Au25-Loaded BaLa4Ti4O15 Water-Splitting Photocatalyst with

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Au25-Loaded BaLa4Ti4O15 Water-Splitting Photocatalyst with Enhanced Activity and Durability Produced Using New Chromium Oxide Shell Formation Method Wataru Kurashige,† Rina Kumazawa,† Daiki Ishii,† Rui Hayashi,† Yoshiki Niihori,† Sakiat Hossain,‡ Lakshmi V. Nair,† Tomoaki Takayama,† Akihide Iwase,†,‡ Seiji Yamazoe,§,∥ Tatsuya Tsukuda,§,∥ Akihiko Kudo,†,‡ and Yuichi Negishi*,†,‡ †

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162−8601, Japan ‡ Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278−8510, Japan § Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113−0033, Japan ∥ Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615−8520, Japan S Supporting Information *

ABSTRACT: We report herein remarkable improvement of activity and stability of an Au25-loaded BaLa4Ti4O15 watersplitting photocatalyst. We first examined the influence of refining the gold cocatalyst on the individual reactions over the BaLa4Ti4O15 photocatalyst in this water-splitting system. The results revealed that refining the gold cocatalyst accelerates not only the hydrogen generation reaction, but also oxygen photoreduction reaction, which suppresses the H2 generation via photoreduction of protons. This finding suggests that photocatalytic activity will be enhanced if the O2 photoreduction reaction can be selectively suppressed by covering Au25 with a Cr2O3 shell which is impermeable to O2 but permeable to H+. Then, we developed new method for the formation of the Cr2O3 shell onto Au25. Our method utilizes the strong metal−support interaction between them. Water-splitting photoactivity of Au25−BaLa4Ti4O15 was improved by 19 times under an optimized coverage of the Cr2O3 shell. The Cr2O3 shell also elongated the lifetime of the photocatalysts by preventing the agglomeration of Au25 cocatalysts.

1. INTRODUCTION A depletion of fossil fuels and global environmental problems are becoming increasingly serious. Thus, our current reliance on fossil fuels needs to be weakened by movement toward the use of clean and renewable energy sources. Because combustion of hydrogen (H2) releases large amounts of energy without any burden on the environment, H2 has attracted considerable interest as a new energy source to solve global issues concerning energy and the environment. In this regard, generation of clean, renewable H2 from water (H2O) by photocatalytic water splitting using solar energy (sunlight) has attracted much attention.1 Efficient semiconductor photocatalysts that promote the water-splitting reaction have been developed extensively in the past decade.2−23 These semiconductor photocatalysts are typically composed of a photocatalyst body and cocatalyst metal nanoparticles as the reaction sites (Figure 1a).2−23 Effective strategies to realize highly active photocatalysts include improvement of the semiconductor photocatalyst and/or the cocatalyst.17,18,23 © XXXX American Chemical Society

The activity of photocatalysts can be enhanced by decreasing the particle diameter and improving the dispersibility of cocatalysts.24−26 The particle size of cocatalysts can be readily controlled when presynthesized nanoparticles/clusters are used as precursors (Figure 1b).24−26 Gold (Au) nanoparticles/ clusters represent promising candidate cocatalysts for H2 generation (Figure 1a) similar to other noble metal cocatalysts such as platinum, rhodium, and ruthenium.27,28 Furthermore, atomically precise and systematic control of size29−37 and atomically precise doping of heteroatoms38−48 are possible for small (∼1 nm) Au clusters protected by thiolate. Such thiolateprotected metal clusters provide an opportunity to study the effects of the size and chemical composition of cocatalysts on the activity of photocatalysts. The molecular-level underSpecial Issue: Prashant V. Kamat Festschrift Received: January 5, 2018 Revised: February 7, 2018 Published: February 7, 2018 A

DOI: 10.1021/acs.jpcc.8b00151 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Schematic of photocatalytic water splitting using a onestep photoexcitation system. CB, conduction band; VB, valence band; Eg, band gap. (b) Schematic of highly regulated metal clusters supported on a photocatalyst surface.

standing of these effects will aid the development of highly active photocatalysts. With this goal in mind, we have been developing techniques to obtain precise Au and alloy clusters as the active sites of water-splitting photocatalysts.49,50 In our studies, we have focused on BaLa4Ti4O15 (Figure S1), one of the most active photocatalysts.51,52 We have already established a method to support size-controlled Au clusters (Aun) on BaLa4Ti4O15.49,50 Furthermore, we found that under flowing carbon dioxide (Figure S2), the Aun cluster-loaded BaLa4Ti4O15 (Aun− BaLa4Ti4O15) thus obtained exhibited higher photocatalytic activity than that of BaLa4Ti4O15 with 8−22 nm Au nanoparticles (denoted AuNP−BaLa4Ti4O15) loaded by a conventional photodeposition method, even though the Au content of Aun−BaLa4Ti4O15 was lower than that of AuNP− BaLa4Ti4O15.49 However, several problems must be resolved to obtain highly active and robust Aun−BaLa4Ti4O15 using this technique. The first problem we must tackle is to understand the effect of cocatalyst miniaturization on the efficiencies of individual reactions over Aun−BaLa4Ti4O15 during the water-splitting reaction (Figure S3).27,28 The second problem is improvement of the stability of the cocatalyst under light irradiation. When Aun−BaLa4Ti4O15 is continuously irradiated with light in water, the Aun clusters aggregate.49 Thus, it is important to suppress the aggregation of Aun clusters during light irradiation. The third obstacle is the need to develop a means to prevent the oxygen (O2) photoreduction reaction,27,28 which causes the depletion of the generation of H2 (Figure S3d). Domen and coworkers demonstrated that protection of the surface of the cocatalyst for reduction with a chromium(III) oxide (Cr2O3) shell dramatically suppresses the contact of the cocatalyst surface with O2 (Figure 2a).6,13,17,24−26,53−56 This observation suggests that the O2 photoreduction reaction over Aun− photocatalyst can be suppressed by forming a Cr2O3 shell on the Aun clusters while retaining their size. However, a Cr2O3 shell has conventionally been formed on the cocatalyst surface by photodeposition (Figure 2a). This photodeposition approach cannot be applied to Aun−BaLa4Ti4O15 because of the instability of Aun clusters under light irradiation, as described above. Thus, it is necessary to develop a new method to form a protective Cr2O3 shell on Aun−BaLa4Ti4O15.

