Atomic-Level Understanding of the Effect of Heteroatom Doping of the

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Atomic-Level Understanding of Effect of Heteroatom Doping of the Cocatalyst on Water-Splitting Activity in AuPd or AuPt Alloy Cluster-Loaded BaLa4Ti4O15 Wataru Kurashige, Rui Hayashi, Kosuke Wakamatsu, Yuki Kataoka, Sakiat Hossain, Akihide Iwase, Akihiko Kudo, Seiji Yamazoe, and Yuichi Negishi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00426 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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ACS Applied Energy Materials

Atomic-Level Understanding of Effect of Heteroatom Doping of the Cocatalyst on WaterSplitting Activity in AuPd or AuPt Alloy ClusterLoaded BaLa4Ti4O15 Wataru Kurashige,1,2 Rui Hayashi,1 Kosuke Wakamatsu,1 Yuki Kataoka,1 Sakiat Hossain,1 Akihide Iwase,1,2 Akihiko Kudo,1,2 Seiji Yamazoe,3,* and Yuichi Negishi1,2,* 1

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1–3 Kagurazaka,

Shinjuku-ku, Tokyo, 162-8601, Japan. 2

Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba,

278–8510, Japan. 3

Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, 1–1 Minami-

Osawa, Hachioji-shi, Tokyo 192-0397, Japan.

KEYWORDS: water-splitting photocatalysts, activation, cocatalyst, alloying, Pt doping, Pd doping, precise control, atomic-level understanding

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ABSTRACT: Various studies on functionalization of water-splitting photocatalysts have been performed toward their practical usage. Control of the cocatalyst has been investigated and recently, in addition to particle size control, alloying has been extensively used to achieve this goal. It is essential to investigate photocatalysts with precisely controlled cocatalysts to obtain a detailed understanding of the effect of heteroatom doping of the cocatalyst on the photocatalytic activity and thereby establish clear design guidelines for functionalization. However, previous studies have investigated photocatalysts with a variety of particle sizes and doping ratios (chemical compositions). In this study, we succeeded in loading precisely controlled Au24Pd and Au24Pt clusters on BaLa4Ti4O15, which is one of the most advanced photocatalysts, using precisely synthesized alloy clusters as the precursor. Experiments of the photocatalysts loaded with the precisely controlled cocatalysts revealed the following three features of heteroatom doping of cocatalysts: (1) Pd is located at the surface of the metal-cluster cocatalyst, whereas Pt is located at the interface between the metal-cluster cocatalyst and the photocatalyst; (2) Pd doping decreases the water-splitting activity, whereas Pt doping improves the water-splitting activity; and (3) this opposite doping effect is strongly related to the doping position of the heteroatom. Furthermore, when Pt doping is combined with surface protection of the cocatalyst with a Cr2O3 shell, a photocatalyst with higher activity and stability can be obtained. These results will lead to clear design guidelines for creating water-splitting photocatalysts with high activity and stability.


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1. INTRODUCTION With the global warming and the depletion of fossil resources, the transition from a society using fossil resources to one using clean and renewable hydrogen (H2) as an energy source is expected. Using a water-splitting photocatalyst (Figure 1),1 it is possible to produce H2 from sunlight and water, which are available in almost unlimited quantities on the earth. However, further improvements are needed to make this practical. In most water-splitting semiconductor photocatalysts, it is necessary to load metal nanoparticles/clusters as active sites (cocatalysts, Figure 1).2–12 In this system, semiconductor photocatalyst absorbs the light and produces electron and hole. The electron excited to the conduction band moves to the cocatalysts and is used for the production of H2. The hole generated in conduction band is used for the production of O2 (Figure 1). In such a system, control of the particle size of cocatalyst is considered to be extremely effective for improving the water-splitting activity.13 When precise ligand-protected metal clusters synthesized in the liquid phase are used as precursors of the cocatalysts, it is possible to load metal clusters on the photocatalyst with a controlled particle size. For some of the most advanced water-splitting photocatalysts, such as (Ga1−xZnx)(N1−xOx),13 BaLa4Ti4O15,14,15 and SrTiO3,16 improvement of the activity has been achieved by controlling the particle size using this method. The electronic structure of metal clusters can change when they are doped with heteroelements.17–29 Thus, it is possible to further improve the activity of the cocatalyst when appropriate heteroatom doping is applied to the cocatalyst. With respect to heteroatom doping of cocatalysts, previous studies have used the photocatalysts loaded with a cocatalyst with a variety of particle sizes and doping ratios (chemical compositions).30–37 However, to obtain a detailed understanding of the effect of heteroatom doping of the cocatalyst on the photocatalytic activity


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Figure 1. Schematic of photocatalytic water splitting using a one-step photoexcitation system (CB, conduction band; VB, valence band; Eg, band gap) showing the processes of i) hydrogen evolution, ii) oxygen evolution, and iii) oxygen photoreduction.

and to establish clear design guidelines for achieving high activity, it is crucial to investigate photocatalysts loaded with a cocatalyst having a precisely controlled chemical composition. BaLa4Ti4O15 (Figure S1) is one of the most advanced photocatalysts,38,39 and an Au cluster40 acts as a cocatalyst on this photocatalyst.14,15 In this study, we succeeded in loading precise 25-mer Au24Pd and Au24Pt clusters, in which one Au atom of the gold 25-mer cluster (Au25) is substituted with Pd or Pt, on BaLa4Ti4O15. Structural analysis of the obtained photocatalysts revealed that Pd and Pt are located at different positions in the metal-cluster cocatalysts and both types of heteroatom doping improve the electron density of Au in the metal cluster. Furthermore, water-splitting activity measurements revealed that Pd doping decreases the activity, whereas Pt doping enhances the activity. We concluded that the opposite effects of Pd and Pt doping can be attributed to the difference in the doping positions of the two elements. For the photocatalyst containing the cocatalyst doped with Pt, we attempted to further enhance the activity. The results showed that a combination of Pt doping and surface protection of the cocatalyst with a Cr2O3 shell


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Figure 2. Geometrical structures of (A) [Au25(PET)18]−,49 (B) [Au24Pd(PET)18]0,42 and (C) [Au24Pt(PET)18]0.42 These metal clusters have been revealed to have a geometrical structure in which Au12M metal core (M = Au, Pd or Pt) is surrounded by six −PET−[Au−PET−]2 staples.42,49 In [Au24Pd(PET)18]0 and [Au24Pt(PET)18]0, both Pd and Pt are located in the central position of Au12M metal core. These figures were reproduced from refs 42 and 49. Copyright 2016 Royal Society of Chemistry and Copyright 2008 American Chemical Society.

Figure 3. Schematic of the experimental procedure: (a) Au24M(PET)18, (b) Au24M(PET)18−y(pMBA)y, (c) Au24M(PET)18−y(p-MBA)y–BaLa4Ti4O15, (d) Au24M–BaLa4Ti4O15 (M = Au, Pd, or Pt), and (e) Au24Pt–Cr2O3–BaLa4Ti4O15.

creates a photocatalyst with higher activity and stability than the photocatalyst formed with only a Pt-doped cocatalyst.

