Visible-Light Activation of a Dissolved Organic Matter-TiO₂ Complex Mediated via Ligand-to-Metal Charge Transfer

Given the widespread use of TiO₂, its release into aquatic systems and complexation with dissolved organic matter (DOM) are highly possible, making it important to understand how such interactions affect photocatalytic activity under visible light. Here, we show that humic acid/TiO₂ complexes (HA/TiO₂) exhibit photoactivity (without significant electron-hole activation) under visible light through ligand-to-metal charge transfer (LMCT). The observed visible-light activities for pollutant removal and bacterial inactivation are primarily linked to the generation of H₂O₂via the conduction band. By systematically considering molecular-scale interactions between TiO₂ and organic functional groups in HA, we find a key role of phenolic groups in visible-light absorption and H₂O₂ photogeneration. The photochemical formation of H₂O₂ in river waters spiked with TiO₂ is notably elevated above naturally occurring H₂O₂ generated from background organic constituents due to LMCT contribution. Our findings suggest that H₂O₂ generation by HA/TiO₂ is related to the quantity and functional group chemistry of DOM, which provides chemical insights into photocatalytic activity and potential ecotoxicity of TiO₂ in environmental and engineered systems.