Effects of Se Incorporation in La5Ti2CuS5O7 by ... - ACS Publications

Forum Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

www.acsami.org

Effects of Se Incorporation in La5Ti2CuS5O7 by Annealing on Physical Properties and Photocatalytic H2 Evolution Activity Swarnava Nandy,†,# Takashi Hisatomi,†,⊥ Song Sun,†,‡ Masao Katayama,† Tsutomu Minegishi,† and Kazunari Domen*,†,§ †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan National Synchrotron Radiation Laboratory, Collaborative Innovation Centre of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230029, China § Center for Energy & Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, Japan ‡

S Supporting Information *

ABSTRACT: Oxysulfoselenide semiconductor photocatalysts absorb light at longer wavelengths than the corresponding oxysulfides. However, the synthesis of oxysulfoselenides is challenging due to excessive particle growth and the limited availability of metal selenide precursors. In this study, a La5Ti2CuS5O7 (LTCSO) oxysulfide was annealed with Se powder in sealed, evacuated quartz tubes to obtain LTCSO:Se photocatalysts, and the properties of these materials were investigated. Se was found to be incorporated into the LTCSO upon heating at 973 K or higher, and the Se/(S + Se) ratio was increased to a maximum of 0.3 upon repeating the heat treatment twice. The addition of Se extended the absorption edge of the LTCSO and thus increased its photocatalytic H2 evolution activity at longer wavelength. Even so, the apparent quantum yield at shorter wavelengths was reduced, which is similar to the results obtained for La5Ti2Cu(S1−xSex)5O7 (LTCS1−xSexO) solid solutions. Overall water splitting was achieved by constructing photocatalyst sheets using LTCSO:Se and LTCS1−xSexO as hydrogen evolution photocatalysts and BiVO4 as an oxygen evolution photocatalyst. Heat treatment with Se is evidently an effective method for the transformation of oxysulfide photocatalysts to oxysulfoselenides that promote photocatalytic H2 evolution and have longer absorption edge wavelengths. KEYWORDS: oxysulfoselenide, thermal diffusion, visible light, hydrogen evolution, photocatalyst sheet

1. INTRODUCTION Photocatalytic water splitting has received significant attention as a means of producing renewable H2 that can subsequently be stored and used as a clean alternative to fossil fuels.1−12 Certain photocatalysts will split water to generate H2 and O2 with high quantum efficiencies,13−15 but most do so only under UV irradiation, which accounts for only 5% of solar energy. Thus, for the effective harvesting of solar energy, it is essential to extend the absorption edge wavelength of such photocatalysts into the visible light (400−800 nm) region. For this reason, throughout the last several decades, many narrow bandgap semiconducting materials have been studied as visible-lightresponsive photocatalysts. La5Ti2Cu(S1−xSex)5O7 (LTCS1−xSexO) solid solutions (0 ≤ x ≤ 1) are visible-light-driven photocatalysts capable of generating H2 from water containing sacrificial electron donors.16 These oxysulfoselenide materials absorb visible light at longer wavelengths than La5Ti2CuS5O7 (LTCSO) oxysulfides because the top of the valence band and the bottom of the conduction band of LTCSO are shifted negatively and positively, respectively, upon incorporating Se2− ions. However, the H2 evolution activity was decreased monotonically with increasing Se2− contents. The H2 evolution activity of these oxysulfoselenides can be improved by coloading NiS and Pt cocatalysts.17 The function of Pt and NiS was discussed using © XXXX American Chemical Society

LTCSO photoelectrodes. It was considered that Pt facilitated the hydrogen evolution reaction while NiS facilitated both the hydrogen evolution reaction and oxidation of sacrificial hole scavengers (S2− and SO32− present in the reaction solution). However, the resulting activity remains inferior to that of the original LTCSO, partly because the driving force for photocatalytic reactions is reduced as the bandgap is narrowed. Another challenge associated with these materials is the difficulty in synthesizing LTCS1−xSexO. The particle size of LTCS1−xSexO tends to be greater than that of LTCSO because metal selenides have lower melting points than metal sulfides.16 Excessive particle growth enhances charge recombination because photoexcited carriers have to migrate longer distances to reach surface active sites. In addition, unlike sulfides, transition metal selenides are not necessarily commercially available, and so metals sensitive to oxygen and moisture must be used. Therefore, the methods and conditions used to synthesize these catalysts must be improved. Our previous work demonstrated that LTCS1−xSexO specimens having smaller Special Issue: Artificial Photosynthesis: Harnessing Materials and Interfaces for Sustainable Fuels Received: February 17, 2018 Accepted: May 10, 2018