2. RESULTS AND DISCUSSION 2.1. Precise Loading of Au24M (M = Au, Pd, and Pt) on BaLa4Ti4O15


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Figure 4. Negative-ion MALDI mass spectra of (a) Au24M(PET)18 and (b) Au24M(PET)18−y(pMBA)y for M = (A) Au, (B) Pd, and (C) Pt. The inserts show a comparison of the experimental and simulated isotope patterns. In these spectra, the asterisks indicate the laser fragments (see Figures S2−S4).

In our previous studies of Au25–BaLa4Ti4O15, we used a thiolate-protected Au25 cluster (Au25(SR)18) as the precursor of loaded Au25.14,15 In recent years, it has become possible to precisely synthesize Au24Pd(SR)18 and Au24Pt(SR)18, in which one Au atom of Au25(SR)18 is substituted by Pd or Pt with atomic precision (Figure 2).17,18,25,41,42 If these clusters can be used as precursor clusters, it is expected that Au24Pd and Au24Pt can also be precisely loaded on the photocatalyst. However, to load metal clusters on BaLa4Ti4O15, it is necessary to initially adsorb the precursor clusters on BaLa4Ti4O15. For Au25(SR)18, precise synthesis is possible using hydrophilic


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Figure

5.

DR

spectra

of

(a)

Au24M(PET)18,

(b)

Au24M(PET)18−y(p-MBA)y,

(c)

Au24M(PET)18−y(p-MBA)y–BaLa4Ti4O15, and (d) Au24M–BaLa4Ti4O15 for M = (A) Au, (B) Pd, and (C) Pt. In (c) and (d), the absorbance of BaLa4Ti4O15 was subtracted from the spectra.

glutathionate43,44 (SG, Figure S2(a)) as the ligand. Au25(SG)18 adsorbs on BaLa4Ti4O15 with a high adsorption efficiency (>99%) because of the interaction between its hydrophilic functional groups (–COOH and –NH2) and the hydrophilic functional groups (–OH) of the photocatalyst surface.14,15 However, for Au24Pd(SR)18 and Au24Pt(SR)18, isolation is only possible when hydrophobic thiolates are used.45 Because metal clusters protected by hydrophobic thiolates have no strong interaction with the hydrophilic surface of BaLa4Ti4O15, it is difficult to adsorb these clusters on BaLa4Ti4O15 with a high adsorption efficiency. To overcome the above problem, we replaced some of the thiolate on the surfaces of Au24Pd(SR)18 and Au24Pt(SR)18 with hydrophilic thiolate (ligand exchange,46,47 Figure 3(a) and


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Figure 6. TEM images and particle-size distributions of (a) Au24M(PET)18, (b) Au24M(PET)18−y(p-MBA)y, (c) Au24M(PET)18−y(p-MBA)y–BaLa4Ti4O15, and (d) Au24M– BaLa4Ti4O15 for M = (A) Au, (B) Pd, and (C) Pt. In this figure, the metal clusters are surrounded by the white or black circles.

(b)). First, Au24Pd(PET)18 (Figures 2B and 4B(a)) and Au24Pt(PET)18 (Figures 2C and 4C(a)) were synthesized with atomic precision using hydrophobic phenylethanethiolate (PET, Figure S2(b)),42,48 as well as Au25(PET)18 (Figures 2A and 4A(a))49−51 for comparison (Figure 3(a)). Then, some of the ligands of the clusters were replaced with hydrophilic para-mercaptobenzoic acid (pMBA,52 Figure S2(c)) by reacting them with p-MBA in solvent (Figure 3(b); see Section 4.3). In the matrix-assisted laser desorption/ionization (MALDI) mass spectra obtained after ligand exchange, there are only peaks attributable to Au25(PET)18−y(p-MBA)y (y = 7–15, Figure 4A(b)), Au24Pd(PET)18−y(p-MBA)y (y = 1–9, Figure 4B(b)), and Au24Pt(PET)18−y(p-MBA)y (y = 3–10, Figure 4C(b)) (Figures S3–S5). There are no substantial differences in the diffuse reflectance (DR)


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Figure 7. Adsorption efficiency of Au24M(SR)18 for M = (A) Au, (B) Pd, and (C) Pt (SR = PET or p-MBA) on BaLa4Ti4O15 (a) before and (b) after the ligand-exchange reaction.

spectra of the clusters (Figures 5A(a) and (b), 5B(a) and (b), 5C(a) and (b), and S5) and the transmission electron microscopy (TEM) images (Figure 6A(a) and (b), 6B(a) and (b), and 6C(a) and (b)) before and after ligand exchange. These results indicate that some of the ligands are replaced in each cluster without any change in the framework structure. The obtained clusters were adsorbed on BaLa4Ti4O15 by stirring each cluster with BaLa4Ti4O15 in solvent (Figure 3(c)). In our previous studies, the highest activity was obtained when the weight ratio of the metal to BaLa4Ti4O15 was 0.1 wt%.14 Thus, in this study, Au25, Au24Pd, and Au24Pt were also mixed with BaLa4Ti4O15 with a weight ratio of Au of about 0.1 wt% and the same number of metal clusters in each sample (see Section 4.3). The adsorption efficiency of Au24M(PET)18−y(p-MBA)y (M = Au, Pd, and Pt) was estimated by measuring the amount of Au contained in the supernatant after mixing by inductively coupled plasma mass spectrometry (ICPMS). The results reveal that each cluster adsorbs on BaLa4Ti4O15 at a high adsorption efficiency (>96%) (Figure 7 and Table S1). There are no significant differences in the DR spectra of the clusters (Figures 5A(b) and (c), 5B(b) and (c), and 5C(b) and (c), and S6) and the TEM images (Figure 6A(b) and (c), 6B(b) and (c), and 6C(b) and (c)) of the clusters before and after adsorption. These results indicate that each cluster adsorbs on BaLa4Ti4O15 at a high adsorption efficiency


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Figure 8. Au L3-edge FT-EXAFS spectra of Au24M−BaLa4Ti4O15 for M = (A) Au, (B) Pd, and (C) Pt together with that of Au foil.

while preserving the original framework structure (hereinafter referred to as Au25(PET)18−y(pMBA)y–BaLa4Ti4O15, Au24Pd(PET)18−y(p-MBA)y–BaLa4Ti4O15, and Au24Pt(PET)18−y(p-MBA)y– BaLa4Ti4O15). Au25(PET)18−y(p-MBA)y–BaLa4Ti4O15,

Au24Pd(PET)18−y(p-MBA)y–BaLa4Ti4O15,

and

Au24Pt(PET)18−y(p-MBA)y–BaLa4Ti4O15 were calcined in an electric furnace at 300 °C (Figure S7, see Section 4.3). The Au L3-edge Fourier transform extended X-ray absorption fine structure (FTEXAFS) spectra of the calcined photocatalysts show almost no peaks at the position of the Au–S bond (~ 1.8 Å, Figures 8, S8 and S9, and Tables S2–S4).53,54 The coordination number of Au–Au (7.1–7.7) of each loaded cluster estimated from the Au L3-edge FT-EXAFS spectrum was largely different from that of the precursor clusters (0.6–0.9)53 and was between the coordination numbers estimated for cuboctahedral structured Au13 (5.5) and Au55 (7.9) (Tables S2–S4). In the TEM images of the photocatalysts after calcination, the particles sizes are similar to those before calcination (after adsorption) (Figure 6A(c) and (d), 6B(c) and (d), and 6C(c) and (d)), but the particle size of only Au25 slightly increases, which will be discussed later. These results indicate that the ligands were removed from Au25(PET)18−y(p-MBA)y, Au24Pd(PET)18−y(p-MBA)y, and Au24Pt(PET)18−y(p-MBA)y, and thereby bare Au25, Au24Pd, and Au24Pt clusters were loaded on


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Figure 9. Pd K-edge FT-EXAFS spectrum of Au24Pd–BaLa4Ti4O15. The pink regions indicate the peak positions assigned to the Pd–S and Pd–Au bonds.

