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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C5CC01728A
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Jiasheng Xu,#a,b Chengsi Pan,a Tsuyoshi Takata*a and Kazunari Domen*a,b, c 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Overall water splitting was achieved on a simple perovskite oxynitride photocatalyst, CaTaO2N, with an absorption edge at 510 nm. This photocatalyst, modified with a Rh-Cr bimetallic oxide cocatalyst, produced stoichiometric H2 and O2 steadily under UV and visible light irradiation after coating of the photocatalyst particles with amorphous Ti oxyhydroxide. Overall water splitting into hydrogen and oxygen on a semiconductor photocatalyst is a potential means to produce clean and renewable hydrogen from solar energy. Hundreds of UV-light-sensitive photocatalysts based on wide-gap oxide semiconductors have been developed to date for water splitting.1 The thrust of this research has centered on realizing visible-lightdriven overall water splitting. Thus, the development of narrower-gap photocatalysts capable of overall water splitting has been studied. Recently, there have been a growing number of overall water splitting systems that are functional under visible light. These are based on direct water splitting on a particulate photocatalyst, Zscheme water splitting and, in a broader sense, photoelectrochemical water splitting.2-4 Among the various approaches, direct water splitting using a single type of photocatalyst is likely to be the most challenging because of a number of more strict requisites compared to in other systems, but it is still attractive for its simplicity, and is expected to be the most cost-effective route toward large-scale solar hydrogen production.5, 6 To date, several photocatalysts have been reported to be capable of overall water splitting under visible light irradiation.711 For example, oxynitrides containing typical metal cations with d10 electron configuration, (Ga1-xZnx)(N1-xOx) and (Zn1+xGe)(N2Ox), are known to be capable of direct water splitting into H2 and O2, and are operable under visible light irradiation below approximately 480 nm.7, 8 For efficient utilization of a large portion of the sunlight spectrum, we have been aiming at developing oxynitrides based on d0-tansition metal cations as next-generation photocatalysts because most of them have a wider absorption band in the visible region compared to d10 metal oxynitride photocatalysts. Among the d0-transition-metal-containing oxynitrides, direct water splitting was first achieved on a modified TaON with an absorption edge at ca. 510 nm.10 Then, very recently, our group reported a newly designed LaMgxTa1-xO1+3xN2-3x (x ≥ 1/3) as a This journal is © The Royal Society of Chemistry [year]
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photocatalyst for overall water splitting that is operable at up to 600 nm.11 In that study, we used two successful approaches: (i) compositional fine tuning to make the bandgap energy and position suitable for water splitting, and (ii) surface modification to control the redox reactions on the photocatalyst surface. In the latter approach, we devised a unique and effective surface coating for the photocatalyst particles: photodeposition of amorphous Ti oxyhydroxide on the whole surface of photocatalyst particles created a core/shell-structured photocatalyst, which helped to prevent a reverse reaction and self-oxidation of the photocatalyst surface, leading to steady overall water splitting. We expect that this new surface coating method can and will be applied to a variety of photocatalysts and enable the fabrication of more efficient photocatalysts. This motivated us to explore using this method with other transition metal oxynitrides. As a result, we found that a Ta5+-based oxynitride, CaTaO2N, became capable of overall water splitting under visible light after surface coating. The present study reports the successful case of overall water splitting on the CaTaO2N photocatalyst. CaTaO2N has a perovskite structure isostructural to that of the LaMgxTa1-xO1+3xN2-3x series, in which sites A and B were occupied by Ca2+ and Ta5+, respectively, as illustrated in Fig. 1a. The ionic radius of Ca2+ (134 pm) is comparable to that of La3+ (136 pm). CaTaO2N is known to have an absorption band up to an onset wavelength of around 500 nm.12 The crystal structure and some physicochemical properties of this material have already been examined,13-15 and its photocatalysis has been studied not only by our group, but by others as well.12, 16-18 However, until now, no one had observed direct water splitting using this material. The present report represents the first achievement of direct water splitting on the CaTaO2N photocatalyst under UV and visible light irradiation. Most of the photocatalyst preparation procedures were carried out according to previous reports, although some minor modifications were made.12,18 CaTaO2N was prepared by thermal ammonolysis of the corresponding oxide precursor under flowing dry ammonia at 1123 K for 6 h. The details of the photocatalyst preparation method are provided in the ESI†. The synthesized sample was characterized by X-ray diffraction (XRD, Cu Kαradiation, D8 Advance, Bruker AXS, Co., Ltd.), UV-Visible diffuse reflectance spectroscopy (UV-Vis DRS, V-560 Spectrophotometer, JASCO International Co., Ltd.) and fieldemission scanning electron microscopy (FE-SEM, SU-8020, Hitachi High-Technologies Corp.). [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
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Photocatalytic Overall Water Splitting on the Perovskite-type Transition Metal Oxynitride CaTaO2N under Visible Light Irradiation
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ChemComm
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DOI: 10.1039/C5CC01728A
Fig.1 (a) Crystal structure of orthorhombic CaTaO2N; (b) XRD patterns of Ca2Ta2O7 (precursor) and CaTaO2N; and SEM images of CaTaO2N particles before (c, d) and after stirring (e, f).