A

DOI: 10.1021/acsami.8b02909 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces

Figure 1. (A) XRD patterns and (B) DR spectra for (a) pristine LTCSO, LTCSO samples heated with Se at (b) 873, (c) 973, (d) 1073, and (e) 1273 K, (f) LTCSO heated without Se at 973 K, and (g) LTCS0.8Se0.2O solid solution (presented for comparison purposes). BVO was synthesized according to a previously published procedure,22 and photocatalyst sheets were prepared by the particle transfer method as described in our previous publication.23 In a typical fabrication procedure, an LTCSO:Se/Au/BVO photocatalyst sheet (composed of LTCSO:Se as the HEP and BVO as the OEP, both embedded in a Au layer), was prepared by dispersing LTCSO:Se (0.01 g) and BVO (0.01 g) in 2-propanol (500 μL) by ultrasonication for 30 min, after which the dispersion was deposited on a clean glass plate (3 × 3 cm). The glass plate coated with the suspension was subsequently allowed to dry overnight. A Au layer thicker than 800 nm was deposited onto the resulting photocatalyst sheet by vacuum evaporation (VFR-200 M/ERH, ULVAC KIKO, Inc.) at a rate of approximately 8−10 nm s−1 as a conductor layer to enable electron transfer from the OEP to the HEP. Another glass plate coated with adhesive carbon tape was then used to remove the Au layer holding the photocatalyst particles from the original glass plate, and any loosely bound particles on the top of the Au layer were removed by ultrasonication in distilled water. 2.2. Loading of Cocatalysts and Photocatalytic Reactions. H2 evolution reactions were conducted in a closed gas circulation system described in previous reports.16−18 In each trial, LTCSO:Se powder (0.20 g) was suspended in 150 mL of an aqueous solution containing 10 mM Na2S and 10 mM Na2SO3. These LTCSO:Se samples were loaded with Pt alone or both Pt and NiS as cocatalysts.17 Pt was loaded by in situ photodeposition using H 2 PtCl 6 as a precursor. Approximately 10 h were required to complete the photodeposition of the Pt and observe a stable H2 evolution rate. When both NiS and Pt cocatalysts were coloaded, the Pt was added first, after which the NiS was loaded onto the Pt-loaded LTCSO:Se by in situ precipitation using a Ni(NO3)2 aqueous solution as the Ni source. To investigate the overall water splitting reaction, a photocatalyst sheet sample (3 × 3 cm, effective area: 8 cm2) was placed at the bottom of the reactor, to which 40 mL of distilled water had been added, and gas evolution was evaluated using the same closed gas circulation system as employed during H2 evolution trials. NiS species was not used as a cocatalyst in overall water splitting because NiS would suffer from corrosion in pure water. Thus, prior to these reactions, shell/core-structured Cr2O3/Ru was deposited onto the photocatalyst sheet samples as a cocatalyst for overall water splitting.23 Ru species were initially photodeposited on the photocatalyst sheet from an aqueous solution containing RuCl3 (0.4 μmol) over a span of 3 h. Cr2O3 was subsequently photodeposited from an aqueous solution containing K2CrO4 (0.2 μmol) for 2 h so as to suppress reverse reactions involving molecular oxygen.24,25 Rh and Pt were also employed instead of Ru from RhCl3 and H2PtCl6, respectively, in the course of screening of the cocatalyst species. Each solution was evacuated prior to the reaction to completely remove air, after which Ar (6 kPa) was introduced into the reaction apparatus. The reaction solution was maintained at room temperature by a flow of cooling water and 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 cutoff filter.

