Cathodic electrodeposition of metal-organic frameworks

Abstract

Metal-organic frameworks (MOFs) represent a class of functional coordination polymers with long-range order, great synthetic flexibility, and exceptionally high internal surface area. Although they have been proposed for a myriad of potential applications, many of these require that MOFs be processed as thin films or other nanostructures to reach peak performance. Thus, a general and facile fabrication process is still much desired. In this thesis, I describe cathodic electrodeposition as an alternative approach to MOF crystallization, which is traditionally achieved solvothermally via ligand deprotonation through in-situ formation of base (often by amine release from the decomposition of an amide solvent). In cathodic electrodeposition, electrochemical reduction of probase molecules produces base equivalents to initiate the metal-ligand bond formation and subsequently the self-assembly process. There are three major advantages to this process in the context of producing MOF films: 1) the formation of base can be controlled more precisely, 2) the acid-base reaction is spatially confined close to the electrode surface, and 3) electrodeposition is, by definition, conformal, and therefore lends itself to electrodes of any geometry, as will be shown in Appendix I. Using cathodic electrodeposition, several Zn-BDC (BDC: 1,4-benzenedicarboxylate) MOFs could be formed as polycrystalline coatings on electrodes. In particular, a microporous composite of Zn₄O(BDC)₃ i.e. MOF-5, and Zn metal could be synthesized at room temperature in less than 15 minutes. This work is described in Chapter 2. By modulating the pH at the electrode surface in the presence of an appropriate probase, biphasic or bilayer structures of different polymorphs could also be accessed with a simple change in the applied potential, thereby providing a facile means of making composite films described in Chapter 4. Lastly, systematic studies of the effects of various variables on the electrodeposition process brought unique mechanistic insights to the early stages of MOF crystallization, as described in Chapter 3. Chapters 1 and 5 provide a context for this research within the larger area of MOF film formation, and a preliminary account on the possible sources of the [mu]₄-O²- atom in the iconic MOF-5 structure, respectively.