Revealing the Structure of Single Cobalt Sites in Carbon Nitride for Photocatalytic CO2 Reduction

Single Co2+ sites in graphitic carbon nitride (C3N4) have demonstrated excellent activity and selectivity in photocatalytic CO2 reduction. 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+2 coordination at the edges of C3N4 flakes. This mode of coordination is further supported by experimental results obtained using single Co2+ sites in C3N4 materials treated with NH3, which contain more edge sites than untreated C3N4. In our experimental observations, doping C3N4 with 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.