Figure 10. Pt L3-edge FT-EXAFS spectrum of Au24Pt–BaLa4Ti4O15. The pink region indicates the peak position assigned to the Pt–O bond.

BaLa4Ti4O15 without any aggregation (hereinafter referred to as Au25–BaLa4Ti4O15, Au24Pd– BaLa4Ti4O15, and Au24Pt–BaLa4Ti4O15). In this way, Au25, Au24Pd, and Au24Pt were loaded on BaLa4Ti4O15 without aggregation (Figure 3(d)) by combining precision synthesis of the alloy clusters, surface hydrophilization using ligand exchange, and calcination under suitable conditions (Figure S9). Loading of the alloy clusters with atomic precision is in principle difficult by conventional methods, such as photoelectrodeposition or impregnation, and, to the best of our knowledge, this is the first example of loading precise alloy clusters (Au24Pd and Au24Pt) on water-splitting photocatalysts.

2.2. Geometrical Structures of Au24M–BaLa4Ti4O15 (M = Au, Pd, and Pt)


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Figure 11. Proposed structures of Au24M–BaLa4Ti4O15 for M = (A) Au, (B) Pd, and (C) Pt before (top) and during the water-splitting reaction (bottom).

2.2.1. Doping Positions of Heteroatoms. The doping positions of the heteroatoms in Au24Pd– BaLa4Ti4O15 and Au24Pt–BaLa4Ti4O15 were investigated by Pd K-edge and Pt L3-edge EXAFS spectroscopy, respectively (Figures 9−11, S10, and S11). The Pd K-edge FT-EXAFS spectrum of Au24Pd–BaLa4Ti4O15 is shown in Figure 9. Peaks attributable to the Pd–Au and Pd–S bonds are observed in the FT-EXAFS spectrum (Table S3). There are almost no peaks that can be attributed to the bond between Pd and photocatalyst surface atoms (e.g., Pd–O bonds) in the FT-EXAFS spectrum. These results indicate that Pd is located at the surface of the loaded metal cluster in Au24Pd–BaLa4Ti4O15 and bonded to S (Figures 11B and S12). In the Au24Pd(PET)18−y(p-MBA)y precursor, Pd is located at the center of the metal core and not bonded to S (Figure 2B).42 It is assumed that this geometry forms because Pd migrates to the surface and subsequently bonds to S detached from Au when structural changes occur during calcination (Figures 5 and 6). The Pt L3-edge FT-EXAFS spectrum of Au24Pt–BaLa4Ti4O15 is shown in Figure 10. Unfortunately, we could only measure the Pt L3-edge EXAFS up to k = 8.3 (Figure 10) because of absorption of Au L3 (>~11900 eV, Figure 12) a little after Pt L3 absorption. Therefore, the Pt-metal bonds do not appear with sufficient strength in the FT-EXAFS spectrum. However, peaks assigned


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Figure 12. Au L3-edge XANES spectra of Au24M–BaLa4Ti4O15 (M = Au, Pd, and Pt) together with that of Au foil. to the Pt–O bond are observed with high intensity in the FT-EXAFS spectrum. This indicates that Pt is bonded to O. In fact, the Pt L3-edge X-ray absorption near edge spectrum (XANES) of Au24Pt–BaLa4Ti4O15 is similar to that of PtO2 (Figure S13). Analysis of the FT-EXAFS spectrum reveals that Pt is bound to about five O atoms in Au24Pt–BaLa4Ti4O15 (Table S4). Previous experiments55 and theoretical calculations56 of AuPt alloy nanoparticle-loaded TiO2 have revealed that Pt can be trapped at surface oxygen defects in TiO2 and it is located at the interface between the alloy nanoparticles and TiO2. In addition, when Pt is located on the surface of the metal cluster, this high coordination number could not be attained since Au doesn’t form the Au–O bond at the surface of the metal cluster. On the basis of these information, we concluded that Pt is also located at the interface and it is bonded to multiple O atoms of BaLa4Ti4O15 in Au24Pt–BaLa4Ti4O15 (Figure 11C). In this way, Pd and Pt occupy different positions in the loaded alloy clusters, although they belong to the same family (X). In the Au24M(PET)18−y(p-MBA)y precursor, both Pd and Pt occupy the central position (Figure 2). This positioning of Pd and Pt can be explained by the difference in the bonding form between M–SR (M = Pd or Pt) and Au–SR; Pd or Pt substitution at the surface of the metal core or staple is considered to cause the deformation of the framework structure of the cluster, leading to the destabilization of the cluster. On the other hand, in the loaded Au24M,


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such a restriction on the location of Pd and Pt should not exist because the ligands have already been removed from the alloy cluster by the calcination. Thus, in the loaded Au24M, Pd and Pt is expected to migrate to the most favorable position. Compared with Au, Pt forms a stronger bond with O (318.4 ± 6.7 kJ/mol for Pt–O vs 223 ± 21 kJ/mol for Au–O)57 and can bind with a large number of O atoms. This seems the reason why Pt migrates to the interface between Au24Pt and BaLa4Ti4O15 in Au24Pt–BaLa4Ti4O15 (Figure 11C). In contrast, Pd forms weaker bonds with O compared with Au (145 ± 11 kJ/mol for Pd–O vs 223 ± 21 kJ/mol for Au–O).57 Therefore, location of Pd at the interface is considered to be thermodynamically undesirable. Because Pd has a larger surface energy than Au (2.003 Jm−2 for the Pd(111) surface vs 1.506 Jm−2 for the Au(111) surface),58 location of Pd at the surface is also thermodynamically disadvantageous. However, it is presumed that this destabilization owing to a high surface energy can be suppressed by combining with the S atom that dissociates from Au (Table S6) during calcination (Figure 11B). This seems to be the reason why Pd migrates to the surface and bonds to S. 2.2.2. Framework Structures of Au24M (M = Au, Pd, and Pt). Among Au25–BaLa4Ti4O15, Au24Pd–BaLa4Ti4O15, and Au24Pt–BaLa4Ti4O15, the framework structures of the loaded metal clusters are assumed to be also different. From the Au L3-edge FT-EXAFS results (Tables S2), it can be estimated that the coordination number of Au in the loaded metal clusters increases in the order Au25–BaLa4Ti4O15 < Au24Pd–BaLa4Ti4O15 < Au24Pt–BaLa4Ti4O15. In addition, the average particle sizes of the loaded metal clusters increase in the order Au24Pd–BaLa4Ti4O15 = Au24Pt– BaLa4Ti4O15 < Au25–BaLa4Ti4O15 (Figures 6A(d), 6B(d), 6C(d), and S14). The particle size of Au25–BaLa4Ti4O15 (~1.2 nm) is also consistent with our previous results obtained using Au25(SG)18 as the precursor. Similar results are also obtained in the high-resolution (HR) TEM images (Figure S15). These results indicate that the alloy clusters (especially Au24Pt) have a more compact structure than Au25 (Figures 11 and S16).


