Synthesis of Nanostructured BaTaO2N Thin Films as Photoanodes for

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Synthesis of Nanostructured BaTaO2N Thin Films as Photoanodes for Solar Water Splitting Chizhong Wang,† Takashi Hisatomi,†,‡ Tsutomu Minegishi,†,‡,§ Qian Wang,†,‡ Miao Zhong,†,‡ Masao Katayama,†,‡ Jun Kubota,†,∥ and Kazunari Domen*,†,‡ †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan ‡ Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), 5-1-5 Kashiwanoha, Kashiwa-shi, 277-8589 Chiba, Japan § Japan Science and Technology Agency/Precursory Research for Embryonic Science and Technology (JST/PRESTO), Kawaguchi Center Building, 4-1-8, Honcho, Kawaguchi-shi, 332-0012 Saitama, Japan S Supporting Information *

ABSTRACT: Synthesis of nanostructured BaTaO2N thin films on metallic Ta substrates, and their application as photoanodes for solardriven photoelectrochemical water oxidation were studied. Ba5Ta4O15 nanosheets vertically grown on Ta substrates by a hydrothermal process were converted into perovskite BaTaO2N with a branching nanostructure by thermal nitridation under an ammonia gas flow. The crystal quality and photoelectrochemical properties of the BaTaO2N thin films were found to improve with increasing nitridation temperature up to 1000 °C. A Ta4N5 interfacial layer was formed between the BaTaO2N thin film and the Ta substrate. Under simulated AM 1.5G light, the BaTaO2N electrode generated a photoanodic current, although it rapidly decreased due to photooxidative corrosion. The degradation of the BaTaO2N electrode could be alleviated by the deposition of a cobalt phosphate layer on its surface. The modified electrode maintained a photoanodic current of 0.75 mA cm−2 at 1.23 V versus the reversible hydrogen electrode with a Faradaic efficiency of almost unity. transfer methods;9−12 however, powder-based photoelectrodes are inefficient in terms of long-range electron transport because of their high porosity and grain-boundary density. This problem could be mitigated by the fabrication of high-quality thin films on conductive substrates.13 Ta3N5 thin films,14 nanorods,15,16 and nanotubes17 have been widely investigated owing to the simplicity of their composition and the mild conditions required to synthesize them. In the case of perovskite oxynitrides, high-temperature nitridation is required because of the low mobility of oxygen and nitrogen atoms.18 So far, few successful attempts have been reported regarding the preparation of thin-film oxynitrides on conductive substrates due to their instability under severe nitridation conditions.19−21 Perovskite BaTaO2N with a band gap energy of 1.9 eV absorbs sunlight at wavelengths of up to 650 nm22,23 and can exhibit a photocurrent of 17 mA cm−2 under standard AM 1.5G sunlight if an incident photon-to-current efficiency (IPCE) of

1. INTRODUCTION Photoelectrochemical (PEC) water splitting is a promising process for converting solar energy to hydrogen energy.1−3 To fabricate p/n-PEC cells, photocathodes and photoanodes are connected in series, and overall water splitting is achieved by combining their driving forces. Although active photocathodes have been widely reported,4,5 highly efficient photoanode materials are scarce. Outstanding issues with such photoanodes are their low performance in the four-electron water oxidation process and poor resistance to self-oxidation. (Oxy)nitride semiconductor materials, such as LaTiO2N,6 Ta3N5,7,8 and BaTaO2N,9,10 have been extensively investigated as photoanodes for PEC oxygen evolution from water under visible light. At present, however, these materials are not being used effectively because of the lack of synthesis methods for (oxy)nitride semiconductor layers with sufficiently high crystallinity and low defect density, together with a structured morphology for effective light absorption and charge carrier transport, on conductive substrates. In previous studies, it was shown that a considerable photoanodic current could be generated by photoanodes composed of particulate (oxy)nitrides fabricated by electrophoretic deposition and particle © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: November 26, 2015 Revised: December 27, 2015

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DOI: 10.1021/acs.jpcc.5b11564 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C unity is assumed. The conduction band minimum (CBM) and valence band maximum (VBM) for BaTaO2N are estimated to be located at −0.4 and 1.5 V versus the normal hydrogen electrode (NHE) at pH 0,18 respectively, theoretically allowing water splitting in the absence of an external bias voltage. BaTaO2N photoanodes prepared by the particle-transfer process yielded a photocurrent >4 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (RHE) under simulated sunlight. In addition, these photoanodes exhibited outstanding stability in the PEC oxygen evolution reaction compared with other oxynitride photoanodes.10 Given the potential advantages of thin-film electrodes over particulate electrodes with regard to charge carrier transport, the present study focused on the development of a new synthesis method for BaTaO2N thin films on metallic substrates. Structured BaTaO2N films were formed on Ta substrates by nitriding hydrothermally grown Ba5Ta4O15 nanosheet layers. The resulting BaTaO 2N photoanode produced a stable photocurrent under simulated AM 1.5G light. This is the first report of film-based BaTaO 2N photoanodes for solar water splitting.

