Controlled Loading of Small Aun Clusters (n = 10â39) onto

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Controlled Loading of Small Au Clusters (n = 10–39) onto BaLaTiO Photocatalysts: Toward an Understanding of Size Effect of Co-Catalyst on Water Splitting Photocatalytic Activity 4

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Yuichi Negishi, Yoshiki Matsuura, Ryota Tomizawa, Wataru Kurashige, Yoshiki Niihori, Tomoaki Takayama, Akihide Iwase, and Akihiko Kudo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5122432 • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on January 23, 2015

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The Journal of Physical Chemistry

Controlled Loading of Small Aun Clusters (n = 10– 39) onto BaLa4Ti4O15 Photocatalysts: Toward an Understanding of Size Effect of Co-Catalyst on Water Splitting Photocatalytic Activity Yuichi Negishi,*,†,‡,# Yoshiki Matsuura,† Ryota Tomizawa,† Wataru Kurashige,† Yoshiki Niihori,† Tomoaki Takayama,† Akihide Iwase,†,‡ and Akihiko Kudo*,†,‡ †

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 Materials Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki, Aichi

444−8585, Japan

ACS Paragon Plus Environment

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ABSTRACT. We have recently succeeded in loading extremely small monodisperse gold clusters (1.2 ± 0.3 nm) as co-catalysts on a water splitting BaLa4Ti4O15 photocatalyst using a glutathione-protected Au25 cluster (Au25(SG)18) as a precursor; an improved photocatalytic activity of 2.6-fold was obtained when compared with that of BaLa4Ti4O15 onto which large gold nanoparticles (10–30 nm) were loaded. In the current study, the controlled loading of a series of ultra small Aun clusters onto BaLa4Ti4O15 using various Aun(SG)m clusters (n = 10, 15, 18, 22, 25, 29, 33, 39) was examined. The results revealed that the use of a highly stable cluster as a precursor is essential to achieve control over the loading of the gold clusters. Additionally, the origin of the improved photocatalytic activity owing to the ultra miniaturization of the cocatalyst was re-considered herein based on the photocatalytic activities of the obtained photocatalysts. The results strongly suggested that the activity per gold atom on the surface decreased significantly owing to the ultra miniaturization of the co-catalyst, and that the origin of the improved activity is the increase in the number of surface gold atoms at a rate that overcomes the reduction effect in their activity.

KEYWORDS. Gold Clusters, Water Splitting Photocatalysts, Size Effect, Co-Catalyst, Ultra Miniaturization, photocatalytic Activity


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1. INTRODUCTION Owing to the depletion of fossil fuel resources and global warming, humans are expected to gradually shift their energy source supplies from fossil fuels to clean and renewable energy resources. Hydrogen discharges a large amount of energy upon combustion. This energy can be converted into electric power by a fuel cell. Furthermore, the combustion of hydrogen generates water only and thus does not have any negative impacts on the environment. Owing to these characteristics, hydrogen has been drawing significant attention as a promising alternative energy source to address current environment and energy-related problems. Currently, hydrogen is mainly produced by reforming fossil fuels. In this approach, fossil fuel is consumed and carbon dioxide is generated as a byproduct. Thus, such a hydrogen production method cannot be employed to efficiently address the current energy and environment-related issues. In contrast, a water splitting photocatalytic reaction can produce clean and renewable hydrogen; in this method, hydrogen is generated from water by solar energy (sunlight). To this effect, water splitting photocatalytic materials that can promote this reaction have been extensively investigated.1–5 However, further improvements are required in this area for the practical use of water splitting photocatalysts. Typically, the loading of metal nanoparticle co-catalysts that behave as reaction sites on the surface of the photocatalyst is an effective strategy to promote the reaction (Scheme 1a).1–5 Improvement of activity in both the co-catalyst nanoparticles and semiconductor photocatalyst is effective to achieve high catalytic activities. Recent studies have shown that the use of nanoparticles, synthesized by the liquid phase method, as a precursor is effective in controlling the properties of the final co-catalyst particles. Liquid phase synthesis enables the synthesis of


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Scheme 1. (a) Schematic of photocatalytic water splitting using a one-step photoexcitation system. CB, conduction band; VB, valence band; Eg, band gap. (b) Schematic of the loading of highly regulated metal clusters onto the photocatalyst surface.

nanoparticles with controlled particle composition and size.6–14 The adsorption of such highly regulated nanoparticles onto the photocatalyst surface followed by their ligand removal enable the loading of controlled nanoparticles onto the photocatalyst (Scheme 1b).15,16 Among the nanoparticles and clusters synthesized by the liquid phase method, thiolateprotected gold clusters (Aun(SR)m)17–37 can be synthesized with atomic precision within an ultra small cluster size region. The use of these clusters as a precursor is expected to enable precise control over the co-catalyst size within a very small cluster region. As exemplified, we have recently succeeded in loading extremely small monodisperse gold clusters (1.2 ± 0.3 nm) onto water splitting BaLa4Ti4O15 photocatalyst using a glutathione-protected Au25 cluster (Au25(SG)18) as a precursor. Furthermore, we have revealed that BaLa4Ti4O15 loaded with such ultra small gold clusters exhibits a photocatalytic activity that is 2.6-fold higher than that of BaLa4Ti4O15


