Nanotubular materials are important building blocks of a future nanotechnology based on synthesis of functional nanoparticles and their assembly into nanoscale devices with novel properties[1]. Several problems in carbon nanotube[2] technology remain to be overcome, e.g. inadequate control over the dimensions and limitations of chemical composition[3, 4]. Inorganic nanotubes offer versatility in the use of suitable chemistries for a variety of applications[4, 5]. Inorganic nanotubes synthesized to date are polydisperse and/or multiwalled materials. To achieve their full potential, nanotechnological applications will ultimately require precise control over nanotube dimensions at length scales below 100 nm.

A suitable candidate to realize this objective is the synthetic version of the 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[7]. An aluminogermanate analog has also been successfully prepared by substitution of silicon with germanium in the synthesis solution[8]. The nanotubes are obtained with high monodispersity in diameter. The aluminogermanate analogs are considerably shorter (< 25 nm) than the aluminosilicate nanotubes and the diameters are about 50% larger. Building upon these previous works, we present here a study of the effect of synthesis temperature, and chemical composition on the dimensions of the nanotubes. Samples were prepared using different molar ratios of Si and Ge at synthesis temperatures of 95ºC, 85ºC and 75ºC. Samples thus obtained were characterized to probe the dimensions, structure and morphology of the nanotubes both in solid state as well as aqueous phase. TEM and XRD data were used to extract information on the morphology of the nanotubes. XRD data also offers quantitative information on the external diameter of the nanotubes. Electron diffraction was used to ascertain the internal structure of the nanotubes. Dynamic light scattering and TEM together provided quantitative information on the length of the nanotubes in solution. X-ray Photoelectron Spectroscopy (XPS) was used to determine the chemical composition of the nanotubes, whereas nitrogen adsorption was used to probe the internal pore diameter of the nanotubes. Our results indicate that the dimensions of the nanotube can be controlled continuously and to a high degree of accuracy, by Si-Ge substitution. This provides a continuously tuned set of well-characterized nanotube materials to investigate the dependence of thermodynamic (e.g. adsorption) and transport (mass and charge transport) on the material dimensions and composition.

References: [1] Tang, Z.Y. and N.A. Kotov, Advanced Materials, 2005. 17(8): p. 951-962. [2]Iijima, S., 354(6348): p. 56-58. [3]Tasis, D., et al., Chemical Reviews, 2006. 106(3): p. 1105-1136. [4]Remskar, M., 2004. 16(17): p. 1497-1504. [5]Rao, C.N.R. and M. Nath, Dalton Transactions, 2003(1): p. 1-24. [6]Russel, J.D., W.J. McHardy, and A.R. Fraser, Clay Minerals, 1969. 8: p. 87-99. [7]Cradwick, P.D., et al., Nature-Physical Science, 1972. 240(104): p. 187-&. [8]Wada, S. and K. Wada, Clays and Clay Minerals, 1982. 30(2): p. 123-128.