particle sizes, obtained by lowering the calcination temperature, exhibited higher photocatalytic H2 evolution activity.18 On the basis of these prior results, we anticipated that further modifications of the synthesis procedures could lead to additional improvements in activity. In research involving chalcogenide thin films, sulfoselenides are often prepared by elemental substitution. As an example, Guo et al. reported the formation of Cu(In1−xGax)(S1−ySey)2 (CIGSSe) nanocrystalline films for solar cell applications by exposing Cu(In1−xGax)S2 nanocrystalline films to Se vapor.19 According to their study, it is possible to replace the majority of the S with Se and obtain a dense CIGSSe absorber film with fewer voids than in the pristine Cu(In1−xGax)S2 film. However, this method has not yet been applied to the synthesis of particulate oxysulfoselenide photocatalysts. Considering that the synthesis and photocatalytic activity of oxysulfides have been studied to a greater extent than those of oxysulfoselenides, the use of a highly crystalline oxysulfide as a starting material could allow the synthesis of active particulate oxysulfoselenide photocatalysts. On this basis, in the present work, LTCSO was annealed in the presence of Se to synthesize LTCS1−xSexO solid solutions, and the properties of the resulting Se-treated LTCSO (LTCSO:Se) specimens were investigated. Here, the chemical formula of LTCSO:Se was employed to stress the use of LTCSO as a starting material unlike our earlier works.16−18 Z-scheme overall water splitting under visible light was also examined using photocatalyst sheets composed of LTCSO:Se and LTCS1−xSexO as the H2 evolution photocatalysts (HEP), BiVO4 (BVO) as the oxygen evolution photocatalyst (OEP), and a thin Au layer as the charge transfer layer.

2. EXPERIMENTAL SECTION 2.1. Preparation of Photocatalyst Samples. LTCSO and LTCS0.8Se0.2O were synthesized by solid state reactions at 1273 K in sealed, evacuated quartz tubes.16,20 LTCSO:Se was prepared by annealing LTCSO (0.25 g) with elemental Se (0.043 g, particle size 2− 3 mm, Nilaco Corp., 99.99%) in sealed quartz tubes (8 mm inner diameter and 150 mm length) at temperatures in the range of 873− 1273 K for 2 h. The mass of Se in these procedures corresponded to approximately half the amount of S in the LTCSO. When heat treatments were repeated, a sample after a heat treatment was cooled to room temperature, taken out, and mixed with an equal amount of fresh Se powder inside a glovebox. Then, the mixture was sealed again in a clean and dry quartz ampule under vacuum and heated. For comparison purposes, Sm2Ti2S2O5 (STSO) was also prepared by a solid state reaction21 and subsequently treated with Se and S (Kojundo Chemical Laboratory Co., Ltd., 99%) in a sealed, evacuated quartz tube at 873 K for 2 h. B


Forum Article


Figure 2. (A) XRD patterns and (B) DR spectra for (a) the pristine LTCSO, LTCSO heated with Se at 973 K (b) one, (c) two, and (d) three times, and (e) an LTCS0.8Se0.2O solid solution (presented for comparison purposes).

Figure 3. SEM images of (a) pristine LTCSO, LTCSO:Se samples heated at (b) 873, (c) 973, (d) 1073, and (e) 1273 K, (f) LTCSO heated without Se at 973 K, and (g) LTCS0.8Se0.2O solid solution (presented for comparison purposes). The apparent quantum yield (AQY) was determined by exposing reaction solutions to monochromatic light from a Xe lamp (300 W, Cermax) equipped with a series of band-pass filters after an induction period under visible light irradiation (λ > 420 nm).16 The AQY was calculated using the equation

for the LTCS0.8Se0.2O solid solution. These results are attributed to the incorporation of only a portion of the Se in the reaction tube into the material, as discussed below. Figure 1B provides DRS data for the LTCSO:Se samples, which demonstrate that LTCSO:Se heated at 873 K and pristine LTCSO had similar absorption onsets. The XRD and DRS results indicate that a temperature of 873 K was too low to allow the substitution of Se2− ions for S2− ions in LTCSO. In contrast, the absorption onsets of LTCSO:Se samples heated at 973 K or higher were red-shifted to value in the range of 650 to 700 nm. The XRD patterns and DR spectra did not change significantly following processing at temperatures above 973 K, suggesting that the incorporation of Se reached a maximum at 973 K. As a control experiment, LTCSO was heated without Se at 973 K. In this case, no changes were observed in the XRD pattern or in the absorption onset compared with the original LTCSO. These results demonstrate that the incorporation of Se into the LTCSO was responsible for the changes in the material properties that were evident in the LTCSO:Se specimens, and that oxysulfoselenide materials were synthesized from the LTCSO by annealing in the presence of Se at 973 K. In an attempt to enhance the degree of Se incorporation, LTCSO was repeatedly heated with Se at 973 K, and XRD patterns and DRS data for LTCSO:Se samples treated once, twice and thrice are presented in Figures 2A and 2B, respectively. All samples generated XRD patterns similar to those for LTCSO as the major phase, although the diffraction peaks were shifted toward lower angles upon repeating the heat

