 and LaFeO3 crystallize in space group No. 62 (Pbnm) and these phases possess lattice parameters that are close to each other.24,25 Therefore, formation of solid solution between NaTaO3 and LaFeO3 is possible and could be potentially beneficial for developing a visible-light active photocatalyst. LaFeO3 is known to be a

1. INTRODUCTION Photocatalytic water splitting is considered to be a clean and sustainable way to harvest solar energy and generate hydrogen as a fuel.1,2 Over the past few years, intense research has been carried out to develop visible-light active photocatalytic materials systems for energy as well as environmental applications. Band engineering of wide band gap semiconductors has been extensively used to achieve visible-light absorption and subsequent photocatalytic activity.3−7 Solid solution photocatalysts such as ZnS−CdS,8 GaN−ZnO,9 ZnS− CuInS2−AgInS2,10 and AgAl1−xGaxO211 have shown promising photocatalytic activities under visible radiation. Further, theoretical and experimental findings have shown that passivated codoping in certain photocatalysts is beneficial for visible-light activity.12−14 As compared to the aliovalent doping, solid solutions or passivated doping causes lesser native point defects which are caused by dopant accommodation. Therefore, passivated doping or preparing solid solutions is a good strategy to develop novel photocatalysts with the visible-light response. Among the various photocatalysts studied so far, NaTaO3 is one of the most efficient photocatalysts for the water-splitting © 2012 American Chemical Society

Received: August 8, 2012 Revised: September 21, 2012 Published: September 25, 2012 22767

dx.doi.org/10.1021/jp307857h | J. Phys. Chem. C 2012, 116, 22767−22773

The Journal of Physical Chemistry C

Article

approximation of the PBE0-functional. PBE0 is defined as the mixing of 75% GGA−PBE functional and 25% Hartree−Fock (HF) (exact) exchange. The resulting exchange-correlation energy expression takes the following form:

visible-light active photocatalyst with a band gap of 2.8 eV and recent investigation on LaFeO3 has gained significant interest in its photocatalytic hydrogen evolution properties.26,27 To the best of our knowledge, the photophysical properties of solid solution of LaFeO3−NaTaO3 or Fe and La codoped NaTaO3 have not been reported in detail and these studies are important for the development of NaTaO3 or LaFeO3 based photocatalysts. In this article, we report electronic structure and photophysical properties of Fe doped NaTaO3 and LaFeO3−NaTaO3 solid solution compounds. It was shown that solid solution photocatalyst showed visible-light absorption up to 450 nm and these samples were active for the photocatalytic hydrogen evolution under visible radiation (λ > 390 nm). Hybrid DFT calculations revealed that Fe doping at the Ta site induced visible-light absorption in NaTaO3, while codoping of La improved its band structure, making it suitable for the photocatalytic hydrogen evolution reaction.

PBE0 E XC =

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

(1)

PBE PBE Here, EHF X is the HF exchange energy and EX and EC are the PBE exchange and correlation energies, respectively. In HSE06, only the local part of the exact exchange energy is treated within HF theory, whereas the nonlocal part is treated by the density-functional approximation (DFT) part, which is calculated within GGA−PBE. In this work, the screening parameter, μ, was set to 0.2, conforming to the HSE06 functional. The Brillion zone was integrated using MonkhorstPack generated sets of k-points.32 k-points mesh 4 × 4 × 7 and 2 × 2 × 3 were found to be sufficient to reach convergence for supercell calculations using GGA−PBE and HSE06 methods, respectively. The plane cut-off of energy of 500 eV was used to describe the electronic wave functions. The PAW potentials with the valence states 3s12p6 for Na, 6s25d3 for Ta, 4s23d6 for Fe, 5s26s25p65d1 for La, and 2s22p4 for O were used. In all the calculations, self-consistency was achieved with a tolerance of 0.01 meV in the total energy values. For the doping cases, a 2 × 2 × 1 supercell with 80 atoms was used. We have substituted one of the Ta atoms in the 80 atoms (2 × 2 × 1) supercell of NaTaO3 by one Fe atom and one of the Na atoms by the La atom. This substitution corresponds to 6.25% of doping concentration, which is close to the present experimental studies. The total and partial density of states of the pristine and doped systems is aligned with respect to the O 2s core states, which are very far from the doped atom.

