Novel Nanostructures Based on Imogolite Nanotubes, Their Derivatives and Applications
Zusammenfassung der Projektergebnisse
In this project, the fundamental properties of clay mineral nanotubes (CMNT) have been investigated by means of atomistic quantum-mechanical calculations. Nanotubes can exist as rolled up form of a single sheet of a layered material. For example, a single layer of graphite is graphene, and directly related carbon nanotubes, with tube walls that have the same structure as graphene, can be formed. Nanotubes exist for most layered material that can be exfoliated to the monolayer. A well-known, yet in this respect under-investigated class of materials are clays, layered aluminosilicates that exist in various forms and are omnipresent in nature. Here, we studied, by means of quantum-mechanical atomistic theory, the structure and stability of nanotubes that are formed by single layers of clay, in particular imogolites, where an alumina and a silica layer are bridged by oxygen atoms. Three main results have been achieved: We have extended an efficient method that allows the efficient calculation of the properties of clay-mineral nanotubes by extending the DFTB methodology to include the elements that are needed to calculate these materials. DFTB is an approximation to density-functional theory (DFT), the most commonly applied method in materials science. DFT calculations are, however, too costly for the investigated CMNTs, as typical structure models that are common in solid-state physics cannot be made here due to lack of symmetry. DFTB is many orders of magnitude faster than DFT, but requires parameters which have to be generated and validated first. We have generated to parameters to treat imogolite and Ge-imogolite nanotubes, the precondition to study the imogolite derivatives reported here, in particular the double-walled Ge-imogolites. It is known for imogolite that these structures form with a well-defined radius. This is uncommon for other nanotubes, as the strain energy (the energy needed to bend the tube) should increase with the tube curvature. Typically, this results in the situation that nanotubes are formed in a large diversity of diameters. In contrast, imogolite and Ge-imogolite (see Figure) have welldefined radii and thus are monodisperse. Similar as for imogolite, Ge-imogolite has a diameter that is defined by the subtle difference between the As-O and Ge-O bond lengths. Ge-imogolite is, however, thicker than the corresponding imogolite moieties. On the other hand, chrysotile and halloysite tubes show the trend that is common in ordinary nanotubes, that is, they do not show a preferred diameter and thus occur only as mixture of tubes with different thickness. Most importantly, we understood the quite unintuitive fact that double-wall Ge-imogolite nanotubes exist, and that also those show a preferred diameter. Double-walled tubes require, obviously, two tubes of different diameter, meaning that at least one of them must be different than the diameter that is preferred for the single-walled system. However, the inter-wall interaction, resulting from electrostatic and London-dispersion forces, mediates between the two tubes, and preferred diameter arrangements are visible.
Projektbezogene Publikationen (Auswahl)
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(2012). Clay Mineral Nanotubes: Stability, Structure and Properties, Stoichiometry and Materials Science - When numbers matter, Dr. Alessio Innocenti (Ed.), ISBN: 978-953-51-0512-1, InTech
H. A. Duarte, M. P. Lourenço, T. Heine, L. Guimarães
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Chem. Theory Comp. 9 (2013), 4006
M. Wahiduzzaman, A. F. Oliveira, P. Philipsen, L. Zhechkov, E. van Lenthe, H. A. Witek, T. Heine, J.
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Phys. Chem. C 118 (2014), 5945
M. P. Lourenço, L. Guimaraes, M. C. da Silva, C. de Oliveirs, T. Heine, H. A. Duarte, J.
