Synthesis, Characterization, and Growth Mechanism of Single-Walled Metal Oxide Nanotubes
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Nanotubes have numerous potential applications in areas such as biotechnology, electronics, photonics, catalysis and separations. There are several challenges to be overcome in order to realize their potential, such as: (1) Synthesis of monodisperse (in diameter and in length) single-walled nanotubes; (2) Quantitative understanding of the mechanism of formation and growth of nanotubes; (3) Capability to engineer the nanotube size; (4) Low temperature synthesis process; and (5) Synthesis of impurity free nanotubes. Our investigation focuses on a class of metal oxide (aluminosilicate/germanate) nanotubes, which are; single walled nanotubes with monodisperse inner and outer diameters, can be synthesized in the laboratory by a low temperature (95ºC) process in mildly acidic aqueous solutions, and their formation timescales is hours, which makes it convenient as a model system to study the mechanisms of nanotube formation. This work is focused on obtaining a qualitative and quantitative understanding of the mechanism of formation of aluminosilicate and aluminogermanate nanotubes. In order to achieve this overall objective, this thesis consists of the following aspects: (1) A systematic phenomenological study of the growth and structural properties of aluminosilicate and aluminogermanate nanotubes. The constant size and increasing nanotube concentration over the synthesis time strongly suggest that these nanotubular are assembled through self-assembly process. (II) Investigation of the mechanism of formation of single-walled aluminogermanate nanotubes provided the central phenomena underlying the formation of these nanostructures: (1) the generation (via pH control) of a precursor solution containing chemically bonded precursors, (2) the formation of amorphous nanoscale (~ 6 nm) condensates via temperature control, and (3) the self-assembly of short nanotubes from the amorphous nanoscale condensates. (III) Synthesis of mixed metal oxide (aluminosilicogermanate) nanotubes with precise control of elemental composition, diameter and length of the product nanotubes. (IV) Preliminary work towards generalization of the kinetic model developed for aluminogermanate nanotubes to a larger class of metal oxide nanotubes. It was found that the size of nanotubes is dependent on the amount of precursors that can be packed in a single ANP and in turn depends on the size of the ANP.