Single-walled metal oxide nanotubes have emerged as an important class of 'building block' materials for molecular recognition-based applications in catalysis, separations, sensing, and molecular encapsulation due to their vast range of potentially accessible compositions and structures, and their unique properties such as well-defined wall structure and porosity, tunable dimensions, and chemically modifiable interior and exterior surfaces. However, their widespread application will depend on the development of synthesis processes that can yield structurally and compositionally well-controlled nanotubes. Moreover, such processes should be amenable to scale-up and preferably operate via benign chemistries under mild conditions. There is currently very little knowledge on the molecular-level 'design rules' underlying the engineering of such materials. The capability to engineer single-walled tubular materials would lead to a range of structures, with novel properties relevant to diverse applications. In this thesis, main objectives are to discover the first molecular-level mechanistic framework governing the formation and growth of single-walled metal-oxide nanotubes, apply this framework to demonstrate the engineering of nanotubular materials of controlled dimensions, and to progress towards a quantitative multiscale understanding of nanotube formation. The class of aluminosilicate (AlSiOH)/germanate (AlGeOH) nanotubes are of particular interest to us, and serve as the exemplar materials for single-walled metal oxide nanotubes. They can be synthesized in pure form from inexpensive and easily accessible reactants at low temperatures (95 ˚C) from aqueous solutions. The synthesis of nanotubes occurs on a time-scale of hours to days, making them an ideal model system to study the nanotube formation mechanism. In Chapter 2, the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature, in the formation of single-walled aluminosilicate nanotubes is reported. The structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm, are characterized by electrospray ionization (ESI) mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. DFT calculations revealed the intrinsic curvature of nanoscale intermediates with bonding environments similar to the structure of the final nanotube product. It is shown that curved nano-intermediates form in aqueous synthesis solutions immediately after initial hydrolysis of reactants at 25 ˚C, disappear from the solution upon heating to 95 ˚C due to condensation, and finally rearrange to form ordered single-walled aluminosilicate nanotubes. Integration of all results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles. Then, in Chapter 3, new molecular-level concepts for constructing nanoscopic metal oxide objects are demonstrated. The diameters of metal oxide nanotubes are shaped with Ångstrom-level precision by controlling the shape of nanometer-scale precursors. The subtle relationships between precursor shape and structure and final nanotube curvature are measured (at the molecular level). Anionic ligands (both organic and inorganic) are used to exert fine control over precursor shapes, allowing assembly into nanotubes whose diameters relate directly to the curvatures of shaped precursors. Having obtained considerable insight into aluminosilicate nanotube formation, in Chapter 4 the complex aqueous chemistry of nanotube-forming aluminogermanate solutions are examined. The aluminogermanate system is particularly interesting since it forms ultra-short nanotubes of lengths as small as ~20 nm. Insights into the underlying important mechanistic differences between aluminogermanate and aluminosilicate nanotube growth as well as structural differences in the final nanotube dimensions are provided. Furthermore, an experimental example of control over nanotube length is shown, using the understanding of the mechanistic differences, along with further suggestions for possible ways of controlling nanotube lengths. Ultimately, it is desired to produce the single-walled aluminosilicate nanotubes on a larger scale (e.g., kilogram or ton scales) for technological application. However, a quantitative multiscale understanding of nanotube growth via a detailed growth model, is critical to be able to predict and control key properties such as the length distribution and concentration of the nanotubes. Such a model can then be used to design liquid-phase reactors for scale-up of nanotube synthesis. In Chapter 5, a generalized kinetic model is formulated to describe the reactions leading to formation and growth of single-walled metal oxide nanotubes. This model is capable of explaining and predicting the evolution of nanotube populations as a function of kinetic parameters. It also allows considerable insight into meso/microscale nanotube growth processes. For example, it shows that two different mechanisms operate during nanotube growth: (1) growth by precursor addition, and (2) by oriented attachment of nanotubes to each other. In Chapter 6, a study of the structure of the nanotube walls is presented. It has usually been assumed in the literature that the nanotube wall is free of defects. A combination of 1H-29Si and 1H-27Al FSLG-HETCOR, 1H CRAMPS, and 1H-29Si CP/MAS NMR experiments were employed to evaluate the proton environments around Al and Si atoms during nanotube synthesis and in the final structure. The HETCOR experiments allowed to track the evolving Si and Al environments during the formation of the nanotubes from precursor species, and relate them to the Si and Al coordination environments found in the final nanotube structure. The 1H CRAMPS spectra of dehydrated aluminosilicate nanotubes revealed the proton environments in great detail. Integration of all the NMR results allows the structural assignment of all the chemical shifts and the identification of various types of defect structures in the aluminosilicate nanotube wall. In particular, five main types of defect structures are identified arising from specific atomic vacancies in the nanotube structure. It is estimated that ~16% of Si atoms in the nanotube inner wall are involved in a defect structure. The characterization of the detailed structure of the nanotube wall is expected to have significant implications for its chemical properties and applications. Chapter 7 contains concluding remarks, as well as suggestions for future directions in the engineering of single-walled nanotube materials.

