Generation and Mechanism of Hydroxyl Radicals in an Aqueous Dopamine/TiO₂ System Independent of Ultraviolet Irradiation

Hydroxyl radicals (•OH), one of the prevalent reactive oxygen species (ROS), are intensively employed in advanced oxidation processes (AOPs). Photoinduction is a common means of generating •OH, but the light wavelengths usually lie in the ultraviolet band, limiting light energy utilization. This study reports a system of •OH generation in water independent of ultraviolet irradiation. Abundant •OH was found under visible illumination (λ ≥ 400 nm) in an aqueous solution with dopamine (DA) and rutile TiO2 (R) at pH = 7–11, which was evidenced by using 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) capture and subsequently secondary radical spin trapping with dimethyl sulfoxide (DMSO) via electron paramagnetic resonance (EPR). Light wavelength and pH significantly affect •OH quantity and formation pathways. The ligand-to-metal charge transfer (LMCT) occurring in the photoexcited DA was the primary formation mechanism of •OH under the visible spectrum. The electrons transferred to the R light trapping sites excited directly to the conduction band (CB) and subsequently reacted with the dissolved oxygen (DO) adsorbed on the R catalytic sites, and then •OH could be generated followed by a multistep reduction. The generation of •OH independent of UV irradiation has a promising prospect for contaminant removal, disinfection, and sterilization by energy-saving methods in the future.