Plate-like Sm2Ti2S2O5 Particles Prepared by a Flux-Assisted One

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Plate-Like SmTiSO Particles Prepared by a Flux-Assisted OneStep Synthesis for the Evolution of O from Aqueous Solutions by Both Photocatalytic and Photoelectrochemical Reactions 2

Guijun Ma, Yongbo Kuang, Dharmapura H. K. Murthy, Takashi Hisatomi, Jeongsuk Seo, Shanshan Chen, Hiroyuki Matsuzaki, Yohichi Suzuki, Masao Katayama, Tsutomu Minegishi, Kazuhiko Seki, Akihiro Furube, and Kazunari Domen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12087 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

Plate-Like Sm2Ti2S2O5 Particles Prepared by a Flux-Assisted One-Step Synthesis for the Evolution of O2 from Aqueous Solutions by Both Photocatalytic and Photoelectrochemical Reactions Guijun Maa,†, Yongbo Kuanga,‡, Dharmapura H. K. Murthyb, Takashi Hisatomia, Jeongsuk Seoa,#, Shanshan Chena, Hiroyuki Matsuzakib, Yohichi Suzukib, Masao Katayamaa, Tsutomu Minegishia, Kazuhiko Sekib, Akihiro Furubeb,c, Kazunari Domen*a

a) Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

b) National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan c) Department of Optical Science, Tokushima University, 2-1 Minamijosanjima-cho, Tokushima 770-8506, Japan †

Current affiliation: School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China

‡

Current affiliation: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

#

Current affiliation: Center for Energy & Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan

Corresponding Author 1

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Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Tel: (+81) -3-5841-1148, Fax: (+81) -3-5841-8838

E-mail address: [email protected] (K. Domen)

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Abstract Sm2Ti2S2O5 (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with applications to water splitting. In this work, plate-like STSO particles were synthesized through a flux-assisted one-step method at various temperatures. The activities of these materials during photocatalytic and photoelectrochemical O2 evolution from aqueous solutions were investigated. Single-phase STSO with a single crystal habit was produced at 923 K, which is approximately 200 K lower than the temperatures required for previously reported methods, such as solid-state reactions and thermal sulfurization under a H2S flow. The STSO sample synthesized at the optimal temperature exhibited an AQE of 1.3±0.2% at 420 nm during photocatalytic sacrificial O2 evolution. This efficiency is twice the values reported for specimens prepared using conventional methods. An STSO/Ti/Sn electrode fabricated by the particle transfer method generated a photoanodic current and evolved O2 by water oxidation with a Faradaic efficiency of approximately 70±7%. The synthesis temperature yielding the highest activity was lower for photocatalytic O2 evolution than for photoelectrochemical O2 evolution. This work demonstrates the applicability of the flux method to the synthesis of well-crystallized oxysulfides having various particle sizes and intended for different uses.

Keywords: Sm2Ti2S2O5; Flux synthesis; Photocatalysis, Photoelectrochemistry, Water splitting

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Introduction Certain oxysulfide semiconductor materials have attracted significant interest during research regarding photocatalytic (PC) and photoelectrochemical (PEC) water splitting, as their band structures are suitable for water reduction and oxidation under visible light.1-3 Among these, Sm2Ti2S2O5 (STSO), having a band gap energy of ca. 2.1 eV, promotes H2 and O2 evolution from aqueous solutions containing sacrificial electron donors and acceptors, respectively, in response to visible light irradiation.4-12 The authors’ group has previously reported that STSO can also function as a H2-evolution photocatalyst in Z-scheme overall water splitting under UV light in conjunction with a TiO2 photocatalyst.11 Oxysulfides are typically synthesized via the solid-state reaction (SSR) of oxide and sulfide precursors in an evacuated and sealed tube.13,14 In the case of STSO, a high temperature of approximately 1273 K and a long duration of at least a week are required for successful synthesis.4,15,16 In 2003, a two-step process was developed that produces STSO exhibiting a level of activity similar to that obtained from the SSR process.5 In this new method, an amorphous Sm2Ti2O7 precursor is initially prepared using a polymerized complex (PC) technique, and subsequently sulfurized under a flow of H2S to yield STSO. This method allows the synthesis of STSO under milder conditions (1 h at 1123 K) than are required by the conventional SSR method and thus has been frequently employed in research concerning STSO.5-11 Unfortunately, the sulfurization step in this process requires a very large excess of toxic H2S gas compared to the amount of S that is actually incorporated into the STSO product.

