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 Bi 2 YO 4 Cl, 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 Bi 2 YO 4 Cl under stoichiometric conditions induces the partial substitution of Bi 3+ into Y 3+ sites, creating localized states near the valence band maximum, which act as hole traps and hinder efficient charge carrier utilization. Using an excess of Y 2 O 3 during the synthesis suppresses the undesired Bi-for-Y substitution, leading to markedly higher H 2 and O 2 evolution rates under visible-light irradiation. This study highlights the critical importance of precise cation placement for maximizing the photocatalytic performance of multicationic photocatalysts.