Nitrogen/oxygen dual-defects modified g-C₃N₄ nanosheets for boosting photocatalytic CO₂ reduction

While thermal air exfoliation is widely used to prepare graphitic carbon nitride (g-C₃N₄) nanosheets, the effects of calcination conditions and atmosphere on their electronic structure and photocatalytic CO₂ reduction reaction (CO₂RR) performance remain systematically unexplored. We prepared g-C₃N₄ nanosheets with varying thickness and defects by controlling exfoliation parameters. The obtained nanosheets calcined longest in air exhibited highest CO₂RR activity, twice that of bulk g-C₃N₄. The comprehensive analysis of structural characterizations indicates the thickness of g-C₃N₄ nanosheets became thinner, and the defects increased as the calcination time increased. The N vacancies (N_v) and O-doping caused by N₂ and O₂ from air, respectively, enable valence band elevation (N_v) and conduction band depression (O-doping) that collectively redistribute the electronic structure. Nitrogen/oxygen dual-defects generated impurity levels, reduced the work function and band gap of g-C₃N₄ nanosheets, and served as shallow traps for photogenerated e⁻. The results of in-situ spectroscopy indicate these increased effective e⁻ are enriched around of N atoms to react with the adsorbed CO₂. During the CO₂ reduction process, the N_v promoted the formation of *COOH, and this dual-defect co-promoted the *CO desorption, resulting in the improved CO₂RR activity. These results comprehensively analyze the regulatory effect of thermal air calcination on the electronic structure of g-C₃N₄, providing valuable insights for designing g-C₃N₄ nanosheets based photocatalysts for CO₂RR.