. Various kinds of photocatalytic systems have been studied for this reaction, with an aim to develop new and effective photocatalysts. [1,2] However, thus far their photocatalytic performances, that is, not only their photoresponse but also their activity, have been insufficient for further applications, such as H 2 production from H 2 O under solar irradiation. Therefore, improving the activity as well as the photoresponse towards H 2 O splitting are important problems that need to be solved.When attempting to improve the photocatalytic activity, the reactivity of photocatalytically produced electrons and holes towards H 2 O should be improved by modifying the bulk and surface states of the photocatalyst. Kudo and co-workers [3][4][5] reported a remarkable improvement of the photocatalytic activity of NaTaO 3 towards H 2 O splitting by adding alkaline-earthmetal and La ions. Specifically, the enhanced activity was achieved by an improved separation and increased reactivity of electrons and holes due to morphological changes, resulting from the addition of the metal ions.Ga 2 O 3 has also been reported as photocatalyst for the overall splitting of H 2 O. [6,7] In earlier work, we investigated the effects of adding metal ions to Ga 2 O 3 on the photocatalytic properties of Ni/Ga 2 O 3 in the H 2 O splitting reaction, and reported that the addition of divalent cations (e.g., Ca 2 + , Sr 2 + , Ba 2 + , and Zn 2 + ) is effective for noticeably improving the photocatalytic activity.[7] The addition of Zn 2 + was the most effective, and markedly improved the photocatalytic activity of Ni/Ga 2 O 3 (by a factor of more than 10). [7] This improvement was due to increases of the mobility and concentration of photocatalytically produced holes, resulting from qualitative changes of Ga 2 O 3 bulk states by the addition of Zn 2 + ions. [7] Beside the addition of metal ions, various other modifications to Ga 2 O 3 photocatalysts can be expected to effectively improve the photocatalytic activity towards H 2 O splitting further. Herein, we investigate two important factors in order to improve the photocatalytic performance of Ga 2 O 3 : surface processes, by applying an effective co-catalyst, and bulk processes, by modifying the particle size of the Ga 2 O 3 photocatalyst. Figure 1 shows the evolution of H 2 and O 2 in time over Ni(1 wt %)/Ga 2 O 3 (P), Rh 0.5 Cr 1.5 O 3 (0.5 wt % Rh)/Ga 2 O 3 (P), and Rh 0.5 Cr 1.5 O 3 (0.5 wt % Rh)/Ga 2 O 3 (M), where Ga 2 O 3 (P) is a commercial material and Ga 2 O 3 (M) a material prepared by a precipitation method. Both Ga 2 O 3 materials have a b-form crystal structure, as confirmed by X-ray diffraction (XRD). The photocatalysts used in the experiment in Figure 1 were prepared according to the conditions described in the Experimental Section and Supporting Information. Notably, the figure shows that H 2 and O 2 are produced in the stoichiometric ratio of H 2 O splitting on all of the photocatalysts. In particular, the Rh 0.5 Cr 1.5 O 3 -loaded Ga 2 O 3 photocatalysts show a markedly higher...

Remarkably high photocatalytic activity for the overall H2O splitting, where the activity was 32 mmol h(-1) for H2 production and 16 mmol h(-1) for O2 production under irradiation from a 450 W high-pressure Hg lamp and the apparent quantum yield (AQY) was 71% under irradiation at 254 nm, was achieved by utilizing a Rh(0.5)Cr(1.5)O3(Rh; 0.5 wt%)/Zn(3 mol%)-Ga2O3 photocatalyst when Ga2O3 was prepared using dilute CaCl2 aqueous solution having a concentration of 0.001 mol l(-1).

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