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photocatalyst loaded with larger gold nanoparticles (10–30 nm) that was prepared using a conventional method (photodeposition method).38 In this study, we examined the controlled loading of a series of ultra small Aun clusters onto BaLa4Ti4O15 using different Aun(SG)m clusters (n = 10, 15, 18, 22, 25, 29, 33, 39). The results demonstrated that the use of a highly stable cluster as a precursor is essential to load highly regulated gold clusters onto BaLa4Ti4O15. Additionally, the origin of the improved photocatalytic activity owing to the ultra miniaturization of the co-catalyst was re-considered in this study based on the photocatalytic activities of the obtained photocatalysts.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Aun(SG)m Clusters. First, a mixture of the Aun(SG)m cluster precursors was prepared according to the reported method.39 Then, the mixture was separated by polyacrylamide gel electrophoresis into each Aun(SG)m cluster (n = 10, 15, 18, 22, 25, 29, 33, 39)39 at a high purity (Figure S1). 2.2. Preparation of Water Splitting BaLa4Ti4O15 Photocatalysts. The water splitting photocatalyst used in this study was BaLa4Ti4O15 on which loaded Au particle acts as a cocatalyst.2 BaLa4Ti4O15 photocatalyst was prepared by a polymerized complex method.3 Briefly, 6.06 g titanium tetrabutoxide and 60.9 g propylene glycol were added to 19 mL ethanol, and the solution was heated to 70 °C. Then, 38.4 g citric acid, 7.71 g lanthanum nitrate hexahydrate, and 0.878 g barium carbonate were sequentially added, and 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. The yield of BaLa4Ti4O15 (Figure S2a) was ~5.0 g.


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2.3. Loading of Small Gold Clusters onto BaLa4Ti4O15. First, each Aun(SG)m cluster was adsorbed onto BaLa4Ti4O15 by mixing 195 mL aqueous solution containing 600 mg BaLa4Ti4O15 and 5 mL aqueous solution containing the Aun(SG)m clusters for 1 h at room temperature (Scheme 1b). According to the previous study on Au25(SG)18, mixing Au25(SG)18 with BaLa4Ti4O15 at a gold content of 0.1 wt% generated a composite photocatalyst with the highest activity when compared with other photocatalyst prepared at mixing gold contents of 0.05-2.0 wt%.38 Therefore, in the current study, the mixing ratio of Aun(SG)m clusters to BaLa4Ti4O15 was fixed at 0.1 wt% Au in all the experiments. The actual amount of gold adsorbed onto BaLa4Ti4O15 was determined by conducting inductively coupled plasma–mass spectrometry (ICP–MS) analysis of the solution after mixing. The Aun(SG)m clusters-adsorbed BaLa4Ti4O15 materials (Aun(SG)m-BaLa4Ti4O15; Figure S2b) were collected by centrifugation, washed with acetone, and dried using a rotary vacuum device. Then, the Aun(SG)m-BaLa4Ti4O15 materials were calcined under vacuum at 300 °C for 2 h to produce the BaLa4Ti4O15 loaded with Aun clusters (Aun-BaLa4Ti4O15; Scheme 1b and Figure S2c).38 2.4. Loading of Gold Nanoparticles onto BaLa4Ti4O15 via Conventional Photodeposition Method. For comparison purposes, gold nanoparticles were loaded onto BaLa4Ti4O15 (AuNPBaLa4Ti4O15) using a conventional photodeposition method. First, 600 mg BaLa4Ti4O15 photocatalyst was added to 350 mL aqueous HAuCl4 solution in a quartz cell. The mixture was stirred for 1 h at room temperature and then irradiated using a high-pressure Hg lamp (400 W) for 1 h. Likewise, the gold ratio was fixed at 0.1 wt%. The average diameter of the particles loaded onto BaLa4Ti4O15 was estimated as 12.3 nm by transmission electron microscopy (TEM). 2.5. Characterization. Optical absorption spectra of the aqueous Aun(SG)m cluster solutions were recorded at room temperature on a JASCO V-630 spectrometer. Diffuse reflectance spectra


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of the Aun(SG)m-BaLa4Ti4O15 and Aun-BaLa4Ti4O15 materials were acquired at room temperature on a JASCO V-670 spectrometer. ICP–MS was conducted using an Agilent 7500c (Agilent Technologies, Tokyo, Japan). Bi was used as an internal standard. ICP–MS analysis of the solutions after stirring was conducted to estimate the quantity of Au that was not adsorbed onto BaLa4Ti4O15. The adsorption efficiency in each experiment was estimated based on this value. X-ray photoelectron spectra were recorded using a JEOL JPS-9010MC electron spectrometer equipped with a chamber at a base pressure of ~2×10−8 Torr. X-rays from the Mg Kα line at 1253.6 eV were used for excitation. The binding energies were corrected by referencing the binding energy of Ti 2p3/2 in BaLa4Ti4O15, which was determined before this study. TEM images were recorded on a Hitachi H-9500 electron microscope operating at 200 kV. A magnification of 150,000× or 200,000× was used. 2.6. Photocatalytic Water Splitting Reactions. Photocatalytic water splitting reactions proceeded at room temperature using an experimental apparatus built in-house that consisted of a high-pressure Hg lamp (400 W) and a quartz cell.38 The reaction was conducted while circulating CO2 gas at 30 mL min−1.38 Before the measurements, the reactant solution, constituting 500 mg Aun-BaLa4Ti4O15 or AuNP-BaLa4Ti4O15 photocatalyst and 350 mL water, was sparged with CO2 gas bubbles for 1 h to ensure complete removal of air in the reaction vessel. The evolved gases were analyzed by gas chromatography (Shimadzu GC-8A equipped with a thermal conductivity detector and MS-5A column; Ar carrier).