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The prepared CaTaO2N sample had an XRD pattern assignable to the perovskite structure (Fig. 1b). The precursor oxide was poorly crystallized to form Ca2Ta2O7, but the corresponding peaks disappeared after nitridation, indicating that the level of nitridation was sufficient. No diffraction peaks attributable to any byproduct phases were detected. Figures 1c-f show SEM images of the synthesized CaTaO2N. The as-nitrided sample consists of primary particles about 50-100 nm in size, which mostly aggregated to form secondary particles. These secondary particles were crushed into small mono-disperse primary particles upon stirring in EtOH. Then, optical absorption properties were examined. The UV-Vis diffuse reflectance spectrum in Fig. 2a revealed a broad absorption that can be divided into two components: (i) a strong absorption band with an onset wavelength at ca. 510 nm, and (ii) a weak absorption tailing to 600 nm. This phenomenon is observed fairly frequently for this material, as reported in previous studies.12, 15, 18 Our current interpretation of the two components is that the former is the absorption band of the CaTaO2N phase, while the latter is attributable to that of a byproduct phase. The onset wavelength of the latter absorption suggests that very fine crystallites of the Ta3N5 phase were present in a very small amount, below the detection limit of the XRD used in our experiments (Fig. S1, ESI†). Since, as mentioned above, the precursor oxide is not a perfect crystalline material, the Ca and Ta components were not distributed entirely homogeneously at the atomic scale. Therefore, partial segregation to CaO and Ta3N5 during nitridation was probable. However, the extent of this segregation, as well as its contribution to photocatalysis, could be negligible. This journal is © The Royal Society of Chemistry [year]
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Fig.2 (a) UV-Vis diffuse reflectance spectrum (inset: Tauc-Plot spectrum), and (b) band position of CaTaO2N, as determined by calculation. CBM and VBM stand for conduction band minimum and valence band maximum, respectively.
By extrapolating the slope of the inset Tauc plot, which shows the spectrum of energy versus [F(R∞)h]n, the bandgap energy and absorption edge of CaTaO2N itself were estimated to be 2.43 eV and 510 nm, respectively. Here, R∞ and F(R∞) are the limiting reflectance and Kubelka-Munk function, respectively. The slope of the Tauc plot fit well at n = 1/2, indicating an indirect transition. The bandgap position was estimated from theoretical calculations by Xu et al (detailed in ESI†).19 As shown in Fig. 2b, conduction band minimum (CBM) and valence band maximum (VBM) are estimated to lie at -0.88 eV and 1.55 eV, respectively, and their energy levels straddle the redox potentials for H+/H2 and O2/H2O. The examined calculation is a semi-empirical method, involving some ambiguity, and the band position obtained may be [journal], [year], [vol], 00–00 | 2
ChemComm Accepted Manuscript
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slightly lacking in accuracy. However, the bandgap position of CaTaO2N was also confirmed to satisfy the thermodynamic requirements for water splitting in other reports.20,21 Therefore, it is expectable that this material will exhibit the ability for overall water splitting. Next, the photocatalytic activity of CaTaO2N was examined. A Rh-Cr bimetallic oxide (denoted by RhCrOy) was loaded as a cocatalyst to promote H2 evolution.11,22 Fig. 3a shows the time course of gas production during an attempt of water splitting under UV+Vis light irradiation ( 300 nm). H2, O2, and N2 evolution was clearly detected, but the evolution rates decreased after 3 h of photoirradiation. According to our previous study, O2 evolution was not possible on this compound even in the presence of a sacrificial electron acceptor.16 However, O2 evolution was observed in AgNO3 aqueous solution on the present sample (Fig. S2, ESI†), which meets the prerequisite for overall water splitting. This difference in the O2 evolution ability resulted from the different nitridation conditions. The severe nitridation condition (NH3 1 L min-1, 1123 K, 15 h) in the previous sample synthesis would generated a defective sample, which likely disabled O2 evolution.16 During the prolonged photoirradiation that lasted for 24 h, the amount of accumulated O2 decreased slightly after 3 h of irradiation, while the amounts of accumulated N2 and H2 continued to increase. In the case of the LaMg1/3Ta2/3O2N photocatalyst, the decrease in accumulated O2 was clearly evident. Thus, we concluded that the reverse reaction on the surface of the photocatalyst was a critical problem hindering water splitting.11 However, for the CaTaO2N photocatalyst, the problem of reverse reaction was found to be insignificant. The accumulated gases were evacuated in 24 h, and the reaction was restarted. The gas production rates in the second run were lower than those in the first run, although H2, O2 and N2 evolved continuously. The N2 evolution was likely attributable to oxidation of the nitrogen species on the photocatalyst surface via the following reaction: 2Ns3- + 6h+ → N2 (3) The above result indicates that holes (h+) oxidized H2O in competition with the surface nitrogen species (Ns). The numbers of consumed electrons and holes, as estimated from the amounts