for 8 min under simulated AM 1.5G light. A CoPi layer was gradually formed on the surface of the BaTaO2N electrode due to oxidation of Co2+ ions by photogenerated holes. Characterization. Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (SEMEDX; SU8020, Hitachi) was used to characterize the morphology and cross-sectional structure of the BaTaO2N thin films. X-ray diffraction (XRD; Rigaku Ultima III) patterns were recorded using Cu Kα radiation (λ = 0.15405 nm, operated at 40 kV and 40 mA). X-ray photoelectron spectroscopy (XPS; JEOL, JPS-90SX) was conducted using Mg Kα radiation at 10 mA and 8 kV. Binding energies were calibrated using the C 1s peak at 284.8 eV as an internal standard. Electrochemical Measurements. The PEC properties of the BaTaO2N photoanodes were measured using a threeelectrode cell in a 0.5 M potassium phosphate electrolyte solution (pH 13) at room temperature. Ag/AgCl (in saturated KCl aq) and Pt wire electrodes were used as the reference and counter electrodes, respectively. The potential with respect to the Ag/AgCl electrode was converted to that relative to the RHE using the Nernst relationship

2. EXPERIMENTAL SECTION Preparation of BaTaO2N Films. Pieces of Ta foil (10 mm × 20 mm × 0.1 mm, 99.95%, Nilaco) were sequentially cleaned by sonication in acetone, isopropanol, and Milli-Q water before being used as a substrate. The preparation process for the BaTaO2N thin films is schematically illustrated in Figure 1. A

E RHE = EAg/AgCl + 0.059 × pH + 0.197

(1)

where ERHE and EAg/AgCl are the potential in units of volts with respect to the RHE and Ag/AgCl electrode, respectively. A solar simulator (SAN-EI Electric, XES-40S2-CE) was used as a light source. The IPCE was measured under monochromatic irradiation (Asahi Spectra, MAX-302) produced by a 300 W Xe lamp and a series of band-pass filters with center wavelengths of 400 to 660 nm at intervals of 20 nm. The measurements were performed in chronoamperometric mode with the applied potential held at 1.23 V versus RHE in a 0.5 M potassium phosphate electrolyte (pH 13). The IPCE as a function of wavelength was calculated using

Figure 1. Synthesis procedure for BaTaO2N films: (i) Vertically assembled Ba5Ta4O15 nanosheets are formed on a Ta substrate via a hydrothermal reaction in an aqueous solution of Ba(OH)2. (ii) Ba5Ta4O15 nanosheets are transformed to a BaTaO2N film by nitriding under a flow of NH3.

IPCE = [1240 × Iλ /(λ × PInput)] × 100%

(2)

where Iλ (mA cm−2) is the steady-state photocurrent density, λ (nm) is the wavelength of the monochromatic irradiation, and Pinput (mW cm−2) is the incident photon density measured using a Si photodiode detector. To determine the Faradaic efficiency of the photocurrent during PEC water splitting, a BaTaO2N photoanode was illuminated by simulated AM 1.5G light in an airtight three-electrode PEC cell at an electrode potential of 1.23 V versus RHE. The amount of H2 and O2 evolved during the illumination was quantified using a micro gas chromatograph (Agilent, 3000A, Micro GC).

hydrothermal method was used to prepare a barium tantalum mixed oxide film on the Ta foil. In this process, 0.03 mol Ba(OH)2 8H2O (98%, Wako Pure Chemical Industries) was added to 30 mL of Milli-Q water bubbled by Ar gas and stirred for 10 min. The mixture was then transferred into an autoclave (100 mL in volume) with a Teflon liner. After the addition of the cleaned Ta foil, the reactor was heated to 200 °C for 24 h. The sample was subsequently rinsed using Milli-Q water and ethanol and dried using a nitrogen gas gun. In this stage, a uniform oxide film with a gray color was formed on the Ta foil. BaTaO2N thin films were obtained by nitriding these oxide films under a flow of NH3 (10 sccm) at 850−1000 °C for 2 h. The resulting BaTaO2N films were immersed in a 0.1 M HNO3 aqueous solution for 30 s and rinsed using Milli-Q water before further tests. For some BaTaO2N films, a thin layer of cobalt phosphate (CoPi) was deposited by a previously reported PEC method.16 In this method, the BaTaO2N photoanode was used as a working electrode in a three-electrode cell with a Ag/AgCl reference electrode and a Pt wire as a counter electrode. For CoPi deposition, 0.5 mM Co(NO3)2 in a potassium phosphate butter solution (0.1 M, pH 7) was prepared and bubbled using Ar gas for 10 min. Deposition was performed in chronopotentiometric mode with the photocurrent held at 10 μA cm−2

