Imogolite Nanotubes: Stability, Electronic, and Mechanical Properties Luciana Guimarães, †,‡ Andrey N. Enyashin, ‡,§ Johannes Frenzel, ‡ Thomas Heine, ‡, * Hélio A. Duarte, † and Gotthard Seifert ‡ † Grupo de Pesquisa em Química Inorgânica Teórica, Departamento de Química-ICEx- Universidade Federal de Minas Gerais, 31.270-901 Belo Horizonte, MG, Brazil, ‡ Department of Physical Chemistry, Technische Universität Dresden, D-01062 Dresden, Germany, and § Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Science, 620219 Ekaterinburg, Russian Federation N anotubes (NTs) have been in- creasingly investigated in the past decade and have become a sym- bol of the new and fast developing area of nanotechnology. 1 The widespread atten- tion can be traced not only to their interest- ing structure but also to their wide range of electrical, chemical, and mechanical properties. Since the discovery of inorganic nanotubes 2 (WS 2 ) in 1992, many other inorganic nano- tubes have been reported, based on transition metal chalcogenides, 2,3 boron nitride- and silicon oxide-based NTs, 4,5 transition metal oxides, 6,7 and others. The aluminosilicate mineral imogolite occurs naturally in soils of volcanic origin and is composed of single-walled nanotubes. The tube walls con- sist of a curved gibbsite-like sheet (Al(OH) 3 ), where the in- ner hydroxyl surface of the gibbsite is substituted by (SiO 3 )OH groups. This structure possesses a composition of (HO) 3 Al 2 O 3 SiOH, 8 which is the sequence of atoms encountered on passing from the outer to the inner surface of the tube (Fig- ure 1a). Imogolite NTs have specific characteris- tics as well as defined tube length and di- ameter that make them unique in compari- son to other NTs. In general, it remains a challenge in the synthesis of nanoparticles to control the dimensions and produce monodisperse NTs. Moreover, various theo- retical studies on several nanotubes, such as C, 9,10 BN, 9,10 BC 2 N, 9 GaS, 11 MoS 2 , 12 and TiO 2 , 13 have shown that the strain energy necessary to roll a monolayer into a tube decreases monotonically with increasing tube radius. Therefore, there is no suitable energy minimum that could be employed to produce nanotubes with a desired diam- eter. 14 However, the imogolite type is an apparent exception. 15–17 As shown in Fig- *Address correspondence to thomas.heine@chemie.tu-dresden.de. Received for review August 21, 2007 and accepted October 11, 2007. Published online November 30, 2007. 10.1021/nn700184k CCC: $37.00 © 2007 American Chemical Society Figure 1. (a) Cross section view of imogolite showing com- position. (b) Optimized structure of hypothetical 2D imog- olite layer with lattice vectors a 1 and a 2 ; views from the top and from the side are shown. White atoms, H; red, O; gray, Al; yellow, Si. ABSTRACT 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. KEYWORDS: inorganic nanotubes · strain energy · DFTB · aluminosilicate · XRD ARTICLE VOL. 1 ▪ NO. 4 ▪ GUIMARÃES ET AL. www.acsnano.org 362