Supporting Information

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1 Supporting Information Au 25 -Loaded BaLa 4 Ti 4 O 15 Water-Splitting Photocatalyst with Enhanced Activity and Durability Produced Using New Chromium Oxide Shell Formation Method Wataru Kurashige, 1 Rina Kumazawa, 1 Daiki Ishii, 1 Rui Hayashi, 1 Yoshiki Niihori, 1 Sakiat Hossain, 2 Lakshmi V. Nair, 1 Tomoaki Takayama, 1 Akihide Iwase, 1,2 Seiji Yamazoe, 3,4 Tatsuya Tsukuda, 3,4 Akihiko Kudo, 1,2 and Yuichi Negishi 1,2,* 1 Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1 3 Kagurazaka, Shinjuku-ku, Tokyo , Japan. 2 Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba , Japan. 3 Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan. 4 Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto , Japan. Corresponding Author negishi@rs.kagu.tus.ac.jp; Tel: ; Fax: Preparation of compounds for comparison NaCrO2 and KCr3O8 were synthesized according to reported methods. 1,2 NaCrO2 was obtained by the following process. First, a mixture of Cr2O3 (6.58 mmol) and Na2CO3 (9.87 mmol) was heated to 900 C at a rate of 2 C/min, and then the mixture was maintained at this temperature for 5 h under N2 atmosphere. Pure NaCrO2 was obtained by adding Na2CO3 (9.87 mmol) to the resulting product and repeating the above heating process. KCr3O8 was obtained by the following process. K2Cr2O7 and CrO3 were mixed with a molar ratio of 1:2. The resulting mixture was heated to 350 C at a rate of 2 C/min and then held at this temperature for 2 h. After washing with water, pure KCr3O8 was obtained by repeating the above calcination process. 2. Additional Table Table S1. Results of Curve Fitting Analysis of Au L3-edge EXAFS Data Photocatalyst Bond C.N. a,b R (Å) a D.W. a,c R factor (%) a Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15 Au S 1.8(2) 2.287(3) 0.008(2) 7.5 Au 25 Cr xo y BaLa 4Ti 4O 15 Au Au 6.8(2) 2.813(3) 0.009(2) 8.9 Au 25 Cr 2O 3 BaLa 4Ti 4O 15 Au Au 7.7(2) 2.834(3) (15) 14.2 Numbers in parentheses are uncertainties; 1.8(2) and 2.287(3) represent 1.8 ± 0.2 and ± 0.003, respectively. a These values were obtained by fitting with Au S and Au Au bonds. b Coordination number c ΔDW = {(σ σ0) 2 }, where σ and σ0 represent Debye Waller factors of the sample and the standard value (0.06), respectively. S1

2 3. Additional Figures Figure S1. Structure of BaLa 4Ti 4O Figure S2. Schematic of the system used to estimate photocatalytic activity in this study. 4,5 Figure S3. Possible reactions that occur over a water-splitting photocatalyst during photocatalytic reaction: (a) H 2 evolution, (b) O 2 evolution, (c) back reaction, and (d) O 2 photoreduction. S2

3 Figure S4. Back reaction between H 2 and O 2 to produce H 2O (Figure S3c) in the gas phase in the dark over (a) Au 25 BaLa 4Ti 4O 15 and (b) Au NP BaLa 4Ti 4O 15. These experiments were conducted using a gas-tight circulation system with a dead volume of ml. In these experiments, 100 mg of photocatalyst was used for each measurement. Figure S5. Schematic of the effect of ultra-miniaturization of the Au cocatalyst on H 2 evolution and the O 2 photoreduction reaction. Figure S6. Cr 2O 3 coating of Au 25 on BaLa 4Ti 4O 15 by the photodeposition of Cr 2O 3 on supported Au 25 clusters. In this procedure, Au 25 BaLa 4Ti 4O 15 (500 mg) was added to aqueous K 2CrO 4 solution (350 ml) in a quartz cell. The mixture was stirred for 1 h at room temperature under Ar flow and then irradiated with UV light from a high-pressure Hg lamp (400 W) for 1 h. The ratio of chromium to BaLa 4Ti 4O 15 was fixed at 0.5 wt%. S3

4 Figure S7. Results for the elemental mapping of BaLa4Ti4O15: (a) scanning electron microscopy (SEM) image, (b) elemental mapping of La Lβ2, (c) elemental mapping of Cr Kα, and (d) SEM energy dispersive X-ray spectrum. These images and spectrum were obtained by a SEM apparatus (JSM-7600F, JEOL) operating at 20 kv using a magnification of 7,000. As shown in (d), the energy position of Cr Kα is quite similar to that of La Lβ2, making it difficult to obtain the exact Cr Kα mapping. For example, Cr Kα mapping could be obtained even for BaLa4Ti4O15 irrespective of the absence of Cr from this sample. Therefore, it was difficult to examine the deposition position of Cr2O3 on BaLa4Ti4O15 and thereby experimentally determine whether the main adsorption site of Au25(SG)18 was the Cr2O3 layer or the exposed BaLa4Ti4O15 surface. S4

