Abstract: Cu metal and its oxides have attracted much attention for photocatalytic CO₂ reduction reaction (CO₂RR), but the stability and effects of Cu oxidation states on CO₂RR are not fully understood. Cu^x+/Cu⁰-loaded graphitic carbon nitride (g-C₃N₄) heterojunctions (Cu-CuO_x/g-C₃N₄) are fabricated via a stepwise calcination method for efficient photocatalytic CO₂RR. Cu₂O is the main component of Cu-CuO_x and the mixed valence Cu includes Cu⁰, Cu⁺, and Cu²⁺, which play the role of charge trapping sites and redox catalytic centers during the photocatalytic CO₂RR process. The main products were CO and CH₄ for the CO₂RR with production rates of 14.45 and 0.66 μmol g^-1 h^-1 for CO and CH₄, which were higher than those for g-C₃N₄ and Cu-CuO_x, respectively. This photocatalytic CO₂RR performance is attributed to the ultrafast switching of “Cu^x+-Cu⁰” and e_CB^-/h_VB⁺ trapping transformation in Cu-CuO_x benefited from the built-in IEF between Cu-CuO_x and g-C₃N₄, increasing the efficient photogenerated e_CB^-, and enabling the stability of Cu-CuO_x/g-C₃N₄. Cu^x+ adsorbed by H₂O works as the electron trapping site to change to Cu⁰ and switch to the hole trapping site; Cu⁰ works as the hole trapping site to change to Cu^x+ and switch to the electron trapping site, causing the CO₂RR of the adsorbed CO₂. Moreover, the coordinated Cu⁰ and Cu⁺ species facilitate the activation of the adsorbed CO₂ and *CO generation, these adsorbed *CO on Cu⁰ and Cu⁺ detected by in-situ DRIFTS quickly transformed to *CHO with a lower energy barrier benefited from the mixed Cu⁰/Cu⁺ active sites during CORR to produce CH₄. This finding provides a new insight into the influence of mixed valence Cu during photocatalytic CO₂RR.

Key words: Photocatalytic CO₂ reduction, g-C₃N₄, Mixed valence Cu, Trapping site, Active-site switches