AQY(%) = AR /I × 100 where A, R, and I represent the number of electrons involved in the photocatalytic reaction (for H2 evolution, A = 2), the rate of gas evolution, and the number of incident photons, respectively. 2.3. Characterization. The LTCSO:Se samples were characterized by X-ray powder diffraction (XRD; RINT-UltimaIII, Rigaku; Cu Kα) and diffuse-reflectance spectroscopy (DRS; V-670, JASCO). Morphologies were observed by field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4700). The elemental proportions of S and Se were determined using energy-dispersive X-ray spectroscopy (EDS, Horiba, EMAX7000) in the SEM system. Prior to EDS analysis, each specimen was heated under vacuum at 373 K for 30 min.

3. RESULTS AND DISCUSSION 3.1. Physical Properties of LTCSO:Se Samples. Figure 1A shows XRD patterns for the original LTCSO, of LTCSO:Se samples heated at different temperatures, and LTCS0.8Se0.2O. Magnified XRD patterns for these samples are also presented in Figure S1. The latter two types of material generated patterns similar to those for LTCSO, and LTCSO:Se heated at 873 K exhibited very little peak shifting compared to pristine LTCSO. In the case of the LTCSO:Se samples heated at temperatures at or above 973 K, the diffraction peaks were shifted to lower angles, although to a lesser extent than was seen in the pattern C


Forum Article


of Se and S and the thermodynamic stability of the LTCS1−xSexO phase at a given temperature. The data appear to show that the most thermodynamically stable composition did not change significantly over the temperature range from 973 to 1273 K. However, further studies are needed to clarify the factors governing the Se/(Se + S) ratio and to determine if LTCSO:Se samples with higher Se levels can be obtained by this method. 3.2. Photocatalytic Activity of LTCSO:Se and LTCSO:Se/Au/BVO Photocatalyst Sheets. Figure 4 summa-

treatment, as shown in the magnified XRD patterns (Figure S2). The DR spectra of the LTCSO:Se samples heated at 973 K twice and thrice show absorption onsets at longer wavelengths than in the case of the sample heated only once and the LTCS0.8Se0.2O solid solution, suggesting that larger amounts of Se were incorporated into these materials upon repeated heating. Figures 3 and S3 show SEM images of the LTCSO:Se samples. The morphology of the original micrometer-sized, rod-like LTCSO particles was found to be largely unchanged by the heat treatment in each case. However, small grains were observed on the surface of the LTCSO:Se sample heated at 873 K, suggesting that unreacted Se had precipitated on the rod-like LTCSO particles. At higher temperatures, there were fewer small grains of unreacted Se, and the LTCSO:Se particles appeared to be smoother. The length of the rod-like LTCSO particles also tended to increase with increasing treatment temperature. The lengths of mature particles were estimated (on the basis of 50 particles) from the SEM images to be 3.4 ± 1.9 μm for the pristine LTCSO, 3.6 ± 1.7, 4.5 ± 1.8, 5.2 ± 2.0, and 6.4 ± 2.6 μm for the LTCSO:Se samples treated at 873, 973, 1073, and 1273 K once, 5.0 ± 1.6 and 5.6 ± 1.6 μm for the LTCSO:Se samples treated at 973 K twice and thrice, and 5.9 ± 2.8 μm for the LTCS0.8Se0.2O solid solution. The Se levels in the LTCSO:Se samples were evaluated by SEM-EDS, and the results are presented in Table 1. The

Figure 4. Photocatalytic H2 evolution activity of (a) original LTCSO, (b) LTCSO heated at 973 K without Se, LTCSO:Se samples heated one time at (c) 873, (d) 973, (e) 1073, and (f) 1273 K, (g) two times at 973 K, and (h) three times at 973 K, and (i) LTCS0.8Se0.2O. Reaction conditions: 0.2 g of photocatalyst loaded with Pt (2.0 wt %) and/or NiS (1.0 wt % for samples a and b and 0.5 wt % for samples c− i), 150 mL of 10 mM Na2S and 10 mM Na2SO3 aq. solution, overhead visible light irradiation with a 300 W Xe lamp through a cutoff filter (λ > 420 nm).