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of H2, O2, and N2 produced, were not completely balanced. There was some H2 deficiency (ca. 8 μmol for 24×2 h); the corresponding number of electrons may have been trapped in the cocatalyst or photocatalyst. Prevention of N2 evolution would lead to stable overall water splitting. Thus, to solve this problem, surface coating of the photocatalyst particles with an amorphous Ti oxyhydroxide layer by in situ photodeposition from a Ti peroxide solution was attempted, since this method has been demonstrated to be effective in preventing N2 evolution on LaMgxTa1-xO1+3xN2-3x.11 Fig. 3b shows the time courses of gas production during the photodeposition of 1 wt% Ti oxyhydroxide and the subsequent water splitting process under UV+Vis light irradiation. In the beginning of the photodeposition process, a large amount of O2 evolved upon photoirradiation, along with some H2 evolution, and nearly terminated after 3 to 6 h of irradiation. The observed O2 evolution was mostly attributable to the decomposition of peroxide species, and the termination of O2 evolution was indicative of the exhaustion of peroxide species. Since the photodeposition ended as a result of the termination of O2 evolution, photoirradiation was restarted after evacuation of the accumulated gases in the reaction system. Then, simultaneous H2 and O2 evolution was observed in the stoichiometric ratio of water splitting. Notably, N2 evolution was prevented almost completely. To determine the effect of H2O2 addition on subsequent water splitting performance, water splitting was investigated on a photocatalyst without Ti oxyhydroxide coating. In this experiment, photoirradiation was conducted in an aqueous H2O2 solution containing an amount of H2O2 comparable to that used in the case of 1 wt% Ti oxyhydroxide photodeposition. As shown in Fig. 3c, a large amount of O2 evolved upon photoirradiation, resulting from the decomposition of H2O2, similarly to the results shown in Fig. 3b. In the subsequent photoirradiation stage, H2 and O2 evolved, while N2 evolution was not prevented, indicating that the deposition of Ti oxyhydroxide was essential for preventing N2 evolution. These results demonstrated that overall water splitting under UV+Vis light irradiation was successful on RhCrOy/CaTaO2N after photodepositing Ti oxyhydroxide. With the present photocatalyst,
Fig.3 (a) Time courses of gas production during water splitting on RhCrOy/CaTaO2N, (b) time courses of gas production during the Ti oxyhydroxide photodeposition process and subsequent water splitting on RhCrOy/CaTaO2N, (c) time courses of gas production during the decomposition of H2O2 and subsequent water splitting on RhCrOy/CaTaO2N. Reaction conditions: catalyst, 0.2 g; reaction solution, pure water (250 mL); light source, Xe lamp (300W, 300 nm); side-irradiation-type reaction vessel made of Pyrex. (■) H2; (●) O2; (▲) N2.
overall water splitting was achieved without any compositional tuning of the CaTaO2N material, which is what distinguishes it from previous results for the LaMgxTa1-xO1+3xN2-3x series. This is likely because CaTaO2N inherently has a suitable bandgap position for overall water splitting. Prolonged irradiation runs were conducted under UV+Vis to
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examine the activity of the surface-modified photocatalyst. As shown in Fig. 4a, H2 and O2 evolved under light irradiation at almost a constant rate, and water splitting was continued for 30 h with two intermediate evacuations of the gases that accumulated in the reaction system. During this stage, no noticeable decrease in activity or N2 evolution was observed. Similarly, under only
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Fig.4 Time courses of gas evolution during water splitting on Tioxyhydroxide-deposited RhCrOy/CaTaO2N under UV + Vis light ( 300 nm) (a) and visible light ( 420 nm) (b). The reaction was continued for 30 h, with evacuation every 10 h (dotted line). Reaction conditions: catalyst, 0.2 g; reaction solution, pure water (250 mL); light source, Xe lamp (300 W); side-irradiation-type reaction vessel made of Pyrex. (■) H2; (●) O2; (▲) N2.
