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 PtO_x 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 ^-.