2. METHODS 2.1. Sample Preparation. Pristine and solid solution photocatalyst powders were prepared by mixing Ta2O5 (Alfa Aesar, >99%), Na2CO3 (Univar 99.8%), Fe2O3 (Alfa Aesar, >99%), and La2O3 (Alfa Aesar) in appropriate stoichiometric proportions. All the samples were prepared with 5−10% excess Na2CO3, as the compounds of Na have the tendency to evaporate at high temperatures. The starting materials were mixed in agate type mortar, ground thoroughly using acetone, pressed into pellets, and heated between 1173 and 1473 K for 10 h using a platinum crucible in ambient atmosphere. In all the cases, the final powders were washed with deionized water and dried below 100 °C. Fe doped NaTaO3 powders prepared with 2%, 4%,and 6% Fe content are referred to 2%Fe, 4%Fe, and 6% Fe, respectively. In the case of solid solution, the samples were synthesized with each 2%Fe−2%La, 4% Fe−4%La, and 6%Fe− 6%La, which are referred to as 2%La−Fe, 4%La−Fe, and 6% La−Fe, respectively. 2.2. Characterization. The crystal structure of the samples was studied by X-ray diffraction technique (XRD) (Bruker D8 Advance; Cu Kα = 1.54 Å). The optical properties of the powders were studied by diffused reflectance spectroscopy (DRS, Perkin-Elmer 900). The morphology of the particles was studied by a field emission electron microscope (JEOL 6400F) and transmission electron microscope (JEOL 2100F). The photocatalytic hydrogen evolution was carried out in a closedgas circulation system consisting of a 800 W high-pressure Xe lamp (Oriole Instruments, USA), a 390 nm cut-off filter, and a gas chromatograph (Shimadzu GC-2014; molecular sieve 5A, TCD detector, Ar carrier gas). Fifty milligrams of catalyst was suspended in 80 mL of deionized (D.I.) water and 20 mL of methanol by magnetic stirring. It was confirmed that no hydrogen was evolved under dark conditions with or without catalyst. 2.3. Computational Details. The first-principles calculations were performed using projected augmented wave (PAW) method28 as implemented in the Vienna ab initio simulation package.29 For our first set of calculations, the exchange-correlation interaction was treated in the level of the GGA using Perdew-Burke-Ernzerhof (GGA−PBE).30 In this work, spin-polarized calculations have been done for all the doped systems. To take into account the effects of nonlocal exchange, we applied the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) functional,31 which is an

3. RESULTS AND DISCUSSIONS 3.1. Crystal Structure. XRD patterns of Fe doped NaTaO3 and La−Fe codoped NaTaO3 showed the presence of monophase (SG No. 62, Pbnm). The representative XRD patterns of La−Fe codoped NaTaO3 powders are shown in Figure 1. The analysis of the XRD patterns showed that Fe doping and La−Fe codoping caused a systematic increment in the unit cell volumes. The increment of the unit cell was significantly higher for La−Fe codoping as compared to that of

Figure 1. XRD patterns of (a) pure and (b) 2%, (c) 4%, and (d) 6% (each) La and Fe codoped NaTaO3 showing the presence of monophase. 22768



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the Fe doping alone (Figure 2). In both cases, accommodation of the dopant ions was successful in the lattice, as no impurities

Figure 2. Variation of the unit cell volumes with Fe and La−Fe content in NaTaO3 powders.

