Effects of Calcination Temperature on the Physical Properties and Hydrogen Evolution Activities of La 5 Ti 2 Cu(S 1-x Se x ) 5 O 7 Photocatalysts

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1 FULL PAPER Photocatalysts Effects of Calcination Temperature on the Physical Properties and Hydrogen Evolution Activities of La 5 Ti 2 Cu(S 1-x Se x ) 5 O 7 Photocatalysts Swarnava Nandy, Takashi Hisatomi, Masao Katayama, Tsutomu Minegishi, and Kazunari Domen* La 5 Ti 2 Cu(S 1-x Se x ) 5 O 7 (LTCS 1-x Se x O) solid solutions are found to function as visible-light-driven photocatalysts to evolve H 2 from aqueous solutions containing sacrificial electron donors. However, this photocatalytic activity is reduced with increasing Se concentrations because of excessive particle growth during calcination at high temperatures. In the present study, the physical properties and photocatalytic H 2 evolution activities of LTCS 1-x Se x O (0 x 0.6) solid solution photocatalysts synthesized by solid-state reactions at varying temperatures are assessed. It is found that the photocatalyst particle sizes are reduced upon lowering the calcination temperature. In addition, the calcination temperature resulting in the highest photocatalytic H 2 evolution rates for NiS-loaded LTCS 1-x Se x O is shown to become lower with increasing Se content. The H 2 evolution activity of LTCS 1-x Se x O (0.2 x 0.6) is improved several-fold by optimizing the calcination temperature because the excessive growth of particles is avoided. The activity of these materials is further improved by coloading Pt and NiS cocatalysts. This work demonstrates the importance of controlling the particle size of narrow bandgap LTCS 1-x Se x O oxysulfoselenides so as to effectively utilize visible light during photocatalytic H 2 evolution. 1. Introduction Photocatalytic water splitting is an ideal means of generating clean-burning, renewable solar hydrogen on a large scale. [1 5] However, this will require the development of photocatalysts with narrow bandgaps so as to obtain effective solar energy conversion. [6 9] Semiconducting (oxy)chalcogenide materials have attracted much attention in this regard because they have narrow band gaps, and thus absorb visible light, and have the ability to produce H 2 in the presence of sacrificial electron donors. [3] In addition, certain oxysulfides have also been found to exhibit improved photocorrosion resistance compared Dr. S. Nandy, Dr. T. Hisatomi, Dr. M. Katayama, Dr. T. Minegishi, Prof. K. Domen Department of Chemical System Engineering The University of Tokyo Hongo, Bunkyo-ku, Tokyo , Japan domen@chemsys.t.u-tokyo.ac.jp The ORCID identification number(s) for the author(s) of this article can be found under DOI: /ppsc to sulfides and can evolve oxygen from aqueous solutions containing sacrificial electron acceptors. [10 14] In 2004, Meignen et al. reported La 5 Ti 2 CuS 5 O 7 (LTCSO), a semiconductor oxysulfide with an absorption edge wavelength of 650 nm. [15] LTCSO can be obtained as rod-shaped particles by solid-state reactions and exhibits 1D conductivity that reflects its crystal structure. [14,16] This material is applicable to photocatalytic and photoelectrochemical H 2 and O 2 evolution reactions with the aid of sacrificial reagents and an external power source, respectively. [14,17] The photocurrent generated by a LTCSO photocathode can be improved by increasing the particle size up to several micrometers as a result of decreases in defect densities [18] and the long-range diffusion of carriers on the order of micrometers. [19] La 5 Ti 2 Cu(S 1-x Se x ) 5 O 7 (LTCS 1-x Se x O) solid solutions can also be synthesized by solidstate reactions. [20] As with many sulfoselenide semiconductors, the absorption edge wavelength