Electrochemical Identification of Surface States Limiting Overall Water Splitting in Particulate Oxynitride Photocatalysts

Oxynitride photocatalysts drive the hydrogen and oxygen evolution reactions under visible light with sacrificial reagents, but overall water splitting remains challenging, pointing to unresolved interfacial energetics. Here we integrate SrTaO 2 N and the (Ba x Sr 1– x )TaO 2 N solid solution into particulate photoelectrodes and quantitatively probe their semiconductor properties at the solid/liquid interface. Beyond the rectifying behavior established for single-crystal photoelectrodes, electrochemical measurements using a reversible Fe 3+ /Fe 2+ redox couple identify a defect-derived surface state located at 0.5 V vs the reversible hydrogen electrode in photocatalysts that mediates irreversible electron transfer. While this state enables half-reactions and rationalizes Z-scheme water splitting with suitable mediators, it promotes carrier recombination and thereby suppresses overall water splitting. This study establishes an electrochemical methodology that bridges photocatalyst powders and interfacial energetics and indicates that eliminating or passivating the defect states should enable overall pure water splitting in oxynitrides.