Imogolite Nanotubes Research Articles

The electronic structure of a single-walled aluminosilicate nanotube

The geometric structure and electronic structure of an imogolite nanotube have beenstudied using density functional theory (DFT). The calculation results indicate that thedeformation of the material leads to structural electric charges on the tube wall. This hydrousaluminosilicate single-walled nanotube is a wide gap semiconductor with a direct band gap,Eg∼3.67 eV atthe Γ point, which may be promising for application in optoelectronic devices. In conjunctionwith the DFT calculations, molecular dynamics simulations based on empirical potentialsare also performed to evaluate the mechanical properties of this material.

Molecular dynamics study of hydrated imogolite : 2. Structure and dynamics of confined water

The behaviour of water confined in an imogolite nanotube was studied by means of molecular dynamics simulations. The results of the study show an important difference between the interaction of water molecules with the internal and external surfaces of the nanotube. The analysis of the density profiles of confined molecules, of their spatial organisation, of the size of molecular clusters, of the lifetime of H-bonds in the system and of dynamical characteristics of molecules permits us to qualify the external imogolite surface as hydrophobic, whereas the internal surface reveals a hydrophilic character.

Imogolite Nanotubes: Stability, Electronic, and Mechanical Properties

The aluminosilicate mineral imogolite is composed of single-walled nanotubes with stoichiometry of (HO)(3)Al(2)O(3)SiOH and occurs naturally in soils of volcanic origin. In the present work we study the stability and the electronic and mechanical properties of zigzag and armchair imogolite nanotubes using the density-functional tight-binding method. The (12,0) imogolite tube has the highest stability of all tubes studied here. Uniquely for nanotubes, imogolite has a minimum in the strain energy for the optimum structure. This is in agreement with experimental data, as shown by comparison with the simulated X-ray diffraction spectrum. An analysis of the electronic densities of states shows that all imogolite tubes, independent on their chirality and size, are insulators.

Ab initiosimulation 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 $[X=\mathrm{Si}∕(\mathrm{Si}+\mathrm{Ge})]$ 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.

Individual dispersion of synthetic imogolite nanotubes via droplet evaporation

Morphology of synthetic imogolite nanotubes formed in droplet evaporation was investigated by transmission electron microscopy and electron diffraction. The nanotubes form a dense entangled network at higher concentrations, while at lower concentrations the nanotubes are liable to form oriented bundles. Under enthanol atmosphere, individual dispersion of nanotubes was observed for the first time, which reveals the length polydispersity of synthetic imogolite nanotubes.

Characterization of synthetic imogolite nanotubes as gas storage

Imogolite is a naturally occurring hydrated aluminumsilicate polymer that consists of a tubular structural unit having an external diameter of 2.3–2.7 nm and an inner diameter of ca. 1 nm. The tube length is from about 400 nm to several micrometers. The tube walls are composed of curved gibbsite-like sheets with SiOH groups on the inside and AlOH groups on the outside, having a composition (HO)3Al2O3SiOH [1]. Imogolite is common in the clay fraction of soils derived from glassy volcanic ashes or pumice beds, and it has also been found in many podzols [2, 3]. Several investigations concerning natural imogolite have been published [4–6]. A synthetic route for production of imogolite was described in 1977 from dilute solutions of aluminum chloride and monosilicic acid [7]. Crystallization takes place at a lower concentration of the starting solutions and low pH under the boiling point treatment [8]. However, the above common process does not produce a high yield of well-crystallized imogolite tubes. Recently, nanotubes composed of various materials such as carbon, boron nitride, and oxides have been investigated [9–11], in particular, carbon nanotubes, which have great potential as materials with novel properties that are not found in conventional graphite or carbon fullerene [12]. For instance, since the suggestion of highly efficient storage of a natural gas with single walled carbon nanotubes, experimental determinations of the storage capacity and the mechanism of the storage have been performed extensively [13]. In the present work, the authors attempt to improve a synthetic method and to characterize an aluminosilicate mineral forming such hollow tubular structures for the purpose of gas storage. Imogolite nanotubes were synthesized from a highly concentrated starting solution under hydrothermal conditions. In a typical synthetic procedure, 66.7 ml of Na4SiO4 solution (concn. 100 mmol/l) was mixed with the same amount of 150 mmol/l AlCl3 solution, and then 1 mol/l NaOH was added dropwise (1 ml/min) into the above homogeneous solution under stirring