Grafting-Induced Dual Electric Fields in Metal-Organic Frameworks for Efficient Piezo-Photocatalytic CO₂ Reduction

Metal-organic frameworks (MOFs), a class of porous crystalline materials with tunable coordination structures, offer unique opportunities for CO₂ photoreduction; however, their application in piezo-photocatalysis remains limited by poor visible-light absorption, high structural symmetry, and low polarity. Herein, we report a CN-grafting strategy that anchors carbon-nitrogen (CN) species onto coordination-unsaturated Ni sites of Ni-BDC, constructing NiM-CN with a grafting-induced internal electric field (IEF). This modification enhances the structural asymmetry and polarity, thereby strengthening piezoelectric polarization and generating a polarization-induced electric field (PEF) under mechanical stimulation conditions, as demonstrated by the increased piezoelectric coefficient (d₃₃, from 42.41 to 82.16 pm V^-1 compared to the reference sample). Meanwhile, CN grafting narrows the band gap and strengthens the IEF in NiM-CN through Ni 3d-N 2p orbital hybridization and asymmetric charge redistribution. The synergistic effect of IEF and PEF accelerates charge separation and transfer, leading to enhanced surface charge accumulation, as confirmed by in situ Kelvin probe force microscopy (KPFM). Consequently, NiM-CN achieves a remarkable CO production rate of 3406.36 µmol g^-1 h^-1 under piezo-photocatalytic conditions. This work establishes grafting-induced IEF engineering as an effective strategy for constructing coupled electric-field systems in MOFs for efficient piezo-photocatalysts.