fig3

Figure 3. ROS identification and the generation mechanisms. (A) Schematic illustration of the reversible change of the shell electrons during the cycling of Cu²⁺ and Cu⁺ on the surface of Cu/TiO₂ NPs; (B) The EPR spectra of radical adducts trapped by DMPO in Cu/TiO₂ under visible light irradiation, where DMPO-^•OH formed in aqueous dispersions and DMPO-O₂^•- formed in methanol dispersions; The EPR spectra of the TiO₂ and Cu/TiO₂ (C) under nitrogen and (D) in air. Inset in (C): the characteristic hyperfine structure of the Cu signal; (E) DO adsorbed on the excited state of Cu/TiO₂ (bearing Cu⁺) in water; (F) The mechanism of superoxide generation via inner sphere electron transfer, including the following steps: (1) formation of charge carriers by photon absorption, whereafter Cu²⁺ is converted to Cu⁺ by trapped photoelectrons; (2) Cu⁺ and DO form a new sigma bond by each donating a single electron; (3) generation of intermediate (Cu²⁺-dioxyenyl); (4) dioxygenyl cation accepts both sigma electrons during heterolytic Cu–O bond cleavage; (5) superoxide radical ion formation; (6) superoxide radical ion diffused off of the Cu/TiO₂ surface; (G) The mechanism of superoxide generation via outer sphere electron transfer, including the following steps: (1) formation of charge carriers by photon absorption, whereafter Cu²⁺ trapped photoelectrons; (2) photoelectrons in the Cu site react with physisorbed oxygen; (3) desorption of reduced O₂ to form superoxide ions; (4) superoxide radical diffused off the Cu/TiO₂ surface; (H) The mechanism of hydroxyl radical generation, including the following steps: (1) formation of charge carriers by photon absorption and homolytic Cu–O bond cleavage wherein h⁺ accepts one electron and the other remains on O with simultaneous nucleophilic attack on Cu by water; (2) formation of Ti-O^• and Cu-OH; (3) proton transfer between Ti-O^• and Cu-OH with displacement of hydroxyl radical by nucleophilic attack of Cu-O onto Ti; (5) hydroxyl radical diffuses away from the Cu/TiO₂ surface. Note: In f, g, and h, a half arrow represents the transfer of one electron, while a full arrow represents a transfer of two electrons. ROS: Reactive oxygen species; NPs: nanoparticles; EPR: electron paramagnetic resonance; DMPO: 5,5-dimethyl-1-pyrroline N-oxide; DO: dissolved oxygen.