3. RESULTS AND DISCUSSION Structure and Morphology. Figure 2a shows a top-view SEM image of a barium tantalum oxide thin film, which is seen to take the form of nanosheets. The upper XRD pattern in Figure 3a indicates that Ba5Ta4O15 was formed as a result of the hydrothermal reaction of Ta and Ba(OH)2, indicating that this was the thermodynamically most stable oxide phase. This is consistent with the fact that Ba5Ta4O5 was the only product when a mixture of BaCO3 and Ta2O5 at a Ba/Ta ratio of 5/4 was calcinated at 1100 °C;24 however, as shown in Figure S1a, the ratio of the (110) to (103) peak intensities is significantly higher for the nanosheet sample than for the standard reference (PDF# 18-0193), indicating highly oriented crystal growth in the former case. The Ba5Ta4O15 unit cell contains five layers of B

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presence of a high concentration of Ba2+ and OH− ions, crystal growth of Ba5Ta4O15 along the c axis has been reported to be inhibited because of the hindering effects of these ions in the space between the third and fourth oxygen-packed layers.26,27 Thus, although the in-plane dimensions along the a and b axes of the nanosheets are on the order of micrometers, the sheet thickness along the c axis is 90% of the values

Figure 7. Hydrogen and oxygen evolution from a CoPi/BaTaO2N photoanode held at 1.23 V versus RHE in a 0.5 M potassium phosphate solution (pH 13) under simulated AM 1.5G light. Solid lines represent the charges estimated from photocurrent.

calculated based on the complete conversion of the photocurrent into hydrogen and oxygen. Therefore, the Faradaic efficiency of the PEC water oxidation process was almost unity. In addition, the ratio of H2 to O2 was close to the stoichiometric value of 2:1. This indicates that most of the photocurrent generated by the BaTaO2N electrode contributed to the water-splitting process.

4. CONCLUSIONS A new method for the fabrication of BaTaO2N thin films on conductive substrates was reported. Ba5Ta4O15 nanosheets were first grown on Ta substrates by a hydrothermal reaction in a Ba(OH)2 aqueous solution. After thermal nitridation in an ammonia gas flow, the oxide nanosheets were converted into perovskite BaTaO2N films with a branching structure. On the basis of SEM and EDX analyses, the films were found to have the layered structure BaTaO2N/Ta4N5/Ta. The sample nitrided at 1000 °C exhibited the highest photocurrent because of a more complete phase transformation from the oxide to the oxynitride and a high degree of crystallinity of the BaTaO2N

Figure 6. (a) Ta 4f and (b) N 1s XPS spectra for Ba5Ta4O15 and BaTaO2N films and a CoPi/BaTaO2N photoelectrode after a PEC measurement for 5 h at 1.23 V versus RHE in 0.5 M potassium phosphate solution (pH 13) under simulated AM 1.5G light. E