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According to the experiments and theoretical calculations performed by other research groups, small Aun clusters tend to have two-dimensional structures.59–64 For Au10–metal oxide, density functional theory (DFT) calculations suggest that the structure in which Au10 has a twodimensional structure on the metal oxide is the most stable.65 Based on these results, Au25 is presumed to have a relatively expansive geometric structure on BaLa4Ti4O15 in Au25–BaLa4Ti4O15 (Figures 11A and S16(a)). From studies of bare alloy clusters, it has been revealed that when Aun clusters are doped with Pd or Pt, the clusters begin to form a steric structure in a smaller size region than that observed for the pure Aun cluster.66,67 Our results indicate that these types of structural changes of the metal clusters also occur in Au24Pd–BaLa4Ti4O15 and Au24Pt–BaLa4Ti4O15 (Figures 11B, 11C, and S16(b)). We have also studied on the framework structure of Au24M (M = Au, Pd, and Pt) by measuring XRD patterns of Au24M–BaLa4Ti4O15 (Figure S17). However, the useful information could not be obtained from XRD patterns because the Au24M was loaded on BaLa4Ti4O15 with only ~0.1 wt% Au to obtain the highest water-splitting activity in this study. 2.2.3. Bonding at The Au24M–BaLa4Ti4O15 (M = Au, Pd, and Pt) Interfaces. As described above, in Au25–BaLa4Ti4O15 and Au24Pd–BaLa4Ti4O15, Au is located at the interface. The bonding energies of Au–O are not very large. In addition, a DFT study of Au10–metal oxide predicts that Au10 on the plane forms Au–O bonds with the metal oxide surface at two points.65 In the Au L3edge FT-EXAFS spectra of Au25–BaLa4Ti4O15 (Figure 8A) and Au24Pd–BaLa4Ti4O15 (Figure 8B), there are no strong peaks at the position attributable to Au–O. On the basis of these results, it is presumed that there are only a few Au–O bonds in Au25–BaLa4Ti4O15 and Au24Pd–BaLa4Ti4O15, and therefore the metal clusters are weakly immobilized on BaLa4Ti4O15 (Figure 11A and 11B). In fact, Au25–BaLa4Ti4O15 and Au24Pd–BaLa4Ti4O15 easily aggregate on the photocatalytic surface during the water-splitting reaction (Figures S18A, S18B, S19A, and S19A). This phenomenon can be interpreted as being related to the weakness of bonding at these interfaces.


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On the other hand, in Au24Pt–BaLa4Ti4O15, Pt is located at the interface and forms multiple Pt–O bonds with BaLa4Ti4O15 (Figure 11C and Table S4). Compared with Au, Pt forms a stronger bond with O. Because of these factors, it is considered that Au24Pt is more strongly immobilized on BaLa4Ti4O15 than Au25 and Au24Pd.55,56 In fact, Au24Pt does not agglomerate on the photocatalyst surface during the water-splitting reaction to the same extent as the other two metal clusters (Figures S18C and S19C). This phenomenon strongly supports our interpretation.

2.3. Electronic Structures of Au24M (M = Au, Pd, and Pt) Loaded on BaLa4Ti4O15 Figure 5A(d), 5B(d), and 5C(d) show the optical absorption spectra of Au25, Au24Pd, and Au24Pt loaded on BaLa4Ti4O15, respectively. These spectra were obtained by subtracting absorption of BaLa4Ti4O15 from that of Au25–BaLa4Ti4O15, Au24Pd–BaLa4Ti4O15, and Au24Pt– BaLa4Ti4O15. All of these optical absorption spectra are greatly different from those of the ligandcontaining clusters (Figure 5A(a)–(c), 5B(a)–(c), and 5C(a)–(c)). As previously mentioned, the geometry of the supported metal clusters (Figure 11) is considerably different from that of the precursor metal clusters (Figure 2). Figure 5 shows that the electronic structure also considerably changes with the change of the geometric structure. Figure 12 shows the Au L3-edge XANES spectra of Au25–BaLa4Ti4O15, Au24Pd– BaLa4Ti4O15, and Au24Pt–BaLa4Ti4O15. The absorption intensities of these photocatalysts immediately after the absorption edge are lower than that of Au foil. This indicates that the electron density of the 5d orbital is higher (the number of d holes decreases) in the loaded metal clusters compared with that of Au foil. Comparing the peaks immediately after the absorption edge among the three photocatalysts, the absorption intensity slightly decreases (the electron density of the 5d orbital increases) in Au24Pd–BaLa4Ti4O15 and Au24Pt–BaLa4Ti4O15 compared with Au25– BaLa4Ti4O15. This can be attributed to the electron densities of Au being higher in Au24Pd and


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Figure 13. Rates of photocatalytic generation of H2 and O2 by water splitting with Au24M−BaLa4Ti4O15 for M = (A) Au, (B) Pd, and (C) Pt. The averages of the values obtained from three experiments are used in the figures (Figure S20).

Au24Pt than in Au25 because charge transfer occurs from Pd (electronic negativity = 2.20) and Pt (electronic negativity = 2.28) to Au (electronic negativity = 2.54) owing to the difference in their electronegativities.

2.4. Effect of Heteroatom Doping on Water-Splitting Activity of Au25–BaLa4Ti4O15 We investigated how the water-splitting activity of the photocatalyst changes owing to heteroatom doping of the cocatalyst. In this experiment, 500 mg of the photocatalyst was dispersed in 350 mL of water and the obtained solution was irradiated with ultraviolet (UV) light from the inside using a high-pressure mercury lamp (400 W) to perform the water-splitting reaction. The rate of generated gas was quantified by gas chromatography (see Section 4.4). The rates of H2 and oxygen (O2) generated from Au25–BaLa4Ti4O15, Au24Pd–BaLa4Ti4O15, and Au24Pt–BaLa4Ti4O15 are shown in Figure 13. The rate of generated gas increases in the order Au24Pd–BaLa4Ti4O15 < Au25–BaLa4Ti4O15 < Au24Pt–BaLa4Ti4O15. Although the activity of the


17


Figure 14. Rates of photocatalytic generation of H2 by Au24M–BaLa4Ti4O15 (M = (A) Au, (B) Pd, and (C) Pt) using methanol as a sacrificial reagent under a flow of Ar gas (labeled “without O2”, Figure S22(a)) or a 7:3 mixture of Ar to air (labeled “with O2”, Figure S22(b)). In this experimental condition, O2 is not generated by light irradiation. Therefore, in the results described as “with O2”, O2 was introduced from outside by a flow of a 7:3 mixture of Ar to air. The averages of the values obtained from three experiments (Figure S23) are used in the figures.