3. RESULTS AND DISCUSSION 3.1. Controlled Loading of Gold Clusters onto BaLa4Ti4O15.


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Figure 1. Diffuse reflectance spectra of the different Aun(SG)m-BaLa4Ti4O15 materials.

Figure 2. Time dependence of the adsorption efficiency of Au25(SG)18 clusters onto BaLa4Ti4O15 studied by ICP–MS. Similar results were obtained for the other Aun(SG)m clusters. 3.1.1. Adsorption. Figure 1 shows the diffuse reflectance spectra of the Aun(SG)m-BaLa4Ti4O15 materials obtained by stirring Aun(SG)m clusters with BaLa4Ti4O15 in water. Each spectrum shows peak characteristics specific to each Aun(SG)m cluster (Figure S1), indicating that all the Aun(SG)m clusters of varying sizes prepared herein were adsorbed onto BaLa4Ti4O15. The glutathione molecule has several polar functional groups i.e., –COOH and –NH2. These polar


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functional groups are expected to form hydrogen bonds with –OH groups on the surface of BaLa4Ti4O15,15 thereby leading to the adsorption of the clusters onto the photocatalyst. The adsorption efficiency values of the Aun(SG)m clusters estimated by ICP–MS were 98.8% (n = 10), 97.8% (n = 15), 96.6% (n = 18), 99.1% (n = 22), 99.2% (n = 25), 97.9% (n = 29), 96.7% (n = 33), and 99.4% (n = 39). These results showed that high adsorption efficiencies of the Aun(SG)m onto BaLa4Ti4O15 were obtained herein at mixing ratio of 0.1 wt% Au regardless of the chemical composition. Conversely, the ICP–MS results additionally revealed that an adsorption efficiency of 100% was not attained, even when the stirring time was extended to 2 h. (Figure 2). This finding implies that under the conditions employed in this study, an equilibrium between the aqueous solution of Aun(SG)m clusters and Aun(SG)m-BaLa4Ti4O15 was almost established for the adsorption efficiencies stated above. These results indicate that the actual amount of gold atoms adsorbed onto BaLa4Ti4O15 cannot be accurately estimated simply from the Aun(SG)m-toBaLa4Ti4O15 mixing ratio. In contrast, an experimental measurement, such as ICP–MS, is essential to accurately estimate the amount of the adsorbed Aun(SG)m clusters. 3.1.2. Calcination. Figure 3a and b shows the Au 4f spectra of the composite materials before and after calcination (i.e., Aun(SG)m-BaLa4Ti4O15 and Aun-BaLa4Ti4O15), respectively. Before calcination, all Aun(SG)m-BaLa4Ti4O15 materials showed an Au 4f peak at a higher binding energy relative to Au(0) (Figure 3a). This peak shift is attributed to a partial charge transfer from Au to S.40 In contrast, after calcination, all Aun-BaLa4Ti4O15 samples showed an Au 4f peak at the Au(0) position, with no peak shift to higher binding energies (Figure 3b). These results indicate that most of the precursor ligands were removed by calcination for all photocatalysts. Similar results were also obtained from analysis of the S 2p spectra of the photocatalysts. Figure 4a and b shows the S 2p spectra of the Aun(SG)m-BaLa4Ti4O15 and Aun-BaLa4Ti4O15


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Figure 3. Au 4f X-ray photoelectron spectra of the composite materials (a) before calcination (i.e., Aun(SG)m-BaLa4Ti4O15) and (b) after calcination (i.e., Aun-BaLa4Ti4O15). For comparison, the X-ray photoelectron spectrum of BaLa4Ti4O15 is shown. The Ba 4d5/2 peak overlaps with the Au 4f peaks at ~89.3 eV. The red lines indicate the energy of Au(0) (i.e., 84.0 eV). samples, respectively, prepared at an Au mixing ratio of 0.5 wt%. (Results relating to samples prepared at an Au mixing ratio of 0.1 wt% are not shown because a distinct peak could not be observed in the S 2p spectra). All Aun(SG)m-BaLa4Ti4O15 samples showed distinct peaks at ~162


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Figure 4. S 2p X-ray photoelectron spectra of the composite materials (a) before calcination (i.e., Aun(SG)m-BaLa4Ti4O15) and (b) after calcination (i.e., Aun-BaLa4Ti4O15). The peak intensities in both Aun(SG)m-BaLa4Ti4O15 and Aun-BaLa4Ti4O15 were normalized using reference Ti 2p3/2 peak. Therefore, the peak intensities of Aun(SG)m-BaLa4Ti4O15 and AunBaLa4Ti4O15 at a given n can be compared. eV (Figure 4a). The peaks are attributed to the S atom in the thiolate moiety.41 In contrast, the Aun- BaLa4Ti4O15 samples did not show any distinct peaks (Figure 4b). These results indicate that most ligands were removed by calcination for all photocatalysts.