Figure 2. Comparison of the procedures of Cr2O3 shell formation in (a) the literature54 and (b) this work.

The goal of this work wsas to produce an Aun−BaLa4Ti4O15 photocatalyst that shows both high activity and high stability by solving the three problems described above. A glutathionate (SG)-protected Au25 cluster (Au25(SG)18)29 was used as the cocatalyst precursor because its size-selective synthesis has been well established.57,58 We first examined the effect of refining the cocatalyst on the H2 production ability (Figure S3a) and O2 photoreduction reactivity (Figure S3d) of Au25−BaLa4Ti4O15 and AuNP−BaLa4Ti4O15. The results revealed that refining the cocatalyst accelerates not only the H2 production reaction, but also the O2 photoreduction reaction. To suppress the O2 photoreduction reaction, we developed a new method to form a Cr2O3 shell on Au25 while suppressing cluster agglomeration. The method we develop consists of three steps, formation of a Cr2O3 layer on BaLa4Ti4O15 by photodeposition, adsorption of Au25(SG)18 on the surface of the Cr2O3-coated BaLa4Ti4O15, and the removal of the SG ligands from Au25 by heating (Figure 2b). This approach solved both the problems of poor cluster stability under light irradiation and the occurrence of the O2 photoreduction reaction. As a result, we achieved the creation of a highly active and stable water-splitting photocatalyst that is about 19 times more active than Au25−BaLa4Ti4O15 without a Cr2O3 shell.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were commercially obtained and used without further purification. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O) was purchased from Tanaka Kikinzoku. Tetraoctylammonium bromide ((C8H17)4NBr), triphenylphosphine ((C6H5)3P), sodium tetrahydroborate (NaBH4 ), titanium(IV) tetrabutoxide (Ti(OC4H9)4), propylene glycol, citric acid, barium carbonate (BaCO3), glutathione (GSH), Cr2O3, chromium(VI) oxide B


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mL/min for the photodeposition of Cr2O3. The mixing ratio of K2CrO4 to BaLa4Ti4O15 was varied from 0.1 to 1.5 wt % Cr. Then, Au25(SG)18 was adsorbed on Cr2O3−BaLa4Ti4O15 with a content of 0.1 wt % Au according to the procedure used to prepare Au25−BaLa4Ti4O15 (Figure 2b). The actual amount of Au adsorbed on Cr2O3−BaLa4Ti4O15 was determined by ICPMS analysis of the aqueous solution after mixing. The obtained Au25(SG)18−Cr2O3−BaLa4Ti4O15 was calcined under reduced pressure (>1.0 × 10−1 Pa) at 300 °C for 2 h to give Au25− CrxOy−BaLa4Ti4O15 (Figure 2b). Au25−Cr2O3−BaLa4Ti4O15 was obtained by irradiating Au25−CrxOy−BaLa4Ti4O15 with a high-pressure Hg lamp (400 W) in aqueous solution for 1 h (Figure 2b). 2.3. Measurement of Photocatalytic Activity. In the following experiments, the evolved gases were analyzed by gas chromatography (Shimadzu GC-8A equipped with a thermal conductivity detector and MS-5A column; Ar carrier gas). Water Splitting. The photocatalytic water-splitting reaction was performed at room temperature using an experimental apparatus built in-house that consists of a high-pressure Hg lamp (400 W) and quartz cell (Figure S2). The reaction was conducted while flowing argon (Ar) gas at a rate of 30 mL/min. Before the measurements, the reaction solution consisting of prepared photocatalyst (500 mg) in water (350 mL) was purged with Ar gas for 1 h to ensure complete removal of air from the reaction vessel. Hydrogen Evolution Using a Sacrificial Reagent. In this experiment, the H2-evolution ability of a photocatalyst (Au25− BaLa4Ti4O15 or AuNP−BaLa4Ti4O15) was estimated using methanol as a sacrificial reagent (Figure 3a). Methanol solution (10%, 350 mL) containing photocatalyst (500 mg) was irradiated with a high-pressure Hg lamp (400 W) under an Ar flow of 30 mL/min at room temperature. Oxygen Photoreduction. In this experiment, the decrease of the quantity of evolved H2 was examined to investigate the

(CrO3), potassium chromate (K2CrO4), potassium dichromate (K2Cr2O7), sodium carbonate (Na2CO3), dichloromethane, bismuth (Bi) standard solution (100 ppm), and Au standard solution (1000 ppm) were obtained from Wako Pure Chemical Industries. Methanol, ethanol, toluene, acetone, hexane, and chloroform were purchased from Kanto Chemical. Pure Milli-Q water (18.2 MΩ·cm) was generated using a Merck Millipore Direct 3 UV system. 2.2. Synthesis. Au25(SG)18. Au25(SG)18 was synthesized by the conversion of (C6H5)3P-protected Aun clusters into Au 25 (SG)18 through a ligand-exchange reaction.57 The (C6H5)3P-protected Aun clusters were prepared by a procedure similar to the method reported by Jin and co-workers.59 Chloroform (75 mL) containing (C6H5)3P-protected Aun clusters (50.4 mg) was mixed with water (75 mL) containing GSH (1456.5 mg, 4.7 mmol). The mixture was heated under reflux at 60 °C for about 10 h. The aqueous phase was then separated from the chloroform phase using a separation funnel. High-purity Au25(SG)18 was obtained by removing excess GSH by ultrafiltration of the aqueous phase. BaLa4Ti4O15. BaLa4Ti4O15 photocatalyst was prepared by a polymerizable complex method.51,52 Briefly, Ti(OC4H9)4 (6.06 g, 17.8 mmol) and propylene glycol (60.9 g, 800 mmol) were added to ethanol (38 mL), and then the solution was heated to 70 °C. Citric acid (38.4 g, 200 mmol), La(NO3)3·6H2O (7.70 g, 17.8 mmol), and BaCO3 (0.878 g, 4.45 mmol) were sequentially added, and then the solution was heated at 120− 130 °C for 5 h. The obtained gray powder was transferred to an electric furnace and heated at 500 °C and then at 1100 °C. About 5.0 g of BaLa4Ti4O15 was obtained by this preparation method. Au25−BaLa4Ti4O15. First, Au25(SG)18 clusters were adsorbed on BaLa4Ti4O15 by mixing an aqueous solution containing Au25(SG)18 with an aqueous solution of BaLa4Ti4O15 (600 mg) for 2 h at room temperature (Figure 1b). The total volume of aqueous solution was fixed at 200 mL, and the mixing ratio of Au25(SG)18 clusters to BaLa4Ti4O15 was fixed at 0.1 wt % Au because it gave a photocatalyst that showed high activity.49 The actual amount of Au adsorbed on BaLa4Ti4O15 was determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis of each aqueous solution after mixing. The obtained Au25(SG)18−BaLa4Ti4O15 was calcined under reduced pressure (>1.0 × 10−1 Pa) at 300 °C for 2 h to give Au25−BaLa4Ti4O15 (Figure 1b). AuNP−BaLa4Ti4O15. AuNP−BaLa4Ti4O15 was prepared by a conventional photodeposition method. First, BaLa4Ti4O15 photocatalyst (600 mg) was added to aqueous HAuCl4 solution (350 mL). The mixture was stirred in a quartz cell at room temperature for 1 h and then irradiated from the inside of the quartz cell using a high-pressure mercury (Hg) lamp (400 W) for 1.5 h (Figure S2). During this process, Ar gas was flowed through the mixture at a rate of 30 mL/min. The Au content was fixed at 0.1 wt %. The actual amount of Au adsorbed on BaLa4Ti4O15 was determined by ICP-MS analysis of the aqueous solution after mixing. Au25−Cr2O3−BaLa4Ti4O15. First, Cr2O3 was loaded on BaLa4Ti4O15 through photodeposition (Figure 2b). In this process, BaLa4Ti4O15 photocatalyst (650 mg) was added to an aqueous K2CrO4 solution (350 mL) in a quartz cell. The mixture was stirred for 1 h at room temperature. The ICP-MS analysis showed that ∼70% of CrO42− was adsorbed on BaLa4Ti4O15.60,61 Then, the solution was irradiated with a highpressure Hg lamp (400 W) under Ar flowing at a rate of 30

