Reactive Chemical Environments Control Charge Carrier Selectivity and Photovoltage at Nanoparticle Electrocatalyst/Semiconductor Junctions in Solar Water Splitting

Interfacial charge transfer at electrocatalyst/semiconductor (EC/SC) junctions is central to the performance of photo(electro)catalysts, yet the influence of the reactive environment on these processes remains poorly understood. This is particularly the case for unburied EC/SC junctions, such as EC nanoparticles anchored on a SC (np-EC/SC), where reacting molecules readily access the EC surface sites and the np-EC/SC interfaces. Herein, we uncover a dynamic, chemically driven mechanism by which the local reaction environment modulates charge transfer at Pt/p-Si interfaces under solar water splitting conditions. We demonstrate that molecular adsorption of H₂ and O₂ at the metal/electrolyte interface induces interfacial dipoles on Pt nanoparticles, effectively tuning their work function and shifting the junction from Ohmic to rectifying behavior. This environment-responsive modulation of the Schottky barrier height governs charge carrier selectivity independently of the commonly cited pinch-off effect, which is found to be negligible. Additionally, a spontaneously evolved thin SiO_x interlayer facilitates tunneling-mediated charge transfer while suppressing recombination, providing an additional degree of control over interfacial energetics. These findings reveal that catalytic surface chemistry can serve as a powerful lever for tuning electronic structure and photovoltage in nanoscale photoelectrode architectures, opening new design strategies for high-efficiency solar fuel systems.