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Photoanodes for Solar Water Splitting. J. Phys. Chem. C 2012, 116, 15918−15924. (9) Higashi, M.; Domen, K.; Abe, R. Fabrication of an Efficient BaTaO2N Photoanode Harvesting a Wide Range of Visible Light for Water Splitting. J. Am. Chem. Soc. 2013, 135, 10238−10241. (10) Ueda, K.; Minegishi, T.; Clune, J.; Nakabayashi, M.; Hisatomi, T.; Nishiyama, H.; Katayama, M.; Shibata, N.; Kubota, J.; Yamada, T.; et al. Photoelectrochemical Oxidation of Water Using BaTaO2N Photoanodes Prepared by Particle Transfer Method. J. Am. Chem. Soc. 2015, 137, 2227−2230. (11) Hisatomi, T.; Katayama, C.; Moriya, Y.; Minegishi, T.; Katayama, M.; Nishiyama, H.; Yamada, T.; Domen, K. Photocatalytic Oxygen Evolution Using BaTaO2N Modified with Cobalt Oxide under Photoexcitation up to 740 nm. Energy Environ. Sci. 2013, 6, 3595− 3599. (12) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520−7535. (13) Nozik, A. J. Photoelectrochemistry - Applications to SolarEnergy Conversion. Annu. Rev. Phys. Chem. 1978, 29, 189−222. (14) Pinaud, B. A.; Vailionis, A.; Jaramillo, T. F. Controlling the Structural and Optical Properties of Ta3N5 Films through Nitridation Temperature and the Nature of the Ta Metal. Chem. Mater. 2014, 26, 1576−1582. (15) Li, Y.; Takata, T.; Cha, D.; Takanabe, K.; Minegishi, T.; Kubota, J.; Domen, K. Vertically Aligned Ta3N5 Nanorod Arrays for SolarDriven Photoelectrochemical Water Splitting. Adv. Mater. 2013, 25, 125−131. (16) Li, Y.; Zhang, L.; Torres-Pardo, A.; González-Calbet, J. M.; Ma, Y.; Oleynikov, P.; Terasaki, O.; Asahina, S.; Shima, M.; Cha, D.; et al. Cobalt Phosphate-Modified Barium-Doped Tantalum Nitride Nanorod Photoanode with 1.5% Solar Energy Conversion Efficiency. Nat. Commun. 2013, 4, 2566. (17) Cong, Y. Q.; Park, H. S.; Wang, S. J.; Dang, H. X.; Fan, F. R. F.; Mullins, C. B.; Bard, A. J. Synthesis of Ta3N5 Nanotube Arrays Modified with Electrocatalysts for Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2012, 116, 14541−14550. (18) Kim, Y. I.; Woodward, P. M.; Baba-Kishi, K. Z.; Tai, C. W. Characterization of the Structural, Optical, and Dielectric Properties of Oxynitride Perovskites AMO(2)N (a = Ba, Sr, Ca; M = Ta, Nb). Chem. Mater. 2004, 16, 1267−1276. (19) Oka, D.; Hirose, Y.; Fukumura, T.; Hasegawa, T. Heteroepitaxial Growth of Perovskite CaTaO2N Thin Films by Nitrogen Plasma-Assisted Pulsed Laser Deposition. Cryst. Growth Des. 2014, 14, 87−90. (20) Oka, D.; Hirose, Y.; Kamisaka, H.; Fukumura, T.; Sasa, K.; Ishii, S.; Matsuzaki, H.; Sato, Y.; Ikuhara, Y.; Hasegawa, T. Possible Ferroelectricity in Perovskite Oxynitride SrTaO2N Epitaxial Thin Films. Sci. Rep. 2014, 4, 4987. (21) Zhong, M.; Ma, Y.; Oleynikov, P.; Domen, K.; Delaunay, J. J. Conductive ZnO-ZnGaON Nanowire-Array-on-a-Film Photoanode for Stable and Efficient Sunlight Water Splitting. Energy Environ. Sci. 2014, 7, 1693−1699. (22) Ravel, B.; Kim, Y. I.; Woodward, P. M.; Fang, C. M. Role of Local Disorder in the Dielectric Response of BaTaO2N. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 184121. (23) Page, K.; Stoltzfus, M. W.; Kim, Y. I.; Proffen, T.; Woodward, P. M.; Cheetham, A. K.; Seshadri, R. Local Atomic Ordering in BaTaO2N Studied by Neutron Pair Distribution Function Analysis and Density Functional Theory. Chem. Mater. 2007, 19, 4037−4042. (24) Shannon, J.; Katz, L. A Refinement of the Structure of Barium Tantalum Oxide, Ba5Ta4O15. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1970, 26, 102−105. (25) Galasso, F.; Katz, L. Preparation and Structure of Ba5Ta4O15 and Related Compounds. Acta Crystallogr. 1961, 14, 647−650. (26) Galasso, F.; George, F. Infra-Red Transmittance of Ba5Ta4O15 Crystals Grown by Czochralski Method. Nature 1965, 206, 1352. (27) Xu, T. G.; Zhang, C.; Shao, X.; Wu, K.; Zhu, Y. F. Monomolecular-Layer Ba5Ta4O15 Nanosheets: Synthesis and Inves-

phase. Under irradiation by simulated AM 1.5G light, a CoPi/ BaTaO2N electrode exhibited a photocurrent of ∼0.75 mA cm−2 at 1.23 V versus RHE and produced oxygen for 5 h with a Faradaic efficiency of >90% without significant deactivation. The reported preparation method can potentially be applied to the synthesis of other perovskite ABO2N (A = Ca, Sr, Ba and B = Nb, Ta) films.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11564. Crystal structures of Ba5Ta4O15 and BaTaO2N, XRD patterns of BaTaO2N films nitrided in different flow rates of NH3 gas, PEC properties of BaTaO2N films nitrided at different temperatures, XPS spectra of Ba5Ta4O15 and BaTaO2N films, and photocurrent density calculated based on the solar simulator spectrum and the IPCE curve. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81 3 5841 8838. Tel: +81 3 5841 1148. E-mail: [email protected]. Present Address ∥

