Phenomenology of the growth of single-walled aluminosilicate and aluminogermanate nanotubes of precise dimensions

We report a detailed phenomenological study of the growth and structural properties of single-walled aluminosilicate and aluminogermanate nanotubes with a structure analogous to the naturally occurring nanotube mineral imogolite. 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. From analysis of the dispersed and solid-state properties of the nanotubes, we obtained several findings: (1) the dimensions of the aluminogermanate nanotubes are approximately 15 nm in length and 3.3 nm in their (outer) diameter, whereas those of the aluminosilicate nanotubes are 100 and 2.2 nm, respectively; (2) nanotube materials are formed at a very early stage in the reaction; (3) the structure of the nanotubes remains essentially identical throughout the synthesis though their concentration increases with synthesis time; (4) their solid-state packing is well-ordered in an apparently monoclinic (and not hexagonal) arrangement; and (5) their dimensions (both diameter and length) appear monodisperse. 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.