Investigation of Metal-Organic Frameworks as Thin Films and Gels

Metal-organic frameworks (MOFs) are porous crystalline materials composed of inorganic metal clusters and organic linkers. The exceptionally high specific surface area and tunable porosity of MOFs make them excellent candidates for applications in catalysis, separation, sensing, and drug delivery. Most applications require the formation of MOF thin films and polymer-MOF composites. For instance, MOF thin films are required for implementing MOFs in catalysis and sensing applications. On the other hand, utilizing MOFs in separation and drug delivery requires the formation of polymer-MOF composites. Therefore, optimizing the synthesis conditions to prepare MOF thin films and polymer-MOF composites with desired properties is of great importance. This work identifies the existing knowledge gaps in the field of MOF thin films and polymer-MOF composites and addresses those issue.

In chapter 2, we discuss the formation of zirconium (Zr)-based anisotropic MOFs NU-901 and NU-1000 thin films with excellent stability and control over crystal orientation. We use a self-assembled monolayer (SAM) method to functionalize the substrate with carboxylic acid (–COOH) groups. We find that the presence of carboxylic acid groups facilitates the formation of NU-901 and NU-1000 crystals on the substrate during solvothermal synthesis, thus obtaining MOF thin films. Furthermore, the SAM method allows excellent adhesion of MOF crystals to the substrate as suggested by scotch tape stability test. We present a unique method to control the orientation of NU-901 and NU-1000 crystals on the substrate. We find that functionalizing the substrate with metal clusters promote crystal growth in the perpendicular direction as opposed to parallel orientation obtained for carboxylic acid functionalized substrate. We hypothesize that the presence of metal clusters on the substrate before MOF formation enhances nucleation density of MOF crystals, resulting in perpendicular growth of the MOF crystals. The method presented here to control the orientation of anisotropic MOF crystals has implications in catalysis, sensing, and separation.

While solvothermal method renders excellent MOF thin films, the method usually takes 24-72 h and is not easily scalable. In chapter 3, we present a solution shearing technique to fabricate large-area (~5 cm2) thin films of NU-901 within 15 minutes. We study the effect of solution shearing parameters (i.e., substrate temperature and linker concentration) in NU-901 thin film properties (i.e., film thickness, crystallinity, and surface coverage). We find the NU-901 crystallinity increases with increasing temperature, while decreases with increasing linker concentration. On the other hand, film thickness increases with increasing temperature and linker concentration. In all cases, high surface coverage (> 90%) of MOF thin films is achieved. To show the generalizability of the solution shearing technique, we fabricate MOF-525 films, a Zr-based MOF, using solution shearing. The metalated MOF-525 films show electrocatalytic reduction of CO2 to CO, which has implications in CO2 capture and utilization. The demonstration of MOF thin film formation using solution shearing can pave the way to roll-to-roll coating of MOF thin films at industrial scale.

In chapter 4, we discuss the formation of polymer-MOF composite gels and shed light on the effects of polymer-MOF interactions on the formation of MOF within the gel. We use a one-pot synthesis method, where metal clusters interact with linker and polymer to allow simultaneous formation of MOF and gel network. We find that polymers containing carboxylic acid groups either inhibit or disrupt MOF formation within the gel compared to MOF formed in the absence of polymer. On the other hand, polymers containing hydroxyl groups do not affect MOF formation within the gel. Surprisingly, using a polymer, which has minimal interactions with metal cluster also allows formation of gel network. This suggests an exciting possibility of polymer entrapment within MOF pores and this entrapment restricts the movement of polymer chains out of the MOF pores, thus rendering a gel structure. We show the formation of polymer-MOF composite gels with various MOFs and polymers with the aim to report the future design of these composite gels for therapeutic applications.