of LTCS 1-x Se x O increases with increasing Se content. [21,22] In addition, LTCS 1-x Se x O loaded with NiS functions as a photocatalyst for H 2 evolution from aqueous solutions containing Na 2 S and Na 2 SO 3 as sacrificial electron donors under visible light. However, this H 2 evolution activity is lowered with the incorporation of Se 2 ions, owing to excessive particle growth during the synthesis process. Se-rich LTCS 1-x Se x O has particle sizes exceeding 10 µm when synthesized using a calcination temperature of 1273 K, likely because selenides have lower melting points than sulfides (since metal selenide bonds are weaker than metal sulfide bonds, reflecting the larger ionic radius of Se 2 than S 2 ). [23] Because the particle size determines the distance that photoexcited carriers in the bulk of photocatalyst materials need to travel to reach active sites on the surface, excessive growth of Se-rich LTCS 1-x Se x O particles is thought to enhance charge recombination. Therefore, it is necessary to control the particle size of these LTCS 1-x Se x O materials to ensure sufficient photocatalytic activity. In the present study, LTCS 1-x Se x O (0 x 0.6) solid solutions were prepared by solid-state reactions in sealed quartz tubes, applying various calcination temperatures in an attempt (1 of 5)

2 to suppress particle growth and thereby maximize the photocatalytic activity. The effects of the calcination temperature on the physical properties and photocatalytic H 2 evolution activity were examined. 2. Result and Discussion 2.1. Physical Properties of LTCS 1-x Se x O Solid Solutions Figure 1A,B presents the X-ray diffraction (XRD) patterns and diffuse-reflectance spectroscopy (DRS) data acquired from the LTCS 0.4 Se 0.6 O samples prepared at K, respectively. A reference XRD pattern of LTCSO obtained from Inorganic Crystal Structure Database is also presented in Figure 1A. [15] Note that the diffraction peaks of LTCS 0.4 Se 0.6 O are located at lower angles because of the expansion of the unit cell. [20] The specimens calcined at K all exhibit similar XRD patterns attributable to an LTCS 0.4 Se 0.6 O phase. The lattice constants of the LTCS 1-x Se x O solid solutions follow Vegard s law. [20] Therefore, the lattice constants of LTCS 0.4 Se 0.6 O samples were calculated by Rietveld refinement as shown in Table S1 in the Supporting Information. It was found that the lattice constants and the unit cell volumes tended to be reduced with decreasing the calcination temperature slightly; suggesting differences in the anion compositions. However, variation in the values of x is estimated to be 0.01 at most taking the Vegard s law into account. LTCS 0.4 Se 0.6 O was not obtained as the major phase in the case of the sample calcined at 1073 K, suggesting the incomplete reaction of the precursors at this relatively low temperature. The DRS data show that lowering the calcination temperature below 1273 K did not affect the light absorption properties of the LTCS 0.4 Se 0.6 O; the absorption onset wavelength remained at 770 nm in each case, including the sample synthesized at 1073 K that was not purely LTCS 0.4 Se 0.6 O. XRD patterns and DRS data for LTCSO calcined at K, LTCS 0.8 Se 0.2 O calcined at K, and LTCS 0.6 Se 0.4 O calcined at K are presented in Figures S1 S3 in the Supporting Information, respectively, along with the reference XRD pattern of LTCSO. [15] In each case, LTCSO, LTCS 0.8 Se 0.2 O, and LTCS 0.6 Se 0.4 O were obtained as the major phases at all temperatures. The lattice constants of the LTCSO, LTCS 0.8 Se 0.2 O, and LTCS 0.6 Se 0.4 O samples calculated by Rietveld refinement were also presented in Tables S2 S4 in the Supporting Information. The lattice constants and the unit cell volumes tended to be