| AVS 62nd International Symposium & Exhibition | |
| Thin Film | Tuesday Sessions |
| Session TF-TuA |
| Session: | ALD for Emerging Applications |
| Presenter: | Paul Lemaire, North Carolina State University |
| Authors: | P.C. Lemaire, North Carolina State University J. Zhao, North Carolina State University C. Oldham, North Carolina State University G.N. Parsons, North Carolina State University |
| Correspondent: | Click to Email |
Metal organic frameworks (MOFs) are microporous materials with chemically functionalized high surface areas. MOFs are attractive for multiple applications including filtration, gas storage, and catalysis. Post-synthetic modification (PSM) is a way to impart additional functionality into these materials and most PSM techniques reported are solution based processes. Vapor-phase PSM methods are highly desired due to the advantageous efficiency in reagent and solvent reagent removal.
In this work we report on a fundamental study on the functionalization of the UiO-66-NH2 MOF through atomic layer deposition (ALD). ALD is a vapor phase self-limiting process that enables the controlled deposition of thin films. The UiO-66-NH2 MOF is of particular interest for its high thermal and chemical stability and catalytic activity. We investigated how UiO-66-NH2 interact with ALD precursors, including titanium tetrachloride (TiCl4), titanium isopropoxide (TTIP), and trimthylaluminum (TMA).
QCM analysis shows that the UiO-66-NH2 consumes a relatively large amount of precursor in the first exposure, with a ~16% and ~20% for TiCl4/TTIP and TMA respectively. It is likely that the majority of the MOF pore volume is filled within the first cycle. For the first ALD cycle, purge time ranging 0.5-2.5 hours are necessary to completely remove excess unreacted precursor and byproducts. Following the first cycle, the mass loading per cycle decreases and becomes relatively linear.
In-situ FTIR analysis of the ALD functionalization of UiO-66-NH2 at 150°C shows that the MOF structure is maintained during the ALD process. Dosing each of the ALD precursors results in a loss of the hydroxyl bands at ~3650, 735, and 675 cm-1, but also a loss of the asymmetric and symmetric carboxylate stretching bands at ~1565 and 1375 cm-1 respectively. Upon water exposure, these bands reappear. These changes in the carboxylate stretching bands was consistent with shifting between the hydroxylated and non-hydroxylated UiO-66-NH2 structure. Finally, the dehydroxylated UiO-66-NH2 at 250°C was significantly less reactive towards the ALD precursors, suggesting that hydroxyl sites in UiO-66-NH2 play a large role in ALD functionalization of the MOF. The ability to deposit metal oxide thin films or nanoparticles within UiO-66-NH2 pores could help improve the MOF catalytic activity.