Interfacial charge redistribution regulates surface hydroxyl dynamics during photocatalytic nitric oxide oxidation

Water can replenish reactive surface hydroxyls during photocatalytic nitric oxide (NO) oxidation, yet excessive hydration blocks adsorption sites. Here, Ag₂O was deposited on hydroxyl-rich zinc hydroxystannate (ZnSn(OH)₆, ZSOH) to couple interfacial charge transfer with dynamic surface-hydroxyl evolution under visible light. The optimized sample removed 69.8% of NO after 30 min and exhibited repeatable activity over five light-on/off cycles. At relative humidities of 5%, 50%, and 95%, NO removal reached 74.3%, 61.8%, and 51.3%, respectively. X-ray photoelectron spectroscopy, work-function calculations, and differential charge density revealed electron redistribution from ZSOH to Ag₂O, while time-resolved photoluminescence, electrochemical impedance spectroscopy, and photocurrent measurements demonstrated improved carrier separation and transport. Electron spin resonance detected enhanced formation of photogenerated electrons, •O₂^-, and •OH. Density functional theory further showed that Ag₂O coupling strengthened H₂O adsorption and elongated the average adsorbed OH bond. In-situ infrared spectroscopy revealed irradiation-induced redistribution and depletion of surface hydroxyls, followed by water-driven partial restoration of the OH environment. These results support an interfacially mediated hydroxyl consumption-regeneration cycle in which charge redistribution promotes carrier utilization and reactive oxygen species generation, whereas adsorbed water restores reactive hydroxyl environments. At high humidity, competitive water adsorption increasingly restricts NO and O₂ access. This work identifies interfacial electronic regulation of dynamic surface hydroxyls as a strategy for photocatalytic NO oxidation under variable humidity.