Figure 3. Schematic of photocatalytic H2 evolution using methanol as a sacrificial reagent under a flow of (a) Ar gas (namely, without O2 in the reaction system) and (b) 7:3 mixture of Ar to air (namely, with O2 in the reaction system). C


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quartz and a 300 W Xe lamp (Asahi Spectra; MAX-302) with attached band-pass filters. The number of incident photons was determined using a photodiode (Ophir: PD300-UV head and NOVA power monitor). The photocatalyst powder (0.1 g) was dispersed in pure water (300 mL) to measure the apparent quantum yield.

likelihood of the O2 photoreduction reaction (Figure 3b). In particular, the H2-evolution ability (see the above section) of the photocatalysts (Au 25 −BaLa 4 Ti 4 O 15 and Au NP −BaLa4Ti4O15) was examined under a gas flow consisting of a 7:3 mixture of Ar to air instead of pure Ar. Under these experimental conditions, sufficient O2 is included in the reaction system for the O2 photoreduction reaction to occur. 2.4. Characterization. ICP-MS was conducted using an Agilent 7500c spectrometer (Agilent Technologies, Tokyo, Japan). Bi was used as an internal standard. The ICP-MS measurements were conducted for the supernatant obtained after mixing Au25(SG)18 with the photocatalyst to estimate the content of Au that was not adsorbed on the photocatalyst. The adsorption efficiency in each experiment was estimated on the basis of this value. Transmission electron microscopy (TEM) and highresolution (HR)-TEM images were recorded with a JEM2100 electron microscope (JEOL) operating at 200 kV, typically using a magnification of 400 000−600 000. Diffuse reflectance (DR) spectra were acquired at ambient temperature using a V-670 spectrometer (JASCO). X-ray photoelectron spectroscopy (XPS) analysis was performed using a JPS-9010MC electron spectrometer (JEOL) equipped with a chamber at a base pressure of ∼2 × 10−8 Torr. X-rays from the Mg Kα line (1253.6 eV) were used for excitation. Binding energies were corrected using the binding energy of Ti 2p3/2 in BaLa4Ti4O15, which was determined before this study. Au L3- and Cr K-edge X-ray absorption fine structure (XAFS) measurements were performed at beamline BL01B1 at the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (proposal number 2016B0910, 2016B1493, and 2016A1436). The incident X-ray beam was monochromatized by a Si(111) double-crystal monochromator. XAFS spectra of Cr2O3, NaCrO2, KCr3O8, K2CrO4, and CrO3 (Cr Kedge: Supporting Information), and Au foil (Au L3-edge) as references were recorded in transmission mode using ionization chambers. Au L3- and Cr K-edge XAFS spectra for photocatalyst samples were measured in fluorescence mode using a 19-element Ge solid-state detector at room temperature. The X-ray energy was calibrated for the Au L3- and Cr K-edges using Cu foil. The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were analyzed using the REX2000 Ver. 2.5.9 program (Rigaku) as follows. The χ spectra were extracted by subtracting the atomic absorption background by cubic spline interpolation and normalized to the edge height. The normalized data were used as XANES spectra. The k3-weighted χ spectra within the k range of 3.0−13.0 Å−1 for the Au L3-edge were Fourier transformed into r-space for structural analysis. The curve fitting analysis was carried out for Au−Au bonds over the r range of 1.9−3.0 Å in the Au L3-edge Fouriertransform (FT)-EXAFS spectra. In the curve fitting analysis, the phase shifts and backscattering amplitude functions of Au−Au were extracted from Au metal (ICSD No. 44362) using the FEFF8 program62 by setting σ2 = 0.0036 (σ is the Debye− Waller factor). This value did not markedly affect the phase shift and backscattering amplitude functions. X-ray diffraction (XRD) measurements were performed on a Rint2500 diffractometer (Rigaku) using Cu Kα radiation (λ = 1.5418 Å). A reflection-free silicon plate was used as a substrate. Apparent quantum yields51 at 270, 300, 320, 340, 400, and 440 nm were estimated using a top-irradiation cell made of

3. RESULTS AND DISCUSSION 3.1. Effect of Ultraminiaturization of the Cocatalyst on Individual Reactions over the Photocatalyst. First, we studied the effect of refining the Au-cluster cocatalyst on the individual reactions over the BaLa4Ti4O15 photocatalyst in water splitting (Figure S3). Au25−BaLa4Ti4O15 was prepared by calcination of Au25(SG)18-adsorbed BaLa4Ti4O15 at 300 °C for 2 h (Figure 1b; see Experimental Section). AuNP−BaLa4Ti4O15 was prepared by irradiating an aqueous solution containing HAuCl4 and BaLa4Ti4O15 with ultraviolet (UV) light (photodeposition method; see Experimental Section). Previous studies revealed that Au25−BaLa4Ti4O15 exhibits the highest watersplitting activity when the amount of Au clusters loaded is 0.1 wt %.49 Therefore, the amount of Au in Au25−BaLa4Ti4O15 was also set to 0.1 wt % in this study. Figure 4a,b show TEM images of Au25−BaLa4Ti4O15 and AuNP−BaLa4Ti4O15, respectively. In the TEM image of Au25−

Figure 4. TEM images and associated core-size distributions of (a) Au25−BaLa4Ti4O15 and (b) AuNP−BaLa4Ti4O15.