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In summary, overall water splitting was achieved on a properly surface-modified CaTaO2N photocatalyst with a bandgap energy of 2.43 eV. This is the second example of overall water splitting among perovskite-type transition metal oxynitrides, but it is the first example among the simple perovskites. Therefore, compositional fine-tuning of this material is expected to lead to further enhancements in photocatalytic activity. The photocatalytic activity of CaTaO2N for water splitting was lower than that of LaMg1/3Ta2/3O2N. Nevertheless, a study exploring the cause of this difference would serve as a guide for fabricating efficient photocatalysts. The CaTaO2N has analogous compounds in which the Ca2+ site is replaced with other alkaline earth metal cations, such as Sr2+ and Ba2+. The absorption band edge shifts toward longer wavelengths in the order of Ca < Sr < Ba forms for this series of perovskites. Notably, BaTaO2N absorbs light up to 660 nm, enabling the utilization of a wide portion of the visible part of the solar spectrum. We believe that the present study represents the first step toward achieving overall water splitting using this highly suitable light absorber. Our future study will expand in these directions. This work was supported in part by the Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Postdoctoral Fellowship for Foreign Researchers, Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Scientific Research (C) (No. 24560953) and for Specially Promoted Research (No. 23000009).
Notes and references a
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Global Research Center for Environment and Energy based on Nanomaterials Science(GREEN), National Institute for Materials Science(NIMS), 1-1 Namiki, Tsukuba-city, Ibaraki 305-0044, Japan b Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
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Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), 5-1-5 Kashiwanoha, Kashiwa-city, Chiba 227-8589, Japan # Present affiliation: Liaoning Province Key Laboratory for Synthesis and Application of Functional Compounds, College of Chemistry, Chemical Engineering and Food Safety, Bohai University, 19 Sci-tech Road, Jinzhou 121013, PR China * Corresponding authors. Fax: +81-3-5841-8838. E-mail address: [email protected], [email protected] † Electronic Supplementary Information (ESI) available: Experimental details and XRD patterns. See DOI: 10.1039/b000000x/ 1. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253. 2. K. Maeda and K. Domen, J. Phys. Chem., 2007, 111, 7851. 3. R. Abe, Bull. Chem. Soc. Jpn., 2011, 84, 1000. 4. T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014, 43, 7520. 5. B. A. Pinaud, J. D. Benck, L. C. Seitz, A. J. Forman, Z. Chen, T. G. Deutsch, B. D. James, K. N. Baum, G. N. Baum, S. Ardo, H. Wang, E. Miller and T. F. Jaramillo, Energy Environ. Sci., 2013, 6, 1983. 6. U. Diebold, Nat. Chem., 2011, 3, 271. 7. K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295. 8. Y. Lee, H. Terashima, Y. Shimodaira, K. Teramura, M. Hara, H. Kobayashi, K. Domen and M. Yashima, J. Phys. Chem. C, 2007, 111, 1042. 9. R. Asai, H. Nemoto, Q. Jia, K. Saito, A. Iwase and A. Kudo, Chem. Commun., 2014, 50, 2543. 10. K. Maeda, D. Lu and K. Domen, Chem. Eur. J., 2013, 19, 4986. 11. C. Pan, T. Takata, M. Nakabayashi, T. Matsumoto, N. Shibata, Y. Ikuhara and K. Domen, Angew. Chem. Int. Ed., 2015, 54, 2955. 12. M. Higashi, R. Abe, T. Takata and K. Domen, Chem. Mater., 2009, 21, 1543. 13. E. Günther, R. Hagenmayer and M. Jansen, Z. Anorg. Allg. Chem., 2000, 626, 1519. 14. S. J. Clarke, K. A. Hardstone, C. W. Michie and M. J. Rosseinsky, Chem. Mater., 2002, 14, 2664. 15. Y. Kim, P. M. Woodward, K. Z. Baba-Kishi, and C. W. Tai, Chem. Mater., 2004, 16, 1267. 16. G. Hitoki, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, Electrochemistry, 2002, 70, 463. 17. M. Higashi, R. Abe, T. Takata and K. Domen, Chem. Mater., 2009, 21, 1543. 18. M. Higashi, R. Abe, K. Teramura, T. Takata, B. Ohtani and K. Domen, Chem. Phys. Lett., 2008, 452, 120. 19. Y. Xu and M. A. A. Schoonen, Am. Mineral., 2000, 85, 543. 20. I. E. Castelli, T. Olsen, S. Datta, D. D. Landis, S. Dahl, K. S. Thygesen and K. W. Jacobsen, Energy Environ. Sci., 2012, 5, 5814. 21. S. Balaz, S. H. Porter, P. M. Woodward and L. J. Brillson, Chem. Mater., 2013, 25, 3337. 22. K. Maeda, K. Teramura, H. Masuda, T. Takata, N. Saito, Y. Inoue and K. Domen, J. Phys. Chem. B, 2006, 110, 13107.
ChemComm Accepted Manuscript
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visible light irradiation ( 420 nm), steady overall water splitting proceeded during the irradiation period (Fig. 4b), which obviously confirmed visible-light-driven overall water splitting on the surface modified CaTaO2N photocatalyst. The quantum efficiency of water splitting on this photocatalyst was estimated to be ca. 3×10-3 % at = 44030 nm.
