Hafnium Metal-Organic Frameworks: Formation Routes and Defect Engineering Francesca Catherine Noriko Firth

Abstract

Metal-organic frameworks (MOFs) consist of metal nodes or clusters linked by multidentate organic ligands. By altering the identity of these components, the properties of the MOF can be tuned. Furthermore, combining the versatility of MOFs with the considerable potential of low-dimensional materials like nanosheets or nanowires gives a wide range of functionalities for real-world applications. The deliberate introduction of defects into a MOF, or ‘defect engineering’, is a particularly significant method of modifying its properties. The UiO family of MOFs, comprising hafnium or zirconium clusters and bidentate linkers, is especially promising due to its stability and its ability to incorporate defects. This thesis explores the effect of controlling the synthesis conditions on the type and distribution of defects in UiO family MOFs, using a range of characterisation techniques including powder X-ray diffraction to identify and understand resultant structural changes, among them the formation of phases with a different hafnium metal cluster. Based on this understanding of defect engineering, synthetic conditions needed to create new related UiO phases are identified and successfully used, enabling the creation of UiO family MOF nanomaterials. The nuclearity and topology of the metal cluster are key to the formation of different MOF phases. This thesis uses in situ X-ray pair distribution function analysis to identify critical hafnium cluster intermediates in the early, pre-crystalline stages of solution formation of UiO family MOFs, and to compare these MOF reaction solutions to reactions forming only the metal clusters. Using solution nuclear magnetic resonance spectroscopy, this work then further explores the interactions in these reaction solutions, both in the solvent and between the solvent molecules and metal clusters, and investigates the effect of changing the composition of the solvent on the formation and behaviour of the hafnium clusters. These insights into the processes occurring during MOF reactions are key to the future of rational design of MOF syntheses.