Highly Dispersed RuO2 Hydrates Prepared via Simple Adsorption as Efficient Cocatalysts for Visible-Light-Driven Z-Scheme Water Splitting with an IO3–/I– Redox Mediator

Loading a cocatalyst onto a photocatalyst is a well-known effective way to improve the efficiency of both one-step water splitting and Z-scheme water splitting with a redox mediator. In Z-scheme water splitting systems with an IO3–/I– redox couple, the reduction of IO3– on O2-evolving photocatalysts via a six-electron process often represents the rate-determining step of the overall process and therefore necessitates effective cocatalysts such as PtOx and RuO2. However, these cocatalysts cannot be loaded onto thermally unstable materials via conventional impregnation processes involving calcination. In the present study, we introduce a Ru-based cocatalyst that can be loaded without calcination and effectively promotes the reduction of IO3– on various photocatalysts, including nonoxide materials. The results reveal that the Ru species adsorbed via simple stirring of photocatalyst particles such as WO3 in an aqueous RuCl3 solution effectively trigger O2 generation in the presence of IO3– as electron acceptor; moreover, the O2 evolution rate on the present Ru-loaded WO3 sample was much higher than that on WO3 loaded with conventional RuO2. The structural analysis indicates that the Ru species adsorbed on WO3 are highly dispersed and characterized by octahedral RuO6 environments similar to those found in RuO2·nH2O. It was confirmed that the loaded Ru species (most likely analogous to RuO2·nH2O) exhibit higher activity in the reduction of IO3– than anhydrous RuO2, whereas RuO2 exhibits much higher activity for the oxidation of water than RuO2·nH2O. The developed Ru-based cocatalyst was also applicable to thermally unstable materials such as H2WO4 and Ta3N5, thus enabling them to generate O2 in the presence of IO3–.