reduced with decreasing the calcination temperature marginally, similarly to the LTCS 0.4 Se 0.6 O samples. The absorption onset wavelengths determined by DRS for the LTCSO, LTCS 0.8 Se 0.2 O, and LTCS 0.6 Se 0.4 O were at 660, 720, and 740 nm, respectively, and were also unaffected by changes in the calcination temperature. These results indicate the successful synthesis of LTCSO, LTCS 0.8 Se 0.2 O, and LTCS 0.6 Se 0.4 O in the respective calcination temperature windows. Figure 2 and Figures S4 S6 in the Supporting Information provide scanning electron microscope (SEM) images of the LTCS 0.4 Se 0.6 O and the other LTCS 1-x Se x O samples (x = 0, 0.2, and 0.4) prepared at various calcination temperatures, respectively. The LTCS 0.4 Se 0.6 O specimen calcined at 1273 K included particles longer than 10 µm in the longitudinal direction. The particle size was reduced as the calcination temperature was lowered from 1273 to 1123 K. In addition, the sample calcined at 1073 K did not exhibit rod-shaped particles because of the incomplete conversion of the precursors into the LTCS 0.4 Se 0.6 O phase. The particle sizes of the other LTCS 1-x Se x O samples (x = 0, 0.2, and 0.4) also tended to become smaller at lower calcination temperatures. These results indicate that the growth of LTCS 1-x Se x O particles can be suppressed without affecting the absorption edge wavelength simply by lowering the calcination temperature H 2 Evolution Activity of LTCS 1-x Se x O Solid Solutions Figure 3 shows the hydrogen evolution activity of NiS-loaded LTCS 1-x Se x O (0 x 0.6) synthesized at different calcination temperatures from aqueous Na 2 S and Na 2 SO 3 solutions, following an induction period. The corresponding H 2 evolution time courses are presented in Figure S7 in the Supporting Information. The LTCSO was modified with 1.0 wt% NiS while the other oxysulfoselenide samples were modified with 0.5 wt% NiS by in situ precipitation, using a process optimized in our previous work. [20] The LTCSO sample prepared at 1273 K showed the highest H 2 evolution rate among those specimens prepared at K. The crystalline phases and the light Figure 1. A) XRD patterns and B) DRS data for LTCS 0.4 Se 0.6 O samples calcined for 48 h at (a) 1273, (b) 1173, (c) 1123, and (d) 1073 K (2 of 5)

3 Figure 2. SEM images of LTCS 0.4 Se 0.6 O calcined for 48 h at A) 1273, B) 1173, C) 1123, and D) 1073 K. absorption properties of the samples were largely unchanged and thus would not be responsible for the observed difference in the activity (Figure S1, Supporting Information). Most likely, calcination below 1273 K was insufficient to reduce defects in LTCSO samples synthesized by solid-state reactions. Our previous study suggested that the synthesis of LTCSO under milder conditions induces defects that prevent charge transfer. [18] By contrast, the LTCSO calcined at 1323 K was accompanied by the generation of white impurities, which are thought to have affected the photocatalytic activity. The LTCS 1-x Se x O solid solutions having higher Se 2 concentrations synthesized at 1273 K exhibited lower H 2 evolution activity under visible light irradiation (λ > 420 nm). [20] However, the H 2 evolution rates were improved upon decreasing the calcination temperature below 1273 K, irrespective of the Se 2 level. The H 2 evolution rates of the LTCS 0.8 Se 0.2 O, LTCS 0.6 Se 0.4 O, and LTCS 0.4 Se 0.6 O were found to be optimized at synthesis temperatures of 1223, 1173, and 1123 K, respectively. These evolution rates were approximately three, five, and four times higher than those of conventional samples synthesized at 1273 K. LTCS 0.4 Se 0.6 O specimens prepared at 1123 and 1273 K exhibited similar XRD patterns and DRS spectra, as shown in Figure 1, although the former