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Title: Crystal growth of metal-organic frameworks investigated by atomic force microscopy
Author: Pambudi, Fajar
ISNI:       0000 0004 8501 4144
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2019
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Porous metal-organic frameworks (MOFs) form the largest family of crystalline porous materials that are attracting great interest due to their diverse array of form and function. To facilitate further design of the properties of such functional material requires a detailed understanding of how these materials form for which real-time in situ studies provide unique opportunity to afford unparalleled insight into the assembly process. Therefore, in situ atomic force microscopy (AFM) and the synthesis components and conditions of MOF crystallisation form an ideal combinations to understand the molecular assembly of this functional material. In situ AFM was applied to understand particular aspects of the formation of various MOFs including Y-fcu-MOFs such as Y-ndc-MOF and Y-fum-MOF and also Zr-fcu-MOFs such as MOF-801 and UiO-66 that remain uncertain, such as the actual pathway that an organic structure directing agent (SDA) assists the formation of crystalline MOFs found in Y-fcu-MOFs and the assembly of the complex metal-containing nodes in MOFs found in both Y-fcu-MOFs and Zr-fcu-MOFs. The crystal growth of both Y- and Zr-fcu-MOFs proceeded through similar type of birth-and-spread mechanism, which indicates a process of nucleation and spreading of meta-stable unenclosed sub-steps to form stable surface terraces of the enclosed frameworks structures. Therefore, the presence of SDA in the form of dimethylammonium (DMA) in Y-fcu-MOFs was observed as meta-stable nuclei with the height of 0.45 ± 0.1 nm, which explain the lateral location of DMA cations during the crystal growth above the Y6 cluster as growth through a process of continuous nucleation. Additionally, the formation of the large inorganic node in Y-fcu-MOFs occurred through addition of simple {Y1-Y3}-containing species indicating no involvement of the pre-formed hexanuclear Y cluster during the low supersaturation crystal growth stage of MOF crystallisation. This was confirmed by dissolution mechanism of Zr-fcu-MOFs (UiO-66), which indicates the dissociation of the hexanuclear Zr nodes via two main steps showing a layer-by-layer dissolution process. The crystal growth of a shell MOF over the surface of a core crystal MOF was carried out for systems that have a lattice mismatch and nearly similar lattice parameters. Here, we report the use of AFM to reveal the growth of a shell of [Zn2(bdc)2(dabco)] on all faces of hexagonal crystal of [Zn2(bdc)2(bpy)] that has similar a, b-lattice parameters but a ~32% mismatch in the c-lattice parameter. The results show the mechanism through which the shell MOF overcomes the core terrace height mismatch depends on that mismatch being reduced before overgrowth of continuous shell layers can occur. This reduction is achieved via a process of growth of non-continuous shell layers that are terminated by terrace edges of the core. Moreover, the results also show that the shell of this lattice-mismatched core-shell MOF was predicted to contain a much larger number of partially coordinated unsaturated metal sites than would be expected in a crystal of the shell MOF only. Additionally, the growth mechanism of a shell of [Cu2(ndc)2(dabco)] over {001} facets of a tetragonal core crystal of [Zn2(ndc)2(dabco)] which has similar lattice parameters was successfully revealed. The work shows the mechanism through which the 2D nuclei of the shell MOF were able to grow over the terrace surface of the core crystals, in which the structure of the shell MOF was rotated about 6.13o relative to the surface structure of the core MOF to give an in-plane rotational defect. The results exemplify the crystal growth versatility of MOFs to accommodate either large lattice mismatch or similar lattice parameters, to give many more functional defects in a core-shell MOF than either of the component MOFs.
Supervisor: Anderson, Michael ; Attfield, Martin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: nanoporous materials ; Core shell MOFs ; Metal-Organic Frameworks ; Crystal growth ; AFM