Simulação computacional das propriedades eletrônicas, estruturais e mecânicas de nanotubos inorgânicos

Abstract

The discovery of the inorganic nanotubes WS2 in 1992 marked the beginning of an age of research related to the synthesis of new nanostructured inorganic materials. Computationalmodeling and simulation have been used to estimate properties, elucidate mechanisms and define strategies of synthesis and technological applications of materials. Chrysotileand imogolite are tubular nanostructured clay minerals which have attracted the attention of researchers in the basic and applied science due to their features of incorporating or retaining organic and inorganic molecules. In addition, they are easily synthesized,characterized, modified and functionalized. In this thesis, chrysotile and synthetic tubular materials from chrysotile and imogolite were investigated through quantum methods providing insights about their electronic, structural and mechanical properties. The Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) method was used to simulate computationally the inorganic nanotubes: chrysotile; silica and silica modified by dimethyl silane; imogolite-like already synthesized: aluminogermanate; and imogolite-likenot synthesized: aluminophosphate, aluminophosphite, aluminoarsenate and aluminoarsenite. In all cases, it was studied different sizes of the zigzag and armchair nanotubes. It was necessary to obtain the SCC-DFTB parameters of the atomic pairs Mg-X (X = Mg,O, Si, H) to study the chrysotile nanotubes and Ge-X (X = Ge, O, Al, H) to simulate the aluminogermanate nanotubes. Furthermore, it was developed the program Framework for Automatization of SLAKO Parameterization (FASP) which was written using thePython programming language and the object oriented programming paradigm which automatizes the DFTB/SCC-DFTB parameterization procedure of the repulsion energy (Erep). This experience allowed us, besides understanding and mastering the DFTB/SCCDFTBmethod, to know in details its parameterization and to consolidate it in the GPQIT. Chrysotile presents several possibilities of applications, its external or internal surface can be functionalized or modified, chrysotile can encapsulate and immobilize ions and moleculesand can be used as support to assembly self-organized materials. SCC-DFTB calculations were carried out to investigate the electronic, structural and mechanical properties of chrysotile. In addition, the nature and the properties of the single-walled nanotubes of silicaand silica modified by dimethyl were simulated. Experimental works have reported the removal of the external layer of chrysotile (brucite) by acid leaching and the reminiscent material, called nano-fibriform silica, is functionalized by dimethyl silane. Computational estimates of the properties of the single- and double-walled aluminogermanate nanotubes were important to understand their nature and stability. Aluminogermanates are imogolitelike nanotubes where the silicate groups (SiO44 ) present in the inner part of imogolite aresubstituted by germanate (GeO44 ). As far we know, aluminogermanate nanotubes are the unique double-walled imogolite-like nanotubes (monodisperse) synthesized. Furthermore, recent works have shown that it is possible to produce single-walled aluminogermanate nanotubes in large scale, in contrast to imogolite. These features of the aluminogermanate nanotubes motivated us to investigate the hypothetical single-walled imogolite-like nanotubes:aluminophosphate, aluminophosphite, aluminoarsenate and aluminoarsenite. The proposal of this works was based only on geometrical analyzes where the next candidates to form imogolite-like nanotubes are those from the substitution of the silicates (SiO44 )present in the inner part of imogolite nanotubes by phosphate (PO34 ), phosphite (PO33 ), arsenate (AsO34 ) and arsenite (AsO33 ). This study allowed us to glimpse and propose possible experimental conditions to synthesize these nanotubes by theoretical models. Finally,by SCC-DFTB computational simulation, it was possible to evaluate the electronic, structural and mechanical properties of several single- and double-walled nanotubes, to understand their stability, nature and possible applications. In addition, we proposed thesynthesis of new materials by the modification of the inner part of imogolite nanotubes expanding the number of compounds which can be used in the synthesis of new materials with special properties such as well defined size and chirality.