Insight and Control of the Crystal Growth of Zeolitic Imidazolate Framework ZIF-67 by Atomic Force Microscopy and Mass Spectrometry

Combination of electrospray ionization mass spectrometry (ESI-MS) and in situ atomic force microscopy (AFM) are applied to provide the first nanoscopic study of the crystal growth of zeolitic imidazolate framework ZIF-67. ZIF-67 is found to form through a process of nucleation and spreading of metastable unenclosed substeps to form stable surface steps of the enclosed framework structure and demonstrates that isostructural MOFs, ZIF-67 and ZIF-8, undergo identical crystal growth mechanisms. The information on the crystal growth species obtained from the AFM experiments correlates well with the solution species identified by ESI-MS indicating that the species involved in the growth under low supersaturation conditions are methylimidazole/methylimidazolate, monomeric nonmethylimidazole/methylimidazolate complexed Co²⁺ ions and monomeric complexed [Co­(methylimidazole/methylimidazolate)_1‑2] ions. Combination of the use of low supersaturation growth solutions and in situ AFM has also allowed the successful extraction of the synthetic conditions necessary for formation of ZIF-67 nanodots that possesses a maximum vertical dimension of 1.2 nm which is the smallest dimension reported for a stable ZIF-67 entity. This methodology may be expanded to understand the formation of, and to form, complex crystal forms of other MOFs for new or improved functionality