BaLa4Ti4O15 (Figure 4a), only particles with a size of about 1 nm were observed (average particle diameter: 1.1 ± 0.2 nm). This size corresponds to the particle diameter of an Au25 cluster,49,50 which indicates that the Au25 clusters were supported on the photocatalyst without agglomeration in Au25−BaLa 4 Ti4O 15 . TEM analysis of Au NP−BaLa 4Ti 4O 15 confirmed that particles with a size of 3 nm or larger (average particle diameter: 5.4 ± 2.8 nm) were supported on BaLa4Ti4O15 (Figure 4b). The water-splitting reaction was conducted by irradiating Au25−BaLa4Ti4O15 or AuNP−BaLa4Ti4O15 suspended in water with UV light from a high-pressure Hg lamp (see Experimental Section).49,50 Figure 5A shows bar graphs indicating the initial activity of the photocatalysts. The ratio of the generated H2 to O2 was nearly 2:1 irrespective of the photocatalyst, indicating that the water-splitting reaction proceeded stoichiometrically. Under flowing Ar, the water-splitting activity decreased a little upon refining the cocatalyst; that is, the photocatalytic activity of Au25−BaLa4Ti4O15 was slightly lower than that of AuNP− BaLa4Ti4O15. Next, we investigated the effect of refining the cocatalyst on individual reactions in the system. First, the H2 generation reaction (Figure S3a) was studied. When methanol was added D


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H2 production because this reaction also consumes the electrons in the conduction band (Figure S3d).27,28 Then, we next investigated the influence of refining the Au cocatalyst on the O2 photoreduction reaction. In this experiment, the photocatalyst was dispersed in 10% aqueous methanol solution and then a 7:3 mixture of Ar to air was flowed through the reaction system (Figure S2). Because a large amount of O2 was present in the system under these experimental conditions, the O2 photoreduction reaction proceeded on the cocatalyst under light irradiation (Figure 3b).27,28 As a result, the electrons in the conduction band were consumed and the total amount of H2 generated decreased. The amount of H2 generated under these conditions is shown in Figure 5B (see the bars labeled “with O2”). The amount of H2 produced upon O2 mixing decreased to about 48% of that without O2 for AuNP−BaLa4Ti4O15, whereas it lowered to about 24% of that without O2 for Au25−BaLa4Ti4O15. These results indicate that the refinement of the Au-cluster cocatalyst also accelerates the O2 photoreduction reaction. In summary, it was found that the refinement of the Aucluster cocatalyst accelerates both the H2 production reaction and O2 photoreduction reaction (Figure S5). Because of these two counteracting effects, the water-splitting activity of Au cocatalyst-loaded BaLa4Ti4O15 was not enhanced merely by refining the cocatalyst (Figure 5A). 3.2. Cr2O3 Shell Formation through a Strong Metal− Support Interaction. The above-mentioned findings suggest that Au25−BaLa4Ti4O15 will behave as a highly active watersplitting photocatalyst if the O2 photoreduction reaction can be prevented. As Domen and co-workers reported, formation of a Cr 2 O 3 shell on the surface of cocatalyst for reduction6,13,17,24−26,53−56,60 is an effective method to prevent the contact of the cocatalyst surface with O2 while allowing the contact of the cocatalyst surface with protons (H+). This behavior is ascribed to the nature of the Cr2O3 shell; it is impermeable to O2 but permeable to H+ approaching from the outside. However, the photodeposition approach typically used to form a Cr2O3 shell cannot be applied to Au25−BaLa4Ti4O15 because of the instability of Au25 clusters under light irradiation (Figure S6). Thus, a new method to form a protective Cr2O3 shell on Au25−BaLa4Ti4O15 is required. It is known that a metal-oxide film is formed on the surface of the metal nanoparticles supported on a metal oxide by heating under reducing atmosphere due to the strong metal− support interaction (SMSI).64−68 This process involves the diffusion of metal oxide onto the surface of the metal nanoparticles to lower the surface energy of the system. Recent studies revealed that an oxide layer is formed by SMSI on Au nanoparticles immobilized on TiO2,69 ZnO,70 and hydroxyapatite (a nonmetal oxide)71 upon heating in an O 2 atmosphere. Similar surface protection mediated by the SMSI is also anticipated between Au nanoparticles and Cr2O3 because the surface energy of Cr2O3 is not high (1.60 J/m2).72,73 Then, we attempted to form a Cr2O3 shell on the surface of Au25 by using the SMSI. In our experiments, a Cr2O3 layer was first supported on BaLa4Ti4O15 by photodeposition before adsorption of Au25(SG)18 (see Experimental Section). An aqueous solution containing BaLa4Ti4O15 and K2CrO4 was stirred and CrO42− ions were adsorbed on BaLa4Ti4O15.60,61 Then, the solution was irradiated with UV light from a highpressure Hg lamp to form a Cr2O3 layer on BaLa4Ti4O15 (Figure 2b). Figure 6 shows the DR spectra of the photocatalysts together with those of related compounds.

