Preparation of ZIF-8-coated silica hard-shell microcapsule by semi-batch operation

This paper reports the selective fabrication of a zeolitic imidazolate framework-8 (ZIF-8) cover layer over the outer surface of silica hard-shell microcapsules (HSMC) to improve the microencapsulation technique of phase change materials for heat storage materials. ZIF-8 was formed not only over the HSMC but also in the synthesis solution side when using a conventional batch operation owing to the rapid formation rate of ZIF-8 in the aqueous solution at 293 K. In this study, a semi-batch operation, which can control the concentration of the components in the synthesis solution by gradually dropping the components, was employed to decrease the formation rate of ZIF-8. X-ray diffraction results suggested that dropping ingredients strongly affected the obtained products; the addition of the Zn 2+ aqueous solution into a 2-methylimidazole (Hmim) aqueous solution was suitable for the formation of ZIF-8, whereas the addition of an aqueous solution of Hmim into the Zn 2+ aqueous solution formed Zn hydroxides as unfavorable by-products. Field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy results confirmed that the semi-batch operation with the addition of Zn 2+ into the Hmim aqueous solution selectively formed a ZIF-8 cover layer over the HSMC. We conclude that the semi-batch operation is an effective procedure for suppressing the unfavorable homogeneous nucleation on the solution side and making heterogeneous nucleation dominant without any additives to fabricate the ZIF-8 cover layer. The semi-batch operation effectively fabricated the ZIF-8 cover layer on silica hard-shell microcapsules.