Controlling Polymorphism and Orientation of NU-901/NU-1000 Metal-Organic Framework Thin Films

NU-1000, a zirconium (Zr)-based metal-organic framework (MOF), is a promising candidate for heterogeneous catalysis, gas storage, electrocatalysis, and drug-delivery applications due to its large pore size and mesoporous structure. However, the synthesis of NU-1000 may produce another polymorph NU-901, which has a smaller average pore size and pore volume than NU-1000. Similarly, the presence of NU-1000 as a phase impurity in NU-901 crystallites is undesired. Although phase-pure NU-901 and NU-1000 have been successfully synthesized in bulk, multiple applications such as electrocatalysis and separation membranes require the formation of thin films. In this study, we utilize self-assembled monolayers and crystal engineering to control the polymorphism and orientation of NU-901/NU-1000 thin films. We report the fabrication of thin films of NU-901 and NU-1000 via a solvothermal method by functionalizing the substrate with carboxyl (-COOH) tail groups. This synthesis produces phase-pure hexagonal rod-shaped NU-1000 crystals and nearly phase-pure prolate-shaped NU-901 crystal as revealed by scanning electron microscope (SEM), powder X-ray diffraction (PXRD), and nitrogen adsorption isotherm analyses. Furthermore, we control the orientation of NU-1000 crystallites on the fluorine-doped tin oxide (FTO) substrate by controlling the nucleation density of the MOFs on the substrate. We hypothesize that heating the functionalized substrate in a Zr-oxo cluster solution preceding solvothermal synthesis results in the coordination of Zr-oxo clusters to the (-COOH) groups of the substrate, which promotes a higher nucleation density of NU-1000 on the substrate, resulting in the perpendicular growth of NU-1000 during crystal formation.