PNNL

Abstract

Understanding the composition and structure of amorphous precursor phases is fundamental for elucidating two-step crystallization mechanisms and designing shape- and size-controlled nanomaterials. However, that understanding is largely lacking for metal-organic framework compounds despite their growing significance as functional materials. Here, we report the crystallization of zeolite imidazolate frameworks-8 (ZIF-8, Zn(C4H5N2)2) via an amorphous ZIF (am-ZIF) solid precursor phase with a rough stoichiometric composition of Zn(C4H5N2)1.86(C4H6N2)0.23(CH3COO)0.14. The formation of am-ZIF is attributed to the incomplete deprotonation of 2-Methylimidazole (HmIm) and the involvement of the hydrogen bond between CH3COO- and –HN, which can further transform into the dense Dia(Zn) structure with a diamondoid crystal topology in pure water. Taking am-ZIF as a precursor, the tunable dissolution and recrystallization kinetics of am-ZIF into ZIF-8, due to the addition of EtOH and CTAB, allows the selective fabrication of dodecahedral, cubic, and hollow ZIF-8. Overall, an in-depth understanding of the differences in composition and structure of am-ZIF from ZIF-8 and the resulting crystallization kinetics suggests a novel approach to designing metal-organic frameworks with controlled crystal morphology.