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

Abstract

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.

Meniscus Guided Coating and Evaporative Crystallization of UiO-66 Metal Organic Framework Thin Films

“…Numerous MOF thin film fabrication processes have been explored over the past decade. Hermes et al solvothermally synthesized MOF-5 thin film by soaking functionalized substrate into the precursor solution at a high temperature, and the thin film of MOFs has been grown on fibers and textiles through both solution-based and atomic-layer deposition-based processes. ,− To improve the grafting of MOFs on any given substrate, carboxylic acid groups can be used to functionalize the substrate through the use of self-assembled monolayers (SAMs), which can have the added benefit of controlling MOF orientation or polymorphism . In addition, the layer-by-layer or liquid-phase epitaxy (LPE) technique has also been used to fabricate many MOF thin films. ,− According to Shekhah et al, Copper benzene-1,3,5-tricarboxylate, called HKUST-1, thin film can be fabricated at room temperature by repeatedly immersing the functionalized substrate into both a metal precursor solution and an organic ligand solution, respectively.…”

“…9,16−18 To improve the grafting of MOFs on any given substrate, carboxylic acid groups can be used to functionalize the substrate through the use of self-assembled monolayers (SAMs), which can have the added benefit of controlling MOF orientation or polymorphism. 19 In addition, the layer-by-layer or liquid-phase epitaxy (LPE) technique has also been used to fabricate many MOF thin films. 6,20−25 According to Shekhah et al, 6 Copper benzene-1,3,5-tricarboxylate, called HKUST-1, thin film can be fabricated at room temperature by repeatedly immersing the functionalized substrate into both a metal precursor solution and an organic ligand solution, respectively.…”

Switchable Ionic Transportation in the Nanochannels of the MOFs Triggered by Light and pH

“…In order to balance the reasonable doping amount and the integrity of the framework, different feed ratios of SP-COOH to NU-1000 were applied for the synthesis of the functional nanochannels, and the XRD technique was utilized to monitor the structural evolution. As shown in Figure 1F, the SP-COOH grafted MOFs (SP-COOH@NU-1000-5/10/15/20) could retain the similar Bragg diffraction peaks in comparison to the parent NU-1000 and the simulated one, 31,32 reflecting their high crystallinity. Noteworthy, since the diffraction peak at 2θ = 4.5°represents the electron density around the (010) plane in which a great portion of Zr 6 clusters populate; the gradually enhanced peak intensity after more and more SP-COOH modification demonstrates the chemical coordination occurring at the metal clusters.…”

Metal–Organic Framework Membranes: Advances, Fabrication, and Applications

“…have been used as supports for MOFs growth and subsequent carbonization. [ 128 ] To obtain better activity, novel MOFs precursors still need to be explored to construct high‐performance HER and OER bifunctional electrocatalysts.…”