sample varied depending on the BaLa4Ti4O15 used in the experiment, the relation of the activity of Au24M–BaLa4Ti4O15 was always this relation (Figure S20). These results indicate that Pd doping decreases the water-splitting activity while Pt doping has an enhancing effect. Improvement of the water-splitting activity by Pt doping has also been reported in the studies using larger Au–Pt alloy nanoparticles as the cocatalyst.30–32 Figure 13 shows that for the photocatalysts loaded with small cocatalysts, a similar effect occurs in the same manner even if only one atom of the Au cocatalyst is replaced by Pt. To determine the reason why the water-splitting activity changes with heteroatom doping, we investigated the effect of heteroatom doping on each reaction that occurs on the photocatalyst (Figure 1). For the photocatalysts used in this study, the water-splitting activity mainly depends on the rate of H2 evolution.38 The rate of H2 evolution is strongly related to the following two elements (Figure S21):15 (1) the peculiar H2-evolution ability of the cocatalyst and (2) the


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likelihood of the O2-photoreduction reaction, which leads to consumption of excited electrons (Figure 1(iii)). Therefore, we investigated the effect of heteroatom doping of the cocatalyst on these two reactions. The H2-evolution ability was estimated by measuring the rate of H2 evolved under conditions where the produced holes were efficiently consumed by the sacrificial reagent (methanol) (Figure S22(a), see Section 4.4).15 An increase in the rate of H2 evolution is observed for both cases of heteroatom doping (Figure 14 and S23). This indicates that both types of heteroatom doping enhance the H2-evolution ability of the cocatalyst (bottom of Figure 11). For the investigation of the likelihood of the O2-photoreduction reaction, O2 was mixed into the reaction system (Ar:air = 7:3) to estimate the extent of the decrease in the rate of H2 production (Figure S22(b), see Section 4.4).15 Although the rate of H2 evolution decreases to 59.1% for Au25–BaLa4Ti4O15, it further decreases to 33.0% and 46.8% for Au24Pd–BaLa4Ti4O15 and Au24Pt–BaLa4Ti4O15, respectively (Figure 14 and S23). These results indicate that both types of heteroatom doping enhance the likelihood of the O2-photoreduction reaction, and this effect is particularly remarkable for Pd doping (bottom of Figure 11). On the basis of these results, we can consider that Pd doping decreases the water-splitting activity because the effect of enhancing the O2-photoreduction reaction is greater than the effect of improving the H2-evolution ability. In addition, Pt doping is considered to increase the watersplitting activity because the effect of improving the H2-evolution ability exceeds the effect of promoting the O2-photoreduction reaction.

2.5. Origin of Different Effects Depending on Kinds of Heteroatom In this way, each heteroatom has a different effect on each reaction occurring on the photocatalyst. We believe that this phonomenon is strongly related to the difference in the doping


19


positions. As mentioned above, both Pd and Pt doping increase the electron density of Au in the cluster (Figure 12). Because both H2 evolution and the O2-photoreduction reaction require electron acceptance (Figure 1), it seems that the increase in the electron density of Au by Pd and Pt doping contributes to acceleration of these two reactions (Figure 11B and 11C). For Au24Pd–BaLa4Ti4O15, Pd, on which the O2-photoreduction reaction is more likely to proceed than on Au (Figure S24),68 is located at the alloy cluster surface (top of Figure 11B). Although Pd is bound to S at the surface immediately after calcination, it seems that S is reduced to sulfur compound, such as H2S, during the water-splitting reaction and subsequently released. Thus, it is assumed that Pd is exposed at the surface of the alloy cluster during the water-splitting reaction (top of Figure 11B). It is considered that the O2-photoreduction reaction is particularly accelerated for Au24Pd–BaLa4Ti4O15 because of the presence of exposed Pd. Conversely, in Au24Pt–BaLa4Ti4O15, Pt is located at the interface between the alloy cluster and BaLa4Ti4O15 (Figure 11C). Because Pt is more effective for electron relay than Au,30 it is expected the photoexcited electrons are a little efficiently transferred from BaLa4Ti4O15 to the metal cluster at the interface in Au24Pt–BaLa4Ti4O15 than in Au24Pd–BaLa4Ti4O15 (Figure 11). Owing to these effects, the H2 evolution and O2-photoreduction reactions on the Au surface seems to be more accelerated for Au24Pt–BaLa4Ti4O15 than for Au24Pd–BaLa4Ti4O15 (Figure 14). As related in section 2.2.3., the difference in the doping positions also affects the stability of Au24M–BaLa4Ti4O15 in addition to their electronic structures. Such a difference in the stability might also a little contribute to different heteroatom effect on each reaction.

2.6. Toward Creation of Highly Active Water-Splitting Photocatalysts As described above, for Au25–BaLa4Ti4O15, the H2-evolution ability can be improved even when the cocatalyst is doped with only one Pt atom, and the water-splitting activity is enhanced


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Figure 15. TEM images, particle-size distributions, and HR-TEM images of (Au24Pt)1–3–Cr2O3– BaLa4Ti4O15 (0.3 wt% Cr, see Section 4.3) before (top) and after (bottom) UV light irradiation for 10 h. The metal clusters are surrounded by the black circles. The thicknesses of the Cr2O3 shells and the particle sizes are indicated by black and yellow double-ended arrows, respectively.

accordingly. We also found that this Pt doping improves the stability of the cocatalyst against agglomeration (Figures S18 and S19). These results indicate that Pt doping is an effective way to improve both the activity and stability of Au25–BaLa4Ti4O15. However, if the O2-photoreduction reaction could be suppressed, a higher water-splitting activity could be obtained. According to previous studies, surface protection of the cocatalyst using a Cr2O3 shell is a powerful way to suppress the O2-photoreduction reaction.13,15 Furthermore, it has been revealed that this type of protective shell has an inhibitory effect on aggregation of metal clusters on the photocatalytic surface.15 To establish design guidelines to produce photocatalysts with high activity and high stability, we attempted to form a Cr2O3 shell on the Au24Pt surface. The Cr2O3 shell was formed by the method previously reported by our group with slight modification (Figure S25).15 In brief, BaLa4Ti4O15 was added to an aqueous solution containing a


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Figure 16. Time course of water splitting over (Au24Pt)1–3–Cr2O3–BaLa4Ti4O15. The time course of water splitting over Au24Pt–BaLa4Ti4O15 is also shown for comparison.

dissolved chromium salt (K2CrO4). In this experiment, the mixture was prepared at a concentration of 0.3 wt% Cr with respect to BaLa4Ti4O15 (Figure S26). UV light was irradiated from the inside of the obtained solution to form a Cr2O3 film on BaLa4Ti4O15 (Cr2O3–BaLa4Ti4O15). The resulting Cr2O3–BaLa4Ti4O15 was mixed with Au24Pt(PET)18−y(p-MBA)y (y = 3–10) in water to adsorb Au24Pt(PET)18−y(p-MBA)y

on

Cr2O3–BaLa4Ti4O15

(Au24Pt(PET)18−y(p-MBA)y–Cr2O3–

BaLa4Ti4O15, Figure S27). The ligand was then removed from Au24Pt(PET)18−y(p-MBA)y by calcination. Finally, the obtained photocatalyst was irradiated with UV light for 1 h to embed the loaded metal cluster into the Cr2O3 film (see Section 4.3). A TEM image and a HR-TEM image of the photocatalyst obtained in this way are shown at the top of Figure 15. Particles with sizes of 1.33 ± 0.31 nm, which are slightly larger than the particle sizes of Au24Pt(PET)18−y(p-MBA)y–Cr2O3–BaLa4Ti4O15 (1.03 ± 0.18 nm, Figure S27), are observed in the TEM image. This indicates that some aggregation of the cocatalyst occurs during Cr2O3 shell formation. From the particle size distribution of the produced particles (top left of Figure 15), it was estimated that the main product is (Au24Pt)1–3 composed of 1–3 Au24Pt units (Au24Pt:(Au24Pt)2:(Au24Pt)3 = 39.0:55.8:5.2, Figure S28), which formed by aggregation of ~77%


