Nanotubos e nanobastões de oxidos e sulfetos de metais de transição obtidos via sistemas bidimensionais (lamelares) : preparação, caracterização e propriedades / Nanotubes and nanorods of transition metal oxides and sulfides obtained from bidimensional systems (layered compunds) : preparation, characteization and properties

Odair Pastor Ferreira

This Thesis reports the preparation, characterization and properties of nanotubes and nanorods of transition metal oxides and sulfides obtained from bidimensional (Iamellar) structures. From the structural point view, nanotubes could be formed from bidimensional compounds by rolling its layers generating scroll-like nanotubes, or by the elimination of dangling bonds in peripheral atoms of the bidimensional clusters, generating nanotubes constructed by perfect cylinders. Nanorods could be formed from the rupture of the layers in specific directions, or by the collapse of the previously formed nanotubes. The following systems were studied: molybdenum sulfide, vanadium oxides, titanium oxides and manganese oxides. A new methodology was proposed to obtain MoS2 nanotubes, which is based on the thermal decomposition of ammonium thiomolybdates [(NH4)2MoS4 and (NH4)2Mo2S12] under inert atmosphere. In this condition, thiomolybdate thermal decomposition occurs via elimination of H2S, NH3 and elemental S, passing through sulfur-rich intermediates before formation of MoS2. The presence of these intermediates in the decomposition of thiomolybdates could be an essential condition to the formation of the nanotubes as well as the template effect of the solid precursor in final products. Some precursor particles present nanorod morphology and its decomposition through the elimination of "gaseous compounds" would result in a volume contraction, generating nanotubes. Autoclaves environment came to be a versatile way of preparing nanotubes and nanorods. Vanadium oxide nanotubes were obtained via solvothermal treatment of lamellar precursor formed by the intercalation of dodecylamine in V2O5. The presence of the amine was extremely important in the preparation of the nanotubes, once when it was substituted by a quaternary ammonium salt (CTAB) it was observed the formation of VO2 nanobelts. Vanadium oxide nanotubes are scroll-like and present outer diameters in the range of 80-120 nm. Its walls are constituted by vanadate units (V7O16) and between the walls there are protonated amines. These amines can be exchanged by metallic cations (Cu, Co and Cd) via ion exchange reactions. To our knowledge, it was found for the first time the bands in the Raman spectra considered the spectral signature of the vanadate nanotubes. However, the laser power density promotes the decomposition of the nanotubes during spectra acquisition. Thermal decomposition of these nanoparticles resulted in the collapse of the tubular morphology accompanied by the formation of V2O5. The decomposition from nanotubes to V2O5 occurs via an intermediary compound that would be isostructural to the V2O5.nH2O xerogel. Nevertheless, the thermal decomposition residue can be converted again to nanotubes through a solvothermal treatment with dodecylamine. Titanium oxide nanotubes were prepared though hydrothermal treatment of TiO2 and NaOH. During the hydrothermal treatment occurs the formation of a layered titanate, in which the layers could be rolled generating scroll-like nanotubes. These nanoparticles present diameter of about 9 nm. In the case of titanium oxide nanotubes, the study was mainly focused on the structure and decomposition of these nanoparticles. Based on the results obtained from the physico-chemical characterization, ion exchange reaction (with H, Ca,Co and Cu) and thermal decomposition, a new composition was proposed for the as-prepared nanotubes: Na2Ti3O7.nH2O. The results also suggest that the nanotube walls have structure similar to those observed in the layers of the bulk Na2Ti3O7. Depending on how the washing process is performed on the nanotubes (water or acid solution) the Na content can be modified via exchange reaction of Na by H. Thus, a general chemical formula was also proposed: Na2-xHxTi3O7.nH2O (0

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