Electrochemical growth of CuBTC : improving the synthesis toolkit through mechanistic understanding
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
License
Abstract
Metal-organic frameworks (MOFs) are crystalline, nanoporous materials that consist of metal cations and organic linker molecules with multiple functional moieties. The sheer infinite amount of possible metal ion-linker combinations makes MOFs in principle ready to be synthesized and tuned to desire for a large variety of applications as gas sensors, gas separation materials or as substrates for catalysis. Lack of synthesis predictability due to missing knowledge about the physicochemical processes during MOF production until now prevents their widespread industrial use.
One specific synthetic pathway, namely the electrochemical synthesis, promises fast and versatile MOF synthesis. The electrochemical potential gives an easily controllable parameter that can be exploited to tune material properties such as the crystal size to desire. To exploit the full range of possibilities the electrochemical synthesis offers, deep understanding of the molecular processes underlying MOF synthesis is necessary.
This thesis aims to gain this understanding on the archetypical MOF CuBTC. CuBTC, as one of only few MOFs, shows direct growth on the surface of the electrode used, a desirable property for facile production of i.e. gas separation membranes. Using a variety of physicochemical methods such as Raman spectroscopy, UV/VIS spectroscopy and cyclic voltammetry the mechanism of electrochemical CuBTC growth is investigated in depth. We investigate the oxidation mechanism of the copper electrode during the synthesis. To facilitate future development of surface-bound MOF coatings, we study the molecular-level reason for the on-surface growth of CuBTC. Finally, we are able to answer the question about the high rate of electrochemical MOF production compared to the classical pathway of homogeneous nucleation from solution.
We unravel a two-step oxidation process from copper to Cu2+ with cuprite as a necessary intermediate. Based on our insight we develop an easy fabrication method for patterned MOF coatings. Additionally, we investigate the specific interaction between the linker molecule BTC and electrified Cu surfaces. We find a strong potential-dependent adsorption of BTC that could explain why CuBTC specifically forms surface-bound MOF coatings contrary to the majority of electrochemically synthesized MOF materials. Lastly, we study the kinetics of CuBTC growth in solution. We find an unexpected inverse relation between the concentration of linker molecules and the growth rate of CuBTC. We develop a chemical reaction model including the formation of an overcoordinated BTC species that shows a good qualitative fit with the experimental results. Our model can yield an explanation for fast electrochemical growth in which the chemical equilibria are shifted to favor CuBTC growth instead of the overcoordinated species. Finally, we summarize our results and give possible directions for future research.