Figure 5. Comparison of the rates of photocatalytic generation of (A) H2 and O2 by water splitting and (B) H2 using methanol as a sacrificial reagent under a flow of Ar gas (labeled “without O2”) or 7:3 mixture of Ar to air (labeled “with O2”) by (a) Au25−BaLa4Ti4O15 and (b) AuNP− BaLa4Ti4O15. Averages of the values obtained from several experiments are used in these figures.

to the reaction solution as a sacrificial reagent, the photogenerated holes in the valence band were not consumed by water oxidation to produce O2, but by the oxidation of methanol (Figure 3a).2 Under these experimental conditions, the back reaction (Figure S3c) and O2 photoreduction reaction (Figure S3d) practically do not occur because the amount of O2 produced is small.27,28 Figure 5B illustrates the amount of H2 generated by UV irradiation when an aqueous solution containing 10% methanol was used as a dispersion solvent (see Experimental Section). As expected, the amount of the produced H2 increased markedly compared with that in Figure 5A. Figure 5B shows that Au25−BaLa4Ti4O15 generated 2.1 times more H2 than AuNP−BaLa4Ti4O15 (4697 vs 2203 μmol/ h; see the bars labeled “without O2”). This indicates that the refinement of the Au-cluster cocatalyst accelerates the H2 production reaction. It has been reported that H atoms are adsorbed on the corner and edge positions of the cluster surface.63 The acceleration of H2 production reaction may be caused by the increase of the ratio of the corner and edge Au atoms on the cluster surface with decrease of cluster size.63 However, as mentioned above, the water-splitting activity was not improved when the Au-cluster cocatalyst was refined (Figure 5A). Thus, it is expected that refinement of the Aucluster cocatalyst also accelerated the other reactions, which suppress H2 production. For photocatalyst with Au cocatalyst, it has been reported that the back reaction (Figure S3c) hardly proceeds in the absence of light irradiation.27,28 In fact, almost no progress of the back reaction was observed without light irradiation for both Au25−BaLa4Ti4O15 and AuNP−BaLa4Ti4O15 (Figure S4). Conversely, previous studies revealed that the photoreduction of O2 occurs under light irradiation for photocatalysts containing Au cocatalysts, which also suppresses E


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Figure 6. DR spectra of (a) Cr2O3−BaLa4Ti4O15 (0.5 wt % Cr), (b) Au25(SG)18−Cr2O3−BaLa4Ti4O15 (0.5 wt % Cr), (c) Au25−CrxOy− BaLa4Ti4O15 (0.5 wt % Cr), and (d) Au25−CrxOy−BaLa4Ti4O15 (0.5 wt % Cr) after UV irradiation for 1 h, which gave Au25−Cr2O3− BaLa4Ti4O15 (0.5 wt % Cr), and those of (e) Cr2O3 and (f) Au25(SG)18 for comparison. In part b, the fitting results (red and green lines) using the spectra of Cr2O3−BaLa4Ti4O15 and Au25(SG)18− BaLa4Ti4O15 are also shown. Figure 7. TEM (a, c, e) and HR-TEM (b, d, f) images of (a, b) Cr2O3−BaLa4Ti4O15 (0.5 wt % Cr), (c, d) Au25(SG)18−Cr2O3− BaLa4Ti4O15 (0.5 wt % Cr), and (e, f) Au25−CrxOy−BaLa4Ti4O15 (0.5 wt % Cr). In parts c and e, the insets show the core-size distributions of the particles. In parts b and f, the thicknesses of the Cr2O3 or CrxOy shells are indicated by yellow double-pointed arrows. In parts f, the particle size is also indicated by a white double-pointed arrow.

The DR spectrum of Cr2O3-coated BaLa4Ti4O15 (Figure 6a) exhibits peaks characteristic of Cr2O3 (Figure 6e), confirming that a Cr2O3 layer was formed by this operation. Parts a and b of Figure 7 display TEM and HR-TEM images of the BaLa4Ti4O15 photocatalyst with a Cr2O3 layer (Cr2O3− BaLa4Ti4O15) formed using 0.5 wt % Cr. These images demonstrate that Cr2O3 layers with a thickness of 0.7−1.3 nm were sparsely formed on BaLa4Ti4O15 by this approach. Then, Au25(SG)18 clusters were adsorbed on Cr2O3− BaLa4Ti4O15 by stirring Cr2O3−BaLa4Ti4O15 and Au25(SG)18 (0.1 wt % Au) in water (see Experimental Section). The color of the solution immediately faded, indicating that almost all the clusters were adsorbed on Cr2O3−BaLa4Ti4O15. ICP-MS measurements of the supernatant confirmed that 99.8% of Au25(SG)18 was adsorbed on Cr2O3−BaLa4Ti4O15. Parts c and d of Figure 7 show TEM and HR-TEM images of the Au25 (SG) 18-adsorbed Cr2 O3 −BaLa4 Ti 4 O15 (Au 25 (SG) 18− Cr2O3−BaLa4Ti4O15), respectively. In the TEM image, only particles with a size of about 1 nm were observed (average particle diameter of 1.1 ± 0.2 nm). The DR spectrum of Au25(SG)18−Cr2O3−BaLa4Ti4O15 (Figure 6b) exhibits absorption peaks originating from Au25(SG)18 (Figure 6f)29 and Cr2O3 (Figure 6e). All these results indicate that Au25(SG)18 was adsorbed on Cr2O3−BaLa4Ti4O15 with high efficiency and without deterioration. For this adsorption process, it is difficult to experimentally judge whether the main adsorption site of Au25(SG)18 is the Cr2O3 layer or the exposed BaLa4Ti4O15 surface (Figure S7). It has been reported that the surface of Cr2O3 in water is in the state of CrO(1.5−m)(OH)2m·xH2O (m =

0, 0.5, or 1.5).56 The polar functional groups, such as −COOH and −NH2, of the SG ligands of Au25(SG)18 are expected to form hydrogen bonds with −OH groups.49,50 In Au25(SG)18− Cr2O3−BaLa4Ti4O15, both the Cr2O3 layer and exposed BaLa4Ti4O15 surface possess −OH groups. Thus, Au25(SG)18 is presumed to be adsorbed on both the Cr2O3 layer and exposed BaLa4Ti4O15 surface in Au25(SG)18−Cr2O3−BaLa4Ti4O15 (Figure 2b). The Au25(SG)18−Cr2O3−BaLa4Ti4O15 thus obtained was calcined in an electric furnace at 300 °C for 2 h under low vacuum (>1.0 × 10−1 Pa) to remove the ligands from Au25 and embed Au25 into the chromium-oxide film. The TEM image of the photocatalyst after calcination depicted particles with an average particle diameter of 1.1 ± 0.3 nm (Figure 7e). An HRTEM image revealed the formation of a thin film with a thickness of about 0.8−0.9 nm around the particles with a size of about 1 nm (Figure 7f). This HR-TEM image also showed that the thickness of the chromium-oxide film on BaLa4Ti4O15 decreased from 0.7−1.3 to 0.7−0.9 nm after calcination (Figure 2b). Assuming that the entire chromium-oxide layer had the same thickness (0.7−0.9 nm), it was estimated that 26%−33% of the BaLa4Ti4O15 surface was covered with a chromium-oxide F


