Directed Substitutional Control Unlocks Intrinsic Water Oxidation and Reduction Activity in an Oxyhalide Photocatalyst

To realize highly optimized properties and performance of semiconductor photocatalysts, precise control over their composition and the site occupancy of multiple cations and anions is essential. This study demonstrates that Bi2YO4Cl, a multicationic oxyhalide photocatalyst, exhibits markedly higher activity when isovalent Bi-for-Y substitution is suppressed by simply controlling the precursor stoichiometry. The flux synthesis of Bi2YO4Cl under stoichiometric conditions induces the partial substitution of Bi3+ into Y3+ sites, creating localized states near the valence band maximum, which act as hole traps and hinder efficient charge carrier utilization. Using an excess of Y2O3 during the synthesis suppresses the undesired Bi-for-Y substitution, leading to markedly higher H2 and O2 evolution rates under visible-light irradiation. This study highlights the critical importance of precise cation placement for maximizing the photocatalytic performance of multicationic photocatalysts.