Nanotubular materials exhibit remarkable physical properties that make them attractive candidates for various applications. The unique properties of nanotubes are due to the combined effects of their nanoscale size, shape, composition, structure, and porosity. The nanotube diameter is an important parameter controlling its electronic, optical, chemical, and transport properties. However, controlling the diameter of a carbon or a related inorganic nanotube is a challenging issue because there is no obvious aspect of their formation mechanism that allows facile control over the nanotube curvature. On the other hand, we are investigating a class of single-walled metal oxide nanotubes whose dimensions can be precisely tuned by varying the nanotube composition via aqueous-phase chemistry. We report here the unique correlation between the diameter, energy, and composition of single-walled aluminosilicogermanate (‘AlSiGeO') nanotubes by employing molecular dynamics (MD) simulations. We also present a simple harmonic force constant model that reproduces the diameter control of the nanotubes by relating the composition of the material to the strain energy of bending into a nanotube.

A series of AlSiGeO nanotubes were synthesized and characterized by XRD, TEM, XPS, nitrogen adsorption, and vibrational spectroscopy to experimentally obtain the nanotube diameter as a function of its composition. Our computational methodology involves construction of atomically detailed models of aluminosilicogermanate nanotubes with varying Si/Ge molar ratios (from zero to infinity) and outer tube diameter for each nanotube composition varying between 1.5-4.8 nm. We then parameterized an atomistic potential model for AlSiGeO nanotubes and validated it by comparison of infrared spectra of the material with calculated normal-mode vibrational frequencies. The optimized structures were subjected to MD simulations to calculate the internal energy as a function of nanotube diameter. The results from the study show progressive minima in the nanotube energy as a function of diameter and composition, a phenomenon that is not observed in carbon nanotubes. This composition-dependent shift in energy minimum diameter of the nanotubes matched closely with the predictions from our harmonic force constant model. Further, the model provided an insight on the changes in the curvature of the nanotubes upon increasing substitution of Ge for Si. Due to difference in Si-O and Ge-O bond lengths and weaker Ge-O bond energy compared to that of Si-O, the outer diameter of the nanotubes increased with increase in Ge content, as corroborated by the experimental results. Concurrently, both the measured infrared absorption spectra and simulated normal-mode vibrational spectra of the nanotubes show progressive shifts in absorption bands due to substitution of Si with Ge. In addition, we also observe computationally the inverse power law dependence of the radial breathing mode (RBM) frequency on the nanotube radius, which is in very good agreement with our theoretical prediction. While the mechanism behind the assembly of diameter-controlled nanotubes is a separate area of investigation, the present correlation indicates that energetic considerations play an important role and that there exists the possibility of engineering nanotubes of controlled dimensions by understanding the influence of these energetic factors.

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