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3.1.3. Particle Size. Figure 5a–c shows typical TEM images (Figures S3 and S4) and associated particle size distributions of the Aun(SG)m clusters, Aun(SG)m-BaLa4Ti4O15, and AunBaLa4Ti4O15 samples, respectively. The Aun(SG)m clusters (Figure 5a) and Aun(SG)mBaLa4Ti4O15 samples (Figure 5b) featured comparable average particle sizes and particle size distributions. This indicates that the particle size of the Aun(SG)m clusters does not change significantly following adsorption. In contrast, the Aun(SG)m-BaLa4Ti4O15 and Aun-BaLa4Ti4O15 samples featured different particle sizes according to the nature of the Aun(SG)m cluster. The clusters could be classified into two groups based on these phenomena: Group 1 (n = 10, 15, 18, 25, 39) and Group 2 (n = 22, 29, 33). For Group 1 clusters, the particle size distribution of the gold clusters remained narrow after calcination, while a slight increase in the average particle size was observed (Figure 5b and c). The slight increase in the average particle size was attributed to the change in the cluster structure owing to the removal of the ligands upon calcination.42 For this sample group, the loaded clusters are expected to maintain the same number of gold atoms as that present in the precursor Aun(SG)m clusters. In contrast, for Group 2 clusters, the average particle sizes increased significantly after calcination. Figure 6 shows TEM images containing small particles (Figure 6a) and large particles (Figure 6b) for Au22BaLa4Ti4O15, Au29-BaLa4Ti4O15, and Au33-BaLa4Ti4O15. The larger-sized particles could also be observed in the TEM images of Au22-BaLa4Ti4O15, Au29-BaLa4Ti4O15, and Au33-BaLa4Ti4O15. Furthermore, the particle size distribution, after calcination, became broader (Figure 5b and c). These results indicate that the loaded gold clusters do not maintain the same number of gold atoms as that present in the precursor Aun(SG)m clusters. This phenomenon was further confirmed by optical absorption spectroscopy. Figure 7 shows the diffuse reflectance spectra of the different Aun-BaLa4Ti4O15 materials. In the diffusion


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Figure 5. TEM images (Figures S3 and S4) and associated core size distributions of the different (a) Aun(SG)m clusters, (b) Aun(SG)m-BaLa4Ti4O15, and (c) Aun-BaLa4Ti4O15.

reflectance spectra of Group 1 samples, the absorption intensity gradually increased as the wavelength decreased. The plasmon absorption effect at ~520 nm that is observed in gold


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Figure 6. TEM images showing (a) small particles and (b) large particles in Au22BaLa4Ti4O15, Au29-BaLa4Ti4O15, and Au33-BaLa4Ti4O15.

Figure 7. Diffuse reflectance spectra of the different Aun-BaLa4Ti4O15 samples. The red line indicates the location of the peak corresponding to the plasmon absorption effect of gold nanoparticles.

nanoparticles of >2 nm was not observed for this group. In contrast, Group 2 samples showed a distinct peak at ~520 nm. This means that the large particles were incorporated in the Aun-


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BaLa4Ti4O15 samples (n = 22, 29, 33; Group 2 clusters). These results additionally indicate that the particle size of the Group 2 clusters increased upon calcination. These groups are well consistent with those of the stability against degradation in a solution. As reported, Aun(SG)m clusters of Group 2 decompose (release of SG or Au-SG oligomers) in aqueous solution in a shorter time when compared with those of Group 1.39 This similarity implies that the decrease in the stability of the Aun(SG)m clusters against decomposition in an aqueous solution is related to the increase in the particle size of Group 2 clusters. In this study, an aqueous solution containing Aun(SG)m clusters and BaLa4Ti4O15 were stirred continuously for 1 h at room temperature that afforded the highest adsorption efficiency of the Aun(SG)m clusters onto BaLa4Ti4O15 (Figure 2). It is presumed that for the Group 2 clusters, part of the Aun(SG)m clusters dissociate during the stirring process, and Au-SG oligomers, formed from the dissociated products, adsorb conjunctively with the Aun(SG)m clusters. The existence of such Au-SG oligomers may promote cohesion of clusters on BaLa4Ti4O15 during calcination. The detailed cluster cohesion mechanism on BaLa4Ti4O15 will be elucidated in a future study. Thus, the loading experiments of a series of Aun(SG)m clusters (n = 10, 15, 18, 22, 25, 29, 33, 39) on BaLa4Ti4O15 revealed that the loaded gold cluster particles exhibited different behaviors depending on the chemical composition of the precursor cluster. The experimental conditions used in this study may not necessarily be optimal to suppress the gold cluster cohesion for Group 2 clusters. A more pronounced suppression of the gold cluster cohesion is expected under appropriate experimental conditions, even for Group 2 clusters. Regardless, the experimental results described herein strongly indicate that it is essential to use highly stable clusters as a precursor to precisely control the size of the gold clusters loaded onto BaLa4Ti4O15.


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Figure 8. Effect of cluster size on activity for water splitting studied using AunBaLa4Ti4O15 and AuNP-BaLa4Ti4O15 photocatalysts. The average values obtained from four measurements are plotted herein.

