Understanding and Controlling Metal Organic Framework Crystallization for Thin Film Fabrication

Metal organic frameworks (MOFs) are a promising class of materials that have seen an exponential growth in research interest in the past few decades. Composed of transition metal nodes with organic linkers, these crystalline, highly porous, and chemically versatile structures show significant promise in applications ranging from drug delivery, data storage, separations, sensing and catalysis. Many of these applications require MOFs to be configured as a thin film for better transport characteristics and to act as a selective barrier between two mediums. In terms of film quality, properties such as grain size, coverage, polycrystallinity, orientation, and thickness must be controlled to optimize thin film performance for a given application. Current fabrication technologies often see a tradeoff between controllability and scalability, where the highest quality films require time-consuming, low-area, and complicated fabrication techniques. This relationship motivates the need to develop a scalable, large area, and rapid fabrication technique capable of producing high quality MOF thin films. The work put forth in this dissertation is an effort to couple MOF crystallization to a thin film deposition technique called solution shearing to fabricate high quality thin films on a seconds to minutes time scale.
In Chapter 2, two methods for thin film fabrication are developed where MOF growth is decoupled from the film fabrication process. MOF morphology and coating parameters were varied to determine effect on thin film properties. Films were characterized using x-ray diffraction and scanning electron microscopy to determine particle and film orientation and morphology, respectively. Results show controlling particle morphology significantly influences crystal orientation in resulting films. Chapter 3 couples MOF formation with a deposition process. This study focuses on understanding the effects of solution shearing on films made from a copper-based MOF, HKUST-1. X-ray diffraction, profilometry and optical microscopy were used to characterize particle and film orientation, thickness, and morphology, respectively. Results indicate solution shearing can control particle size, thickness and orientation during the thin film deposition process. The study is extended by using machine learning to create a virtual experimental space to understand solution shearing parameter relationship with film coverage and thickness. This model is used to identify parameters that minimize film thickness while maintaining a fully covered film.
Coupling other MOFs, such as the highly stable zirconium based UiO-66, to solution shearing proved to be a difficult task. This is because the crystallization process is not well understood. Thus, Chapter 4 and 5 study MOF crystallization. Chapter 4 uses x-ray diffraction and scanning electron microscopy to study the influence of reactant speciation on the rapid crystallization kinetics of MOFs. We show that tuning pH and forming the correct metal node topology prior to synthesis allows MOFs to crystallize on a seconds time scale at room temperature for several prototypical MOFs and conclude each system can be treated as a reactive-crystallization. Chapter 5 extends these concepts to study MOF formation using in situ wide angle x-ray scattering. The influence of synthesis parameters including reactant and modulator concentration, temperature, and addition of heterogeneous nucleation sites on crystallization kinetics is observed. Insight into a potential formation mechanism is discussed and used to motivate a new crystallization model developed by collaborators.
Chapter 6 highlights two applications using films developed from previous chapters. Rate of gas adsorption is measured using an adsorption analyzer. Kinetic selectivity between CO2/CH4 is shown to be orientation dependent for a zinc based anisotropic MOF developed in Chapter 2. Finally, the rapid synthesis developed in Chapter 4 is adapted to create MOF coated fabric. Pollution filtration is measured using a particle counter to show the addition of MOF enhances filtration efficiency for PM1-4 particles.