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The Journal of Physical Chemistry C layer after calcination (Figure S8). Figure 8 shows the XPS analysis of the photocatalyst before and after calcination. The

Figure 9. Comparison of Au L3-edge FT-EXAFS spectra for (a) Au25(SG)18−Cr2O3−BaLa4Ti4O15 (0.5 wt % Cr), (b) Au25−CrxOy− BaLa4Ti4O15 (0.5 wt % Cr), and (c) Au25−CrxOy−BaLa4Ti4O15 (0.5 wt % Cr) after UV irradiation for 1 h, which gave Au25−Cr2O3− BaLa4Ti4O15 (0.5 wt % Cr). Because these spectra were collected at room temperature, weak peaks attributable to the Au−Au bond (2.5− 3.0 Å) were observed in the spectrum of Au25(SG)18−Cr2O3− BaLa4Ti4O15 (0.5 wt % Cr).74

calculated for cuboctahedral Au13 (5.5) and Au55 (7.9), suggesting that the Au13(Au2(SG)3)6 core−shell structure was transformed into Au25 by calcination. In fact, the DR spectrum of the photocatalyst after calcination shows no substantial plasmon absorption near 520 nm, which is typically observed for Au nanoparticles with a size larger than 2 nm77 (Figure 6c). These results indicate that Au25 hardly agglomerated during the calcination process. As shown above, we succeeded in embedding Au25 in the chromium-oxide film with nearly unchanged particle size even after removing the ligand by calcination. The surface energy of Au (1.35 J/m2) is lower than that of Cr2O3 (1.60 J/m2).72,73,78 However, the surface energy of superfine Au25 is expected to be high because of the low coordination number of its surface atoms. Actually, a molecular dynamics simulation indicated that the surface energy of Au is expected to increase to ∼1.7 J/m2 when its particle size decreases to ∼1.0 nm.79 This increase of surface energy with decreasing particle size may be the reason why chromium oxide migrated to the surface of Au25 during heating. Considering the mechanism of SMSI, the ligand removal is likely to occur first in this process, and then chromium oxide coats the surface of Au25 because of the increased surface energy of Au25 following ligand removal (Figure S11). 3.3. Morphology of the Cr2O3 Shell. The chemical composition of the chromium-oxide film obtained by the above operation was probably similar to Cr2O3 immediately after photodeposition on BaLa4Ti4O15, as previously reported for Cr2O3 shell formation on Rh nanoparticles.6,13,17,24−26,54−56,60 In fact, the peak structure observed in the DR spectrum of Cr2O3−BaLa4Ti4O15 (Figure 6a) was very similar to that of Cr2O3 (Figure 6e). Because a similar peak structure was also observed for Au25(SG)18−Cr2O3−BaLa4Ti4O15 (Figure 6b), the adsorption of Au25(SG)18 is considered to have almost no effect on the chemical composition of the chromium-oxide film. Conversely, there was no peak from Cr2O3 around 600 nm in the DR spectrum of Au25−CrxOy−BaLa4Ti4O15 after calcination (Figure 6c), indicating that the charge state and/or the

Figure 8. Comparison of (a) Cr 2p3/2, (b) Au 4f7/2, and (c) S 2p spectra of Au25(SG)18−Cr2O3−BaLa4Ti4O15 (top) and Au25−CrxOy− BaLa4Ti4O15 (bottom). In these figures, the red and green curves are the fitting results and the gray curves are the baselines. In each figure, the values in red indicate the relative area compared with the area of Cr 2p3/2 in part a.

area ratio of chromium (Cr) to Au determined from the Cr 2p3/2 and Au 4f7/2 peaks was 1:0.33 before calcination. This ratio changed to 1:0.13 after calcination (Figure 8a,b). These results indicate that the Au25 clusters moved into the inside of the the chromium-oxide layer after calcination. The results of the energy dispersive X-ray spectrometry line profile of the calcined sample were also consistent with this interpretation (Figure S9). From these results, we conclude that Au25 was covered with a chromium-oxide film following calcination. The removal of the SG ligands from Au25(SG)18−Cr2O3− BaLa4Ti4O15 by calcination was confirmed from the Au L3-edge FT-EXAFS spectrum (Figure 9 and S10) and XPS analysis (Figure 8c). In the Au L3-edge FT-EXAFS spectrum of the photocatalyst before calcination, a peak attributed to the Au−S bond74−76 was observed around 2.0 Å (Figure 9a). This peak disappeared from the spectrum of the photocatalyst after calcination; only a peak attributable to the Au−Au bond74−76 was observed near 2.5−3.0 Å (Figure 9b). Similarly, a weak peak attributable to S 2p was observed at ∼163 eV in the XPS curve of the sample before calcination, whereas this peak was hardly detectable after calcination (Figure 8c). These results demonstrate that the SG ligands were almost completely removed from Au25 during calcination. Analysis of the Au L3-edge FT-EXAFS spectrum of Au25 in the photocatalyst after calcination (Figure 9b) also gave an estimated coordination number for Au−Au bonds of about 6.8 (Table S1). This coordination number is between those G


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The Journal of Physical Chemistry C geometrical structure of the chromium-oxide film changed during calcination. To obtain more detailed information on the chemical composition of the chromium-oxide film in this process, we further measured the Cr K-edge XANES spectra of Cr2O3− BaLa4Ti4O15, Au25(SG)18−Cr2O3−BaLa4Ti4O15, and Au25− CrxOy−BaLa4Ti4O15, as depicted in Figure 10a,b,c, respectively.