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Only about 3% of the sulfur in the H2S is incorporated into the STSO in a typical sulfurization process. The flux (or molten salt) method has been widely used in the synthesis of oxide, (oxy)nitride, and (oxy)chalcogenide photocatalyst materials as a means of improving the photocatalytic and PEC activity of these compounds during water splitting.12,17-20 Recently, the authors reported a flux-assisted one-step synthesis of STSO. This technique allows STSO to be obtained at a relatively low temperature and over a short reaction duration, and results in a product that exhibits higher photocatalytic activity than that synthesized by sulfurization.12 Visible light-driven Z-scheme water splitting into H2 and O2 was realized utilizing the resulting STSO photocatalyst as a H2-evolution photocatalyst in conjunction with a WO3 photocatalyst. The above suggests that STSO prepared by the flux method may also demonstrate high activity during the O2 evolution reaction from aqueous solution. Based on the above, the present work assessed the effects of synthesis temperature on the properties of STSO powders obtained by the flux-assisted one-step method. The photocatalytic O2 evolution activities of the STSO specimens thus obtained were subsequently evaluated, using Ag+ as an electron scavenger. In addition, PEC O2 evolution from an aqueous solution over STSO/Ti/Sn photoanodes was investigated. Factors affecting activity during the photocatalytic and PEC O2 evolution processes are discussed herein based on the results.

Experimental section

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Preparation of STSO powder Sm2O3 (99.9%, Wako Chemicals), TiO2 (rutile, 99.0%, Kanto Chemical), TiS2 (99%, Kojundo Chemical) and CsCl (99.9%, Wako Chemicals) were mixed at a molar ratio of 1:0.95:1.05:8 in a glove box filled with N2.12 Subsequently, 10 mol% sulfur powder was added to the precursors to obtain a sulfur-rich environment. The mixture was sealed in an evacuated quartz tube and calcined at 873–1273 K for 5 h, then cooled at 0.5 K min-1. The sintered samples were dispersed in water to dissolve the CsCl flux. The resulting STSO powder was reclaimed by filtration and then calcined in air at 573 K for 2 h.

Loading of cocatalysts on the STSO The STSO powder was loaded with IrO2, acting as an oxidation cocatalyst, in preparation for photocatalytic and PEC reactions. STSO (0.2 g) was dispersed in ethylene glycol (20 mL) followed by the addition of the necessary amount of an aqueous IrCl3 solution (2 wt% Ir with respect to STSO). The mixture was transferred into a glass vial and heated in a microwave reactor (Monowave 300, Anton Paar Company) at 423 K for 0.5 h.

Fabrication of STSO photoelectrodes STSO photoelectrodes were fabricated by a particle transfer (PT) method using Ti and Sn contact and conductor layers, respectively.21-23 The STSO powder was densely deposited on a glass plate. Following this, a Ti layer (0.5 μm) and a Sn layer (10 μm)

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were deposited sequentially on top of the STSO powder by thermal vapor deposition. During this process, the top layer of the STSO particles was embedded in the Ti film. The metal films were bonded to a second glass plate using double-sided tape and then peeled away from the primary glass plate, after which STSO particles physically attached to the second glass plate were removed by ultrasonication in water. Thus, photoelectrodes with the composition STSO/Ti/Sn/tape/glass plate were obtained.

Photocatalytic O2 evolution reactions A Pyrex overhead irradiation reaction vessel connected to a closed gas circulation system was used for reaction trials. Sacrificial O2 evolution reactions were carried out in an aqueous solution containing AgNO3 as an electron acceptor. Prior to each reaction, the solution was evacuated to completely remove dissolved air and then irradiated using a Xe lamp (300 W) equipped with an L-42 cut-off filter (λ > 420 nm). A flow of cooling water was used to maintain the reactant solution at room temperature. The evolved gases were analyzed using a gas chromatograph (thermal conductivity detector, molecular sieve 5 Å column and Ar carrier gas). The apparent quantum efficiency (AQE) was measured using the same apparatus described above. A 300 W xenon lamp fitted with a 420 nm band-pass filter was used for light irradiation. The number of photons reaching the solution was measured with a Si photodiode. Quantum efficiency (Φ) values were calculated using the equation Φ (%) = (AR/I) × 100%

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where A, R, and I represent the coefficient corresponding to the number of electrons to evolve an O2 molecule from water (four), the O2 evolution rate, and the rate of photons incident on the reactor, respectively. Here, Φ is referred to as the AQE because not absorbed photons but incident photons are considered. The error margins related to PC activity were given by the standard deviation of experimental data.

PEC assessments of STSO/Ti/Sn photoelectrodes Current-potential and current-time curves were acquired using a typical three-electrode system. A glass cell was filled with a 1 M aqueous H3BO3 solution and the pH of the electrolyte was adjusted to 12 by the addition of KOH. The valence band edge of STSO was estimated to be ca. 0.9 V (vs. NHE) but almost independent of pH.4 Therefore, the use of a basic electrolyte solution shifts the water oxidation potential more negative to the valence band edge of STSO and allows this material to evolve oxygen. Pt wire and a Ag/AgCl electrode were employed as the counter and reference electrodes during PEC measurements, respectively. A solar simulator (AM 1.5 G) was used to irradiate the photoanode sample immersed in the electrolyte solution through a flat window. The error margins related to PEC data were assumed to be 10%, respectively. The amounts of evolved H2 and O2 gases were measured with an on-line micro gas chromatograph (Agilent 3000A) and an Ar-filled three-electrode system. The electrolyte solution and light source used were the same with those described above. The Faradaic efficiency (ηF) was calculated by the equation

