Synthesis, characterization, and growth mechanism of single-walled metal oxide nanotubes

This work is focused on obtaining a qualitative and quantitative understanding of the mechanism of formation of aluminosilicate and aluminogermanate nanotubes. Understanding of the self-assembly, nucleation and growth of such a model system would enable precise predictive control of synthesis parameters for a wider range of nanoscale materials. This work is also focused on precise control of nanotube dimensions (length and diameter). In order to achieve this overall objective, this thesis consists of the following aspects:; I. A systematic phenomenological study of the growth and structural properties of aluminosilicate and aluminogermanate nanotubes. The evolution of the aqueous-phase nanotube synthesis process over a period of 5 days, was carefully analyzed by a number of qualitative and quantitative characterization tools. In particular, the time-dependence of the nanotube size, structure, and solid-state packing was followed using electron microscopy, electron diffraction, X-ray diffraction, and dynamic light scattering. The essentially constant size and structure of the nanotubes over their entire synthesis time, the increasing nanotube concentration over the synthesis time, and the absence of significant polydispersity, strongly suggest that these nanotubular inorganic macromolecules are assembled through a thermodynamically controlled self-assembly process, rather than a kinetically controlled growth/polymerization process.; II. Investigation of the mechanism of formation of single-walled aluminogermanate nanotubes and development of key insights into the process of hydrolysis and self-assembly of metal oxides in mildly acidic aqueous solutions. Here we employ solution-phase and solid-state characterization tools to elucidate such a mechanism, particularly that governing the formation of short (20 nm), ordered, monodisperse (3.3 nm diameter), aluminum-germanium-hydroxide ('aluminogermanate') nanotubes in aqueous solution. The central phenomena underlying this mechanism are: (1) the generation (via pH control) of a precursor solution containing aluminate and germanate precursors chemically bonded to each other, (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. This mechanism provides a model for controlled low-temperature assembly of small, monodisperse, ordered nanotube objects.; III. Synthesis of mixed metal oxide (aluminosilicogermanate) nanotubes with precise control of elemental composition of the product nanotubes. Here we demonstrate that with the use of compositional control, we can precisely control the dimensions (external diameter and length) of the 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. With a simple calculation we have estimated that the diameter of ANP required to pack enough precursor material to make a 90 nm nanotube is 10 nm, and which is close to the value predicted by the kinetic model (12 nm). Furthermore, it is found that nanotube concentration is independent of nanotube composition since the activation energy barrier for the formation of nanotubes from ANP is independent of composition. (Abstract shortened by UMI.)...