Facet Engineering in Ultrathin Two-Dimensional NiFe Metal–Organic Frameworks by Coordination Modulation for Enhanced Electrocatalytic Water Oxidation

Crystal facet engineering has been proved as a dramatically efficient strategy to optimize the intrinsic catalytic property of catalysts. However, because of the difficulty in precise construction, facet engineering of two-dimensional metal–organic frameworks (2D MOFs) is rarely reported. Herein, we propose a new and facile coordination modulation method to realize the controllable synthesis of 2D NiFe-MOFs with a highly exposed (001) crystal facet. Specifically, acetate ions, which possess the same coordination groups with the participated terephthalic ligand, have been adopted into the MOF construction as the morphology and facet regulator. Through competitive coordination effect and ion-induced self-assembly, the fabricated NiFe-MOFs reveal a hierarchical microsphere architecture consisting of ultrathin nanosheets with thickness ∼5 nm. Further benefiting from the structural advantages, e.g., abundant active sites, highly exposed catalytic facet, and enhanced mass/charge transport, the hierarchical NiFe-MOF nanosheets exhibit an attractive overpotential of 240 mV at the current density of 10 mA cm–2, a small Tafel slope of 73.44 mV dec–1, and satisfactory stability for 16 h toward oxygen evolution reaction (OER). Besides, the increased exposure of the (001) facet will increase the proportion of high-valent nickel oxyhydroxides during OER, which will enhance the electrocatalytic performance. The universality of the modulation method is further verified by the successful synthesis of a series of 2D MOFs (Ni, Co, Cu, Zn, and corresponding bimetal MOFs). This method may serve as an important inspiration for the preparation and crystal facet engineering of 2D MOFs.