Soft seed-mediated dimensional control of metal–organic framework nanocrystals through oil-in-water microemulsions

Abstract

Metal–organic framework (MOF) nanocrystals with high surface area and morphological tunability have shown great potential for a wide range of industrial applications when compared with their bulk counterparts. However, achieving environment-friendly synthesis of MOF nanocrystals with minimal use of hazardous chemicals and low energy consumption remains a significant challenge. Herein, we developed a soft seed-mediated approach to synthesise nanocrystals of zeolitic imidazolate frameworks (ZIFs) in water without any assistance of chemical additives such as surfactants and organic solvents. An oil-in-water microemulsion, made from a trace amount of oil with water, was introduced to the reaction medium as the soft seed, and precise size control of ZIF-8 crystals from nanometres to micrometres was enabled by varying the volume of the oil-in-water microemulsion. Scanning electron microscopy and cryo-transmission electron microscopy analyses of the reaction intermediates revealed that the surface of the oil-in-water microemulsion functions as a nucleation site for ZIF-8 crystals, which dissipates over time. Furthermore, aromatic compounds with different numbers and lengths of alkyl chains for the oil phase were used to examine the chemical effect of the oil-in-water microemulsion on ZIF-8 crystal size, which led to fine-size control of ZIF-8 nanocrystals. Our microemulsion-induced synthesis could also be used to dimensionally control ZIF-67 crystals as well as ZIF-derived hollow nanostructures. ZIF-8 nanocrystals in our work exhibited enhanced catalytic activity for Knoevenagel condensation and dye adsorptivity with methyl orange and rhodamine B. We anticipate that our soft seed-mediated method will help improve the efficiency of MOF nanocrystal production for industrial applications.