3.2. Size Effect of Co-Catalyst on Water Splitting Photocatalytic Activity. The water splitting photocatalytic studies were performed using Aun-BaLa4Ti4O15 of Group 1 samples (n = 10, 15, 18, 25, 39) onto which highly regulated gold clusters were loaded. The photocatalytic activity changed between measurements (Figure 8). However, the ratio of the generated amounts of hydrogen to oxygen remained at 2:1 (Figure S5), which agrees with the stoichiometric proportion of the water splitting reaction. Figure 8 shows the rate of the gas evolution. As observed, the photocatalytic activity increased with decreasing Aun cluster sizes in Aun-BaLa4Ti4O15. Thus Aun-BaLa4Ti4O15 samples that contained smaller gold clusters exhibited higher photocatalytic activities than those containing larger gold clusters at a given mixing concentration of gold (0.1 wt%). Because a moderate increase in activity was observed as the

size decreased, it is

reasonable to attribute this change in activity primarily to the change in the ratio of the surface atoms in the gold clusters with decreasing sizes,43 i.e., the change in the number of gold atoms reacting with hydrogen. Note that Figure 8 does not necessarily imply that smaller gold clusters are superior co-catalysts to larger gold clusters. To determine the optimal gold cluster size, it is


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Figure 9. TEM image and associated core size distribution of AuNP-BaLa4Ti4O15.

necessary to compare the highest activities of all Aun-BaLa4Ti4O15. Though active sites were introduced upon loading of the co-catalyst particles, the latter also instigated a decrease in the light absorption by the photocatalyst. Therefore, the optimal loading amount required to achieve the highest activity is influenced by both factors; thus, the optimal co-catalyst loading differs for each photocatalyst.44 In Figure 8, the activity of each Aun-BaLa4Ti4O15 photocatalysts is compared for a given gold loading (i.e., 0.1 wt%), and the highest activity of each system is not necessarily compared. To determine the highest activity of each Aun-BaLa4Ti4O15 photocatalyst, the activity of each system must be measured by gradually changing the amount of the loaded gold. It is expected that the optimal cluster size for attaining activity will be determined by future studies following the approach described above. Herein, the origin of the improvement in photocatalytic activity owing to the ultra miniaturization of co-catalyst using the activities obtained for the Aun-BaLa4Ti4O15 samples (Figure 8) is re-considered. In a previous study, we reported that (1) the highest photocatalytic activity of Au25-BaLa4Ti4O15 was obtained at a gold loading of ~0.1 wt%; (2) the highest photocatalytic activity of AuNP-BaLa4Ti4O15, onto which gold nanoparticles of 10–30 nm were loaded, was obtained at a gold loading of ~0.5 wt%; and (3) Au25-BaLa4Ti4O15 exhibited a higher activity (2.6-fold) than AuNP-BaLa4Ti4O15.38 These results demonstrated that ultra miniaturization of the co-catalyst was effective to increase the photocatalytic activity. However, because the


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amount of loaded gold differed in these systems, direct comparison was difficult, and a deeper insight in the origin of the improved activity could not be obtained. To clarify this issue, in the current study, gold nanoparticles (8–22 nm) were loaded onto BaLa4Ti4O15 using a conventional photodeposition method (AuNP-BaLa4Ti4O15; Figure 9) at a gold loading amount of 0.1 wt%, which is the same as that used for the preparation of Aun-BaLa4Ti4O15. The photocatalytic activity of the obtained AuNP-BaLa4Ti4O15 was compared with that of Aun-BaLa4Ti4O15 (Figure 8). As observed, Au10-BaLa4Ti4O15 that contained the ultra small gold clusters exhibited a higher photocatalytic activity of ~4.2-fold than AuNP-BaLa4Ti4O15. However, this difference in the photocatalytic activities cannot be simply explained by the difference in the number of cluster surface atoms. As observed in Figure 9, AuNP has a hemispherical geometrical structure4 on BaLa4Ti4O15, implying that only ~3.6–5.9% of the total gold atoms is located on the gold cluster surface in AuNP-BaLa4Ti4O15 (Tables S1 and S2). In contrast, most gold atoms are expected to be located on the gold cluster surface in Au10-BaLa4Ti4O15. Assuming that all the gold atoms are located on the surface (Scheme S1),45 the total number of surface gold atoms in Au10BaLa4Ti4O15 can be estimated to be ~17–28-fold larger than that in AuNP-BaLa4Ti4O15 (Scheme S1 and Table S2). However, the photocatalytic activity of Au10-BaLa4Ti4O15 was higher by a factor of ~4.2 only when compared with that of AuNP-BaLa4Ti4O15 (Figure 8). Thus, the activity per surface gold atom in Au10-BaLa4Ti4O15 was lower by 75–85% relative to that in AuNPBaLa4Ti4O15 (Table S2). This value is only an estimate. The precise structural determination is yet to be conducted to accurately determine the activity per surface gold atom in each system. Furthermore, every surface atom might not necessarily provide the same catalytic activity, as in the case of catalysis for oxidation of carbon monoxide.46 However, this apparent difference in the


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Figure 10. Comparison of the photocatalytic activities of BaLa4Ti4O15, Au25(SG)18BaLa4Ti4O15, and Au25-BaLa4Ti4O15.

activity per surface gold atom strongly suggests that the main cause of the improved activity owing to the ultra miniaturization of co-catalysts is the increase in the number of surface gold atoms at a rate exceeding the decrease in the activity per surface gold atom. 3.3. Calcination Requirement. Finally, the requirement for calcination of the precursor ligands in this composite system is briefly discussed. In recent years, some studies have investigated composites consisting of Aun(SG)m clusters and semiconductor photocatalysts. These studies reported that an electron relay is observed even in the presence of precursor ligands.47–50 Likewise, in our study, Au25(SG)18-BaLa4Ti4O15 that was not calcined exhibited a higher photocatalytic activity than BaLa4Ti4O15 that did not contain any loaded clusters (Figure 10). This indicates that an electron relay can occur between Au25(SG)18 and BaLa4Ti4O15 even when the precursor ligands are not removed and that Au25(SG)18 acts as a co-catalyst. However, the photocatalytic activity of Au25(SG)18-BaLa4Ti4O15 was only 30% of that of Au25-BaLa4TiO15 (Figure 10). This implies that the efficiency of the electron relay between the gold clusters and BaLa4Ti4O15 decreases or the water reduction performance of an individual surface gold atom decreases when ligands are present compared with that in the Au25-BaLa4TiO15 system. Thus,


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these results indicate that calcination (removal) of the precursor ligands is essential to obtain high water splitting photocatalytic activities for the current composite system.

