Optimizing charge transfer pathways in nickel metal-organic framework for enhanced CO₂ photoreduction

Nickel Metal-Organic Frameworks (MOFs) Ni-BDC (BDC: 1,4-benzenedicarboxylic acid) with coordination-unsaturated metal sites (CUMs) exhibits remarkable CO₂ reduction activity under visible-light irradiation. However, its photocatalytic mechanism remains a challenging due to complex charge-transfer pathways. Herein, a series of CUMs-tunable MOFs (Ni-BDC-D_x) were synthesized via a pyrazine-assisted aqueous-phase strategy for photocatalytic CO₂ reduction. A positive correlation is found between the CUMs density and photocatalytic activity, and Ni-BDC-D_1.0 with highest CUMs density achieves a remarkable CO production rate of 3503.1 μmol·g^-1·h^-1, which is 5.2 times higher than conventional bulk Ni-BDC. Experimental and theoretical results reveal that CUMs induce the formation of defect level and enhance CO₂ adsorption. The defect level serves as electron trap to promote localized charge accumulation by the optimizing charge transfer pathway. This pathway is achieved by facilitating non-radiative relaxation pathway to inhibit radiative carrier recombination behavior. These findings provide insights into electron transfer mechanism on CUMs-rich Ni-BDC for photocatalytic CO₂ reduction.