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ηF (%) = (AnF/Q) × 100% where A, n, F and Q represent the coefficient (4 for O2 evolution), the amount of O2 evolved (mol), the Faraday's constant (96485 C mol−1) and the total charge observed as a photocurrent in Coulomb, respectively. Since the STSO showed an anodic photocurrent, the half-cell solar-to-hydrogen (HC-STH) energy conversion efficiency was calculated using the equation HC − STH =

2𝑅O2 × Δ𝐺H° 2 O 𝑃sun × 𝑆

×

𝐸O° 2 /H2 O − 𝐸RHE 𝐸O° 2 /H2 O

where 𝑅O2 , Δ𝐺H° 2 O , 𝑃sun and 𝑆 represent the O2 evolution rate (mol s−1), Gibbs free energy of water splitting (237 kJ mol-1), the power of solar simulator (0.1 W cm-2) and the irradiation area of the photoelectrode, respectively. 𝐸𝑂° 2 /H2 O (1.23 V vs. RHE) and 𝐸RHE are the standard H2O oxidation potential and the electrode potential in the unit of V vs. RHE, respectively.

Characterization of materials UV-vis diffuse reflectance spectroscopy (DRS) data were recorded using a spectrophotometer (JASCO V-670) equipped with an integration sphere, employing a commercial material (Spectralon) as a reference. X-ray powder diffraction (XRD) patterns were acquired with a Rigaku MiniFlex 300 powder diffractometer. The morphology of each photocatalyst was examined using scanning electron microscopy (SEM, Hitachi SU8020), in conjunction with a silicon drift X-Ray detector (Horiba, X-max). The thickness of particles was evaluated from more than 10 particles observed in SEM images. Specific surface areas were calculated by applying the 9



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Brunauer-Emmett-Teller (BET) method to nitrogen absorption–desorption isotherms obtained with a dedicated apparatus (BELSORP-mini, BEL Japan). Transient absorption spectroscopy (TAS) was employed to investigate the charge carrier dynamics of the STSO. Nanosecond TAS was performed using the third harmonic of a 10 Hz Nd3+:YAG laser (Ekspla, SL311; λ = 1064 nm, pulse width ≈ 150 ps (full width at half maximum, FWHM)) acting as the pump light source. A Xe flash lamp (Hamamatsu, L7684) was used as the probe light source. The dynamics at specific wavelengths in the visible region were examined by passing diffusely reflected light from the sample through a monochromator (Spectral products, CM 110) to a Si photodetector (New Focus, 1601FS-AC). The signal from the detector was subsequently sent to a digital oscilloscope (LeCroy, WaveRunner 6200A). It should be noted that, during the present TAS experiments, the transient absorption (TA) signal was monitored in the diffuse reflection mode because the opaque photocatalyst powders did not allow the transmission of light. During these analyses, the powder samples were held in 1 mm path length quartz cuvettes. The TA intensity in the diffuse reflection mode is presented herein in units of percentage absorption, calculated as 100  (1-R/R0), where R and R0 are the intensities of the diffusely reflected light with and without excitation, respectively.

Results and discussion Preparation and characterization of STSO Fig. 1 shows the XRD patterns of the STSO powders synthesized at 873–1273 K.

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At 873 K (Fig. 1a), diffraction peaks attributable to the (004), (006) and (103) crystal planes of the STSO are evident, along with other peaks originating from impurities. These impurities are not attributable to any of the precursors used, which indicates that reactions occurred among the precursors themselves at this temperature to generate various intermediate species. Single-phase STSO was produced when the synthesis temperature was raised to 923 K (Fig. 1b), representing the lowest temperature yet reported for the formation of STSO. This low-temperature method effectively prevented the production of Sm2Ti2O7, an impurity that is readily generated during the synthesis of STSO, because amorphous Sm2Ti2O7 crystallizes at 1073 K.5 These patterns also demonstrate that pure STSO was obtained at temperatures from 923 to 1273 K. The full-width at half maximum (FWHM) values of the (103) peak decreases from 0.236° to 0.136° as the temperature increases from 923 to 1273 K. In addition, the peaks associated with the (004) and (006) crystal planes of the STSO are more intense than the (110) diffraction peak compared with the ratio observed in the reference pattern, suggesting preferential two-dimensional crystal growth along the (00l) planes during the crystallization process. The morphologies of STSO samples synthesized at the various temperatures were observed by SEM. As shown in Fig. 2, the STSO samples were composed of plate-like particles, reflecting the crystal structure of the material. With increases in the synthesis temperature from 923 to 1273 K, the thickness of the STSO particles increased from 22 ± 7 to 330 ± 100 nm, although the in-plane dimensions were largely unchanged. STSO has a layered crystal structure in which Sm2S2 slabs parallel