Table 1. Compositions of LTCSO:Se Samples as Determined by SEM-EDS sample LTCSO LTCSO:Se LTCSO:Se LTCSO:Se LTCSO:Se LTCSO:Se LTCSO:Se LTCS0.8Se0.2O

Se-treatment temperature (K) 873 973 1073 1273 973 973

Setreatment times 1 1 1 1 2 3

S/Cu 4.9 4.8 4.3 4.3 4.3 3.7 3.6 3.8

Se/Cu Se/(S + Se) 0.0 0.1 0.7 0.7 0.7 1.3 1.3 0.9

0.0 0.0 0.1 0.1 0.1 0.3 0.3 0.2

rizes the photocatalytic H2 evolution activity of LTCSO:Se samples modified with Pt and NiS in aqueous solutions containing sacrificial electron donors. Coloading with NiS in addition to Pt effectively enhanced the H2 evolution rate regardless of whether the samples had been heated with Se or not. These data show that the H2 evolution activity of LTCSO (Figure 4a) remained unchanged when it was heated without Se at 973 K (Figure 4b). Also, the LTCSO:Se specimen heated at 873 K (Figure 4c) showed comparable activity to the pristine LTCSO, confirming that Se was not incorporated at this relatively low temperature. The H2 evolution activity of LTCSO:Se samples annealed at temperatures above 873 K was found to decrease, such that the activity of LTCSO:Se heated once at 973 K was decreased by 30% compared to the original LTCSO. This effect is attributed to the narrowing of the band gap, because a decrease in the H2 evolution rate with increasing Se/(Se + S) ratios has also been observed for LTCS1−xSexO solid solutions synthesized at optimum temperatures.18 In addition, the H2 evolution activity of LTCSO:Se decreased monotonically as the heating temperature increased, even though the composition as determined by SEM-EDS was unchanged within the accuracy of the measurement technique. This loss of activity may have been be due to the growth of LTCSO:Se particles at higher temperatures, which restrict the migration of excited charge carriers to surface active sites.161718 Figure 5 shows the action spectra, which demonstrate the effect of wavelength on the AQY values for LTCSO:Se samples prepared at 973 K and modified with Pt and NiS. The AQY at 420 nm was decreased by repeating the Se treatment, from

amount of Se in the LTCSO:Se sample treated at 873 K was virtually negligible, while the Se/(Se + S) ratio in LTCSO:Se increased to 0.1 (based on Vegard’s law), following treatment at 973 K, with no further increases at higher temperatures. The compositions of the samples as determined by SEM-EDS were consistent with the diffraction peak angles in the corresponding XRD patterns as well as the absorption edge wavelengths estimated from DRS data.16 The absorption edge wavelengths for these samples were all approximately 690 nm, which was in good agreement with the Se/(Se + S) ratio of 0.1 in the LTCS1−xSexO solid solution.16 These data also confirm that the incorporation of Se was enhanced by heating the LTCSO twice with Se at 973 K. However, it should be noted that all the Se added to the tubes was not fully incorporated into the LTCSO phase. The Se/(Se + S) ratio did not exceed 0.3 even after repeated treatments at temperatures from 973 to 1273 K. The incorporation of Se into the LTCSO evidently plateaued following heating at 973 K for 2 h. This is attributed to the presence of both Se and the S removed from the LTCSO (as a result of Se incorporation) in the same closed tube. Under these conditions, the upper limit of the Se content will be determined by thermodynamics, based on the vapor pressures D


Forum Article


strate that the diffusion of Se into oxysulfides during annealing is an effective route to the synthesis of narrow bandgap oxysulfoselenide materials from a variety of starting compounds. Figure 6 shows the water splitting activity of LTCSO/Au/ BVO, LTCS0.8Se0.2O/Au/BVO, and LTCSO:Se/Au/BVO

