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

Given the widespread use of TiO, 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/TiO complexes (HA/TiO) 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 HO the conduction band. By systematically considering molecular-scale interactions between TiO and organic functional groups in HA, we find a key role of phenolic groups in visible-light absorption and HO photogeneration. The photochemical formation of HO in river waters spiked with TiO is notably elevated above naturally occurring HO generated from background organic constituents due to LMCT contribution. Our findings suggest that HO generation by HA/TiO is related to the quantity and functional group chemistry of DOM, which provides chemical insights into photocatalytic activity and potential ecotoxicity of TiO in environmental and engineered systems.