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to the a-b plane are separated by a two-dimensional network of corner sharing Ti(S,O)6 octahedra.5,15,16,24 It is believed that the crystallization of STSO starts along the a-b plane and that the anisotropic growth of crystallites is enhanced in a molten salt environment. At higher temperatures, the STSO particles were found to become thicker while their aspect ratio was reduced. Consequently, the sample heated at the highest temperature (1273 K) generated an XRD pattern similar to the simulated pattern shown in Figure 1. UV-vis DRS data for the STSO samples synthesized at various temperatures are shown in Fig. 3, along with the spectra of the precursors (Sm2O3, TiO2 and TiS2) for comparison. The STSO absorption edge is evidently located at approximately 600 nm. This wavelength is longer than the values for the Sm2O3 and TiO2 because of the contribution of the S 3p orbitals to the valence band.4 The STSO synthesized at 923 K does not exhibit an absorption shoulder that would suggest the presence of impurities or dopants, demonstrating that single-phase STSO was obtained at this temperature. As the synthesis temperature is raised, the absorption background at wavelengths longer than 600 nm becomes more intense, suggesting higher defect densities. The effect of the preparation temperature on the STSO carrier dynamics was assessed using nanosecond TAS. This technique has been widely applied to the examination of carrier dynamics in various types of photocatalytic materials and has also been employed to correlate photophysical parameters with photocatalytic efficiency.25-28 Fig. 4 summarizes the carrier dynamics upon 620 nm probing of STSO specimens prepared at 923 and 1223 K. To further understand the nature of the

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charges being probing at 620 nm, trials were also performed in the presence of methanol as a hole scavenger (Fig. S1). The addition of methanol caused the TA signal to decay more rapidly, suggesting that photogenerated holes were predominantly probed at 620 nm. Comparing the decay profiles of the samples prepared at 923 and 1223 K, it is clear that the material synthesized at 1223 K shows a longer hole lifetime. A similar result was obtained at a probe wavelength of 570 nm (Fig. S2). The nature of the charges probed at 620 nm was also analyzed by assessing the effect of the pump intensity on the decay behavior. Interestingly, neither of the STSO samples (prepared at 923 or 1223 K) showed any significant changes in time characteristics with increasing pump intensity (Fig. S3). This result demonstrates that the decay of holes via direct recombination with electrons was not the major decay pathway on the timescale of tens of nanoseconds in these experiments. This observation can be attributed to efficient electron deep trapping that in turn reduces the direct recombination of holes with trapped electrons. It is known that an appropriate defect density can suppress electron-hole recombination through trapping and detrapping of charge carriers.29 The decay dynamics of the STSO prepared at 1223 K suggest two decay pathways: a faster decay ( 80 ns). We are presently attempting to elucidate the origin of the rapid decay using ultrafast TAS and by studying the electronic nature of the STSO defect states. Nevertheless, the current nanosecond TAS results clearly indicate that the STSO preparation temperature has a significant effect on the hole lifetime, which in turn is expected to affect the efficiency of the water oxidation reaction.

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Photocatalytic water oxidation on particulate STSO Table 1 summarizes the activities of particulate STSO specimens synthesized at different temperatures during photocatalytic O2 production from an aqueous solution containing AgNO3 as a sacrificial electron acceptor. This table also provides BET surface areas for these samples. The activity evidently improved along with increases in the synthesis temperature, and the maximum O2 evolution rate of 83±8 µmol h-1 was obtained from the STSO synthesized at 1148 K. The AQE for this sample was 1.3±0.2% in response to monochromatic light at 420 nm. This level of activity is twice that reported for STSO prepared by the two-step PC-sulfurization process and loaded with the same cocatalyst.12 Time profiles of the photocatalytic O2 evolution over STSO synthesized from 923 to 1273 K were shown in Fig. S4. A control experiment (Fig. S4-g) showed that no O2 was detected in 5 h in the absence of light, which excluded the possibility of catalytic O2 evolution from aqueous AgNO3 solution on IrO2. As shown in Fig. 1, the crystallinity of the STSO was enhanced as the synthesis temperature was elevated, which increases photocatalytic reactions because the degree of crystallinity affects the migration of charge carriers. In addition, the recombination of photoexcited holes was suppressed upon increasing the synthesis temperature from 923 to 1223 K (Fig. 4). The prolonged lifetime of photogenerated holes is believed to favor the photocatalytic water oxidation reaction. However, the excessive growth of particles at high temperatures is detrimental because electrons must be scavenged by

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Ag+ ions adsorbed on the surface during the O2 evolution reaction. Therefore, the photocatalytic activity per unit surface area may be a good measure of the efficiency of a sacrificial reaction.30-32 The O2 production rates per unit surface area were calculated based on the BET surface areas and are listed in Table 1. The activity per surface area was improved upon raising the synthesis temperature from 923 to 1223 K, reflecting the increasing crystallinity of the STSO and the probability of survival of photoexcited holes. In contrast, the BET surface area decreased monotonically along with increases in the calcination temperature, such that the STSO samples synthesized above 1148 K exhibited lower activities. At 1273 K, the activity per surface area also decreased, presumably because the defect density became too great, as suggested by the DRS spectra, meaning that charge recombination was enhanced.