of Fe2O3 or La2O3 were seen in the XRD patterns. The ionic radius of Fe3+ (0.65 Å) is close to that of Ta5+ (0.64 Å) for sixfold oxygen coordination. Similarly, the ionic radii of Na1+ and La3+ are close to each other for 12-fold oxygen coordination. Therefore, based on the values of ionic radii and the monophase XRD patterns, it was proposed that La−Fe codoped samples possess La ions at Na sites but Fe ions at Ta sites. Further, the reported unit cell volume of NaTaO3 (V = 235.53 Å3, a = 5.4768 Å, b = 5.5212 Å, c = 7.789 Å)24 is significantly smaller than that of LaFeO3 (V = 242.92 Å3, a = 5.55702 Å, b = 5.56521 Å, c = 7.85426 Å).25 Thus, the addition of LaFeO3 to NaTaO3 increased its unit cell volume significantly, indicating that the solid solution of these two phases has been formed. In the DFT calculations, the unit cell volume of the Fe doped NaTaO3 was found to be smaller than La−Fe codoped NaTaO3 by 2.95 Å3. The unit cell volumes of the samples from XRD studies agree well with those calculated by DFT. Doping of Fe in NaTaO3 is an aliovalent type of doping. Thus, based on the charge neutrality equations (eqs 2−7), the following native point defects are likely to be generated in the lattice. (Ta 5 +) = Fe3 + − O2 −

(2)

(Na1 +) = Fe3 + − 2Na1 +

(3)

(Ta 5 +) = Fe3 + + 2Na1 +

(4)

(Na1 +) = La 3 + − 2Na1 +

(5)

(Na1 +) = La 3 + − 2/5 Ta 5 +

(6)

(Na1 +) + (Ta 5 +) = (La 3 +) + (Fe3 +)

(7)

Figure 3. (a) Diffused reflectance spectra of the pristine and Fe doped NaTaO3 samples showing intense absorption in the visible region. (b) Diffused reflectance spectra of the pristine and La−Fe codoped NaTaO3 samples showing extension in the visible region.

value of 2.24 eV was obtained. Similarly, the absorption spectra of LaFeO3−NaTaO3 solid solution samples showed shifts toward the visible region with increase in the dopant content (Figure 3b). In this case, a peak around 450 nm was prominently observed. It was noted that both Fe and La−Fe doping in NaTaO3 induced visible-light absorption in this phase; however, the extent of visible-light absorption was larger for Fe doping alone. The calculated band gap values of the Fe doped system (2.85, 2.45, and 2.24 eV for 2%, 4% and 6%, respectively) were found to be smaller than the corresponding values in the La−Fe doped systems (3.05, 2.95, and 2.88 eV for 2%, 4%, and 6%, respectively). Figure 4 shows the surface morphologies of the pristine and solid solution particles. The presence of surface nanosteps was

On the other hand, the ionic charge in the lattice is maintained when Fe3+ is doped at the Ta5+ site and La3+ is doped at the Na1+ site. Therefore, the solid solution samples are likely to have reduced point defects in the lattice as compared to that of Fe monodoped NaTaO3. 3.2. Optical Properties and Microstructure. The optical absorption spectra of the pristine and Fe doped samples are shown in Figure 3a. The pristine NaTaO3 showed absorption only in the ultraviolet region with a band gap value of 4.01 eV as estimated by the Kubelka−Munk plot. On the other hand, all the doped samples showed extension in the visible region (λ > 420 nm). Fe doped NaTaO3 samples showed a prominent peak at around 500 nm. It was observed that the band gap narrowing increased with Fe content, and for 6% Fe doping, a band gap

Figure 4. Field emission scanning electron microscope images of (a) 0%, (b) 2%, (c) 4%, and (d) 6% La−Fe codoped NaTaO3 powders showing surface nanosteps. 22769



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observed for all LaFeO3−NaTaO3 solid solution samples (Figure 4b−d). It was found that the prominence of surface nanosteps increased with an increase in the doping concentration. Figure 5a shows the enlarged view of the

is the most favorable, followed by La−Fe codoping and Fe monodoping, respectively. Figure 6a shows the total density of states (TDOS) of Fe doped NaTaO3 and pristine NaTaO3. These plots reveal that

Figure 5. (a) Enlarged view of the surface nanosteps; (b) bright field image of 2% La−Fe codoped NaTaO3.