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DOI: 10.1039/C5CC01728A
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Xu, C. Pan, T. Takata and K. Domen, Chem. Commun., 2015, DOI: 10.1039/C5CC01728A.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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ChemComm
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DOI: 10.1039/C5CC01728A
ARTICLE TYPE
Jiasheng Xu,#a,b Chengsi Pan,a Tsuyoshi Takata*a and Kazunari Domen*a,b, c 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Overall water splitting was achieved on a simple perovskite oxynitride photocatalyst, CaTaO2N, with an absorption edge at 510 nm. This photocatalyst, modified with a Rh-Cr bimetallic oxide cocatalyst, produced stoichiometric H2 and O2 steadily under UV and visible light irradiation after coating of the photocatalyst particles with amorphous Ti oxyhydroxide. Overall water splitting into hydrogen and oxygen on a semiconductor photocatalyst is a potential means to produce clean and renewable hydrogen from solar energy. Hundreds of UV-light-sensitive photocatalysts based on wide-gap oxide semiconductors have been developed to date for water splitting.1 The thrust of this research has centered on realizing visible-lightdriven overall water splitting. Thus, the development of narrower-gap photocatalysts capable of overall water splitting has been studied. Recently, there have been a growing number of overall water splitting systems that are functional under visible light. These are based on direct water splitting on a particulate photocatalyst, Zscheme water splitting and, in a broader sense, photoelectrochemical water splitting.2-4 Among the various approaches, direct water splitting using a single type of photocatalyst is likely to be the most challenging because of a number of more strict requisites compared to in other systems, but it is still attractive for its simplicity, and is expected to be the most cost-effective route toward large-scale solar hydrogen production.5, 6 To date, several photocatalysts have been reported to be capable of overall water splitting under visible light irradiation.711 For example, oxynitrides containing typical metal cations with d10 electron configuration, (Ga1-xZnx)(N1-xOx) and (Zn1+xGe)(N2Ox), are known to be capable of direct water splitting into H2 and O2, and are operable under visible light irradiation below approximately 480 nm.7, 8 For efficient utilization of a large portion of the sunlight spectrum, we have been aiming at developing oxynitrides based on d0-tansition metal cations as next-generation photocatalysts because most of them have a wider absorption band in the visible region compared to d10 metal oxynitride photocatalysts. Among the d0-transition-metal-containing oxynitrides, direct water splitting was first achieved on a modified TaON with an absorption edge at ca. 510 nm.10 Then, very recently, our group reported a newly designed LaMgxTa1-xO1+3xN2-3x (x ≥ 1/3) as a This journal is © The Royal Society of Chemistry [year]
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photocatalyst for overall water splitting that is operable at up to 600 nm.11 In that study, we used two successful approaches: (i) compositional fine tuning to make the bandgap energy and position suitable for water splitting, and (ii) surface modification to control the redox reactions on the photocatalyst surface. In the latter approach, we devised a unique and effective surface coating for the photocatalyst particles: photodeposition of amorphous Ti oxyhydroxide on the whole surface of photocatalyst particles created a core/shell-structured photocatalyst, which helped to prevent a reverse reaction and self-oxidation of the photocatalyst surface, leading to steady overall water splitting. We expect that this new surface coating method can and will be applied to a variety of photocatalysts and enable the fabrication of more efficient photocatalysts. This motivated us to explore using this method with other transition metal oxynitrides. As a result, we found that a Ta5+-based oxynitride, CaTaO2N, became capable of overall water splitting under visible light after surface coating. The present study reports the successful case of overall water splitting on the CaTaO2N photocatalyst. CaTaO2N has a perovskite structure isostructural to that of the LaMgxTa1-xO1+3xN2-3x series, in which sites A and B were occupied by Ca2+ and Ta5+, respectively, as illustrated in Fig. 1a. The ionic radius of Ca2+ (134 pm) is comparable to that of La3+ (136 pm). CaTaO2N is known to have an absorption band up to an onset wavelength of around 500 nm.12 The crystal structure and some physicochemical properties of this material have already been examined,13-15 and its photocatalysis has been studied not only by our group, but by others as well.12, 16-18 However, until now, no one had observed direct water splitting using this material. The present report represents the first achievement of direct water splitting on the CaTaO2N photocatalyst under UV and visible light irradiation. Most of the photocatalyst preparation procedures were carried out according to previous reports, although some minor modifications were made.12,18 CaTaO2N was prepared by thermal ammonolysis of the corresponding oxide precursor under flowing dry ammonia at 1123 K for 6 h. The details of the photocatalyst preparation method are provided in the ESI†. The synthesized sample was characterized by X-ray diffraction (XRD, Cu Kαradiation, D8 Advance, Bruker AXS, Co., Ltd.), UV-Visible diffuse reflectance spectroscopy (UV-Vis DRS, V-560 Spectrophotometer, JASCO International Co., Ltd.) and fieldemission scanning electron microscopy (FE-SEM, SU-8020, Hitachi High-Technologies Corp.). [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
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Photocatalytic Overall Water Splitting on the Perovskite-type Transition Metal Oxynitride CaTaO2N under Visible Light Irradiation
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ChemComm
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DOI: 10.1039/C5CC01728A
Fig.1 (a) Crystal structure of orthorhombic CaTaO2N; (b) XRD patterns of Ca2Ta2O7 (precursor) and CaTaO2N; and SEM images of CaTaO2N particles before (c, d) and after stirring (e, f).
