High-Entropy Spinel Gallate Photocatalyst for Visible-Light-Driven Water Oxidation

High-entropy metal oxides represent an emerging and conceptually distinct platform for developing innovative visible-light-active photocatalysts for solar-driven water splitting. Here, we report the rational synthesis of a gallate-based high-entropy spinel oxide photocatalyst, Fe_0.3Co_0.3Ni_0.3Cu_0.3Zn_0.3Ga_1.5O₄ (Ga_1.5-HES), for a visible-light-driven oxygen-evolution reaction (OER). Through deliberately reducing the ratio of Ga to transition metals below the conventional stoichiometry of spinel gallates, the OER-active Fe/Co/Ni elements are driven to occupy both tetrahedral and octahedral sublattices of the spinel lattice, resulting in the mixed valence states that effectively optimize the catalytically active centers for oxygen evolution. The resulting Ga_1.5-HES exhibits a tailored electronic structure with a narrow bandgap of ∼2.15 eV and band-edge positions suitable for water oxidation. Under visible-light irradiation and without any cocatalyst, Ga_1.5-HES achieves efficient oxygen evolution with an apparent quantum yield (AQY) of ∼1.7% at 450 nm, and further enables stoichiometric overall water splitting when integrated into a mediator-assisted indirect Z-scheme.