PEC water oxidation on STSO photoanodes The current-potential curves acquired under chopped solar light irradiation from photoanodes based on STSO powder synthesized at 1223 K (as a representative sample) are shown in Figure 5. The STSO electrodes generated an anodic photocurrent, indicating n-type semiconductor behavior, and this current ceased immediately at the point at which the illumination was stopped. The onset potential of the photoanodic current when using an IrO2-loaded STSO photoanode was ca. +0.64 V vs. RHE (VRHE), and the photocurrent reached 0.45 mA cm-2 under simulated sunlight at 1.23 VRHE. Fig. 6 summarizes the wavelength dependencies of the IPCE on the IrO2-loaded

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STSO photoanode, measured at 1.0 and 1.23 VRHE. At 1.23 VRHE, the IPCE decreased with increases in the irradiation wavelength, going from 6.9% at 420 nm to 0.1% at 600 nm. The onset wavelengths for the photocurrents were also found to closely coincide with the light absorption edge of the STSO, appearing at approximately 600 nm. These data demonstrate that background light absorption above 600 nm did not contribute to the oxidation photocurrent. Fig. 7 presents the current-time profiles of STSO photoelectrodes synthesized at different temperatures under simulated sunlight in an aqueous solution at pH 12. The maximum photocurrent was produced by the STSO synthesized at 1223 K, in accordance with the trend exhibited by the photocatalytic activity per surface area data in Table 1. The stability of the plate-like STSO samples in PEC water oxidation was comparable with other oxysulfides.33 In the case of PEC O2 evolution on a photoanode fabricated by the particle transfer method, a photocatalyst material with moderately large particle sizes is preferred. This is because a photocatalyst layer made of small particles absorbs incident light poorly as a result of the limited amount of the photocatalyst on the photoelectrode. A thick particle layer may absorb incident light sufficiently but suffers from grain boundary resistance. In contrast, in the case of the sacrificial O2 evolution reaction, photocatalysts with small particle sizes and high specific surface areas may be preferable because adsorption of the sacrificial reagent is essential. In addition, the use of small particles does not cause problems with respect to light absorption because light penetrating through or scattered by a particle can be absorbed by other particles in the solution. Because of the different particle

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size requirements for the PEC and PC O2 evolution reactions, the maximum photocurrent was obtained from STSO synthesized at 1223 K. This is higher than the temperature used to produce the STSO exhibiting the maximum PC activity during sacrificial O2 evolution (1148 K). O2 production over the photoelectrode made using STSO synthesized at 1223 K was assessed in an air-tight three-electrode system. This apparatus consisted of an IrO2/STSO/Ti/Sn working electrode, Ag/AgCl reference electrode and Cr 2O3-coated Pt wire as a counter electrode. Assuming a Faradaic efficiency of unity, one quarter of the total charge passing through the system as photoanodic current should theoretically correspond to the amount of O2 generated by PEC water splitting. A comparison of the amount of O2 generated over the IrO2/STSO/Ti/Sn photoanode at 1.0 VRHE and the amount calculated from the total charge is shown in Fig. 8. Based on these data, the Faradaic efficiency of the IrO2/STSO/Ti/Sn photoanode was 73±7% during PEC O2 production over 4 h. Taking the Faradaic efficiency into account, the HC-STH energy conversion efficiency during the initial hour of operation was calculated to be 0.017±0.002%. The low Faradic efficiency of this photoanode is attributed to the partial photooxidation of sulfide ions in the STSO, which is commonly observed in oxysulfide materials.12,33 However, the photoanodic current and the O2 evolution rate obtained in this work are still among the highest values yet achieved with oxysulfide photoanodes under simulated sunlight.33,34 These results therefore suggest that STSO may play an important role in the research of oxysulfides for application to the O2 evolution reaction.

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Conclusion STSO was synthesized via the calcination of Sm2O3, TiO2, TiS2 and CsCl in sealed, evacuated tubes at temperatures from 923 to 1273 K. The threshold temperature of 923 K for producing single-phase STSO in this work is approximately 200 K lower than that required for conventional methods based on sulfurization using H2S. Both crystallinity and photoexcited hole lifetimes improved upon increasing the synthesis temperature, while the specific surface area decreased. The temperature required to obtain STSO optimized for sacrificial PC O2 evolution was found to be lower than that needed to obtain STSO suited for PEC O2 evolution. An AQE of 1.3±0.2% (at 420 nm) was obtained during PC O2 evolution over the STSO powder synthesized at 1148 K, while the highest photocurrent was obtained over a photoanode containing STSO synthesized at 1223 K, in conjunction with a Faraday efficiency of approximately 70±7%, during PEC O2 evolution. This report suggests a useful method for the preparation of oxysulfide materials applicable to PC and PEC water oxidation.

Acknowledgements This work was financially supported by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO) and by Grants-in-Aid for Scientific Research (A) (No. 16H02417) and for Young Scientists (A) (No. 15H05494) from the Japan Society for the Promotion of Science (JSPS).

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Supporting Information Transient absorption spectra and time courses of photocatalytic O2 evolution for Sm2Ti2S2O5 samples.

