Surface Deposition and Characterization of Metal-Organic Frameworks As Thin Films

Metal-organic frameworks (MOFs) are a premier candidate material for next-generation functional materials in basic science and industry. These porous crystalline materials can be designed around the coordination geometry of metal ions or clusters and organic linker molecules to yield materials with controlled pore size and chemical functional groups. Surface deposited MOFs are of particular interest to applications in gas sensors. However, relatively little work has been done to understand the growth mechanism of surface deposited MOFs and how best to measure their adsorption properties relative to bulk materials. MOF thin films were deposited by a vapor-assisted conversion (VAC) technique onto a variety of substrates. Specifically, MOF thin films of UiO-67-X where X is H and NH2 and M- MOF-74 where M is Co and Ni were growth on quartz crystal microbalance (QCM) surfaces, which allowed for gravimetric measurements to determine adsorption capacity. In addition, the films were characterized by powder X-ray diffraction, vibrational spectroscopy and scanning electron microscopy (SEM). The influence of the synthesis conditions (precursor concentration, temperature, and time) on the growth mechanism of UiO-67 was studied in detail. SEM studies were able to identify a preference for solvent phase or surface growth modes by quenching the VAC process over time. The precursor concentration and modulator were found to influence the growth mode most significantly.Vapor phase adsorption experiments were conducted on compounds from the BTEX family, namely, benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p- xylene isomers. Adsorption experiments on UiO-67-H films at 30 °C yielded saturation adsorption capacities from 22.4 – 26.9 % by mass, for the BTEX compounds.Similar adsorption experiments were conducted on the BTEX compounds, except toluene, at 25 °C on M-MOF-74 films. The maximum adsorption capacity was ≈17 – 20% by mass and 36 – 42 % by mass for Ni-MOF-74 and Co-MOF-74 films, respectively. These saturation values measured in thin film materials closely match the values measured on bulk materials. The difference in adsorption capacity between Ni-MOF-74 and Co-MOF-74 has been attributed to missing linker defects caused by the decomposition of the DMF solvent.