was composed of smaller particles. The LTCS 0.8 Se 0.2 O and LTCS 0.6 Se 0.4 O synthesized at the optimum temperatures also showed similar properties. Therefore, the higher H 2 evolution rates of LTCS 1-x Se x O (0.2 x 0.6) samples synthesized at lower temperatures are most likely associated with their smaller particle sizes. Excited charge carriers in smaller particles have shorter distances to cover when traveling from the bulk of the particles to surface active sites to participate in surface chemical reactions. They are therefore able to reach the surface before undergoing recombination more frequently than in larger particles. The LTCS 1-x Se x O samples calcined at temperatures lower than the optimal values showed lower H 2 evolution rates, most likely because of insufficient calcination, as was also the case with the LTCSO. Figure 4 presents the action spectra obtained for LTCS 0.8 Se 0.2 O and LTCS 0.6 Se 0.4 O samples synthesized at the optimum temperatures and modified with 0.5 wt% NiS. Lowering the calcination temperature evidently increased the apparent quantum yield (AQY) values without changing the photoactivity onset wavelengths. The LTCS 0.8 Se 0.2 O synthesized at 1223 K showed an AQY of 0.44% at 420 ± 10 nm, which was three times higher than that of the LTCS 0.8 Se 0.2 O calcined at 1273 K. This material was also able to utilize visible light up to 720 nm for H 2 evolution, in keeping with our previous observations. [20] The LTCS 0.6 Se 0.4 O synthesized at 1173 K also exhibited higher AQY values during the H 2 evolution reaction. These data demonstrate that the particle size can be effectively tuned for each LTCS 1-x Se x O composition by selecting the appropriate calcination temperature. The photocatalytic activity of these LTCS 1-x Se x O (0 x 0.6) solid solutions could be further enhanced by coloading with Pt and NiS, as shown in Figure S8 in the Supporting Information. [24] 3. Conclusion Figure 3. H 2 evolution activities of LTCS 1-x Se x O (0 x 0.6) solid solutions calcined for 48 h at various temperatures. Squares, circles, triangles, and inverted triangles represent LTCSO, LTCS 0.8 Se 0.2 O, LTCS 0.6 Se 0.4 O, and LTCS 0.4 Se 0.6 O. Reaction conditions: 0.2 g LTCS 1-x Se x O loaded with 1.0 wt% NiS for x = 0 and 0.5 wt% NiS for 0.2 x 0.6, 150 ml of an aqueous solution containing m Na 2 S and m Na 2 SO 3, visible light overhead irradiation with a 300 W Xe lamp through a cut-off filter (λ > 420 nm). This work studied the effects of the calcination temperature on the physical properties and photocatalytic H 2 evolution activities of LTCS 1-x Se x O solid solutions. LTCS 1-x Se x O (0.2 x 0.6) showed higher photocatalytic activity when synthesized at temperatures less than the value of 1273 K that is optimal for LTCSO. This effect is attributed to a reduction in the particle size that promotes the migration of photoexcited carriers to surface active sites, because both the crystalline phases and the absorption edge wavelengths were unchanged. The optimum temperature for maximizing the photocatalytic activity became (3 of 5)

4 Figure 4. The action spectra for A) LTCS 0.8 Se 0.2 O synthesized at (a) 1273 and (b) 1223 K, and B) LTCS 0.6 Se 0.4 O synthesized at (a) 1273 and (b) 1173 K. Reaction conditions: 0.2 g LTCS 1-x Se x O loaded with 0.5 wt% NiS, 150 ml of an aqueous solution containing m Na 2 S and m Na 2 SO 3, visible light overhead irradiation with a 300 W Xe lamp through a band pass filter. lower with increasing Se 2 content in the LTCS 1-x Se x O, reflecting changes in the relative rates of particle growth. Specifically, LTCS 0.8 Se 0.2 O, LTCS 0.6 Se 0.4 O, and LTCS 0.4 Se 0.6 O calcined at 1223, 1173, and 1123 K, respectively, exhibited H 2 evolution rates several times higher than those of a sample calcined at 1273 K under visible light irradiation. However, specimens calcined at lower temperatures were found to have lower activities, presumably because of insufficient progress of the solid-state synthesis reactions. This study highlights the importance of controlling the particle size of narrow band gap photocatalysts such as LTCS 1-x Se x O solid solutions when attempting to improve the H 2 evolution activity of such materials while harvesting longer wavelength photons. 