shows the Cr K-edge XANES spectrum of Au25−CrxOy− BaLa4Ti4O15 after UV irradiation. No peaks related to the tetrahedral chromium-oxide species seen for higher oxides of chromium were observed in this spectrum. This result demonstrates that the photoexcited electrons in the conduction band reduce the higher oxides of chromium to trivalent Cr. The Cr 2p3/2 XPS analysis (Figure S13c) also supported this interpretation. In the DR spectrum of the photocatalyst after UV irradiation, a peak characteristic of Cr2O3 was observed at around 600 nm (Figure 6d). These results demonstrate that chromium oxide can return to a chemical composition and geometrical structure similar to those of Cr2O3, which should prevent the contact of the cocatalyst surface with O2. Previous studies showed that the Cr2O3 protective film has an amorphous structure.56 To investigate the structure of the obtained Cr2O3 film, powder X-ray diffraction (XRD) patterns of Cr2O3−BaLa4Ti4O15, Au25(SG)18−Cr2O3−BaLa4Ti4O15, Au25−CrxOy−BaLa4Ti4O15, and Au25−CrxOy−BaLa4Ti4O15 after UV irradiation (Au25−Cr2O3−BaLa4Ti4O15) were also measured. Similar XRD patterns were obtained for all the photocatalysts, and the patterns resembled that of BaLa4Ti4O15 (Figure S14). No diffraction peaks attributable to Cr2O3 or CrO3 were observed in the XRD patterns (Figure S14 and S15). These results indicate that BaLa4Ti4O15 maintained its crystal structure during the protective coating formation process and none of the formed chromium-oxide films had a crystalline structure. In other words, the chromium-oxide films in all of the photocatalysts in this work were also in an amorphous state. 3.4. Effect of Cr2O3 Shell Formation on Photocatalytic Activity. The Au25−Cr2O3−BaLa4Ti4O15 photocatalyst thus obtained showed much higher water-splitting activity than that of Au25−BaLa4Ti4O15. Figure 11 compares the water-splitting

Figure 10. Comparison of Cr K-edge XANES spectra for (a) Cr2O3− BaLa4Ti4O15 (0.5 wt % Cr), (b) Au25(SG)18−Cr2O3−BaLa4Ti4O15 (0.5 wt % Cr), (c) Au25−CrxOy−BaLa4Ti4O15 (0.5 wt % Cr), and (d) Au25−CrxOy−BaLa4Ti4O15 (0.5 wt % Cr) after UV irradiation for 1 h to form Au25−Cr2O3−BaLa4Ti4O15 together with those of (e) Cr2O3, (f) NaCrO2, (g) KCr3O8, (h) K2CrO4, and (i) CrO3 for comparison.

A small peak near 5992 eV was observed only for Au25−CrxOy− BaLa4Ti4O15 (Figure 10c). This peak is attributed to tetrahedral chromium-oxide species, which is characteristic of chromium oxides with a high degree of oxidation (KCr3O8, K2CrO4, or CrO3; Figure 10g−i and S12), indicating that some of the chromium-oxide film was further oxidized during calcination. From the peak area, it was estimated that Au25−CrxOy− BaLa4Ti4O15 contained about 20% chromium oxide with a high degree of oxidation immediately after calcination. A similar interpretation was also obtained from the Cr 2p3/2 XPS analysis (Figure S13). During calcination, the photocatalyst was heated under low vacuum (>10 × 10−1 Pa) to efficiently remove the ligand. However, a low concentration of O2 would be present near the photocatalyst even under these conditions. This residual O2 may oxidize the Cr2O3 film, leading to the change in the oxidation state of the chromium-oxide film and the formation of a shell on Au25 by SMSI. Thus, the charge state of some of the chromium-oxide film was modified during calcination. Then, Au25−CrxOy−BaLa4Ti4O15 was irradiated with UV light for 1 h to reduce the chromium oxide with a high degree of oxidation. Figure 10d

Figure 11. Comparison of rates of H2 and O2 evolution by photocatalytic water splitting over Au25−BaLa4Ti4O15 and Au25− Cr2O3−BaLa4Ti4O15 (0.5 wt % Cr). Averages of values obtained from several experiments are shown.

activities of Au25−BaLa4Ti4O15 and Au25−Cr2O3−BaLa4Ti4O15. The ratio of the amounts of H2 to O2 generated was nearly 2:1 for Au25−Cr2O3−BaLa4Ti4O15, indicating that the watersplitting reaction progressed ideally. Au25−BaLa4Ti4O15 produced 155.7 μmol/h of H2, while Au25−Cr2O3−BaLa4Ti4O15 generated 3032 μmol/h of H2.80 This indicates that the watersplitting activity of the photocatalyst was improved by about 19 times by Cr2O3 shell formation. The apparent quantum yield of this improved photocatalyst was estimated to be 6.3%, 4.1%, and 1.4% at 270, 300, and 320 nm, respectively (Figure S16). The supported Cr2O3 itself provided a negligible improvement of water-splitting activity (Figure S17). Furthermore, there was H


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The Journal of Physical Chemistry C no marked change in the electronic state of Au25 by the Cr2O3 coating (Figure S18), which is different from the case of mixing of Fe2O3 cocatalyst with Cr2O3; the Fe2O3 and Cr2O3 forms a Fe2−xCrxO3 solid solution.81 Therefore, it is reasonable to attribute the improved activity induced by Cr2O3 shell formation to inhibition of the O2 photoreduction reaction. In fact, it was experimentally confirmed that the O2 photoreduction reaction was largely suppressed by the formation of the Cr2O3 shell (Figure S19). Thus, we succeeded in producing a highly active water-splitting photocatalyst utilizing the characteristics of superfine Au25 by embedding naked Au25 in a Cr2O3 shell (Figure S20). During these experiments, we also found that it was necessary to optimize the amount of Cr used to achieve high photocatalyst activation. Figure 12 shows the correlation

Figure 13. Proposed surface structure of the Au 25−Cr2O3− BaLa4Ti4O15 photocatalysts with (a) 0.1, (b) 0.5, (c) 1.0, and (d) 1.5 wt % Cr.

photocatalyst with increase of Cr content is considered to induce the above-mentioned decrease in water-splitting activity. 3.5. Enhancement of Stability Caused by Cr2O3 Shell Formation. The aggregation of the cocatalyst during light irradiation was also suppressed by forming this type of Cr2O3 shell. In the Cr2O3 shell formation process described herein, Au25 was embedded in Cr2O3 (Figure 2b). In this case, Au25 is unlikely to move readily on the photocatalyst surface. The cocatalyst-particle size remained similar after UV irradiation (Au25−CrxOy−BaLa4Ti4O15 vs Au25−Cr2O3−BaLa4Ti4O15; Figure S23). The Au L3-edge FT-EXAFS spectra also confirmed that the particle size was almost maintained during UV irradiation. As shown in Figure 9b,c and S10b,c, there was no substantial change in the shape of the Au L3-edge EXAFS and FT-EXAFS spectra before and after UV irradiation for 1 h. For the sample after irradiation, the estimated coordination number of Au (7.7) was close to that reported for the Au25 cluster76 (Table S1). Although we checked the particle size even after the water-splitting reaction of 10 h, only a little increase of the particle size was observed for Au25−Cr2O3−BaLa4Ti4O15 in contrast with Au25−BaLa4Ti4O15 (Figure 14). Such stable Au25−Cr2O3−BaLa4Ti4O15 maintained high water-splitting activity for a long time (Figure 15). These results demonstrate that the surface protection of cocatalyst particles achieved by our method improves not only the water-splitting activity but also the stability of the Au25 cocatalyst under UV irradiation.

