Tuning the size, morphology, and composition of ZIF-8 and Co-doped ZIF-8 crystals through the application of an electric field in a reaction-diffusion framework

Abstract

Recently in our lab, some metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) were successfully synthesized at room temperature through the reaction-diffusion framework (RDF) in a hydrogel medium. We pro- pose a development of this technique through the application of an electric field during ZIF crystal formation to further control size, morphology, and dopant in- corporation. We use a 2-D circular reactor and circular tungsten electrodes to apply potential differences in the range of ±2.0 V to a ZIF-8 system in agar gel throughout the course of precipitation. All solids are collected and characterized using scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD). Doped samples are also characterized using energy-dispersive X-ray (EDX) analysis. Our study shows that the effect of the external electric field on the size and morphology of the ZIF-8 particles depends on the nature of the metal salts used. Applying a positive field reduces the size of the particles in the sulfate salt system, and leaves the nitrate system unaffected while applying a negative field increases the size and enhances the morphology of the particles in the nitrate system, while leaving the sulfate system unaffected. In both systems, the application of a negative filed induces the formation of an unidentified spherical co-precipitate in areas close to the diffusion front. Similar trends are observed in a Zn-Co mixed-metal ZIF-8 system. The application of an external field also affects the extent of cobalt incorporation into the ZIF-8 framework in both sulfate and nitrate salt systems; the application of a negative field encourages the cobalt incorporation, and the positive field drives cobalt out of the framework.