Oxygen-Doped Red Carbon Nitride: Enhanced Charge Separation and Light Absorption for Robust CO₂ Photoreduction

Utilizing artificial photosynthesis for the conversion of CO₂ into value-added fuels has been recognized as a promising strategy for the ever-increasing energy crisis and the greenhouse effect. Herein, the element doping engineering of red spherical g-C₃N₄ having oxygen bonded with compositional carbon (C-O-C) for CO₂ photoreduction has been explored to address this challenge. The C-O bond was formed by hydrothermal treatment with dicyandiamide and 1,3,5-trichlorotriazine. The experimental and DFT results displayed the optimum oxygen substitution sites and demonstrated that the oxygen doping greatly improved the light utilization efficiency, CO₂ affinity, and charge carrier transfer, which enhanced photoreduction efficiency of CO₂. The evolution rates of CO (47.2 μmol g^-1) and CH₄ (9.1 μmol g^-1) using O-CN were much higher than that of bulk-CN without a cocatalyst. The main reason was the contribution of the O 2p orbital to the conduction band (CB) and valence band of O-CN, which effectively reduced the electron mass, facilitating electron/hole separation and enhancing its fluidity. Furthermore, the Fermi level also shifted to the bottom of the CB, leading to higher electron density, which further improved the CO₂ reduction ability.