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Zhang, F.; Maeda, K.; Takata, T.; Hisatomi, T.; Domen, K. Investigation of Cocatalsysts on Silver Modified Sm2Ti2S2O5 Photocatalyst for Water Reduction and Oxidation under Visible Irradiation. Catal. Today 2012, 185, 253–258.

10 Li, R.; Chen, Z.; Zhao, W.; Zhang, F.; Maeda, K.; Huang, B.; Shen, S.; Domen, K.; Li, C. Sulfurization-Assisted Cobalt Deposition on Sm2Ti2S2O5 Photocatalyst for Water Oxidation under Visible Light Irradiation. J. Phys. Chem. C 2013, 117, 376–382. 11 Zhao, W.; Maeda, K.; Zhang, F.; Hisatomi, T.; Domen, K. Effect of Post-treatments on the Photocatalytic Activity of Sm2Ti2S2O5 for the Hydrogen Evolution Reaction. Phys. Chem. Chem. Phys. 2014, 16, 12051–12056. 12 Ma, G.; Chen, S.; Kuang, Y.; Akiyama, S.; Hisatomi, T.; Nakabayashi, M.; Shibata, N.; Katayama, M.; Minegishi, T.; Domen, K. Visible Light-Driven Z-Scheme Water Splitting Using Oxysulfide H2 Evolution Photocatalysts. J. Phys. Chem. Lett. 2016, 7, 3892−3896.

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13 Guittard, M.; Benazeth, S.; Dugue, J.; Jaulmes, S.; Palazzi, M.; Laruelle, P.; Flahaut, J. Oxysulfides and Oxyselenides in Sheets, Formed by a Rare Earth Element and a Second Metal. J. Solid State Chem.1984, 51, 227−238. 14 Cody, J. A.; Ibers, J.A. Synthesis and Characterization of the New Rare-Eath/Transition-Metal Oxysulfides La6Ti2S8O5 and La4Ti3S4O8. J. Solid State Chem. 1995, 114, 406−412. 15 Boyer, C.; Deudon, C.; Meerschaut, A. Synthesis and Structure Determination of the New Sm2Ti2O5S2 Compound. C.R. Acad. Sci. 2 (Se´ rie IIc) 1999, 2, 93–99 (Paris). 16 Goga, M.; Seshadri, R.; Ksenofontov, V.; Gütlich, P.; Tremel, W. Ln2Ti2S2O5 (Ln = Nd, Pr, Sm): A Novel Series of Defective Ruddlesden–Popper Phases. Chem. Commun. 1999, 979−980. 17 Ham, Y.; Hisatomi, T.; Goto, Y.; Moriya, Y.; Sakata, Y.; Yamakata, A.; Kubota, J.; Domen, K. Flux-Mediated Doping of SrTiO3 Potocatalysts for Efficient Overall Water Splitting. J. Mater. Chem. A 2016, 4, 3027−3033. 18 Boltersdorf, J.; Sullivan, I.; Shelton, T. L.; Wu, Z.; Gray, M.; Zoellner, B.; Osterloh, F. E.; Maggard, P. A. Flux Synthesis, Optical and Photocatalytic Properties of n-type Sn2TiO4: Hydrogen and no Evolution under Visible Light. Chem. Mater. 2016, 28, 8876–8889. 19 Mu, L.; Zhao, Y.; Li, A.; Wang, S.; Wang, Z.; Yang, J.; Wang, Y.; Liu, T.; Chen, R.; Zhu, J. et al. Enhancing Charge Separation on High Symmetry SrTiO3

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Exposed with Anisotropic Facets for Photocatalytic Water Splitting. Energy Environ. Sci. 2016, 9, 2463–2469. 20 Goto, Y.; Minegishi, T.; Kageshima, Y.; Higashi, T.; Kaneko, H.; Kuang, Y.; Nakabayashi, M.; Shibata, N.; Ishihara, H.; Hayashi, T. et al. A Particulate (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 Photocathode Modified with CdS and ZnS for Sunlight-Driven Overall Water Splitting. J. Mater. Chem. A, 2017, DOI: 10.1039/c7ta06663e. 21 Minegishi, T.; Nishimura, N.; Kubota, J.; Domen, K. Photoelectrochemical Properties of LaTiO2N Electrodes Prepared by Particle Transfer for Sunlight-Driven Water Splitting. Chem. Sci. 2013, 4, 1120–1124. 22 Kuang, Y.; Jia, Q.; Ma, G.; Hisatomi, T.; Minegishi, T.; Nishiyama, H.; Nakabayashi, M.; Shibata, N.; Yamada, T.; Kudo, A. et al. Ultrastable Low-Bias Water Splitting Photoanodes via Photocorrosion Inhibition and in situ Catalyst Regeneration. Nat. Energy 2016, 2, 16191. 23 Ma, G.; Liu, J.; Hisatomi, T.; Minegishi, T.; Moriya, Y.; Iwase, M.; Nishiyama, H.; Katayama, M.; Yamada, T.; Domen, K. Site-Selective Photodeposition of Pt on A Particulate Sc-La5Ti2CuS5O7 Photocathode: Evidence for One-Dimensional Charge Transfer. Chem. Commun. 2015, 51, 4302–4305. 24 Yashima, M.; Ogisu, K.; Domen, K. Structure and Electron Density of Oxysulfide Sm2Ti2S2O4.9, A Visible-Light-Responsive Photocatalyst. Acta Cryst. 2008, B64, 291–298.