Figure 5. Action spectra of (a) pristine LTCSO, LTCSO heated with Se at 973 K (b) once and (c) twice, and (d) an LTCS0.8Se0.2O solid solution (presented for comparison purposes). Reaction conditions: 0.2 g photocatalyst loaded with 2.0 wt % Pt via photodeposition and 1.0 wt % (LTCSO) or 0.5 wt % (LTCSO:Se) NiS loaded via in situ precipitation, 150 mL 10 mM Na2S and 10 mM Na2SO3 aq. solution, overhead visible light irradiation with a 300 W Xe lamp through a cutoff filter (λ > 420 nm). Figure 6. Water-splitting activity of photocatalyst sheets having compositions (a) LTCSO/Au/BVO, (b, c) LTCSO:Se/Au/BVO, and (d) LTCS0.8Se0.2O/Au/BVO. The LTCSO:Se samples in panels b and c were treated with Se at 973 K once and twice, respectively. All samples were loaded with Ru and Cr2O3. Reaction conditions: distilled water (40 mL); light source, 300 W Xe lamp equipped with a cutoff filter (λ > 420 nm); irradiation area, 8 cm2.

1.3% for pristine LTCSO to 1.1% and 0.42% for LTCSO:Se samples heated once and twice, respectively. However, LTCSO:Se heated once was able to utilize visible light up to 700 nm, representing an increase of 50 nm compared to the original LTCSO, reflecting the red-shift of the light absorption onset. This extended light absorption would contribute to the extent of photocatalytic H2 evolution. The LTCSO:Se sample heated twice exhibited approximately 1.5 times higher AQY values than LTCS0.8Se0.2O, while utilizing visible light to a similar extent. Here, LTCS0.8Se0.2O was chosen as a reference because it had a similar Se content and an absorption edge wavelength to the LTCSO:Se sample. In fact, LTCS0.8Se0.2O had slightly a smaller Se content and shorter absorption edge wavelength. Therefore, underestimation of the activity of the reference was avoided. This enhancement of the activity is likely attributable to the difference in the particle sizes of the materials. A comparison of the SEM images in Figures 3c and 3f shows that the LTCSO:Se sample had smaller particles than LTCS0.8Se0.2O, which is to be expected as these materials were heated in the presence of Se at 973 and 1273 K, respectively. Optimizing the particle size of LTCS0.8Se0.2O has been shown to increase the activity of this material by as much as a factor of 3.18 Accordingly, we conclude that the higher activity of LTCSO:Se relative to LTCS0.8Se0.2O was within the range of the effect that could be obtained by reducing the particle size. Nevertheless, the present results demonstrate that the synthesis of LTCSO:Se via the heat treatment of LTCSO with Se is a viable approach to preparing active oxysulfoselenide photocatalysts. This represents an alternative to the standard one-step solid state reaction and suggests the possibility of synthesizing oxysulfoselenide materials from well-researched oxysulfide photocatalysts. This work also confirms that narrow band gap oxysulfoselenides can be prepared from Sm2Ti2S2O5 by calcination with Se. The XRD pattern for STSO:Se following treatment at 873 K (Figure S4A) exhibits peak shifted toward lower diffraction angles compared to the pristine STSO. In addition, the DR spectra indicate a significant extension of the absorption onset (to 650 nm) with respect to the original STSO, which had an absorption onset of 550 nm (Figure S4B). The absorption onset was unchanged following processing with S at 873 K, which rules out the possibility of either heat treatment or S loss being responsible for the redshifting of the absorption onset. These observations demon-

photocatalyst sheets modified with noble metals and Cr2O3 cocatalysts. In these trials, LTCSO:Se heated at 973 K either once and twice was applied. The evolution of gaseous H2 and O2 at the stoichiometric molar ratio of 2:1 was observed over these photocatalyst sheets, similar to the results obtained in our previous study.23 LTCSO:Se/Au/BVO photocatalyst sheets modified with Ru/Cr2O3 exhibited a somewhat higher water splitting rate than those modified with Rh/Cr2O3 and Pt/Cr2O3 (Figure S5) in our preliminary study. It is difficult to directly observe cocatalyst particles by transmittance electron microscopy because the fine structure of samples is destroyed during the processing for observation. Nevertheless, judging from the functionality to suppress backward reactions, it is believed that the cocatalyst species form a core−shell cocatalyst.24,25 The water splitting rates employing different HEPs decreased in the order of LTCSO > LTCSO:Se heated with Se once, LTCSO:Se heated with Se twice > LTCS0.8Se0.2O, reflecting the H2 evolution activities of the HEP materials. Although significant increases in the H2 evolution activities of these systems would be required for practical applications, the present results suggest the potential of oxysulfoselenide materials with narrower band gaps for use in unassisted photocatalytic water splitting driven by long-wavelength visible light.