surface nanosteps for 2% La−Fe doping concentration, indicating that the nanosteps were around 20−50 nm in size. The bright field images of the sample confirmed the presence of nanosteps, as seen in Figure 5b. The surface nanosteps were first observed by Kudo et al. in M (M = La, Pr, Nd) doped NaTaO3 systems.17,33 It was proposed that doping of these ions at the A site in the lattice suppresses the crystal growth due to the difference in the ionic radii. The surface nanosteps in the present samples indicate that La doping at the Na site has occurred. This could be secondary proof of codoping being successful. The microstructure studies further showed that Fe doped NaTaO3 particles were agglomerated and their shapes deviated from perfect cuboid. These particles did not show any surface features in electron microscope studies. 3.3. Band Structure Calculations. To understand the origin of the visible-light absorption, spin-polarized DFT calculations were performed on the pristine and doped NaTaO3 systems. The calculated band gap using the GGA− PBE method was 2.65 eV, which is smaller than the experimental value of 4.01 eV.34 The GGA−PBE method generally underestimates the band gap for metal oxides, which is a very well-known problem within DFT.35 The hybrid functional calculations (HSE06) are known to be good for the band gap calculations of semiconducting and insulating materials. Therefore, we have employed the HSE06 method to calculate the exact band gap of the pristine NaTaO3. It was found that the HSE06 method produced accurate band gap value for the pristine NaTaO3 (4.0 eV), which is in excellent agreement with the experimental value of 4.01 eV. To understand the effect of dopants on the electronic structure and optical absorption spectra of the samples, we have analyzed the density of states for La, Fe, and both La−Fe doped NaTaO3 structures. Further, the formation energy (ΔEf) for each dopant is calculated using the following equation:36 ΔEf = E T(D) − E T(H) + nμ X − nμ Y

Figure 6. (a) The calculated (HSE06 method) total density of states (TDOS) of NaTaO3 (black lines) and NaTa1−xFexO3 (red lines); (b) partial density of states (pDOS) of NaTa1−xFexO3 (x = 0.0625); the vertical dashed lines represent the Fermi energy.

substitution of Fe at the Ta site produces extra energy states in the band gap which are unoccupied in nature. The siteprojected DOS analysis (Figure 6b) shows that intermediate states are mainly composed of Fe 3d and O 2p orbitals and they are located at around 1.1 and 2.0 eV above the VBM. The appearance of extra states due to Fe doping reduces the effective band gap of NaTaO3 and thus this compound can absorb the visible light as seen in the optical absorption spectra of the synthesized samples (Figure 3a). Earlier work on doping indicate that the intermediate unoccupied states may facilitate the electron−hole recombination losses.37 In addition to these mid-gap states, Fe 3d induced unoccupied energy states also appear at the bottom of the CBM. It is noted that Zhou et al. have studied Fe doping in cubic NaTaO3 by DFT calculations, which also shows a similar trend.38 The present investigations are carried out on the orthorhombic NaTaO3 using hybrid DFT method and therefore the present DFT calculations provide an accurate description of the band structure. In the case of Fe−La codoped NaTaO3, dopants at “near” and “far” configurations were studied. The distances between La and Fe atoms in near and far configurations were set to 3.16 and 6.50 Å, respectively. The calculated (using GGA−PBE) formation energies for the Fe−La codoped NaTaO3 for far and near configurations are 0.151 and 0.141 eV/f.u, respectively. The relative stability in a codoped system (compared to the monodoping systems) is calculated by the defect pair binding energy calculations using the following equation:39 ΔE b = E(Fe) + E(La) − E(Fe−La) − E(pure)

(8)

Here, ET(H) and ET(D) are the total energies of pristine and doped NaTaO3, respectively, and n is the number of the doped atoms. μx and μy are the atomic potential of the host and dopant atoms, respectively. The calculated formation energy values for Fe doped NaTaO3, La doped NaTaO3, and Fe−La codoped NaTaO3 are 0.497, −0.141, and 0.153 eV/f.u, respectively. These values indicate that La doping in NaTaO3

(9)

Here E(Fe), E(La), and E(Fe, La) are the total energies of Fe, La, and Fe−La doped systems, respectively, and E(pure) is the total energy of the pristine NaTaO3. The relative binding energies of codoped NaTaO3 for near and far configurations are 3.32 and 3.13 eV, respectively. The positive value indicates that codoping is more favorable as compared to the monodoping in NaTaO3. Figure 7a shows the total and site projected DOS of 22770



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Figure 8. Calculated optical absorption curves for pristine, Fe doped, and La−Fe codoped NaTaO3.

