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Metal-Organic Frameworks, Disorder and Defects

The Grey Group also works on metal-organic frameworks (MOFs). With their incredible tunability and versatility, arising from the wide range of metal nodes and organic linkers available from which to build a multi-dimensional porous network, MOFs are the source of great excitement in the search for materials for all kinds of applications, from carbon capture to gas and liquid separation, catalysts, sensors and even energy storage materials.

However, changing the identity of the metal or linker is not the only method of altering the properties of the framework – the formation of defects, such as missing linkers, missing nodes, or stacking faults, in a structure can also significantly change the properties of a material. While in conventional framework materials, the importance of understanding and characterising defects is generally well-recognised, the effect of disorder in MOFs has been generally less widely addressed.

In the Grey Group, we work to understand how control over the synthesis conditions of a MOF enables control over the number, type and correlation of defects within the framework. We primarily investigate UiO family MOFs, which are ideal for studying defects as their strong metal-linker coordination and high connectivity allow them to incorporate linker and metal-cluster vacancies to a high degree.

We have been able to create and exploit both missing-cluster and missing-linker defects in UiO MOFs, expanding the range of known defect structures from random and nano-regions of correlated defects to include the formation of new bulk crystal structures derived from long-range defect ordering. Tuning the synthesis therefore allowed us to control the crystal phase selection and purity as well as the concentration of defects, and hence the properties of the MOF.

Figure: Different MOF structures obtained by controlling the defects via the synthesis conditions.

In particular, this method has enabled us to directly synthesise highly-crystalline MOF nanosheets, which represent a potential route towards the synthesis of MOF membranes which could be used as separator films in energy storage devices.

We are continuing to explore the formation of defects and defect-mediated MOF structures, with the aim of increasing our synthetic control in order to obtain other new MOFs and design MOF-based materials with interesting properties suitable for a variety of real-world applications.

List of selected publications:

"Metal-Organic Nanosheets Formed via Defect-Mediated Transformation of a Hafnium Metal-Organic Framework.", MJ Cliffe, E Castillo-Martínez, Y Wu, J Lee, AC Forse, FCN Firth, PZ Moghadam, D Fairen-Jimenez, MW Gaultois, JA Hill, OV Magdysyuk, B Slater, AL Goodwin, CP Grey - Journal of the American Chemical Society (2017) 139, 5397. (DOI: 10.1021/jacs.7b00106)

"Engineering New Defective Phases of UiO Family Metal-Organic Frameworks with Water", FCN Firth, MJ Cliffe, D Vulpe, M Aragones-Anglada, PZ Moghadam, D Fairen-Jimenez, B Slater, CP Grey - Journal of Materials Chemistry A (2019), 7, 7459-7469. (DOI: 10.1039/C8TA10682G)