5 Figure S8. Comparison of (a) Cr 2p 3/2 and (b) Ti 2p 3/2 spectra obtained for Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15 (top) and Au 25 Cr xo y BaLa 4Ti 4O 15 (bottom). In these figures, the red and green curves show the fitting results and the gray curves show the baselines. In each figure, the values in red indicate the relative area compared with the area of Cr 2p 3/2 in (a). The area ratio of Cr to Ti determined from the Cr 2p 3/2 and Ti 2p 3/2 peaks was 1:0.64 before calcination. This ratio changed to 1:0.56 after calcination. These ratios indicate that the area of the chromium-oxide layer on BaLa 4Ti 4O 15 increased following calcination. S5

6 Figure S9. Energy dispersive X-ray spectrometry line profile of Au 25 Cr xo y BaLa 4Ti 4O 15; (a) Scanning TEM (STEM) image and (b) Energy dispersive X-ray spectra at point 1 4 in (a). This image and spectra were obtained by a TEM apparatus (JEM-2100F, JEOL) operating at 200 kv using a magnification of 1,000,000. In (b), the clear peak related to Au (~2 kev) can be seen only at point 2 and 3, consistent with the interpretation that the Au 25 clusters moved inside the chromium-oxide layer after calcination. Figure S10. Au L 3-edge EXAFS oscillations of (a) Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15, (b) Au 25 Cr xo y BaLa 4Ti 4O 15, and (c) Au 25 Cr xo y BaLa 4Ti 4O 15 after UV irradiation for 1 h to form Au 25 Cr 2O 3 BaLa 4Ti 4O 15. S6

7 Figure S11. Proposed mechanism for Cr xo y coating formation on Au 25. Figure S12. XRD patterns for the compounds used for comparison. Powder diffraction file (PDF) numbers of the samples are (Cr 2O 3), (NaCrO 2), (KCr 3O 8), (K 2CrO 4), and (CrO 3). S7

8 Figure S13. Cr 2p 3/2 XPS analysis of (a) Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15, (b) Au 25 Cr xo y BaLa 4Ti 4O 15, and (c) Au 25 Cr xo y BaLa 4Ti 4O 15 after UV irradiation for 1 h to form Au 25 Cr 2O 3 BaLa 4Ti 4O 15. Comparison of (a) and (b) shows that a new peak appears at high binding energy (578.2 ev) after calcination, demonstrating that chromium oxide with a high degree of oxidation forms during calcination. This peak disappears after UV irradiation of the sample for 1 h, indicating that the chromium oxide with a high oxidation state returned to the chemical composition of Cr 2O 3 after UV irradiation. S8

9 Figure S14. Comparison of XRD patterns for (a) Cr 2O 3 BaLa 4Ti 4O 15, (b) Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15, (c) Au 25 Cr xo y BaLa 4Ti 4O 15, and (d) Au 25 Cr xo y BaLa 4Ti 4O 15 after UV irradiation for 1 h to form Au 25 Cr 2O 3 BaLa 4Ti 4O 15 together with those of (e) BaLa 4Ti 4O 15, (f) Cr 2O 3, and (g) CrO 3 for comparison. Assignments of peaks of (f) Cr 2O 3 and (g) CrO 3 are shown in Figure S12. The PDF number of BaLa 4Ti 4O 15 is S9

10 Figure S15. Enlarged XRD patterns for (a) Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr), (b) Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr), (c) Au 25 Cr xo y BaLa 4Ti 4O 15 (0.5 wt% Cr), and (d) Au 25 Cr xo y BaLa 4Ti 4O 15 (0.5 wt% Cr) after UV irradiation for 1 h to form Au 25 Cr 2O 3 BaLa 4Ti 4O 15. In the preparation of this figure, each XRD pattern was first normalized with the strongest peak of BaLa 4Ti 4O 15 (106), and then vertically expanded with the same magnitude. In (a) (d), the peak patterns attributed to crystalline Cr 2O 3 or CrO 3 are hardly observed, implying that the Cr 2O 3 shell does not have a crystalline structure. (e) XRD pattern of Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr) after calcination at 800 C for 6.5 h. In this XRD pattern, a series of weak peaks assigned to BaCrO 4, which is likely to be formed by the reaction between Cr 2O 3 and BaLa 4Ti 4O 15, was observed. This means that the absence of the peak pattern of crystal Cr 2O 3 in (a) (d) is not caused by the small amount of Cr on the surface but by the absence of crystalline structure. Thus, it can be interpreted that the Cr 2O 3 protective film has an amorphous structure similar to that in a previous study. 6 The PDF number of BaCrO 4 is Figure S16. Comparison of the apparent quantum yields at each wavelength and DR spectrum of BaLa 4Ti 4O 15. The apparent quantum yields are consistent with the DR spectrum, indicating that the photocatalytic reaction is caused by the light absorption by BaLa 4Ti 4O 15. S10