4. CONCLUSION In this study, we have investigated the controlled loading of a series of Aun clusters onto BaLa4Ti4O15 using a series of Aun(SG)m clusters (n = 10, 15, 18, 22, 25, 29, 33, 39). The use of a highly stable cluster as a precursor was essential to achieve control over the loading of the gold clusters onto BaLa4Ti4O15. The findings of this study are expected to be useful for attaining high activities not only for the current composite system, but also for other photocatalytic systems1 and fuel cells,51,52 whereby the supported metal clusters act as the activity sites, similarly to those in the current system. Furthermore, this study strongly suggests that the activity per gold atom on the surface in Aun-BaLa4Ti4O15 decreases significantly when compared with that in AuNPBaLa4Ti4O15. We therefore conclude that the origin of the improved activity owing to the ultra miniaturization of co-catalysts is the increase in the number of surface gold atoms at a rate that overcomes the reduction effect in their activity. Regarding this composite system, future studies should focus on comparing the highest activity values of the different Aun-BaLa4Ti4O15 photocatalysts. The optimal cluster size for achieving high photocatalytic activities can be subsequently determined. Furthermore, for the precisely loaded gold clusters, the geometric and electronic structures can be clarified using theoretical calculations45 and experimental techniques such as scanning TEM.42 Accordingly, it is expected that the correlation between the geometric/electronic structures of the loaded clusters and catalytic activity of the current composite system can be clarified, subsequently enabling clear design guidelines to be established for further improved photocatalytic activities.


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ASSOCIATED CONTENT Supporting Information. Calculation methods for estimating the number of (surface) gold atoms (and associated tabulated data) loaded onto the composite materials, schematics of the geometries of the surface gold atoms, optical absorption spectra of the gold clusters in solution, and TEM photographs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Mr Michiyo Hirayama, Ms Mamiko Omatoi, Ms Miku Matsuzaki, Mr Takumi Terui, Mr Daiki Ishii, and Mr Tomohisa Murayama for technical assistance. This work was financially supported by the Grants-in-Aid for Scientific Research (Nos. 25288009 and 25102539), the Canon Foundation, Kumagai Foundation, Kanto Electrical Safety Services Foundation, and the SEI Group CSR Foundation.


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REFERENCES (1) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253–278. (2) Miseki, Y.; Kato, H.; Kudo, A. Water Splitting into H2 and O2 over Niobate and Titanate Photocatalysts with (111) Plane-Type Layered Perovskite Structure. Energy Environ. Sci. 2009, 2, 306–314. (3) Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A. Photocatalytic Reduction of Carbon Dioxide over Ag Cocatalyst-Loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) Using Water as a Reducing Reagent. J. Am. Chem. Soc. 2011, 133, 20863–20868. (4) Iwase, A.; Kato, H.; Kudo, A. The Effect of Au Cocatalyst Loaded on La-Doped NaTaO3 on Photocatalytic Water Splitting and O2 Photoreduction. Appl. Catal. B: Environ. 2013, 136– 137, 89–93. (5) Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655–2661. (6) Schmid, G.; Chi, L. F.; Metal Clusters and Colloids. Adv. Mater. 1998, 10, 515–526. (7) Toshima, N.; Yonezawa, T. Bimetallic Nanoparticles–Novel Materials for Chemical and Physical Applications. New. J. Chem. 1998, 22, 1179–1201. (8) Sakamoto, M.; Tanaka, D.; Tsunoyama, H.; Tsukuda, T.; Minagawa, Y.; Majima, Y.; Teranishi, T. Platonic Hexahedron Composed of Six Organic Faces with an Inscribed Au Cluster. J. Am. Chem. Soc. 2012, 134, 816–819.


22

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) McKenzie, L. C.; Zaikova, T. O.; Hutchison, J. E. Structurally Similar TriphenylphosphineStabilized Undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, Exhibit Distinct Ligand Exchange Pathways with Glutathione. J. Am. Chem. Soc. 2014, 136, 13426–13435. (10) Shichibu, Y.; Zhang, M.; Kamei, Y.; Konishi, K. [Au7]3+: A Missing Link in the FourElectron Gold Cluster Family. J. Am. Chem. Soc. 2014, 136, 12892–12895. (11) Zhang, H.; Watanabe, T.; Okumura, M.; Haruta, M.; Toshima, N. Catalytically Highly Active Top Gold Atom on Palladium Nanocluster. Nat. Mater. 2012, 11, 49–52. (12) Puls, A.; Jerabek, P.; Kurashige, W.; Förster, M.; Molon, M.; Bollermann, T.; Winter, M.; Gemel, C.; Negishi, Y.; Frenking, G.: Fischer, R. A. A Novel Concept for the Synthesis of Multiply Doped Gold Clusters [(M@AunM’m)Lk]q+. Angew. Chem. Int. Ed. 2014, 53, 4327– 4331. (13) Yamamoto, K.; Imaoka, T.; Chun, W.-J.; Enoki, O.; Katoh, H.; Takenaga, M.; Sonoi, A. Size-Specific Catalytic Activity of Platinum Clusters Enhances Oxygen Reduction Reactions. Nat. Chem. 2009, 1, 397–402. (14) Zheng, J.; Zhang, C.; Dickson, R. M. Highly Fluorescent, Water-Soluble, Size-Tunable Gold Quantum Dots. Phys. Rev. Lett. 2004, 93, 077402. (15) Sakamoto, N.; Ohtsuka, H.; Ikeda, T.; Maeda, K.; Lu, D.; Kanehara, M.; Teramura, K.; Teranishi,

T.;

Domen,

K.

