Materials discovery and crystal growth of zeolite A type zeolitic-imidazolate frameworks revealed by atomic force microscopy.

A new zeolitic-imidazolate framework (ZIF), [Zn(imidazolate)2-x(benzimidazolate)x], that has the zeolite A (LTA) framework topology and contains relatively inexpensive organic linkers has been revealed using in situ atomic force microscopy. The new material was grown on the structure-directing surface of [Zn(imidazolate)1.5(5-chlorobenzimidazolate)0.5] (ZIF-76) crystals, a metal-organic framework (MOF) that also possesses the LTA framework topology. The crystal growth processes for both [Zn(imidazolate)2-x(benzimidazolate)x] and ZIF-76 were observed using in situ atomic force microscopy; it is the first time the growth process of a nanoporous material with the complex zeolite A (LTA) framework topology has been monitored temporally at the nanoscale. The results reveal the crystal growth mechanisms and possible surface terminations on the {100} and {111} facets of the materials under low supersaturation conditions. Surface growth of these structurally complex materials was found to proceed through both "birth-and-spread" and spiral crystal-growth mechanisms, with the former occurring through the nucleation and spreading of metastable and stable sub-layers reliant on the presence of non-framework species to bridge the framework during formation. These results support the notion that the latter process may be a general mechanism of surface crystal growth applicable to numerous crystalline nanoporous materials of differing complexity and demonstrate that the methodology of seeded crystal growth can be used to discover previously unobtainable ZIFs and MOFs with desirable framework compositions.