Revealing the Structure of Single Cobalt Sites in Carbon Nitride for Photocatalytic CO₂Reduction

Single Co²⁺sites in graphitic carbon nitride (C₃N₄) have demonstrated excellent activity and selectivity in photocatalytic CO₂reduction. In this work, we combine computational and spectroscopic tools, including X-ray absorption spectroscopy, to probe the structure of such single Co²⁺sites. Our results indicate that the Co²⁺sites exist in the Co-N₂₊₂coordination at the edges of C₃N₄flakes. This mode of coordination is further supported by experimental results obtained using single Co²⁺sites in C₃N₄materials treated with NH₃, which contain more edge sites than untreated C₃N₄. In our experimental observations, doping C₃N₄with carbon is found to be important for the photocatalytic properties of the single Co²⁺sites. A simplified model is proposed to explain the origin of the observed enhancement effect of C doping. In this model, the presence of C dopant near the metal centers results in shorter Co-N bond length and stronger Co-N binding energy. In addition to enhanced light absorption and charge separation in C-doped C₃N₄, the stronger Co-N binding upon C doping likely contributes to the improved catalytic properties of the single Co²⁺sites.