Figure 7. (a) The calculated (HSE06 method) total density of states (TDOS) of NaTaO3 (black lines) and Na1−xLaxTa1−xFexO3 (red lines); (b) partial density of states (pDOS) of Na1−xLaxTa1−xFexO3 (x = 0.0625); the vertical dashed lines represent the Fermi energy.

the La−Fe codoped NaTaO3 system (near configuration). The substitution of La at the Na site and Fe at the Ta site removes the unwanted impurity states generated on Fe monodoping. Doping of Fe3+ at the Ta5+ cation site causes lack of two positive charges, which would be reflected as unoccupied Fe 3d induced states in DOS (dangling bonds). After La3+ (at Na1+ cation site) is introduced in this system, the charge transfer from La ion to Fe ion is likely in the lattice, removing the dangling bonds and making the extra energy states occupied completely. Therefore, La−Fe codoping improves the electronic structure of NaTaO3 for photocatalysis. The occupied states at the VBM are mainly contributed by Fe 3d and O 2p and also by a fraction of La 5d orbitals, which are shown in the projected density of states (Figure 7b). The DOS analysis shows that the conduction band edge position is shifted downward as compared to the pristine NaTaO3, which could be attributed to different levels of interactions between Fe and Ta atoms. The effective band gap of Fe−La codoped NaTaO3 was found to be 2.7 eV. The computational results indicate that doping of Fe and codoping of La and Fe would reduce the effective band gap of NaTaO3, which will induce the visiblelight absorption in photocatalyst. It is noted that the electronic structures of the La−Fe codoped system was similar for near and far configurations. The optical absorption spectra of the pristine and doped NaTaO3 systems were calculated from the electronic structures (Figure 8). It is seen that La−Fe codoped NaTaO3 can harvest wavelength up to 450 nm, while Fe doped NaTaO3 can absorb the longer part of the visible-light spectra as compared to the pristine NaTaO3. Comparison between Figure 8 and Figure 3 reveals that the calculated absorption spectra for doped structures match well with the experimental measurements. 3.4. Photocatalytic Hydrogen Evolution. Figure 9 shows the photocatalytic hydrogen generation from La−Fe solid solution samples (0.05% Pt loading) under the visible-light radiation. It was seen that a maximum yield of 0.81 μ·mol·h−1·g−1 was obtained for 2% La−Fe doping, while H2 evolution dropped for higher doping concentration. The higher concentration of La doping (>2%) may have increased lattice distortion and point defects, resulting in the decrease in the photocatalytic activity. The pristine NaTaO3 sample did not show any significant hydrogen evolution under similar

Figure 9. Photocatalytic hydrogen evolution of 0.05% Pt loaded La− Fe codoped samples under visible radiation (20 mL of CH3OH, 80 mL of D.I. water; λ > 390 nm, 150 mW/cm2).

conditions. It is interesting to note that, though Fe doped NaTaO3 samples showed intense absorption in the visible region, these samples did not show any significant hydrogen evolution under illumination of the visible light. The photocatalytic behavior of the doped samples could be explained by the results of hybrid DFT studies. The DFT results reveal that, the band structure of Fe doped NaTaO3 contained unoccupied intermediate energy states (in the band gap). In this case, the electronic transitions from VB to mid-gap states induce the visible-light absorption; however, the photoexcited electrons do not have sufficient potential for reduction of protons. Further, the localized nature of the mid-gap states reduce the mobility of the electrons and promote electron−hole recombination. Therefore, the band structure of Fe doped NaTaO3 is not found suitable for visible-light photocatalysis and the samples did not show significant photocatalytic activity. As discussed earlier, the accommodation of Fe3+ would create native points defects in the lattice. These defects are produced to maintain the ionic charge balance and thus creation of these defects may fill the unoccupied electronic states to a certain degree. Although these states would become occupied, the photoexcited electron−hole pairs would be confined to defect sites and would be highly localized in nature. Further, the defect sites themselves act as electron−hole recombination center, thereby reducing the qunatum yield. On the other hand, in the case of the solid solution photocatalysts, the mid-gap states are completely occupied in nature. Upon visible-light excitation, these electronic states allow transition from mid-gap states to the conduction band, 22771



