Adam Cahn

Isotropic and Anisotropic Growth of UiO-66(Zr/Hf) and MIL-68(Al/In) Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a family of porous, crystalline materials comprising metal nodes or clusters and organic linkers. The diversity of available nodes and linkers and the ability to fine-tune physical and chemical properties of MOFs make them promising candidates for gas storage, heterogeneous catalysis, drug delivery, and chemical sensing, among others. Systematically and reproducibly tuning these properties for various applications, however, requires good understanding of MOF growth mechanisms, which remain poorly studied. In this work, hybrid zirconium/hafnium UiO-66 and aluminum/indium MIL-68 were synthesized to observe how and where metal nodes are incorporated into the MOF structure during growth. By adding spikes of metal nodes and linker during the synthesis, the compositions of the hybrid particles could be visualized using scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDS). UiO-66(Zr/Hf) particles resulted in a Zr core surrounded by a thin Hf shell, representing isotropic (not facet-dependent) growth in accordance with the symmetric morphology of UiO-66. Because of the rod-like structure of MIL-68, it was expected to grow anisotropically, whereby In nodes grow only at the tips of an Al particle. However, low crystallinities and small particle sizes made it impossible to determine the distribution of metals in MIL-68(Al/In), so future work is needed to obtain discrete particles and highly crystalline samples. These results will help elucidate the facet-dependencies of MOF growth, allowing for effective size control and offering access to a variety of complex morphologies for industrial and academic applications.

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