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

TLDR

The study theoretically investigates imogolite‑like single‑wall nanotubes across varying silicon‑germanium compositions and tube radii, mapping structural and electronic properties for models with 9–13 gibbsite‑like units. The authors constructed periodic DFT‑optimized models of the nanotubes, performing geometry optimizations with GGA‑PW91 at Γ and selected k‑points, and analyzed structural evolution via XRD, surface charge via Hirshfeld mapping, electronic properties through total and local density of states, band gaps, and vibrational frequencies. They found a linear correlation between tube radius and the positions of the first two XRD peaks, observed that reactivity and band gaps increase with silicon‑germanium content, and confirmed that calculated O–H vibrational frequencies agree with literature values.

Abstract

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 $[X=\mathrm{Si}∕(\mathrm{Si}+\mathrm{Ge})]$ 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 $\ensuremath{\Gamma}$ point and for various $k$ points, using the GGA-PW91 functional, finding an optimal unit cell length of 8.72 and $8.62\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ 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 $\ensuremath{\Gamma}$ point finding a good agreement in the $\mathrm{O}\text{\ensuremath{-}}\mathrm{H}$ vibrations with those reported in the literature.