Investigating Controlled Thin Films of Metal Organic Frameworks through Solution Processing

Metal organic frameworks (MOFs) are highly porous, crystalline materials with numerous applications ranging from medicine to energy. Many MOF based applications requiring a thin film or membrane configuration, especially those related to transport, filtration, and species selectivity, have been limited to non-ideal morphologies or isotropic pore alignment. Thus far, MOF thin film morphologies have largely taken the form of randomly packed small particles, non-oriented polycrystalline films, and oriented small-area uniform films. While significant work has been performed to produce highly oriented thin film MOFs, a compromise between synthesis time and film quality is often made.
Here we present using a well-controlled deposition technique to produce oriented, close-packed thin films of a prototypical MOF, HKUST-1 (HKUST=Hong Kong University of Science and Technology), where film formation occurs on the order of seconds. Techniques such as grazing incidence wide angle X-ray diffraction (GIWAXS), Brannauer-Emmet-Teller (BET) Surface Area analysis and Fourier Transform Infrared Spectroscopy using Attenuated Total Reflectance (FTIR-ATR) are used to characterize and confirm the rapid formation of HKUST-1 in drop cast and solution sheared samples. Control of HKUST-1 film thickness is demonstrated by performing a preliminary search of processing parameters and conventional flow regimes are observed as described in other crystallization and polymer systems. Classical nucleation theory (CNT) is called upon to explain observed morphologies as they relate to solution shearing parameters. The development of instrumentation for in situ optical microscopy and microbeam grazing incidence wide angle x-ray scattering (μGIWAXS) is discussed along with instrumentation limitations and recommended improvements. In situ μGIWAXS of select solution shearing conditions are analyzed to gain insight into how crystallization kinetics of HKUST-1 couple with solution shearing fluid dynamics. It is found that precursor concentration ratio and deposition speed play a role in crystallization kinetics. The instrumentation and analysis techniques put forth are intended to lay groundwork for future solution shearing in situ studies of MOFs and other crystalline materials.