Highly

Dispersed

Noble-Metal/Chromia

(Core/Shell)

Nanoparticles as Efficient Hydrogen Evolution Promoters for Photocatalytic Overall Water Splitting under Visible Light. Nanoscale 2009, 1, 106–109.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

(16) Ikeda, T.; Xiong, A.; Yoshinaga, T.; Maeda, K.; Domen, K.; Teranishi, T. Polyol Synthesis of Size-Controlled Rh Nanoparticles and Their Application to Photocatalytic Overall Water Splitting under Visible Light. J. Phys. Chem. C 2013, 117, 2467–2473. (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of ThiolDerivatized Gold Nanoparticles in a Two-Phase Liquid–Liquid System. J. Chem. Soc., Chem. Commun. 1994, 7, 801–802. (18) Tsukuda, T. Toward an Atomic-Level Understanding of Size-Specific Properties of Protected and Stabilized Gold Clusters. Bull. Chem. Soc. Jpn. 2012, 85, 151–168. (19) Weissker, H.-Ch.; Escobar, H. B.; Thanthirige, V. D.; Kwak, K.; Lee, D.; Ramakrishna, G.; Whetten, R. L.; López-Lozano, X. Information on Quantum States Pervades the Visible Spectrum of the Ubiquitous Au144(SR)60 Gold Nanocluster. Nat. Commun. 2014, 5, 37853792. (20) Parker, J. F.; Fields-Zinna, C. A.; Murray, R. W. The Story of a Monodisperse Gold Nanoparticle: Au25L18. Acc. Chem. Res. 2010, 43, 1289–1296. (21) Mathew, A.; Natarajan, G.; Lehtovaara, L.; Häkkinen, H.; Kumar, R. M.; Subramanian, V.; Jaleel, A.; Pradeep, T. Supramolecular Functionalization and Concomitant Enhancement in Properties of Au25 Clusters. ACS Nano 2014, 8, 139–152. (22) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Quantum Sized Gold Nanoclusters with Atomic Precision. Acc. Chem. Res. 2012, 45, 1470–1479. (23) Kumara, C.; Zuo, X.; Ilavsky, J.; Chapman, K. W.; Cullen, D. A.; Dass, A. Super-Stable,


24

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Highly Monodisperse Plasmonic Faradaurate-500 Nanocrystals with 500 Gold Atoms: Au~520(SR)~120. J. Am. Chem. Soc. 2014, 136, 7410–7417. (24) Luo, Z.; Nachammai, V.; Zhang, B.; Yan, N.; Leong, D. T.; Jiang, D.-e.; Xie, J. Toward Understanding the Growth Mechanism: Tracing All Stable Intermediate Species from Reduction of Au(I)−Thiolate Complexes to Evolution of Au25 Nanoclusters. J. Am. Chem. Soc. 2014, 136, 10577–10580. (25) Bigioni, T. P.; Whetten, R. L.; Dag, Ö. Near-Infrared Luminescence from Small Gold Nanocrystals. J. Phys. Chem. B 2000, 104, 6983–6986. (26) Knoppe, S.; Wong, O. A.; Malola, S.; Häkkinen, H.; Bürgi, T.; Verbiest, T.; Ackerson, C. J. Chiral Phase Transfer and Enantioenrichment of Thiolate-Protected Au102 Clusters. J. Am. Chem. Soc. 2014, 136, 4129–4132. (27) Kwak, K.; Kumar, S. S.; Pyo, K.; Lee, D. Ionic Liquid of a Gold Nanocluster: A Versatile Matrix for Electrochemical Biosensors. ACS Nano 2014, 8, 671–679. (28) Nardi, M. D.; Antonello, S.; Jiang, D.-e.; Pan, F.; Rissanen, K.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Maran, F. Gold Nanowired: A Linear (Au25)n Polymer from Au25 Molecular Clusters. ACS Nano 2014, 8, 8505–8512. (29) Zhang, P. X-ray Spectroscopy of Gold-Thiolate Nanoclusters. J. Phys. Chem. C 2014, 118, 25291–25299. (30) Kurashige, W.; Niihori, Y.; Sharma, S; Negishi, Y. Recent Progress in the Functionalization Methods of Thiolate-Protected Gold Clusters. J. Phys. Chem. Lett. 2014, 5, 4134–4142.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