11 Figure S17. Comparison of the rates of gas evolution by photocatalytic water-splitting reactions of BaLa 4Ti 4O 15 and Cr 2O 3 BaLa 4Ti 4O 15. Figure S18. Comparison of Au L 3-edge XANES spectra of Au 25(SG) 18 BaLa 4Ti 4O 15, Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15, Au 25 BaLa 4Ti 4O 15, Au 25 Cr xo y BaLa 4Ti 4O 15, and Au 25 Cr xo y BaLa 4Ti 4O 15 after UV irradiation for 1 h to form Au 25 Cr 2O 3 BaLa 4Ti 4O 15 together with that of Au foil. This comparison shows that the Cr 2O 3 coating has little effect on the electronic state of Au 25, even though the ligand elimination causes a marked change of the electronic state of Au 25. S11

12 Figure S19. Comparison of the rate of gas evolution by photocatalytic water-splitting reaction over Au 25 Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr) with and without O 2. The O 2 photoreduction reaction is largely suppressed compared with the case of Au 25 BaLa 4Ti 4O 15 (Figure 5Ba). However, the suppression of the O 2 photoreduction reaction was not necessarily perfect in this experiment. Thus, it seems that a small number of unprotected Au 25 clusters still exist in Au 25 Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr). Figure S20. Comparison of the rates of H 2 and O 2 evolution by photocatalytic water-splitting reaction over Au 25(SG) 18 Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr) and Au 25 Cr 2O 3 BaLa 4Ti 4O 15 (0.5 wt% Cr). This comparison demonstrates the importance of calcination to remove the ligands. S12

13 Figure S21. HR-TEM images of Au25 CrxOy BaLa4Ti4O15 prepared with different contents of Cr: (a) 0.1, (b) 0.5, (c) 1.0, and (d) 1.5 wt%. In these images, film thicknesses are indicated by yellow double-pointed arrows. White double-pointed arrows indicate the Au particle size. Figure S22. TEM images and associated core-size distributions of Au25 CrxOy BaLa4Ti4O15 prepared with different contents of Cr: (a) 0.1, (b) 0.5, (c) 1.0, and (d) 1.5 wt%. S13

14 Figure S23. TEM image and associated core-size histograms of Au 25 Cr xo y BaLa 4Ti 4O 15 (a) before and (b) after UV irradiation for 1 h (Au 25 Cr xo y BaLa 4Ti 4O 15 vs. Au 25 Cr 2O 3 BaLa 4Ti 4O 15). 4. References (1) Komaba, S.; Nakayama, T.; Ogata, A.; Shimizu, T.; Takei, C.; Takada, S.; Hokura, A.; Nakai, I. Electrochemically Reversible Sodium Intercalation of Layered NaNi 0.5Mn 0.5O 2 and NaCrO 2. ECS Transactions 2009, 16, (2) Koksbang, R.; Fauteux, D.; Norby, P.; Nielsen, K. A. Lithium Insertion in LiCr 3O 8, NaCr 3O 8, and KCr 3O 8 at Room Temperature and at 125. J. Electrochem. Soc. 1989, 136, (3) Ohsato, H.; Tohdo, Y.; Kakimoto, K.; Okawa, T.; Okabe, H. Crystal Structure and Microwave Dielectric Properties of Ba nla 4Ti 3+nO 12+3n Homologous Compounds with High Dielectric Constant and High Quality Factor. Cerami. Eng. Sci. Proc. 2003, 24, (4) Negishi, Y.; Mizuno, M.; Hirayama, M.; Omatoi, M.; Takayama, T.; Iwase, A.; Kudo, A. Enhanced Photocatalytic Water Splitting by BaLa 4Ti 4O 15 Loaded with ~1 nm Gold Nanoclusters Using Glutathione- Protected Au 25 Clusters. Nanoscale 2013, 5, (5) Negishi, Y.; Matsuura, Y.; Tomizawa, R.; Kurashige, W.; Niihori, Y.; Takayama, T.; Iwase, A.; Kudo, A. Controlled Loading of Small Au n Clusters (n = 10 39) onto BaLa 4Ti 4O 15 Photocatalysts: Toward an Understanding of Size Effect of Cocatalyst on Water-Splitting Photocatalytic Activity. J. Phys. Chem. C 2015, 119, (6) Yoshida, M.; Takanabe, K.; Maeda, K.; Ishikawa, A.; Kubota, J.; Sakata, Y.; Ikezawa, Y.; Domen, K. Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall Water Splitting Studied by Model Electrodes. J. Phys. Chem. C 2009, 113, S14