Regulating Surface Electron Dynamics Enables Selective C─N Coupling in Photoelectrochemical Oxime Synthesis

A central challenge in photoelectrochemical multi-electron transformations is simultaneously achieving efficient charge separation and precise control over reaction selectivity. Herein, we construct a p-n junction photocathode composed of an atomic-layer-deposited ZnO_x layer and black silicon (b-Si) to regulate interfacial electron dynamics and reaction selectivity. Upon illumination, oxygen vacancies in the ZnO_x surface serve as electron-buffering sites that facilitate reversible charge storage and release, as confirmed by in situ Kelvin probe force microscopy. This modulation of electron chemical potential suppresses competing hydrogen evolution and over-reduction pathways, while directing nitrogen-centered intermediates toward selective hydroxylamine formation and subsequent C─N coupling. The optimized photocathode delivers a photocurrent density of 38.6 mA cm^-2 with a Faradaic efficiency of 94.6% for cyclohexanone oxime production and a cyclohexanone-to-oxime selectivity of 99.9%. Mechanistic studies combining operando spectroscopy, density functional theory, and microkinetic analysis reveal that coverage-dependent site competition governs surface site availability, thereby inhibiting the further reduction of hydroxylamine and promoting its desorption for solution-phase coupling. Integration of the microkinetic model with the photovoltaic response establishes a quantitative design principle based on the matching between semiconductor photoresponse and interfacial catalytic kinetics, providing a general framework for selectivity control in photoelectrochemical systems.