Controlling Metal-Organic Framework/ZnO Heterostructure Kinetics through Selective Ligand Binding to ZnO Surface Steps

Abstract

Metal-organic framework (MOF) heterostructures exhibit unique properties beyond those of individual components, but their design requires an understanding of energetic and kinetic controls at MOF-substrate interfaces. Although the structural relationship has been widely used in heterostructure design, it overlooks the interplay between the organic ligand and the substrate which controls the kinetics and energetics of growth of the final structure. Herein we used zeolitic imidazolate frameworks (ZIF-8) on ZnO as a model system to evaluate this interplay via in situ monitoring and simulations. Our results demonstrate multiple roles of the 2-methyl-imidazole (2-MIM) ligand as "dissolution-promoter", "step-pinner", and "terrace-binder"on the ZnO (001) face and "dissolution-promoter"and "terrace-binder"on the ZnO (100) face. Through these multiple face-specific roles, 2-MIM modulates ZnO dissolution kinetics and, hence, the Zn2+ release rate, tuning local supersaturations that dictating the characteristic ZIF-8 crystallization kinetics on different substrate faces. The critical thickness for transition from 2D to 3D growth is dictated by competition between interfacial and strain energies. The atomic-scale mechanism of the coupled substrate dissolution and MOF growth, mediated by selective linker-substrate binding, furnishes a new synthesis pathway for other complex heterostructures.