The Growth of Metal-Organic Framework Films on Calcium Fluoride and Their Interaction With Reactive Molecules

Spectroscopy on metal-organic framework (MOF) films and the molecular phenomena associated to them is mostly limited to grazing incidence methods. To allow for transmission-based characterization and highlight the applicability of MOFs on transparent substrates, UiO-67, UU-1, and ZIF-8 MOFs are synthesized on CaF2 windows. It is revealed that the growth of the UiO-67 MOF follows a Volmer-Weber mechanism using scanning electron microscopy (SEM). This growth is assisted by growing seeds in solution, which anchor on the film and become part of the intergrown film itself. UU-1, a copper-based MOF, is formed after spin coating the Cu-BTC precursors, showing its characteristic fiber-like morphology and resulting inter-fiber macroporosity. ZIF-8 is formed using a "flash"-synthesis, and it is shown that this approach resembles a Volmer-Weber growth mode as well. Last, CO probe molecule adsorption FT-IR spectroscopy is utilized to study the effect of methanol exposure. UiO-67 becomes inaccessible toward CO, due to the formation of methoxy species, whereas UU-1 undergoes a topotactic transformation to HKUST-1. ZIF-8 is the most stable as methanol only removes impurities from the framework. This approach opens the venue for other film materials to be studied in situ synthesis, sorption, or catalysis using transmission-based spectroscopy.