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thereby making the band structure suitable for photocatalysis. Due to the suitable band structure and reduced native point defects, the solid solution samples showed photocatalytic hydrogen evolution under the visible-light irradiation. It is understood that further investigations are needed to optimize and enhance the performance of the doped powders to consider for water-splitting applications. Studies on codoping of Co−La23 and Cr−La40 in the NaTaO3 system have shown a similar trend in the photocatalytic hydrogen evolution. The visible-light driven hydrogen yield from these systems ranges from 2.8 to 4.5 μ·mol·h−1·g−1. Though the hydrogen yield in the present experiments (0.81 μ·mol·h−1·g−1) is smaller than the other reports, compared to these codoped systems, La−Fe codoping may offer an advantage of solid solution formation as well as surface nanosteps which could be useful in the development of both the phases. The present study throws light on the origin of the photocatalytic activities of codoped NaTaO3 systems by providing insights on the electronic structure, optical absorption properties, and morphology of the doped materials system. It further opens a possibility of preparing perovskite solid solutions and demonstrates that such materials systems are active photocatalysts under the visible radiation.

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4. CONCLUSIONS A solid solution of photocatalysts Na1−xLaxFe1−xTaxO3 (x up to 0.06) was developed and its photophysical properties and the electronic structure were studied in detail. (1) Both the compounds studied in this work, that is, Fe doped NaTaO3 and LaFeO3−NaTaO3 solid solution, show absorption of the visible radiation (λ > 420 nm). The solid solution photocatalyst shows photocatalytic hydrogen evolution (0.81 μ·mol·h−1·g−1) under the visible-light radiation. (2) The DFT studies reveal that substitution of Fe at the Ta site causes band gap narrowing on account of Fe 3d and O 2p induced energy states. The optical properties predicted by DFT calculations agree well with those obtained from the experimental investigations. Further, the calculated relative binding energy values show that La−Fe codoped systems are more stable than their respective monodoped systems. (3) Hybrid DFT calculations reveal that codoping of La and Fe in NaTaO3 produces bandlike occupied states near the valence band which are beneficial for the visible-light driven photocatalysis. On the other hand, Fe doped NaTaO3 possesses Fe 3d unoccupied mid-gap states that are not favorable for the photocatalytic reduction process.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +65-6790-4256. Fax: +65-6790 9081. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from MOE Singapore (Grant RG 112/05 and RG 14/03) is gratefully acknowledged. Financial support from the Swedish Research Council (VR and FORMAS) and Stiffelsen J. Gust Richerts Minne (SWECO) is acknowledged. J. Nisar is thankful to the Higher Education Commission (HEC) of Pakistan. SNIC and UPPMAX are acknowledged for providing computing time. 22772



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(37) Gai, Y.; Li, J.; Li, S. S.; Xia, J. B.; Wei, S. H. Phys. Rev. Lett. 2009, 102, 3. (38) Zhou, X.; Shi, J.; Li, C. J. Phys. Chem. C 2011, 115 (16), 8305− 8311. (39) Yin, W. J.; Wei, S. H.; Al-Jassim, M. M.; Yan, Y. Phys. Rev. Lett. 2011, 106 (6), 066801. (40) Yang, M.; Huang, X.; Yan, S.; Li, Z.; Yu, T.; Zou, Z. Mater. Chem. Phys. 2010, 121 (3), 506−510.

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