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of the Au24Pt. From the HR-TEM image (top right of Figure 15), it was confirmed that a Cr2O3 shell covered these alloy clusters (Figures S29–S32 and Tables S5 and S6, hereafter (Au24Pt)1–3– Cr2O3–BaLa4Ti4O15). Figure 16 shows the amount of gas evolved from (Au24Pt)1–3–Cr2O3–BaLa4Ti4O15. This photocatalyst produces gas approximately 20 times more than Au24Pt–BaLa4Ti4O15. The total amount of gas increases with time. After 10 h UV light irradiation, the average particle size slightly increases compared with that before UV light irradiation (Figure 15). However, the extent of the increase in the particle size (1.33 ± 0.31 nm to 1.56 ± 0.57 nm) is considerably lower than the case of Au24Pt–BaLa4Ti4O15 without the Cr2O3 shell (1.11 ± 0.19 nm to 2.84 ± 0.93 nm, Figures 6C(d) and S18C). These results indicate that photocatalysts with high activity and high stability can be created by combining Pt doping with Cr2O3 shell formation (Figures S33 and S34). Unfortunately, in this experiment, ~77% of the Au24Pt aggregated during formation of the Cr2O3 shell. p-MBA (Figure S2(c)) has less hydrophilic functional groups than SG (Figure S2(a)). Furthermore, in Au24Pt(PET)18−y(p-MBA)y (y = 3–10), not all of the ligands have hydrophilic functional groups. Therefore, Au24Pt(PET)18−y(p-MBA)y (y = 3–10) should more weakly adsorb to the Cr2O3 surface than Au25(SG)18. Probably because of this weaker adsorption, slight aggregation of Au24Pt occurred during calcination, unlike past experiments using Au25(SG)18 as the precursor. Furthermore, there is a slight increase in the particle size after 10 h of the water-splitting reaction even for the photocatalyst with a Cr2O3 shell (Figure 15). This increase is probably because all of the alloy clusters were not covered with a Cr2O3 shell.15 In the future, if these problems can be overcome, photocatalysts with both higher activity and stability can be created based on Pt doping of the cocatalyst and complete protection of the cocatalyst by formation of a Cr2O3 shell.

4. CONCLUSIONS


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We have established a method using hydrophobic metal clusters as the precursor of the cocatalyst. Using this method, we succeeded in loading precise Au24Pd and Au24Pt clusters onto BaLa4Ti4O15. Investigation of the photocatalysts loaded with these precisely controlled cocatalysts revealed the following five features concerning heteroatom doping of the cocatalyst. (1) Pd is located at the surface of the metal cluster, whereas Pt is located at the interface between the metal cluster and the photocatalyst. (2) The electron density of Au in the metal cluster is improved by both Pd and Pt doping. (3) Pt doping improves the water-splitting activity, whereas Pd doping reduces the water-splitting activity. (4) The doping position of the heteroatom considerably contributes to the doping effect. (5) It is possible to create a photocatalyst with both higher activity and stability by combining Pt doping with surface protection of the cocatalyst with a Cr2O3 shell. These observations are expected to lead to clear design guidelines for creation of water-splitting photocatalysts with both high activity and high stability. In addition, when hydrophobic thiolate is used as the ligand, not only Au25 but also other gold and alloy clusters with different constitutive atom numbers can be precisely synthesized.17,18,20–28 Therefore, using this method, in addition to the gold and alloy clusters investigated in this study, metal clusters with various chemical compositions can be loaded on BaLa4Ti4O15. Furthermore, in principle, this method can be applied to different water-splitting photocatalysts with hydrophilic surfaces.16 It is expected that more extensive knowledge about the correlation between the chemical composition of the cocatalyst and the water-splitting activity can be gained using the method established in this study.

4. EXPERIMENTAL SECTION


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4.1. Chemicals All of the chemicals were commercially obtained and used without further purification. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O) and chloroplatinic acid hexahydrate (H2PtCl6·6H2O) were purchased from Tanaka Kikinzoku. Palladium acetate (Pd(O2CCH3)2), tetraoctylammonium bromide ((C8H17)4NBr), titanium tetrabutoxide (Ti(OC4H9)4), lanthanum nitrate hexahydrate (La(NO3)3), barium carbonate (BaCO3), bismuth standard solution (100 ppm), and gold standard solution (1000 ppm) were obtained from Fujifilm Wako Pure Chemical Corporation. Propylene glycol, citric acid, potassium chromate (K2CrO4), dichloromethane (CH2Cl2), methanol, toluene, acetone, ethanol, hexane, tetrahydrofuran (THF), acetonitrile, and hydrogen peroxide (H2O2) were purchased from Kanto Chemical Co., Inc. Trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]-malononitrile (DCTB) and sodium tetrahydroborate (NaBH4) were purchased from Tokyo Kasei Co., Ltd. p-MBA and 2-phenylethanethiol (PhC2H4SH) were obtained from Aldrich. Deionized water with a resistivity of >18 MΩ cm was used.

4.2. Synthesis Au25(PET)18: Au25(PET)18 was synthesized by the method reported by Jin and co-workers50 with slight modification. First, 0.75 mmol of HAuCl4·4H2O was dissolved in 25 mL of a THF solution containing 0.76 mmol (C8H17)4NBr. After stirring for 15 min, 4.7 mmol of PhC2H4SH was added to the solution. After a further 15 min, 5.8 mL of a cold (0 ℃) aqueous solution containing 8.7 mmol NaBH4 was rapidly added. After 12 h, THF was evaporated and the remaining red brown powder was washed with methanol to remove excess thiol and other byproducts. Finally, pure [Au25(PET)18][N(C8H17)4] was extracted from the precipitate of the mixture using acetonitrile.