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25 Furube, A.; Asahi, T.; Masuhara, H.; Yamashita, H.; Anpo, M. Charge Carrier Dynamics of Standard TiO2 Catalysts Revealed by Femtosecond Diffuse Reflectance Spectroscopy. J. Phys. Chem. B. 1999, 103, 3120−3127. 26 Furube, A.; Wang, Z. S.; Sunahara, K.; Hara, K.; Katoh, R.; Tachiya, M. Femtosecond Diffuse Reflectance Transient Absorption for Dye-Sensitized Solar Cells under Operational Conditions: Effect of Electrolyte on Electron Injection. J. Am. Chem. Soc. 2010, 132, 6614−6615. 27 Furube, A.; Maeda, K.; Domen, K. Transient Absorption Study on Photogenerated Carrier Dynamics in Visible Light Responsive Photocatalysts GaN:ZnO. Proc. SPIE 8109, Solar Hydrogen Nanotechnol. VI 2011, 810904. 28 Singh, R. B.; Matsuzaki, H.; Suzuki, Y.; Seki, K.; Minegishi, T.; Hisatomi, T.; Domen, K.; Furube, A. Trapped State Sensitive Kinetics in LaTiO2N Solid Photocatalyst with and without Cocatalyst Loading. J. Am. Chem. Soc. 2014, 136, 17324. 29 Suzuki, Y.; Murthy, D. H. K.; Matsuzaki, H.; Furube, A.; Wang, Q.; Hisatomi, T.; Domen, K.; Seki, K. Rational Interpretation of Correlated Kinetics of Mobile and Trapped Charge Carriers: Analysis of Ultrafast Carrier Dynamics in BiVO4. J. Phys. Chem. C 2017, 121, 19044−19052. 30 Agrios, A. G.; Pichat, P. Recombination Rate of Photogenerated Charges Versus Surface Area: Opposing Effects of TiO2 Sintering Temperature on Photocatalytic Removal of Phenol, Anisole, and Pyridine in Water. J. Photochem. Photobio. A: Chem. 2006, 180, 130−135.

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31 Zhang, J.; Xu, Q.; Feng, Z.; Li, M.; Li, C. Importance of the Relationship between Surface Phases and Photocatalytic Activity of TiO2. Angew. Chem. Int. Ed. 2008, 47, 1766−1769. 32 Yan, J.; Wu, G.; Guan, N.; Li, L.; Li, Z.; Cao, X. Understanding the Effect of Surface/Bulk Defects on the Photocatalytic Activity of TiO2: Anatase Versus Rutile. Phys. Chem. Chem. Phys. 2013, 15, 10978−10988. 33 Ma, G.; Suzuki, Y.; Singh, R. B.; Iwanaga, A.; Moriya, Y.; Minegishi, T.; Liu, J.; Hisatomi, T.; Nishiyama, H.; Katayama, M. et al. Photoanodic and Photocathodic Behaviour of La5Ti2CuS5O7 Electrodes in the Water Splitting Reaction. Chem. Sci., 2015, 6, 4513–4518. 34 Goto, Y.; Seo, J.; Kumamoto, K.; Hisatomi, T.; Mizuguchi, Y.; Kamihara, Y.; Katayama, M.; Minegishi, T.; Domen, K. Crystal Structure, Electronic Structure, and Photocatalytic Activity of Oxysulfides: La2Ta2ZrS2O8, La2Ta2TiS2O8, and La2Nb2TiS2O8. Inorg. Chem. 2016, 55, 3674–3679.

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Table/Figure/Scheme captions Table 1. Photocatalytic O2 Evolution Activities over STSO Specimens Synthesized at Temperatures from 923 to 1273 K. Figure 1. XRD patterns of STSO samples synthesized at (a) 873, (b) 923, (c) 998, (d) 1073, (e) 1148, (f) 1223 and (g) 1273 K. Figure 2. SEM images of STSO samples synthesized at (a) 923, (b) 998, (c) 1073, (d) 1148, (e) 1223 and (f) 1273 K. The insets show the thickness of each sample. The scale bars in the main and inset images are 2 µm and 300 nm, respectively. Figure 3. UV-vis DRS spectra of STSO samples synthesized at (a) 923, (b) 998, (c) 1073, (d) 1148, (e) 1223 and (f) 1273 K, and of the starting materials (Sm2O3, TiO2, and TiS2). Figure 4. Normalized nanosecond transient time profiles of STSO powders synthesized at (a) 923 and (b) 1223 K, probed at 620 nm (λexc = 355 nm). Figure 5. Current–potential curves acquired from STSO photoanodes modified with IrO2 (2 wt%), measured under chopped simulated sunlight in an aqueous solution of KBi (1 M, pH 12). The STSO was synthesized at 1223 K and the electrode potential was swept from positive to negative potentials at 10 mV s-1. Figure 6. Wavelength dependences of the IPCE values of an IrO2 (2 wt%)/STSO photoanode, measured at an electrode potential of 1.0 (red) and 1.23 (blue) V vs. RHE over the wavelength range from 420 to 640 nm. The STSO was synthesized at 1223 K and the measurements were carried out in an aqueous 1 M KBi solution at pH 12. The

