Crystal\nGrowth of ZIF-8, ZIF-67, and Their Mixed-Metal\nDerivatives

Abstract

A facile method to\nproduce zeolitic imidazolate frameworks (ZIF-8,\nZIF-67, and solid–solution ZIFs (mixed Co and Zn)) is reported.\nZIF crystals are produced via a reaction–diffusion framework\n(RDF) by diffusing an outer solution at a relatively high concentration\nof the 2-methyl imidazole linker (HmIm) into an agar gel matrix containing\nthe metal ions (zinc­(II) and/or cobalt­(II)) at room temperature.\nAccordingly, a propagating supersaturation wave, initiated at the\ninterface between the outer solution and the gel matrix, leads to\na precipitation front with a gradient of crystal sizes ranging between\n100 nm and 55 μm along the reaction tube. While the precipitation\nfronts of ZIF-8 and ZIF-67 travel the same distance for the same initial\nconditions, ZIF-8 crystals therein are consistently smaller than the\nZIF-67 crystals due to the disparity of their rate of nucleation and\ngrowth. The effects of the temperature, the concentration of the reagents,\nand the thickness of the gel matrix on the growth of the ZIF crystals\nare investigated. We also show that by using RDF we can envisage the\nformation mechanism of the ZIF crystals, which consists of the aggregation\nof ZIF nanospheres to form the ZIF-8 dodecahedrons. Moreover, using\nRDF, the formation of a solid–solution ZIF via the incorporation\nof Co­(II) and Zn­(II) cations within the same framework is achieved\nin a controlled manner. Finally, we demonstrate that doping ZIF-8\nby Co­(II) enhances the photodegradation of methylene blue dye under\nvisible light irradiation in the absence of hydrogen peroxide.