Figure 12. Quantities of evolved H2 and O2 over Au25−Cr2O3− BaLa4Ti4O15 with different Cr contents. Averages of values obtained from several experiments are shown.

between the amount of Cr and activity of Au25−Cr2O3− BaLa4Ti4O15. Au25−Cr2O3−BaLa4Ti4O15 with 0.5 wt % Cr exhibited the highest activity of the photocatalysts containing 0.1−1.5 wt % Cr. The activity of the photocatalyst with 0.1 wt % Cr was slightly lower than that of the one with 0.5 wt % Cr (Figure 12). The HR-TEM images of Au 25 −Cr x O y − BaLa4Ti4O15 with 0.1−1.5 wt % Cr revealed that the film thicknesses on the surface of Au25 were similar (0.6−0.9 nm; Figure S21), even though the film thickness on the surface of BaLa4Ti4O15 increased with Cr content (0.1 wt % Cr, 0.4−0.6 nm (Figure S21a); 0.5 wt % Cr, 0.7−0.9 nm (Figure S21b); 1.0 wt % Cr, 0.9−1.8 nm (Figure S21c); 1.5 wt % Cr, 1.2−2.0 nm (Figure S21d). Slight aggregation of Au25 was observed only for Au25−CrxOy−BaLa4Ti4O15 with 0.1 wt % Cr (Figure S22a). Thus, photocatalyst with 0.1 wt % Cr might contain some Au25 without a Cr2O3 shell because of the low Cr content (Figure 13a). In contrast, agglomeration of Au25 was hardly observed in the photocatalysts containing ≥0.5 wt % Cr (Figure S22b−d). Most of the surface of Au25 is considered to be protected by a Cr2O3 shell in these photocatalysts (Figure 13b−d). The photocatalytic activity of these samples decreased with increasing Cr content (Figure 12). In this type of photocatalyst, the photocatalyst surface not covered with the Cr2O3 shell functions as both a light absorption site and O2 generation site (Figure 1a). Addition of excessive Cr decreases the area of exposed photocatalyst surface, thereby diminishing the area of light absorption and O2 generation sites (Figure 13b−d). Assuming that the entire Cr2O3 layer had the same thickness, it was estimated that 26%−33%, 26%−51%, and 34%−57% of the BaLa4Ti4O15 surface was covered with the Cr2O3 layer for the photocatalysts containing 0.5, 1.0, and 1.5 wt % Cr, respectively. This increase of the covered area of the

Figure 14. TEM images of (a) Au25−BaLa4Ti4O15 and (b) Au25− Cr2O3−BaLa4Ti4O15 after UV irradiation for 10 h. I


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The Journal of Physical Chemistry C ORCID

Akihide Iwase: 0000-0002-6395-9556 Tatsuya Tsukuda: 0000-0002-0190-6379 Yuichi Negishi: 0000-0003-3965-1399 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Yutaro Mori, Marika Aoki, Shunya Yoshino, and Taichi Tsuchiya for technical assistance and Yuki Koyama for drawing schematics. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Numbers JP16H04099, 17H05385, and 16K21402), and Scientific Research on Innovative Areas “All Nippon Artificial Photosynthesis Project for Living Earth (AnApple)” (Grant Numbers 15H00883 and 24107994). Funding from the Takahashi Industrial and Economic Research Foundation, Futaba Electronics Memorial Foundation, Iwatani Naoji Foundation, and Ube Industries Foundation is also gratefully acknowledged.

Figure 15. Time dependence of the evolution of H2 and O2 over Au25−Cr2O3−BaLa4Ti4O15 with 0.5 wt % Cr.

4. CONCLUSIONS The influence of refining the Au-cluster cocatalyst on the watersplitting reaction catalyzed by Aun−BaLa4Ti4O15 was clarified on the individual reaction level. We found that suppressing the O2 photoreduction reaction is an important factor to achieve high activity by refining the cocatalyst. To prevent the O2 photoreduction reaction, we formed a Cr2O3 shell on the Au25 clusters by a new method based on SMSI. We embedded Au25 in a Cr2O3 film formed on BaLa4Ti4O15 photocatalyst layers while nearly maintaining the size of the Au clusters. The resulting water-splitting photocatalyst was highly active and stable, displaying activity about 19 times higher than that of Au25−BaLa4Ti4O15 without a Cr2O3 shell. The apparent quantum yield of Au25−Cr2O3−BaLa4Ti4O15 (6.3%) is still lower than that of NiOx-loaded BaLa4Ti4O15 (15%).51 However, considering the versatility of SMSI, it is anticipated that the shell film formation method developed herein will be applicable to not only Au and alloy clusters using Au as a base element but also other metal clusters. The shell film composed of other metal oxide82 might also be formed by this shell formation method. Regarding the liquid-phase synthesis of metal clusters, both single-element metal clusters (Au, silver, platinum, and nickel)83−89 and alloy clusters38−48,86,90−92 using these base elements can now be precisely synthesized with atomic accuracy. The precise biomolecule-protected metal nanoclusters could also be a candidate as a precursor of cocatalyst.93 By combining these two techniques, it should be possible to obtain design guidelines at the atomic level to realize strong activation through cocatalyst modification. We believe that deeper understanding of the high activity obtained by such studies will lead to the realization of highly active photocatalysts with practical utility.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00151. Preparation method of the compounds used for comparison, results of curve fitting analysis of Au L3edge EXAFS data, schematic of the system, results for the O2 photoreduction reaction, assignments of the peaks in XRD patterns, comparison of Au L3edge XANES spectra, and other additional figures (PDF)

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REFERENCES

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*(Y.N.) E-mail: [email protected]. Telephone: +81-35228-9145. J


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