Disentangling the Activity-Charge Coupling in Cocatalyst-Semiconductor Heterojunctions for Optimized Oxygen Evolution

Abstract

Decoupling cocatalyst effects in photocatalysis, often intertwined between enhancing photocarrier accumulation and boosting intrinsic catalytic activity (ICA), is essential yet challenging for rational photocatalyst design. Using cocatalyst-IrO2/TiO2 heterojunction as a prototype, we integrate ab initio molecular dynamics with first-principles microkinetic modeling to elucidate the oxygen evolution reaction (OER) mechanism at the aqueous interface under varying light intensities. Our analysis reveals how IrO2 modulates photo-hole harvesting to regulate water activation and O-O coupling, enabling a quantitative separation of ICA from cocatalyst-mediated effects. Surprisingly, IrO2/TiO2 does not exceed bare TiO2 in ICA, as excessively strong oxygen binding at surface Ir sites hinders O-O bond formation. Instead, the enhanced O2 turnover originates from an increased surface hole concentration, facilitated by the superior hole accumulation capacity (HAC) of IrO2 and the formation of Schottky junction, which compensate for its limited ICA. Extending this analysis to rutile-type metal oxide cocatalysts with varying oxygen affinities uncovers an intrinsic ICA-HAC seesaw relationship that gives rise to a volcanotype OER activity trend. We further identify the work function (ɸ) as a unifying descriptor, with moderate-ɸ cocatalysts (e.g., RhO2 and CrO2 ) predicted to deliver optimal performance by balancing these competing effects. This study provides the first quantitative disentanglement of cocatalyst functions, establishing a general design principle for efficient OER cocatalysts.