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IPCEs are compared to the UV–vis DRS spectrum of the corresponding STSO photocatalyst. Figure 7. Current-time profiles acquired from STSO photoelectrodes synthesized at (a) 923, (b) 998, (c) 1073, (d) 1148, (e) 1223 and (f) 1273 K, measured at +1.0 V vs. RHE under simulated sunlight (AM 1.5) in an aqueous solution of KBi (1 M, pH 12). IrO2 (2 wt%) was loaded as an O2-evolution cocatalyst. Figure 8. PEC water oxidation under simulated sunlight irradiation on an IrO2 (2 wt%)/STSO photoanode (3.7 cm2) in an aqueous solution of KBi (1 M, pH 12). The STSO was synthesized at 1223 K and the electrode potential was +1.0 V vs. RHE.

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Table 1. Photocatalytic O2 Evolution Activities over STSO Specimens Synthesized at Temperatures from 923 to 1273 K.a

a

Temp.

Activityb

BET surface area

Activity per surface area

/K

/ µmol h-1

/ m2 g-1

/ µmol h-1 m-2

923

24 ± 4

9.5

25 ± 4

998

35 ± 5

8.4

42 ± 6

1073

62 ± 6

5.8

103 ± 10

1148

83 ± 8

4.0

208 ± 20

1223

53 ± 8

2.4

223 ± 33

1273

16 ± 5

1.0

157 ± 50

Reaction conditions: STSO: 0.1 g (IrO2 2 wt%); 0.01 M AgNO3 aqueous solution

(200 mL); La2O3 (0.20 g) was added as a pH buffer; light source, xenon lamp (300 W) equipped with a L-42 cutoff filter; reaction vessel, Pyrex top-irradiation type;

b

Estimated from the amount of gases evolved in first hour.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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TOC graphic

Plate-like Sm2Ti2S2O5 particles prepared by a flux-assisted one-step synthesis evolve O2 from aqueous solutions both photocatalytically and photoelectrochemically.

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Figure 1. XRD patterns of STSO samples synthesized at (a) 873, (b) 923, (c) 998, (d) 1073, (e) 1148, (f) 1223 and (g) 1273 K.



SEM images of STSO samples synthesized at (a) 923, (b) 998, (c) 1073, (d) 1148, (e) 1223 and (f) 1273 K. The insets show the thickness of each sample. The scale bars in the main and inset images are 2 µm and 300 nm, respectively. 67x70mm (300 x 300 DPI)


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UV-vis DRS spectra of STSO samples synthesized at (a) 923, (b) 998, (c) 1073, (d) 1148, (e) 1223 and (f) 1273 K, and of the starting materials (Sm2O3, TiO2, and TiS2). 173x135mm (300 x 300 DPI)



Normalized nanosecond transient time profiles of STSO powders synthesized at (a) 923 and (b) 1223 K, probed at 620 nm (λexc = 355 nm). 219x167mm (300 x 300 DPI)


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Current–potential curves acquired from STSO photoanodes modified with IrO2 (2 wt%), measured under chopped simulated sunlight in an aqueous solution of KBi (1 M, pH 12). The STSO was synthesized at 1223 K and the electrode potential was swept from positive to negative potentials at 10 mV s-1.



Figure 6. Wavelength dependences of the IPCE values of an IrO2 (2 wt%)/STSO photoanode, measured at an electrode potential of 1.0 (red) and 1.23 (blue) V vs. RHE over the wavelength range from 420 to 640 nm. The STSO was synthesized at 1223 K and the measurements were carried out in an aqueous 1 M KBi solution at pH 12. The IPCEs are compared to the UV–vis DRS spectrum of the corresponding STSO photocatalyst.


Page 42 of 45

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Current-time profiles acquired from STSO photoelectrodes synthesized at (a) 923, (b) 998, (c) 1073, (d) 1148, (e) 1223 and (f) 1273 K, measured at +1.0 V vs. RHE under simulated sunlight (AM 1.5) in an aqueous solution of KBi (1 M, pH 12). IrO2 (2 wt%) was loaded as an O2-evolution cocatalyst.



Figure 8. PEC water oxidation under simulated sunlight irradiation on an IrO2 (2 wt%)/STSO photoanode (3.7 cm2) in an aqueous solution of KBi (1 M, pH 12). The STSO was synthesized at 1223 K and the electrode potential was +1.0 V vs. RHE.


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224x173mm (300 x 300 DPI)


Plate-like Sm2Ti2S2O5 Particles Prepared by a Flux-Assisted One

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