Modulating Coordination Environment of Single Nickel Atom in Covalent Organic Framework to Enhance Photocatalytic Hydrogen Evolution Reaction

Covalent organic frameworks (COFs) have emerged as fascinating platforms for photocatalytic hydrogen evolution reactions in recent years. However, the relationship between the coordination environment of metals in a covalent organic framework and catalytic properties is still rarely studied. In this study, a covalent organic framework (COF-BP), containing a benzothiazole unit with typical electron-withdrawing properties, was successfully synthesized from 4,4'-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline and 3,3',5,5'-tetraformyl-4,4'-biphenyldiol. The free hydroxyl and imine groups in COF-BP were used for postmodification loading of Ni²⁺ to form COF-BP-Ni1, and then the coordination environment of Ni was further modulated through ligand exchange, replacing acetate anions by salicylideneaniline to construct COF-BP-Ni2 containing Schiff base-Ni complexes. The experimental results demonstrated that modulating the ligands of metal within the COF to alter its coordination environment enhanced the synergy with the COF framework, thereby promoting photoelectron separation and transfer, further significantly improving its photocatalytic activity. COF-BP-Ni2 showed good photocatalytic hydrogen evolution performance with a rate of 12.21 mmol g^-1 h^-1 in the presence of Pt as cocatalyst, which was 2.2-fold that of COF-BP-Ni1 at the same condition. Interestingly, the hydrogen evolution rate of COF-BP-Ni2 reaches 1.27 mmol g^-1 h^-1 even without Pt as a cocatalyst. This work provides new insights into how to improve the catalytic performance of COF-based catalysts.