Regulation of the molecular structure of carbon nitride by dual carboxylic group and nitrogen vacancy modulation for photocatalytic carbon dioxide reduction

The photoconversion efficiency of CO₂ is hindered by severe charge recombination and sluggish multi-electron kinetics at the surface reaction. For strengthening CO₂ adsorption and activation process, abundant nitrogen vacancy (N_v) and -COOH groups are introduced into potassium doped carbon nitride (KCN) by mechanical ball milling and acid treatment for construting KCN treated by Mg-assisted (KCN-M2). Compared with pristine KCN (2.13 μmol·g^-1·h^-1), the photocatalytic CO evolution rates of KCN-M1, KCN-M2, and KCN-M3 materials increase to 6.79, 11.32, and 7.06 μmol·g^-1·h^-1, respectively. KCN-M2 materials also show favorable photocatalytic CO evolution at low CO₂ concentrations (10% CO₂ (8.91 μmol·g^-1·h^-1) and air (0.04% CO₂ (6.16 μmol·g^-1·h^-1)). The comprehensive analysis reveals that N_v and -COOH groups can synergistically facilitate photogenerated electron transfer, CO₂ adsorption, and water affinity. Furthermore, density functional theory (DFT) computations for CO₂ conversion pathway reveal that KCN-M2 exhibits a lower energy change for the rate-determining step (*COOH → *CO) energy changes (0.82 eV) than pristine KCN (1.03 eV), confirming that N_v and -COOH groups facilitate the transformation of *COOH to *CO intermediate. The results provide insights into photocatalyst design, contributing not only to the artificial carbon cycle but also to the realization of the critical dual-carbon goals.