(570b) Effects Of Composition On The Mechanism Of Formation Of Single-Walled Mixed-Oxide Nanotubes

Nanotubes are important ?building block' materials for nanotechnology, but short (sub-100-nm) nanotubes with structural order and monodisperse diameter has remained elusive. To achieve this goal, it is critical to possess a definitive mechanistic framework for control over nanotube dimensions and structure at very small length scales. While no general strategy has thus far been proposed, we are interested in a unique model system that offers mechanistic insights into the assembly of nanotubular objects that could lead to a more general synthesis process. We recently employed solution-phase and solid-state characterization tools to elucidate the mechanism governing the formation of short (20 nm), ordered, monodisperse (3.3 nm diameter), aluminum-germanium-hydroxide(?aluminogermanate' or ?AlGe') nanotubes in aqueous solution. We also observed, that by changing the Si/Ge ratio during the synthesis, the resulting aluminosilicogermanate (?AlSiGe') nanotubes undergo changes in their dimensions (both length and diameter ) as a function of composition. The profound influence of composition on the nanotube dimensions motivates the present study, which is aimed at generalizing our proposed mechanism (originally developed for AlGe nanotubes) to the entire class of AlSiGe nanotubes.

Here we combine mechanistic synthesis experiments, dynamic light scattering, UV-Vis/Raman/IR spectroscopy, microscopy, and diffraction techniques to investigate the compositional dependence of the proposed nanotube assembly mechanism. We find that for all the AlSiGe compositions investigated, the central phenomena underlying the mechanism remain similar, viz. the generation (via pH control) of a precursor solution containing aluminate, silicate, and germanate precursors chemically bonded to each other, the formation of amorphous nanoscale condensates via temperature control, and the self-assembly of nanotubes from the amorphous nanoscale condensates. However, we discuss important differences in the details of the mechanism, particularly the size distribution of the intermediate amorphous nanoparticles, the kinetics of their evolution into nanotubes, and the degree of influence of aggregation processes. We discuss the extraction of quantitative parameters (reaction rate constants and activation energies) as well as their physicochemical significance in the composition-dependence of the mechanism. In separate work, we have used atomistic simulations to show that the formation of ordered diameter-monodisperse nanotubes is strongly related to the existence of unique energy minima in the nanotube structure as a function of diameter. Taken together, the results of the present investigation indicate a generalizable set of mechanistic principles that could be used as a basis for engineering of single-walled metal oxide nanotubes.