Investigations of the Growth Mechanism of Metal-Organic Nanotubes

Metal-organic nanotubes (MONTs) are the one-dimensional variant of metal-organic frameworks (MOFs), where the nanotubular framework propagates in one-dimension coincident with the pore. Presently, MONTs are studied by conventional solid-state characterization techniques, similar to MOFs. Bulk MONT materials are composed of thousands of nanotubes stacked together and display similar structures to 3D MOFs with tubular pores. Metal-organic nanotubes need to be isolated from this bulk material in order to study their unique one-dimensional properties and truly become a new unique class of materials. Metal-organic nanotube are synthesized through solvothermal reactions of organic ligands and metal salts, where the design of the ligand plays a large role in the dimensionality of the framework. There were several concurrent goals for improved MONT synthesis described herein. The organic ligand design can be divided in four categories including those ligands that produce (1) large pore MONTs, (2) fluorescent MONTs, (3) reduced aggregation MONTs, and (4) a series of isostructural MONTs. One ligand from each of these categories was incorporated in MONT reactions, but only one new MONT was synthesized. The challenges in predicting the dimensionality of a framework was shown by these MONT syntheses. To show that MONTs are unique materials, the fundamental chemistry on the nanoscale, the colloidal phase, needs to be understood. This idea revolutionizes the way MONTs are typically studied. Liquid-cell transmission electron microscopy (LCTEM) and small angle neutron scattering (SANS) were used as key nanoscale characterization techniques. LCTEM was able to visualize the formation and growth of nanoscale MONTs in real time for the first time and match the colloidal product to the bulk MONT solid. Small angle neutron scattering provided detail of the initial nanostructures formed within the MONT reactions. The combination of ligand design and colloidal MONT analysis is vital for future research of these materials. Once the fundamental chemistry of formation is understood, directed syntheses and applications can be applied to MONTs.