(275c) Phenomenology of the Growth of Single Walled Aluminosilicate and Aluminogermanate Nanotubes

Nanotubular materials are important building blocks of a future nanotechnology based on synthesis of functional nanoparticles and their assembly into nanoscale devices with novel applications in areas such as electronics, biotechnology, sensing, separations, energy storage/management and catalysis. The discovery of carbon nanotubes[1] has generated a great deal of interest, and extensive research is presently being done on the synthesis, properties and applications of carbon nanotubes[2, 3]. Several problems in carbon nanotube technology remain to be overcome, e.g. the development of a low-temperature synthetic process with high yield as well as precise control over the nanotube dimensions and chirality, limitations of chemical composition, and the production of ?three-dimensionally nanoscale' carbon nanotube objects. To achieve their full potential, nanotechnological applications will ultimately require precise control over nanotube dimensions and monodispersity at length scales below 100 nm.

Inorganic nanotubes[4], however, offer greater versatility in the use of suitable chemistries for a variety of applications. However, all the inorganic nanotubes synthesized to date are polydisperse and/or multiwalled materials with micron-scale lengths[5]. An apparent exception is the synthetic version of the naturally occurring nanotube mineral imogolite[6]. Imogolite is a single-walled nanotube whose wall structure is identical to a layer of aluminum (III) hydroxide (gibbsite); with isolated silicate groups bound on the inner wall. An aluminogermanate analog has also been successfully prepared by substitution of silicon with germanium in the synthesis solution[7]. However, the aluminogermanate analogs are considerably shorter (< 50 nm) than the aluminosilicate nanotubes and the diameters are about 50% larger. We are particularly interested in the potential nanotechnological applications of inorganic nanotubes with precise length and diameter, that can be synthesized via relatively mild chemistry and which have technologically useful properties different from carbon nanotubes. Systematic studies delving into the mechanism of growth of these nanotubes is largely absent in literature. Thus a study of the nanotube growth would be invaluable in understanding the mechanisms leading to nanoscale materials formation and the insights gained could be used to synthesize new functional materials.

Here we present a systematic study of the growth of aluminosilicate (Al-Si) and aluminogermanate (Al-Ge) nanotubes as a function of synthesis time. We have used of a number of complementary characterization techniques to probe the dimensions, structure and morphology of the nanotubes both in solid state as well as aqueous phase, as a function of synthesis time. TEM and XRD data were used to extract information on the morphology of the nanotubes and to propose a model for their packing in the solid state. SAED was used to ascertain the internal structure of the nanotubes as a function of growth time. Detailed mathematical analysis of DLS data provided quantitative information on the dimensions of the nanotubes in solution. X-ray Photoelectron Spectroscopy (XPS) was used to determine the chemical composition of the nanotubes. The growth of the nanotubes as a function of synthesis time could be either kinetically or thermodynamically controlled. Formation of nucleation sites in the early stage of the reaction and subsequent addition of oligomers and monomers as growth units to these sites indicates kinetic control. This mechanism involves a substantial increase in length of the nanotubes with synthesis time. On the other hand thermodynamic control involves self-assembly of the monomers and oligomers leading to spontaneous formation of the nanotube. Thus, presence of nanotubes in an early stage of the reaction, coupled with quantitative proof that the nanotubes does not increase in length with reaction time would indicate a thermodynamically controlled nanotube formation reaction.

Our study leads to the following important conclusions: (1) the dimensions of the Al-Ge NTs are approximately 10 nm in length and 3.3 nm in diameter, whereas those of the Al-Si NTs are 100 nm and 2.2 nm respectively. (2) Nanotube are formed at a very early stage in the reaction; (3) the structure and dimensions of the nanotubes remain 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 length and diameter) 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.

References

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