4. CONCLUSION LTCSO:Se oxysulfoselenide materials were synthesized by treating an LTCSO oxysulfide with Se at 973 K or higher in sealed, evacuated tubes. Shifts in XRD peaks toward lower angles and the extension of the absorption onset toward longer wavelengths, as determined by DRS, indicate the formation of the desired oxysulfoselenide solid solutions. The Se/(Se + S) ratio in the LTCSO:Se could be increased up to a maximum value of 0.3. These materials were able to utilize visible light at longer wavelengths than pristine LTCSO during photocatalytic H2 evolution, and also functioned as the HEP in a photocatalyst sheet system during unassisted water splitting. The LTCSO:Se E


Forum Article


(5) Zhang, K.; Guo, L. Metal Sulphide Semiconductors for Photocatalytic Hydrogen Production. Catal. Sci. Technol. 2013, 3, 1672−1690. (6) Maeda, K. Z-Scheme Water Splitting Using Two Different Semiconductor Photocatalysts. ACS Catal. 2013, 3, 1486−1503. (7) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520−7535. (8) Li, J.; Wu, N. Semiconductor-based Photocatalysts and Photoelectrochemical Cells for Solar Fuel Generation: A Review. Catal. Sci. Technol. 2015, 5, 1360−1384. (9) Lewis, N. S. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351, aad1920. (10) Qureshi, M.; Takanabe, K. Insights on Measuring and Reporting Heterogeneous Photocatalysis: Efficiency Definitions and Setup Examples. Chem. Mater. 2017, 29, 158−167. (11) Hisatomi, T.; Domen, K. Introductory Lecture: Sunlight-Driven Water Splitting and Carbon Dioxide Reduction by Heterogeneous Semiconductor Systems as Key Processes in Artificial Photosynthesis. Faraday Discuss. 2017, 198, 11−35. (12) Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D. J.; Higashi, M.; Kong, D.; Abe, R.; Tang, J. Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems. Chem. Rev. 2018, DOI: 10.1021/acs.chemrev.7b00286. (13) Ham, Y.; Hisatomi, T.; Goto, Y.; Moriya, Y.; Sakata, Y.; Yamakata, A.; Kubota, J.; Domen, K. Flux-Mediated Doping of SrTiO3 Photocatalysts for Efficient Overall Water Splitting. J. Mater. Chem. A 2016, 4, 3027−3033. (14) Kato, H.; Asakura, K.; Kudo, A. Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure. J. Am. Chem. Soc. 2003, 125, 3082−3089. (15) Sakata, Y.; Hayashi, T.; Yasunaga, R.; Yanaga, N.; Imamura, H. Remarkably High Apparent Quantum Yield of the Overall Photocatalytic H2O Splitting Achieved by Utilizing Zn Ion Added Ga2O3 Prepared using Dilute CaCl2 Solution. Chem. Commun. 2015, 51, 12935−12938. (16) Nandy, S.; Goto, Y.; Hisatomi, T.; Moriya, Y.; Minegishi, T.; Katayama, M.; Domen, K. Synthesis and Photocatalytic activity of La5Ti2Cu(S1‑xSex)5O7 Solid Solutions for H2 Production under Visible Light Irradiation. ChemPhotoChem. 2017, 1, 265−272. (17) Nandy, S.; Hisatomi, T.; Ma, G.; Minegishi, T.; Katayama, M.; Domen, K. Enhancement of the H2 Evolution Activity of La5Ti2Cu(S1‑xSex)5O7 Photocatalysts by Coloading Pt and NiS Cocatalysts. J. Mater. Chem. A 2017, 5, 6106−6112. (18) Nandy, S.; Hisatomi, T.; Katayama, M.; Minegishi, T.; Domen, K. Effects of Calcination Temperature on the Physical Properties and Hydrogen Evolution Activities of La5Ti2Cu(S1‑xSex)5O7 Photocatalysts. Part. Part. Syst. Charact. 2018, 35, 1700275. (19) Guo, Q.; Ford, G. M.; Hillhouse, H. W.; Agrawal, R. Sulfide Nanocrystal Inks for Dense Cu(In1‑xGax)(S1‑ySey)2 Absorber Films and Their Photovoltaic Performance. Nano Lett. 2009, 9, 3060−3065. (20) Meignen, V.; Cario, L.; Lafond, A.; Moëlo, Y.; Guillot-Deudon, C.; Meerschaut, A. Crystal Structures of Two New Oxysulfides La5Ti2MS5O7 (M=Cu, Ag): Evidence of Anionic Segregation. J. Solid State Chem. 2004, 177, 2810−2817. (21) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Oxysulfide Sm2Ti2S2O5 as a Stable Photocatalyst for Water Oxidation and Reduction under Visible Light Irradiation (λ ≤ 650 nm). J. Am. Chem. Soc. 2002, 124, 13547−13553. (22) Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. J. Am. Chem. Soc. 2011, 133, 11054−11057. (23) Sun, S.; Hisatomi, T.; Wang, Q.; Chen, S.; Ma, G.; Liu, J.; Nandy, S.; Minegishi, T.; Katayama, M.; Domen, K. Efficient RedoxMediator-Free Z-scheme Water Splitting Employing Oxysulfide Photocatalysts under Visible Light. ACS Catal. 2018, 8, 1690−1696.