25


Au24Pd(PET)18: Au24Pd(PET)18 was synthesized according to the method reported by Wu and coworkers42 with slight modification. First, 0.80 mmol of HAuCl4·4H2O and 0.067 mmol of Pd(O2CCH3)2 were dissolved in 30 mL of a THF solution containing 1.16 mmol (C8H17)4NBr. After stirring for 1 h, 2.58 mmol of PhC2H4SH was added to the solution. After a further 10 min stirring, 5.0 mL of a cold (0 ℃) aqueous solution containing 10 mmol NaBH4 was rapidly added. After 5 h, THF was evaporated and the remaining black powder was washed with methanol to remove excess thiol and other byproducts. The product was fractionated using an open column loaded with silica gel as the stationary phase and a mixture of solvents (toluene:hexane = 5:3) as the eluent, and the third brown fraction was collected. This fraction contained Au24Pd(PET)18 and Au25(PET)18. Au25(PET)18 was selectively decomposed in the mixture to obtain high-purity Au24Pd(PET)18. The product (50 mg) was dissolved in 5 mL of CH2Cl2 and then 5 mL of H2O2 was added.48 After stirring for 3 h, the H2O2 phase was removed and CH2Cl2 was removed by evaporation. The byproduct produced during the decomposition process using H2O2 was removed by column chromatography using silica gel as the stationary phase and a mixture of toluene and hexane (5:3) as the mobile phase. The second fraction contained the objective cluster and ~5 mg of green [Au24Pd(SC2H4Ph)18]0 was obtained. Au24Pt(PET)18: Au24Pt(PET)18 was synthesized according to the method reported by Wu and coworkers42 with slight modification. First, 0.80 mmol of HAuCl4·4H2O and 0.20 mmol of H2PtCl6·6H2O were dissolved in 30 mL of a THF solution containing 1.16 mmol (C8H17)4NBr. After stirring for 1 h, 2.58 mmol of PhC2H4SH was added to the solution. After a further 10 min stirring, 5.0 mL of a cold (0 ℃) aqueous solution containing 10 mmol NaBH4 was rapidly added. After 5 h, THF was evaporated and the remaining black powder was washed with methanol to remove excess thiol and other byproducts. The product was fractionated using an open column loaded with silica gel as the stationary phase and a mixture of solvents (toluene:hexane = 5:3) as


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the eluent, and the second green fraction was collected. This fraction contained Au24Pt(PET)18 and Au25(PET)18. Au25(PET)18 was selectively decomposed in the mixture to obtain high-purity Au24Pt(PET)18. Specifically, 50 mg of the product was dissolved in 5 mL of CH2Cl2 and then 5 mL of H2O2 was added. After stirring for 10 h, the H2O2 phase was removed and CH2Cl2 was removed by evaporation. The byproduct formed in the decomposition process using H2O2 was removed by column chromatography using silica gel as the stationary phase and a mixture of toluene and hexane (5:3) as the mobile phase. The first fraction contained the objective cluster, and ~5 mg of green [Au24Pt(PET)18]0 was obtained.

4.3. Preparation BaLa4Ti4O15: In this study, BaLa4Ti4O15, which shows high activity by loading Au particles, was used as the water-splitting photocatalyst. The BaLa4Ti4O15 photocatalyst was prepared by a polymerized complex method.38 In brief, 6.06 g (17.8 mmol) of Ti(OC4H9)4 and 60.9 g (800 mmol) of propylene glycol were added to 19 mL of ethanol, and the solution was then heated to 70 °C. Subsequently, 38.4 g (200 mmol) of citric acid, 7.71 g (17.8 mmol) of La(NO3)3, and 0.878 g (4.45 mmol) of BaCO3 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. About 5.0 g of BaLa4Ti4O15 was obtained. Au24M–BaLa4Ti4O15: First, some of the ligands (PET) of Au24M(PET)18 (M = Au, Pd, and Pt) were exchanged with p-MBA (ligand-exchange reaction). For Au25(PET)18, 3 mg of [Au25(PET)18][N(C8H17)4] was dissolved in 1 mL of acetone. To this solution, 3 mg of p-MBA was added and the solution was left to sit at room temperature for 2 h. For Au24M(PET)18 (M = Pd, and Pt), 3 mg of [Au24M(PET)18]0 was dissolved in 1 mL of THF. To this solution, 3 mg of pMBA was added and the solution was left to sit at room temperature for 2 h. The obtained product


27


was evaporated to dryness and then the dried product was washed with a mixture of methanol and water (7:3) to remove excess thiol and other byproducts. This operation was repeated five times. The obtained Au24M(PET)18−y(p-MBA)y (M = Au, Pd, and Pt) with a lower number of p-MBA ligands was then reacted again with p-MBA under the same experimental conditions to increase the number of p-MBA ligands in Au24M(PET)18−y(p-MBA)y (M = Au, Pd, and Pt). The product was washed with a mixture of methanol and water (7:3) and highly pure Au24M(PET)18−y(p-MBA)y (M = Au, Pd, and Pt) was isolated from the dried product with THF. The obtained Au24M(PET)18−y(p-MBA)y (M = Au, Pd, or Pt) was adsorbed on BaLa4Ti4O15. For Au25(PET)18−y(p-MBA)y, the cluster was adsorbed on BaLa4Ti4O15 by mixing an acetone solution containing Au25(PET)18−y(p-MBA)y with an acetone solution of BaLa4Ti4O15 (600 mg) for 1 h at room temperature. The total volume of the solution was fixed at 200 mL, and the mixing ratio of Au25(PET)18−y(p-MBA)y to BaLa4Ti4O15 was fixed at 0.100 wt% Au because this gives a photocatalyst with high activity.14 The amount of Au in the solution was determined by ICP-MS analysis of each aqueous solution. For Au24M(PET)18−y(p-MBA)y (M = Pd and Pt), the cluster was adsorbed on BaLa4Ti4O15 by mixing a THF solution containing Au24M(PET)18−y(p-MBA)y with a THF solution of BaLa4Ti4O15 (600 mg) for 1 h at room temperature. The total volume of the solution was again fixed at 200 mL. The mixing ratios of Au24M(PET)18−y(p-MBA)y (M = Pd and Pt) to BaLa4Ti4O15 were fixed at 0.098 and 0.096 wt% Au for M = Pd and Pt, respectively. Under these experimental conditions, the numbers of Au24M(PET)18−y(p-MBA)y (M = Pd and Pt) in the solution are same as that of Au25(PET)18−y(p-MBA)y in the Au25(PET)18−y(p-MBA)y solution with 0.100 wt% Au. The obtained Au24M(PET)18−y(p-MBA)y (M = Au, Pd, and Pt) was calcined under reduced pressure (>1.0 × 10−1 Pa) at 300 °C for 80 min to give Au24M–BaLa4Ti4O15. The elimination of all the ligands included in Au24M(PET)18−y(p-MBA)y by the calcination at 300 °C for 80 min was confirmed from both TGA data of Au24M(PET)18−y(p-MBA)y and Au L3-edge FT-


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EXAFS spectra of Au24M–BaLa4Ti4O15. For the calcination, we have also conducted it at 250 °C for 80 min. However, the surviving of S was observed in Au L3-edge FT-EXAFS spectra for the Au24M–BaLa4Ti4O15 obtained by calcination at 250 °C, demonstrating that 250 °C is not high enough for complete elimination of all of the ligands of Au24M(PET)18−y(p-MBA)y.

