Nanotubular materials have gained significant importance as building blocks of nanoscale and nanostructured devices for new technological applications. The synthesis, characterization, and applications of carbon-based and inorganic nanotube materials have been pursued extensively over the last decade. We are investigating a class of single-walled metal oxide nanotubes whose dimensions and compositions can be precisely tuned by aqueous-phase chemistry [1,2]. In particular, the current study focuses on aluminosilicate nanotubes that are synthesized from mildly acidic aqueous precursor solutions between 25-100°C. The structure consists of hexagonally arranged aluminum atoms connected by double oxygen bridges, with pendant silanol groups in the nanotube interior. It is known that these nanotubes are produced in highly monodisperse form despite a substantial variety of synthesis conditions [1,3]. This is not observed in carbon nanotubes and related nanotube materials. Here we investigate the physics governing control over the nanotube diameter via a harmonic force-constant model and molecular dynamics (MD) simulations.

First, atomically detailed models of electrically neutral aluminosilicate nanotubes were built with the outer tube diameter varying between 1.5-4.5nm. The nanotube structures were subjected to energy minimization simulation to calculate the normal-mode vibrational frequencies of Al-O and Si-O bonds. The optimized structures were subjected to MD simulations to calculate the internal energy, and Al-O and Si-O bond strain energies as a function of nanotube diameter. The results from the study show a minimum in the internal energy per atom at a nanotube diameter of 2.26nm, a phenomenon that is not observed in the carbon nanotubes. We model the total energy of the nanotube based on harmonic bond stretching energies of Al-O and Si-O bonds, with the assumption of semi-rigid AlO₆ octahedra and SiO₄ tetrahedra. Note that this assumption is less restrictive than the rigid-body SiO₄ models used for predicting flexibility in zeolite frameworks [4]. The bond strain energy - calculated based on the harmonic force constants obtained by nonlinear least squares fit to the simulation data - was found to decrease monotonically for Al-O bonds with increasing nanotube radius, while that of the Si-O increased. Due to the difference in Si-O and Al-O bond energies, and functionalization of the inner wall of the nanotube with silanol groups, a strain energy minimum is thus observed. Further, the model correctly predicted the linear decrease in distances between the aluminum atoms as a function of nanotube radius seen in our simulations. In addition, we also observed a power law dependence of the radial breathing mode (RBM) frequency on the nanotube radius, which is in very good agreement with our theoretical prediction and is in accord with mid-infrared spectroscopic characterization. The present study thus serves as an important starting point for understanding – and manipulating – the dimensions of this class of metal oxide nanotube materials.

References: [1] Mukherjee, S.; Bartlow, V.M.; Nair, S. Chem. Mater. 2005, 17, 4900. [2] Konduri, S.; Mukherjee, S.; Nair, S. Phys. Rev. B (Submitted) 2006. [3] Ackerman, W. C.; Smith, D. M.; Huling, J. C.; Kim, Y. W.; Bailey, J. K.; Brinker, C. J. Langmuir 1993, 9, 1051. [4] Hammonds, K. D.; Deng, H.; Heine, V.; Dove, M. T. Phys. Rev. Lett. 1997, 78, 3701.