Remarkable Improvement of the Photocatalytic Activity of Ga₂O₃ Towards the Overall Splitting of H₂O

Abstract

. 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...

Understanding synergetic effects of Zn and Rh–Cr promotion to wide-bandgap Ga, Ta and Ti oxides in photocatalytic water splitting

“…The results suggest a strong synergetic promoting effect between the doping and the co-catalytic components, which cannot be explained by simple additive combination of the two contributions. The catalytic activities reported here are in good agreement with those reported for Sakata's commercial Ga 2 O 3 modification series 7 (ca. 50% of the reported batch reactor activity for Zn/Rh-Cr modified sample) with the exception of a lower oxygen evolution for bare Ga 2 O 3 and Ga 2 O 3 Zn samples in our study (very close to the detection limit as shown in Table 1), although differences in reactor design, catalyst amount and lamp power do not allow a strict comparison.…”

“…A common productivity trend was consistently found in the order of bare oxide < Zn < Rh-Cr < Zn/Rh-Cr modified materials. The intrinsic activity of the metal oxides was also found to be extremely important, as the photocatalytic activity 7 were reached testing Ga 2 O 3 materials. Tantalum based oxides also displayed excellent activity but with an order of magnitude below the promoted Ga 2 O 3 materials.…”

“…7 In this work, we aim to (i) investigate the general validity of the promotional strategy using Zn and Rh-Cr for water splitting, (ii) examine the activity of Zn-and Rh-Cr-promoted materials in CO 2 reduction, and importantly (iii) elucidate the role of Zn and Rh-Cr promotors. Ga 2 O 3 , Ta 2 O 5 , NaTaO 3 and TiO 2 were used as starting materials.…”

Photocatalytic Water-Splitting Reaction from Catalytic and Kinetic Perspectives

“…The contrast in the characterization and interpretation may arise from the use of different types of SrTiO 3 as supports and the different amounts and temperatures used for the loading and treatment of nickel species as cocatalysts. NaTaO 3 doped with La [44] and Ga 2 O 3 doped with Zn [45] exhibit the highest known QY water-splitting rates under UV irradiation after successful catalyst loading with NiO and Rh 2-y Cr y O 3 , respectively. Both NiO and Rh 2-y Cr y O 3 cocatalysts improved hydrogen evolution activity, which is essential for achieving overall water splitting.…”

Effects of Zn2+ and Pb2+ dopants on the activity of Ga2O3-based photocatalysts for water splitting

“…[25][26][27] Recently, it was found that the activity of Ga 2 O 3 -based photocatalyst could be enhanced by doping with foreign ions such as Zn 2+ . 21,28 However, doping Ga 2 O 3 with Zn 2+ ions can also result in a decrease of photocatalytic H 2 evolution activity. 23 Doping Ga 2 O 3 with other ions often leads to a decrease of photocatalytic activity.…”