Ab initio simulation of the structural and electronic properties of aluminosilicate and aluminogermanate natotubes with imogolite-like structure

A theoretical study of imogolite-like single wall nanotubes as a function of silicon and germanium content and their tubular radius is presented along with mapping of silicon-germanium content properties in models that contain from 9 gibbsite-like units (N-u = 9) to 13 gibbsite-like units (N-u = 13) with a silicon-germanium content [X = Si/(Si+Ge)] of 0, 0.20, 0.40, 0.60, 0.80, and 1.00. The imogolite nanotubes were built setting periodic boundary conditions to density functional theory (DFT) geometry-optimized models along both radial and axial directions in order to obtain the stable structure. The DFT calculations were carried out on the Gamma point and for various k points, using the GGA-PW91 functional, finding an optimal unit cell length of 8.72 and 8.62 A using a double numeric (DN) and double numeric with polarization (DNP) basis set, respectively, along the axial direction. The structural properties were analyzed through the evolution of the x-ray diffraction pattern as a function of both N-u and X. A linear correlation between the tube radius and the position of the first two peaks on the x-ray diffraction is found. The imogolite surface charge was mapped with the Hirshfeld charge showing the characteristic acid tendency experimentally reported. The reactivity of imogolite-like structures was studied employing the total density of states and the band gap evolution as a function of N-u and X showing an increasing behavior with X. The local reactivity was analyzed by looking at the local density of states in models with X = 0.0 and 1.0 with N-u = 10. Finally, the frequency analysis was carried out in the optimized structure on the Gamma point finding a good agreement in the O-H vibrations with those reported in the literature.