4. Experimental Section LTCS 1-x Se x O solid solutions (0 x 0.6) were prepared by solid-state reactions in sealed quartz tubes, employing a method described in the previous report. [20] Briefly, precursors were prepared by mixing La 2 O 3 (Wako, 99.99%), La 2 S 3 (High Purity Chemicals, 99.9%), TiO 2 (rutile, Rare Metallic Co., Ltd., 99.99%), Cu 2 S (Kojundo Chemical Laboratory Co., Ltd., 99%), S (Kojundo Chemical Laboratory Co., Ltd., 99%), La (Rare Metallic Co., Ltd., 20 mesh, 99.9%), Se (grain size 2 3 mm, Nilaco Corp., 99.99%), and Cu 2 Se (Kojundo Chemical Laboratory Co., Ltd., 99.9%) in a nitrogen-filled glove box. The La 2 O 3 and TiO 2 were both freshly calcined at 1273 K for 1 h prior to use to eliminate moisture and CO 2. In the case of the S and Se, a 5% excess relative to the required stoichiometric amount was added to the precursor mixtures to suppress the formation of S and Se defects in the products. The precursor mixtures were sealed in evacuated quartz tubes and heated from room temperature to 473 K over 9 min, from 473 to 673 K over 100 min and from 673 to K at 0.2 K min 1, then maintained at their respective temperatures for 48 h. After the completion of the calcination process, each sample was ground into powder. It should be noted that the material calcined at 1323 K exhibited white impurities on the sample edges, presumably as a result of reaction with the quartz container. These impurities were removed from the sintered sample before further use. The crystal structures of the LTCS 1-x Se x O specimens were examined using XRD (RINT-UltimaIII, Rigaku; Cu Kα) and refined by the Rietveld method, employing the RIETAN-FP program. [20,25] Light absorption properties were evaluated by DRS with a UV visible-infrared spectrometer (V-670, JASCO) equipped with an integration sphere. The morphologies of the synthesized samples were observed by fieldemission scanning electron microscopy (Hitachi, S-4700). H 2 evolution reactions were conducted in a Pyrex reaction vessel connected to a closed gas circulation system. In each trial, 0.20 g of LTCS 1-x Se x O powder was suspended in 150 ml of an aqueous solution containing m Na 2 S and m Na 2 SO 3, both of which acted as sacrificial electron donors. [26] Some LTCS 1-x Se x O samples were also modified with NiS and/or Pt cocatalysts. [24] NiS was loaded by the in situ precipitation of Ni(NO 3 ) 2, following our previously reported method. [27] Herein, the loading amount of the NiS catalyst is expressed in terms of the amount of metallic Ni. Pt was loaded via photodeposition using H 2 PtCl 6. In the case that both NiS and Pt cocatalysts were coloaded, LTCS 1-x Se x O samples were first modified with Pt, after which NiS was added to the Pt-loaded LTCS 1-x Se x O, using the in situ precipitation technique. Photocatalytic water splitting reactions were conducted in reaction solutions containing the LTCS 1-x Se x O sample and S 2 /SO 3 2 as sacrificial electron donors. The reaction system was evacuated several times prior to the reaction to remove air and subsequently filled with Ar (6 kpa). The sample was irradiated with visible light (λ > 420 nm) from the top of the reaction vessel using a Xe lamp (300 W, Cermax) in conjunction with a cut-off filter. The suspension was