J.K.: Department of Chemical Engineering, Fukuoka University 8-19-1 Nanakuma, Jonan-ku, 814-0180 Fukuoka, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Grants-in-Aid for Specially Promoted Research (No. 23000009), for Young Scientists (A) (No. 15H05494) and Young Scientists (B) (No. 5K17895), and the A3 Foresight Program of Japan Society for the Promotion of Science (JSPS). This work was partly supported by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO).

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Nozik, A. J. p-n photoelectrolysis cells. Appl. Phys. Lett. 1976, 29, 150−153. (3) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (4) Paracchino, A.; Laporte, V.; Sivula, K.; Gratzel, M.; Thimsen, E. Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat. Mater. 2011, 10, 456−461. (5) Ji, L.; McDaniel, M. D.; Wang, S.; Posadas, A. B.; Li, X.; Huang, H.; Lee, J. C.; Demkov, A. A.; Bard, A. J.; Ekerdt, J. G.; et al. A SiliconBased Photocathode for Water Reduction with an Epitaxial SrTiO3 Protection Layer and a Nanostructured Catalyst. Nat. Nanotechnol. 2015, 10, 84−90. (6) Minegishi, T.; Nishimura, N.; Kubota, J.; Domen, K. Photoelectrochemical Properties of LaTiO2N Electrodes Prepared by Particle Transfer for Sunlight-Driven Water Splitting. Chem. Sci. 2013, 4, 1120−1124. (7) Dang, H. X.; Hahn, N. T.; Park, H. S.; Bard, A. J.; Mullins, C. B. Nanostructured Ta3N5 Films as Visible-Light Active Photoanodes for Water Oxidation. J. Phys. Chem. C 2012, 116, 19225−19232. (8) Pinaud, B. A.; Vesborg, P. C. K.; Jaramillo, T. F. Effect of Film Morphology and Thickness on Charge Transport in Ta3N5/Ta F

DOI: 10.1021/acs.jpcc.5b11564 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C tigation of Photocatalytic Properties. Adv. Funct. Mater. 2006, 16, 1599−1607. (28) Moriya, Y.; Takata, T.; Domen, K. Recent Progress in the Development of (Oxy)Nitride Photocatalysts for Water Splitting under Visible-Light Irradiation. Coord. Chem. Rev. 2013, 257, 1957− 1969. (29) Yang, M.; Oro-Sole, J.; Rodgers, J. A.; Jorge, A. B.; Fuertes, A.; Attfield, J. P. Anion Order in Perovskite Oxynitrides. Nat. Chem. 2011, 3, 47−52. (30) Dabirian, A.; van de Krol, R. High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties. Chem. Mater. 2015, 27, 708− 715. (31) Stampfl, C.; Freeman, A. J. Stable and Metastable Structures of the Multiphase Tantalum Nitride System. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 024111. (32) Terao, N. Structure of Tantalum Nitrides. Jpn. J. Appl. Phys. 1971, 10, 248−259. (33) Brophy, M. R.; Pilgrim, S. M.; Schulze, W. A. Synthesis of BaTaO2N Powders Utilizing NH3 Decomposition. J. Am. Ceram. Soc. 2011, 94, 4263−4268. (34) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (35) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347−57. (36) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (37) Gamelin, D. R. Water Splitting: Catalyst or Spectator? Nat. Chem. 2012, 4, 965−967. (38) Hafez, A. M.; Salem, N. M.; Allam, N. K. Unravelling the Correlated Electronic and Optical Properties of BaTaO2N with Perovskite-Type Structure as a Potential Candidate for Solar Energy Conversion. Phys. Chem. Chem. Phys. 2014, 16, 18418−18424. (39) Lin, F.; Boettcher, S. W. Adaptive Semiconductor/Electrocatalyst Junctions in Water-Splitting Photoanodes. Nat. Mater. 2014, 13, 81−86. (40) Chen, S.; Wang, L. Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution. Chem. Mater. 2012, 24, 3659−3666.

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