specimens did not exhibit significantly improved H2 evolution activity compared with a conventional LTCS0.8Se0.2O solid solution, taking the difference in particle sizes into account. However, heat treatment with Se as demonstrated in this study could be applicable to other oxysulfides and offers a facile means of preparing narrow band gap oxysulfoselenide materials without the use of pure metals or metal sulfides that are difficult to handle. Accordingly, this work is expected to promote the development of photocatalytic materials and systems based on oxysulfoselenides intended to harvest long-wavelength visible light.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02909. XRD patterns and DR spectra of synthesized materials and water splitting activity of photocatalyst sheets loaded with different cocatalyst species (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Swarnava Nandy: 0000-0001-5089-5840 Takashi Hisatomi: 0000-0002-5009-2383 Tsutomu Minegishi: 0000-0001-5043-7444 Kazunari Domen: 0000-0001-7995-4832 Present Addresses #

S.N.: Laboratory of Renewable Energy Science and Engineering, Institute of Mechanical Engineering, Swiss Federal Institute of Technology in Lausanne (EPFL), Station 9, Lausanne 1015, Switzerland. ⊥ T.H.: Center for Energy & Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, Japan. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by Grant-in-Aids for Scientific Research (A) (Nos. 16H02417 and 17H01216) and for Young Scientists (A) (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 to Dr. Guijun Ma of the University of Tokyo (currently at Shanghai Tech University) for providing the Sm2Ti2S2O5 powder.

■

REFERENCES

(1) Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 2008, 20, 35−54. (2) Inoue, Y. Photocatalytic Water Splitting by RuO2-loaded Metal Oxides and Nitrides with d0- and d10-related Electronic Configurations. Energy Environ. Sci. 2009, 2, 364−386. (3) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (4) Abe, R. Recent Progress on Photocatalytic and Photoelectrochemical Water Splitting under Visible Light Irradiation. J. Photochem. Photobiol., C 2010, 11, 179−209. F


Forum Article

ACS Applied Materials & Interfaces (24) Yoshida, M.; Takanabe, K.; Maeda, K.; Ishikawa, A.; Kubota, J.; Sakata, Y.; Ikezawa, Y.; Domen, K. Role and Function of Noble-Metal/ Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall Water Splitting Studied by Model Electrodes. J. Phys. Chem. C 2009, 113, 10151−10157. (25) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. Nobel-Metal/Cr2O3 Core/Shell Nanoparticles as a Cocatalyst for Photocatalytic Overall Water Splitting. Angew. Chem., Int. Ed. 2006, 45, 7806−7809.

G
