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ANALYTICAL APPROACH TO THE STUDY OF CRYSTAL GROWTH IN NANOPOROUS MATERIALS

Wagia, Raghidah

[Thesis]. Manchester, UK: The University of Manchester; 2018.

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Abstract

Understanding the crystallization of MOFs is critical to transforming them from novel to applied functional materials. Such a complex process poses numerous questions concerning how the structure forms and determining the actual growth species involved in the process. Real-time nanoscale observations during crystal growth obtainable using atomic force microscopy (AFM) combined with electrospray ionization mass spectrometry (ESI-MS) can reveal answers to these questions. Combination of ESI-MS and in-situ AFM is applied to provide the first nanoscopic study of the crystal growth of zeolitic imidazolate framework ZIF-67 with the sodalite framework topology. ZIF-67 is found to form through a process of nucleation and spreading of meta-stable unenclosed sub-steps 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 for ZIF-67 and ZIF-8 (from previous work) correlate 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 non-methylimidazole/ methylimidazolate complexed Co2+/ Zn2+ ions and monomeric complexed [Co/ Zn (methylimidazole/ methylimidazolate)1-2] ions. Combination of the use of supersaturation growth solutions with different levels and in-situ AFM has also allowed the successful extraction of the synthetic conditions for the formation of ZIF-67 layers and nanodots grown on the isostructural ZIF-8 and ZIF-8 layers grown on ZIF-67 substrate crystal. Combination of in-situ AFM and ESI-MS was used to identify preformed, complex, cadmium 2-ethylimidazole containing solution species in the growth solution of the cadmium imidazolate framework CdIF-4 with the RHO framework topology, and show that they are critical in the surface nucleation for the crystal growth of this material. Surface nucleation appears to be instigated by these [Cdx(CH3CO2)y(C5H7N2/C5H8N2)z]-containing solution species. The in-situ AFM technique was used to study the dissolution mechanism of CdIF-4, and to extract the synthetic conditions to form layers of isostructural CdIF-9 with a step height 2.2 nm on the surface of CdIF-4 crystal. The crystal growth and dissolution mechanism on zeolitic imidazolate framework ZIF-302 with the chabazite framework topology were investigated for the first time for this topology using in-situ AFM method. It is revealed that growth process of ZIF-302 occurs through the nucleation and spreading of successive metastable sub-steps to form stable terrace steps of the framework structure with an initial sub-step 0.4 nm that corresponds to the height of linker 5(6)-methylbenzimidazolate (mbIm) on (001) facet. The formation of step bunching was observed on ZIF-302 crystal surface, the step bunching edges have a direction parallel to <100> and <010> to the (001) facet edges of the crystal.