Fig. 9. Proposed mechanism of formation and evolution of Cu/TiO₂ photocatalysts upon thermal treatment, showing the thermally induced densification and diffusion of the Cu species. The bottom scheme illustrates the state of Cu-based co-catalysts during the wet-impregnation synthesis with the outer layer enriched with Cu²⁺, while the Cu species at the TiO₂ surface partially turn into Cu⁺ and Cu⁰via direct electron reduction.