(Au24Pt)1–3–Cr2O3–BaLa4Ti4O15: The Cr2O3 shell was formed by our previously reported method15 with slight modification (Figure S25). First, Cr2O3 was loaded on BaLa4Ti4O15 by photodeposition. In this process, the BaLa4Ti4O15 photocatalyst (650 mg) was added to aqueous K2CrO4 solution (350 mL) in a quartz cell. The mixture was stirred for 1 h at room temperature. The solution was then irradiated with a high-pressure Hg lamp (400 W) under Ar flowing at a rate of 30 mL/min for photodeposition of Cr2O3. The mixing ratio of K2CrO4 to BaLa4Ti4O15 was set to 0.3 wt% Cr (Figure S26). Au24Pt(PET)18−y(p-MBA)y was then adsorbed on Cr2O3–BaLa4Ti4O15 with a Au content of 0.096 wt%. The amount of Au in the solution was determined by ICP-MS analysis of the aqueous solution. The obtained Au24Pt(PET)18−y(p-MBA)y–Cr2O3–BaLa4Ti4O15 was calcined under reduced pressure (>1.0 × 10−1 Pa) at 300 °C for 2 h to eliminate the ligands. In a previous study of Au25–Cr2O3–BaLa4Ti4O15 using Au25(SG)18 as the precursor, Au25 was embedded into the chromium oxide shell during this calcination process by the strong metalsupport interaction (SMSI) effect.15 However, in this study, the metal clusters were not covered by a chromium oxide shell during this calcination process, and slight aggregation of Au24Pt occurred during calcination (Figure 15), probably because of the weak interaction between Au24Pt(PET)18−y(p-MBA)y and Cr2O3. An increase in the diameter decreases the surface energy of the metal cluster.70 This seems to be the reason why the metal clusters could not be covered by a chromium oxide shell after removal of the ligands by calcination. According to our previous study,15 part of the chromium oxide in the calcined photocatalyst is further oxidized to higher than


29


Cr3+. Such chromium oxide with higher oxidation states can be reduced to Cr3+ by irradiation with a high-pressure Hg lamp (400 W) in aqueous solution. The obtained photocatalyst was then irradiated with a high-pressure Hg lamp (400 W) in aqueous solution for 1 h to reduce the chromium oxide to Cr2O3. This procedure also led to formation of a Cr2O3 shell on (Au24Pt)1–3, probably because of the decrease in the surface energy of chromium oxide. In this way, (Au24Pt)1– 3–Cr2O3–BaLa4Ti4O15

with (Au24Pt)1–3 covered by a Cr2O3 shell was obtained (top of Figure 15).

4.4. Measurement of the Photocatalytic Activity In the following experiments, the evolved gases were analyzed by gas chromatography (Shimadzu GC-8A equipped with a thermal conductivity detector and a 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 consisting of a high-pressure Hg lamp (400 W) and a quartz cell.15 The reaction was performed with a flowing Ar gas rate of 30 mL/min. Before the measurements, the reaction solution containing the 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. H2 Evolution Using a Sacrificial Reagent: In this experiment, the H2-evolution ability of the photocatalyst was estimated using methanol as a sacrificial reagent (Figure S22(a)). A methanol solution (10%, 350 mL) containing the photocatalyst (500 mg) was irradiated with a high-pressure Hg lamp (400 W) under Ar flow of 30 mL/min at room temperature.15 O2 Photoreduction: In this experiment, the decrease of the amount of evolved H2 was examined to investigate the likelihood of the O2-photoreduction reaction (Figure S22(b)). In particular, the H2-evolution ability (see the above section) of the photocatalysts was investigated under a gas flow


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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.15

4.5. Characterization. The MALDI mass spectra were recorded with a spiral time-of-flight mass spectrometer (JMSS3000, JEOL) equipped with a semiconductor laser (λ = 349 nm). DCTB was used as the MALDI matrix. To minimize cluster dissociation induced by laser irradiation, the cluster-to-matrix ratio was fixed at 1:1000. The DR spectra were acquired at ambient temperature with a V-670 spectrometer (JASCO). The wavelength-dependent optical data (I(w)) were converted to energy-dependent data (I(E)) by the following equation that conserved the integrated spectral areas: I(E) = I(w)/|∂E/∂w| ∝ I(w) × w2. ICP-MS was performed with an Agilent 7500c spectrometer (Agilent Technologies, Tokyo, Japan). Bi was used as the internal standard. The ICP-MS measurements were performed for the supernatant obtained after mixing Au24M(PET)18 or Au24M(PET)18−y(p-MBA)y (M = Au, Pd, and Pt) with the photocatalyst to estimate the unadsorbed Au content. The adsorption efficiency in each experiment was estimated on the basis of this value. The TEM and HR-TEM images were recorded with a JEM-2100 electron microscope (JEOL) operating at 200 kV, typically using magnification of 400,000–600,000. Thermogravimetric analysis (TGA) was performed with a Bruker TGA2000SA thermogravimetric analyzer at a heating rate of 7 °C/min under Air flow over the temperature range 25–300 °C using 3–5 mg samples of Au24M(PET)18–y(p-MBA)y (M = Au, Pd, and Pt). The Au L3-, Pd K-, and Pt L3-edge XAFS measurements were performed at beamlines BL01B1 and BL37XU of the SPring-8 facility of the Japan Synchrotron Radiation Research


31


Institute (proposal numbers 2017A0910, 2017B0910, 2017B0919, 2018B0919, and 2018B1422). The incident X-ray beam was monochromatized by a Si(111) double-crystal monochromator. The XAFS spectra of Au foil (Au L3-edge), Pd foil (Pd K-edge), and Pt foil (Pt L3-edge) as references were recorded in transmission mode using ionization chambers. The Au L3-, Pd K-, and Pt L3-edge XAFS spectra of the photocatalyst samples were measured in fluorescence mode using a 19element Ge solid-state detector at room temperature. The X-ray energies for the Au L3- and Pt L3edges were calibrated using Au foil and that for the Pd K-edge was calibrated using Pd foil. The XANES and EXAFS spectra were analyzed using the REX2000 program (version 2.5.9, 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 the XANES spectra. The k3-weighted χ spectra in the k range 3.0–13.0 Å−1 for the Au L3-edge, 3.0– 8.3 Å−1 for the Pt L3-edge, and 3.0–12.0 Å−1 for the Pd K-edge were Fourier transformed into r space for structural analysis. The curve fitting analysis was performed in the ranges 1.9–3.1 Å, 0.9–2.8 Å, and 1.5–3.2 Å for the Au L3-, Pt L3- and Pd K-edge, respectively. In the curve fitting analysis, the phase shifts and backscattering amplitude functions of Au–S, Au–Au, Pt–O, Pt–Au, Pd–S and Pd–Au were extracted from Au2S, Au metal, PtO2, PdS, and Au3Pd, respectively, using the FEFF8 program by setting σ2 = 0.0036, where σ is the Debye–Waller factor.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Y. Negishi) Tel.: +81-3-5228–9145 E-mail: [email protected] (S. Yamazoe) Tel.: +81-42-677–2553


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Ms. Shihori Kajino, Mr. Takumi Terui, Mr. Shun Yoshino, Mr. Yutaro Mori, Mr. Kota Hamada, and Mr. Shuhei Ozaki for technical assistance. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (grant numbers JP16H04099 and 16K21402), Scientific Research on Innovative Areas “Coordination Asymmetry” (grant number 17H05385), and Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion” (grant number 18H05178). Funding from the Takahashi Industrial and Economic Research Foundation, Futaba Electronics Memorial Foundation, Iwatani Naoji Foundation, and Asahi Glass Foundation is also gratefully acknowledged.

Supporting Information. Results of curve fitting analysis of the EXAFS data, MALDI mass spectra, DR spectra, TGA data, additional EXAFS and XANES data, additional TEM and HRTEM images, schematic of Cr2O3 shell formation, additional activity data, and other additional figures. The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.

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