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The prepared CaTaO2N sample had an XRD pattern assignable to the perovskite structure (Fig. 1b). The precursor oxide was poorly crystallized to form Ca2Ta2O7, but the corresponding peaks disappeared after nitridation, indicating that the level of nitridation was sufficient. No diffraction peaks attributable to any byproduct phases were detected. Figures 1c-f show SEM images of the synthesized CaTaO2N. The as-nitrided sample consists of primary particles about 50-100 nm in size, which mostly aggregated to form secondary particles. These secondary particles were crushed into small mono-disperse primary particles upon stirring in EtOH. Then, optical absorption properties were examined. The UV-Vis diffuse reflectance spectrum in Fig. 2a revealed a broad absorption that can be divided into two components: (i) a strong absorption band with an onset wavelength at ca. 510 nm, and (ii) a weak absorption tailing to 600 nm. This phenomenon is observed fairly frequently for this material, as reported in previous studies.12, 15, 18 Our current interpretation of the two components is that the former is the absorption band of the CaTaO2N phase, while the latter is attributable to that of a byproduct phase. The onset wavelength of the latter absorption suggests that very fine crystallites of the Ta3N5 phase were present in a very small amount, below the detection limit of the XRD used in our experiments (Fig. S1, ESI†). Since, as mentioned above, the precursor oxide is not a perfect crystalline material, the Ca and Ta components were not distributed entirely homogeneously at the atomic scale. Therefore, partial segregation to CaO and Ta3N5 during nitridation was probable. However, the extent of this segregation, as well as its contribution to photocatalysis, could be negligible. This journal is © The Royal Society of Chemistry [year]
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Fig.2 (a) UV-Vis diffuse reflectance spectrum (inset: Tauc-Plot spectrum), and (b) band position of CaTaO2N, as determined by calculation. CBM and VBM stand for conduction band minimum and valence band maximum, respectively.
By extrapolating the slope of the inset Tauc plot, which shows the spectrum of energy versus [F(R∞)h]n, the bandgap energy and absorption edge of CaTaO2N itself were estimated to be 2.43 eV and 510 nm, respectively. Here, R∞ and F(R∞) are the limiting reflectance and Kubelka-Munk function, respectively. The slope of the Tauc plot fit well at n = 1/2, indicating an indirect transition. The bandgap position was estimated from theoretical calculations by Xu et al (detailed in ESI†).19 As shown in Fig. 2b, conduction band minimum (CBM) and valence band maximum (VBM) are estimated to lie at -0.88 eV and 1.55 eV, respectively, and their energy levels straddle the redox potentials for H+/H2 and O2/H2O. The examined calculation is a semi-empirical method, involving some ambiguity, and the band position obtained may be [journal], [year], [vol], 00–00 | 2
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slightly lacking in accuracy. However, the bandgap position of CaTaO2N was also confirmed to satisfy the thermodynamic requirements for water splitting in other reports.20,21 Therefore, it is expectable that this material will exhibit the ability for overall water splitting. Next, the photocatalytic activity of CaTaO2N was examined. A Rh-Cr bimetallic oxide (denoted by RhCrOy) was loaded as a cocatalyst to promote H2 evolution.11,22 Fig. 3a shows the time course of gas production during an attempt of water splitting under UV+Vis light irradiation ( 300 nm). H2, O2, and N2 evolution was clearly detected, but the evolution rates decreased after 3 h of photoirradiation. According to our previous study, O2 evolution was not possible on this compound even in the presence of a sacrificial electron acceptor.16 However, O2 evolution was observed in AgNO3 aqueous solution on the present sample (Fig. S2, ESI†), which meets the prerequisite for overall water splitting. This difference in the O2 evolution ability resulted from the different nitridation conditions. The severe nitridation condition (NH3 1 L min-1, 1123 K, 15 h) in the previous sample synthesis would generated a defective sample, which likely disabled O2 evolution.16 During the prolonged photoirradiation that lasted for 24 h, the amount of accumulated O2 decreased slightly after 3 h of irradiation, while the amounts of accumulated N2 and H2 continued to increase. In the case of the LaMg1/3Ta2/3O2N photocatalyst, the decrease in accumulated O2 was clearly evident. Thus, we concluded that the reverse reaction on the surface of the photocatalyst was a critical problem hindering water splitting.11 However, for the CaTaO2N photocatalyst, the problem of reverse reaction was found to be insignificant. The accumulated gases were evacuated in 24 h, and the reaction was restarted. The gas production rates in the second run were lower than those in the first run, although H2, O2 and N2 evolved continuously. The N2 evolution was likely attributable to oxidation of the nitrogen species on the photocatalyst surface via the following reaction: 2Ns3- + 6h+ → N2 (3) The above result indicates that holes (h+) oxidized H2O in competition with the surface nitrogen species (Ns). The numbers of consumed electrons and holes, as estimated from the amounts