maintained at room temperature by a flow of cooling water during this irradiation. Apparent quantum yields (AQYs, φ) were determined by exposing the reaction solutions to monochromatic light (λ = 420 ± 10 nm, 460 ± 30 nm, 540 ± 30 nm, 600 ± 30 nm, 640 ± 30 nm, 660 ± 30 nm, 680 ± 30 nm, 700 ± 30 nm, or 720 ± 30 nm) from a Xe lamp (300 W, Cermax) after completion of an induction period under visible light irradiation (λ > 420 nm). The AQYs were calculated using the equation φ (%) = AR/ I 100 (1) where A, R, and I represent the number of electrons involved in the photocatalytic reaction (for H 2 evolution, A = 2), the rate of gas evolution, and the number of incident photons, respectively. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was financially supported by Grants-in-Aids for Scientific Research (A) (Grant Nos. 16H02417 and 17H01216) and for Young (4 of 5)

5 Scientists (A) (Grant No. 15H05494). S.N. is grateful to the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, for financial support during his stay in Tokyo and also wishes to thank Atsushi Furuki of the University of Tokyo for assisting in the synthesis of the LTCS x Se 1-x O solid solutions. Conflict of Interest The authors declare no conflict of interest. Keywords hydrogen evolution, oxysulfoselenide, particle size, semiconductors, visible-light photocatalysis Received: July 28, 2017 Revised: September 11, 2017 Published online: November 2, 2017 [1] T. Hisatomi, J. Kubota, K. Domen, Chem. Soc. Rev. 2014, 7, [2] T. Hisatomi, K. Domen, Faraday Discuss. 2017, 198, 11. [3] K. Maeda, K. Domen, J. Phys. Chem. C 2007, 111, [4] J. Li, N. Wu, Catal. Sci. Technol. 2015, 5, [5] K. Zhang, L. Guo, Catal. Sci. Technol. 2013, 3, [6] Y. Ham, T. Hisatomi, Y. Goto, Y. Moriya, Y. Sakata, A. Yamakata, J. Kubota, K. Domen, J. Mater. Chem. A 2016, 4, [7] K. Domen, J. N. Kondo, M. Hara, T. Takata, Bull. Chem. Soc. Jpn. 2000, 73, [8] F. E. Osterloh, Chem. Mater. 2008, 20, 35. [9] Y. Inoue, Energy Environ. Sci. 2009, 2, 364. [10] A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Am. Chem. Soc. 2002, 124, [11] A. Ishikawa, T. Takata, T. Matsumura, J. N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Phys. Chem. B 2004, 108, [12] K. Ogisu, A. Ishikawa, K. Teramura, K. Toda, M. Hara, K. Domen, Chem. Lett. 2007, 36, 854. [13] K. Ogisu, A. Ishikawa, Y. Shimodaira, T. Takata, H. Kobayashi, K. Domen, J. Phys. Chem. C 2008, 112, [14] T. Suzuki, T. Hisatomi, K. Teramura, Y. Shimodaira, H. Kobayashi, K. Domen, Phys. Chem. Chem. Phys. 2012, 40, [15] V. Meignen, L. Cario, A. Lafond, Y. Moëlo, C. Guillot-Deudon, A. Meerschaut, J. Solid State Chem. 2004, 177, [16] K. Sakata, T. Hisatomi, Y. Goto, B. Magyari-Köpe, P. Deák, T. Yamada, K. Domen, J. Phys. D: Appl. Phys. 2017, 50, [17] G. Ma, Y. Suzuki, R. B. Singh, A. Iwanaga, Y. Moriya, T. Minegishi, J. Liu, T. Hisatomi, H. Nishiyama, M. Katayama, K. Seki, A. Furube, T. Yamada, K. Domen, Chem. Sci. 2015, 6, [18] J. Liu, T. Hisatomi, M. Katayama, T. Minegishi, J. Kubota, K. Domen, J. Mater. Chem. A 2016, 4, [19] Y. Suzuki, R. B. Singh, H. Matsuzaki, A. Furube, G. Ma, T. Hisatomi, K. Domen, K. Seki, Chem. Phys. 2016, 476, 9. [20] S. Nandy, Y. Goto, T. Hisatomi, Y. Moriya, T. Minegishi, M. Katayama, K. Domen, ChemPhotoChem 2017, 1, 265. [21] S. Chen, X. G. Gong, S. Wei, Phys. Rev. B. 2007, 75, [22] J. Ge, Y. Yu, Y. Yan, J. Mater. Chem. A 2016, 4, [23] C.-H. Leung, L. H. Van Vlack, Metall. Trans. A 1981, 12A, 981. [24] S. Nandy, T. Hisatomi, G. Ma, T. Minegishi, M. Katayama, K. Domen, J. Mater. Chem. A 2017, 5, [25] F. Izumi, K. Momma, Solid State Phenom. 2007, 130, 15. [26] J. F. Reber, K. Meier, J. Phys. Chem. 1984, 88, [27] M. Tabata, K. Maeda, T. Ishihara, T. Minegishi, T. Takata, K. Domen, J. Phys. Chem. C. 2010, 114, (5 of 5)