(31) Negishi, Y.; Kurashige, W.; Niihori, Y.; Nobusada, K. Toward the Creation of Stable, Functionalized Metal Clusters. Phys. Chem. Chem. Phys. 2013, 15, 18736–18751. (32) Malola, S.; Lehtovaara, L.; Enkovaara, J.; Häkkinen, H. Birth of the Localized Surface Plasmon Resonance in Monolayer-Protected Gold Nanoclusters. ACS Nano 2013, 7, 10263– 10270. (33) Jiang, D.-e.; Kühn, M.; Tang, Q.; Weigend, F. Superatomic Orbitals Under Spin-Orbit Coupling. J. Phys. Chem. Lett. 2014, 5, 3286–3289. (34) Guidez, E. B.; Aikens, C. M. Quantum Mechanical Origin of the Plasmon: from Molecular Systems to Nanoparticles. Nanoscale 2014, 6, 11512–11527. (35) Tlahuice-Flores, A.; Whetten, R. L.; Jose-Yacaman, M. Vibrational Normal Modes of Small Thiolate-Protected Gold Clusters. J. Phys. Chem. C 2013, 117, 12191–12198. (36) Liu, C.; Lin, S.; Pei, Y.; Zeng, X. C. Semiring Chemistry of Au25(SR)18: Fragmentation Pathway and Catalytic Active site. J. Am. Chem. Soc. 2013, 135, 18067–18079. (37) Tofanelli, M. A.; Ackerson, C. J. Superatom Electron Configuration Predicts Thermal Stability of Au25(SR)18 Nanoclusters. J. Am. Chem. Soc. 2012, 134, 16937–16940. (38) Negishi, Y.; Mizuno, M.; Hirayama, M.; Omatoi, M.; Takayama, T.; Iwase, A.; Kudo, A. Enhanced Photocatalytic Water Splitting by BaLa4Ti4O15 Loaded with ~1 nm Gold Nanoclusters using Glutathione-Protected Au25 Clusters. Nanoscale 2013, 5, 7188–7192. (39) Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-Protected Gold Clusters Revisited: Bridging the Gap between Gold(I)-Thiolate Complexes and Thiolate-Protected Gold


26

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261–5270. (40) Kurashige, W.; Yamazoe, S.; Kanehira, K.; Tsukuda, T.; Negishi, Y. Selenolate-Protected Au38 Nanoclusters: Isolation and Structural Characterization. J. Phys. Chem. Lett. 2013, 4, 3181–3185. (41) Ohta, T.; Shibuta, M.; Tsunoyama, H.; Negishi, Y.; Eguchi, T.; Nakajima, A. Size and Structure Dependence of Electronic States in Thiolate-Protected Gold Nanoclusters of Au25(SR)18, Au38(SR)24, and Au144(SR)60. J. Phys. Chem. C 2013, 117, 3674–3679. (42) Bruma, A.; Negreiros, F. R.; Xie, S.; Tsukuda, T.; Johnston, R. L.; Fortunelli, A.; Li, Z. Y. Direct Atomic Imaging and Density Functional Theory Study of the Au24Pd1 Cluster Catalyst. Nanoscale 2013, 5, 9620–9625. (43) Wang, L.-M.; Wang, L.-S. Probing the Electronic Properties and Structural Evolution of Anionic Gold Clusters in the Gas Phase. Nanoscale 2012, 4, 4038–4053. (44) Maeda,K.; Sakamoto, N.; Ikeda, T.; Ohtsuka, H.; Xiong, A.; Lu, D.; Kanehara, M.; Teranishi, T.; Domen, K. Preparation of Core–Shell-Structured Nanoparticles (with a Noble-Metal or Metal Oxide Core and a Chromia Shell) and Their Application in Water Splitting by Means of Visible Light. Chem. Eur. J. 2010, 16, 7750–7759. (45) Jiang, D.-e.; Overbury, S. H.; Dai, S. Interaction of Gold Clusters with a Hydroxylated Surface. J. Phys. Chem. Lett. 2011, 2, 1211–1215. (46) Haruta, M. Role of perimeter interfaces in catalysis by gold nanoparticles. Faraday Disc. 2011, 152, 11-32.


27


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Page 28 of 29

(47) Sakai, N.; Tatsuma, T. Photovoltaic Properties of Glutathione-Protected Gold Clusters Adsorbed on TiO2 Electrodes. Adv. Mater. 2010, 22, 3185–3188. (48) Lee, J.; Shim, H. S.; Lee, M.; Song, J. K.; Lee, D. Size-Controlled Electron Transfer and Photocatalytic Activity of ZnO-Au Nanoparticle Composites. J. Phys. Chem. Lett. 2011, 2, 2840–2845. (49) Chen, Y.-S.; Kamat, P. V. Glutathione-Capped Gold Nanoclusters as Photosensitizers. Visible Light-Induced Hydrogen Generation in Neutral Water. J. Am. Chem. Soc. 2014, 136, 6075–6082. (50) Nakata, K.; Sugawara, S.; Kurashige, W.; Negishi, Y.; Nagata, M.; Uchida, S.; Terashima, C.; Kondo, T.; Yuasa, M.; Fujishima, A. Cosensitization Properties of Glutathione-Protected Au25 Cluster on Ruthenium Dye-Sensitized TiO2 Photoelectrode. Int. J. Photoenergy 2013, 2013, 456583. (51) Oezaslan, M.; Hasché, F.; Strasser, P. Pt-Based Core−Shell Catalyst Architectures for Oxygen Fuel Cell Electrodes. J. Phys. Chem. Lett. 2013, 4, 3273–3291. (52) Chen, W.; Chen, S. Oxygen Electroreduction Catalyzed by Gold Nanoclusters: Strong Core Size Effects. Angew. Chem. Int. Ed. 2009, 48, 4386–4389.


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Controlled Loading of Small Aun Clusters (n = 10â39) onto

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Controlled Loading of Small Aun Clusters (n = 10â39) onto