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of H2, O2, and N2 produced, were not completely balanced. There was some H2 deficiency (ca. 8 μmol for 24×2 h); the corresponding number of electrons may have been trapped in the cocatalyst or photocatalyst. Prevention of N2 evolution would lead to stable overall water splitting. Thus, to solve this problem, surface coating of the photocatalyst particles with an amorphous Ti oxyhydroxide layer by in situ photodeposition from a Ti peroxide solution was attempted, since this method has been demonstrated to be effective in preventing N2 evolution on LaMgxTa1-xO1+3xN2-3x.11 Fig. 3b shows the time courses of gas production during the photodeposition of 1 wt% Ti oxyhydroxide and the subsequent water splitting process under UV+Vis light irradiation. In the beginning of the photodeposition process, a large amount of O2 evolved upon photoirradiation, along with some H2 evolution, and nearly terminated after 3 to 6 h of irradiation. The observed O2 evolution was mostly attributable to the decomposition of peroxide species, and the termination of O2 evolution was indicative of the exhaustion of peroxide species. Since the photodeposition ended as a result of the termination of O2 evolution, photoirradiation was restarted after evacuation of the accumulated gases in the reaction system. Then, simultaneous H2 and O2 evolution was observed in the stoichiometric ratio of water splitting. Notably, N2 evolution was prevented almost completely. To determine the effect of H2O2 addition on subsequent water splitting performance, water splitting was investigated on a photocatalyst without Ti oxyhydroxide coating. In this experiment, photoirradiation was conducted in an aqueous H2O2 solution containing an amount of H2O2 comparable to that used in the case of 1 wt% Ti oxyhydroxide photodeposition. As shown in Fig. 3c, a large amount of O2 evolved upon photoirradiation, resulting from the decomposition of H2O2, similarly to the results shown in Fig. 3b. In the subsequent photoirradiation stage, H2 and O2 evolved, while N2 evolution was not prevented, indicating that the deposition of Ti oxyhydroxide was essential for preventing N2 evolution. These results demonstrated that overall water splitting under UV+Vis light irradiation was successful on RhCrOy/CaTaO2N after photodepositing Ti oxyhydroxide. With the present photocatalyst,
Fig.3 (a) Time courses of gas production during water splitting on RhCrOy/CaTaO2N, (b) time courses of gas production during the Ti oxyhydroxide photodeposition process and subsequent water splitting on RhCrOy/CaTaO2N, (c) time courses of gas production during the decomposition of H2O2 and subsequent water splitting on RhCrOy/CaTaO2N. Reaction conditions: catalyst, 0.2 g; reaction solution, pure water (250 mL); light source, Xe lamp (300W, 300 nm); side-irradiation-type reaction vessel made of Pyrex. (■) H2; (●) O2; (▲) N2.
overall water splitting was achieved without any compositional tuning of the CaTaO2N material, which is what distinguishes it from previous results for the LaMgxTa1-xO1+3xN2-3x series. This is likely because CaTaO2N inherently has a suitable bandgap position for overall water splitting. Prolonged irradiation runs were conducted under UV+Vis to
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examine the activity of the surface-modified photocatalyst. As shown in Fig. 4a, H2 and O2 evolved under light irradiation at almost a constant rate, and water splitting was continued for 30 h with two intermediate evacuations of the gases that accumulated in the reaction system. During this stage, no noticeable decrease in activity or N2 evolution was observed. Similarly, under only
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Fig.4 Time courses of gas evolution during water splitting on Tioxyhydroxide-deposited RhCrOy/CaTaO2N under UV + Vis light ( 300 nm) (a) and visible light ( 420 nm) (b). The reaction was continued for 30 h, with evacuation every 10 h (dotted line). Reaction conditions: catalyst, 0.2 g; reaction solution, pure water (250 mL); light source, Xe lamp (300 W); side-irradiation-type reaction vessel made of Pyrex. (■) H2; (●) O2; (▲) N2.
