Understanding the Crystallization of Solution Sheared Metal-Organic Frameworks (MOFs) Thin Film

Metal-organic frameworks (MOFs) have emerged in the scientific community as a promising candidate in research interest due to their remarkable tunability and structural diversity as well as ultrahigh porosity. MOFs are highly-ordered porous crystalline materials that are formed via coordination bonding between metal ions or secondary building units (SBUs) as nodes and multitopic organic ligands as linkers. These characteristics enable MOFs for a wide range of applications such as sensor, gas separation and storage, membrane, and catalysis. To utilize MOFs in these applications, MOFs should be grown as thin films or membranes. Thin films can lower the mass transfer resistances, and the porous nature of MOF can increase its selective performance.
Among various thin film fabrication techniques, solution shearing is used to fabricate MOF thin films. Conventional MOF fabrication techniques, including solvothermal growth or layer-by-layer (LbL) growth, provide high-quality thin films, but they suffer from slow crystallization kinetics. On the other hand, solution shearing that belongs in the same class of the meniscus-guided coating techniques has been studied as a versatile as well as a concise approach that enables to fabricate thin film rapidly and control film thickness, crystal orientation, and film coverage. Therefore, this dissertation focuses on fabricating different types of MOF thin films and infiltrating guest molecules into their pores using solution shearing to observe the thin film property changes from the pristine MOF thin films.
In Chapter 2, solution shearing is used to fabricate a large area, continuous and insulating Cu(II)-based MOF, called HKUST-1, thin film. However, it is hard to synthesize a large area and continuous HKUST-1 thin film using solution shearing. Therefore, we found out that repeating solution shearing cycles enhance the film coverage. The initial solution shearing cycle is used to deposit HKUST-1 crystals, and the secondary crystallization may occur in the subsequent solution shearing cycles on the substrate. The film coverage of the large-area and continuous HKUST-1 thin film is confirmed by incorporating a redox-active molecule, called 7,7,8,8-Tetracyanoquinodimethane (TCNQ). Upon TCNQ infiltration, the electrical conductivity of the thin film increases seven orders of magnitude electrical conductivity than the pristine HKUST-1. Furthermore, the active learning approach is applied in solution shearing parameters to find an optimized parameter that provides the thinnest fully covered HKUST-1 thin film. In this chapter, the solution shearing technique shows its feasibility that can be used as a large area and continuous MOF thin film.
Chapter 3 highlights that the zirconium 1,4-dicarboxybenzene MOF, UiO-66, can be fabricated using evaporative crystallization during solution shearing. Solution shearing parameters (type of solvent, coating speed, substrate temperature, and concentration of the precursor solution) are varied to observe the changes in the film thickness, coverage, and crystallinity. In addition, the oriented crystal structure of UiO-66 thin film is grown when dimethylformamide (DMF) is used as a solvent. Last, solution-sheared UiO-66 thin film is fabricated on the porous substrate (anodic alumina oxide, known as AAO) to use the thin film for separation applications. To the best of our knowledge, this is the first time that UiO-66 crystals are formed via solution shearing.
In Chapter 4, based upon what we have found in Chapter 3, the pores of UiO-66 are incorporated by polymer molecules. The polymer and the MOF composite, called polyMOF, are synthesized. In this study, the thin film composite of piezoelectric polymer, known as poly(vinyl difluoride) (PVDF), and UiO-66 is fabricated via solution shearing. Unlike the conventional polyMOF thin film fabrication process, the precursor solution that contains node, linker, and polymer is deposited as a thin film using solution shearing. By using the concept of secondary crystallization, a large area, continuous and freestanding thin film of PVDF@UiO-66 is fabricated within a minute. Due to the addition of PVDF, the film coverage reaches up to 100% under the resolution of SEM, and its addition inside the UiO-66 pores is confirmed by observing the changes in piezoelectric performance. This is the first instance that shows the fabrication of polyMOF thin film using solution shearing. This dissertation provides the feasibility towards expanding the application of solution shearing in the MOF thin film field.