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

Abstract

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 H2O2 via 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.

In Situ Growth of Cs3Bi2Br9 Quantum Dots on Bi-MOF Nanosheets via Cosharing Bismuth Atoms for CO2 Capture and Photocatalytic Reduction

“…In addition, in Bi-MOFs, the Bi species acted as an electron shuttle channel for charge transfer between the ligands . This ligand-to-ligand charge transfer (LLCT) differs from the widely accepted ligand-to-metal charge transfer (LMCT) mechanism in most MOFs and can prolong the lifetime of photogenerated charge carriers, making it significantly advantageous to be applied in photo- or photoelectric catalysis. − It has been reported that layered Bi-MOFs exhibited great activity and selectivity for electrochemical reduction of CO 2 to HCOOH due to the lower Gibbs free energy of the *COOH intermediates . Using surface-mounted Bi-MOFs as specific sites on the two-dimensional (2D) BiOBr support, the p band center of Bi nodes shows large downshifts and their unsaturated state is intensified, enabling the orbitals of CO 2 2p to shift downward its Fermi level, stimulating the rapid adsorption and activation of CO 2 .…”

Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C–C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light

“…The modification of heterogeneous photocatalysts, such as TiO 2 , in order to shift their photoactivity spectrum from UV to visible light [ 10 , 34 , 35 , 36 , 37 ] is the key task for expanding the scope of their applications in organic synthesis, increasing selectivity and making the of use cheap and available light sources for catalyst activation possible. At present, the following modification approaches have been proposed: the immobilization of dyes (organic compounds or metal complexes) on the photocatalyst surface [ 34 , 38 , 39 , 40 , 41 ], doping with metal ions or non-metal elements [ 42 , 43 ], semiconductor coupling [ 7 , 44 , 45 , 46 , 47 , 48 , 49 ] and modification with organic molecules bearing hydroxyl or carboxyl groups [ 34 , 50 , 51 , 52 , 53 , 54 , 55 , 56 ], which demonstrate the occurrence of visible light absorption when adsorbed on the surface of a semiconductor.…”

Visible-Light-Driven Photocatalytic Dehydrogenation of Alcohols on TiO2 via Ligand-to-Metal Charge Transfer for Coproduction of H2 and Aldehydes

“…To broaden the light-absorption range of TiO 2 , some modification methods such as element doping, − semiconductor coupling, − and dye sensitization , have been developed. Besides these popular methods, another promising approach to visible-light activation of wide band-gap TiO 2 is the formation of surface complexation to enable ligand-to-metal charge transfer (LMCT). − In such an approach, organic or inorganic compounds are adsorbed and coordinated to TiO 2 , forming surface complexes and introducing new absorbance bands in the visible-light region, even though the organic or inorganic molecules themselves do not absorb visible light alone. The formed surface complexes enable the direct photoexcitation of electrons from the highest occupied molecular orbital (HOMO) of the adsorbate (ground state) to the conduction band of TiO 2 (Scheme ).…”