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In summary, overall water splitting was achieved on a properly surface-modified CaTaO2N photocatalyst with a bandgap energy of 2.43 eV. This is the second example of overall water splitting among perovskite-type transition metal oxynitrides, but it is the first example among the simple perovskites. Therefore, compositional fine-tuning of this material is expected to lead to further enhancements in photocatalytic activity. The photocatalytic activity of CaTaO2N for water splitting was lower than that of LaMg1/3Ta2/3O2N. Nevertheless, a study exploring the cause of this difference would serve as a guide for fabricating efficient photocatalysts. The CaTaO2N has analogous compounds in which the Ca2+ site is replaced with other alkaline earth metal cations, such as Sr2+ and Ba2+. The absorption band edge shifts toward longer wavelengths in the order of Ca < Sr < Ba forms for this series of perovskites. Notably, BaTaO2N absorbs light up to 660 nm, enabling the utilization of a wide portion of the visible part of the solar spectrum. We believe that the present study represents the first step toward achieving overall water splitting using this highly suitable light absorber. Our future study will expand in these directions. This work was supported in part by the Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Postdoctoral Fellowship for Foreign Researchers, Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Scientific Research (C) (No. 24560953) and for Specially Promoted Research (No. 23000009).
Notes and references a
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Global Research Center for Environment and Energy based on Nanomaterials Science(GREEN), National Institute for Materials Science(NIMS), 1-1 Namiki, Tsukuba-city, Ibaraki 305-0044, Japan b Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
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Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), 5-1-5 Kashiwanoha, Kashiwa-city, Chiba 227-8589, Japan # Present affiliation: Liaoning Province Key Laboratory for Synthesis and Application of Functional Compounds, College of Chemistry, Chemical Engineering and Food Safety, Bohai University, 19 Sci-tech Road, Jinzhou 121013, PR China * Corresponding authors. Fax: +81-3-5841-8838. E-mail address: [email protected], [email protected] † Electronic Supplementary Information (ESI) available: Experimental details and XRD patterns. See DOI: 10.1039/b000000x/ 1. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253. 2. K. Maeda and K. Domen, J. Phys. Chem., 2007, 111, 7851. 3. R. Abe, Bull. Chem. Soc. Jpn., 2011, 84, 1000. 4. T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014, 43, 7520. 5. B. A. Pinaud, J. D. Benck, L. C. Seitz, A. J. Forman, Z. Chen, T. G. Deutsch, B. D. James, K. N. Baum, G. N. Baum, S. Ardo, H. Wang, E. Miller and T. F. Jaramillo, Energy Environ. Sci., 2013, 6, 1983. 6. U. Diebold, Nat. Chem., 2011, 3, 271. 7. K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295. 8. Y. Lee, H. Terashima, Y. Shimodaira, K. Teramura, M. Hara, H. Kobayashi, K. Domen and M. Yashima, J. Phys. Chem. C, 2007, 111, 1042. 9. R. Asai, H. Nemoto, Q. Jia, K. Saito, A. Iwase and A. Kudo, Chem. Commun., 2014, 50, 2543. 10. K. Maeda, D. Lu and K. Domen, Chem. Eur. J., 2013, 19, 4986. 11. C. Pan, T. Takata, M. Nakabayashi, T. Matsumoto, N. Shibata, Y. Ikuhara and K. Domen, Angew. Chem. Int. Ed., 2015, 54, 2955. 12. M. Higashi, R. Abe, T. Takata and K. Domen, Chem. Mater., 2009, 21, 1543. 13. E. Günther, R. Hagenmayer and M. Jansen, Z. Anorg. Allg. Chem., 2000, 626, 1519. 14. S. J. Clarke, K. A. Hardstone, C. W. Michie and M. J. Rosseinsky, Chem. Mater., 2002, 14, 2664. 15. Y. Kim, P. M. Woodward, K. Z. Baba-Kishi, and C. W. Tai, Chem. Mater., 2004, 16, 1267. 16. G. Hitoki, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, Electrochemistry, 2002, 70, 463. 17. M. Higashi, R. Abe, T. Takata and K. Domen, Chem. Mater., 2009, 21, 1543. 18. M. Higashi, R. Abe, K. Teramura, T. Takata, B. Ohtani and K. Domen, Chem. Phys. Lett., 2008, 452, 120. 19. Y. Xu and M. A. A. Schoonen, Am. Mineral., 2000, 85, 543. 20. I. E. Castelli, T. Olsen, S. Datta, D. D. Landis, S. Dahl, K. S. Thygesen and K. W. Jacobsen, Energy Environ. Sci., 2012, 5, 5814. 21. S. Balaz, S. H. Porter, P. M. Woodward and L. J. Brillson, Chem. Mater., 2013, 25, 3337. 22. K. Maeda, K. Teramura, H. Masuda, T. Takata, N. Saito, Y. Inoue and K. Domen, J. Phys. Chem. B, 2006, 110, 13107.
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visible light irradiation ( 420 nm), steady overall water splitting proceeded during the irradiation period (Fig. 4b), which obviously confirmed visible-light-driven overall water splitting on the surface modified CaTaO2N photocatalyst. The quantum efficiency of water splitting on this photocatalyst was estimated to be ca. 3×10-3 % at = 44030 nm.
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DOI: 10.1039/C5CC01728A
