Molybdenum-substituted polyoxometalate as stable shuttle redox mediator for visible light driven Z-scheme water splitting system

Abstract

In this study, a Mo-substituted Keggin-type polyoxometalate [SiW11MoVIO40]4− was examined as a shuttle redox mediator in Z-scheme water splitting, and its electrochemical and chemical stability were also investigated. The redox cycle of [SiW11MoVIO40]4−/[SiW11MoVO40]5−, which corresponds to the valence change between the incorporated MoVI/MoV species, was confirmed to possess appropriate redox potential (+0.58 V vs. SHE at pH 2.7) for use as a shuttle redox (i.e. between water reduction and oxidation potentials). A sufficiently stable redox cycle in acidic (pH 2.7) aqueous solutions was indeed exhibited, whereas slow but obvious degradation was observed under mildly acidic conditions (pH 4.5). Under visible light, the oxidized form, [SiW11MoVIO40]4−, was found to function as an electron acceptor over WO3-based photocatalysts (i.e., an O2-evolving system), and the reduced form, [SiW11MoVO40]5−, was found to function as an electron donor over Ru-loaded SrTiO3 doped with Rh species (i.e., a H2-evolving system). Finally, two-step water-splitting into H2 and O2 was demonstrated under visible light using [SiW11MoVIO40]4−/[SiW11MoVO40]5− couples as a shuttle redox mediator between Ru/SrTiO3:Rh and PtOx/WO3 photocatalyst particles, under acidic conditions with pH 2.7.