Orientador: Oswaldo Luiz Alves / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-11T15:46:56Z (GMT). No. of bitstreams: 1 Vieira_LucianaLima_M.pdf: 10012169 bytes, checksum: 3526b70650ff03af91d229d38c75c1e4 (MD5) Previous issue date: 2008 / Resumo: Esta dissertação visa à obtenção de nanoestruturas de sulfeto de molibdênio e sulfeto de tungstênio partindo dos respectivos óxidos com morfologia de nanobastões. Os óxidos precursores foram preparados via rota hidrotérmica (MoO3 e W18O49) via rota térmica (WO3). Os sulfetos foram preparados a partir da reação sólido-gás dos óxidos em atmosfera de H2S. Os nanobastões de óxido de molibdênio foram preparados através do tratamento hidrotérmico do MoO3·2H2O em solução aquosa de ácido acético a 180 °C durante 7 dias. Os nanobastões de MoO3 com diâmetro médio de 150 nm foram submetidos à atmosfera de H2S e H2 5% / N2 95% a 800 °C, obtendo-se como produto nanobastões de sulfeto de molibdênio. Nanobastões de W18O49 foram preparados pelo tratamento hidrotérmico do ácido túngstico (WoO3·nH2O) na presença de sulfato de sódio (0-30 g) e possuem diâmetro de 5- 15 nm. Foi observado que o sulfato de sódio possui um papel importante como direcionador de fase e morfologia do óxido de tungstênio. A sulfidização dos nanobastões de W18O49 foi realizada na presença e na ausência de atmosfera redutora. Em ambos os casos foram obtidos como produto WS2 com morfologia de nanobastões e/ou nanopartículas. Por ser uma fase parcialmente reduzida, não foi necessária a presença de gás hidrogênio para a preparação de nanoestruturas de WS2 através desta rota. Nanobastões de WO3 triclínico foram obtidos através do tratamento térmico de óxido de tungstênio não cristalino (proveniente da reação entre WCl6 e metanol) em atmosfera de nitrogênio a 600-1000 °C. Os nanobastões foram sulfidizados a 800 °C em atmosfera de H2S obtendo-se como produto nanotubos de WS2 com diâmetro entre 20 e 180 nm. Tal rota se mostrou eficiente quanto ao rendimento morfológico, e também interessante na medida que temos um número menor de etapas de síntese envolvidas na obtenção da morfologia e fase desejada. Também foram feitos experimentos de sulfidização alternativa para óxidos de molibdênio e tungstênio sobre condições hidrotérmicas. Tais reações foram realizadas em meio aquoso e utilizando agentes sulfidizantes (compostos que podem gerar H2S ou íons S2-), tais como enxofre, tioacetamida e tiouréia. A sulfidização do óxido de molibdênio foi efetiva, formando sulfeto de molibdênio. Os sulfetos apresentam uma interessante mudança de morfologia do agregado dependendo do tipo de preparação de amostras para microscopia eletrônica, apresentando-se na forma de flores quando a preparação é feita através do tratamento da suspensão aquosa do sólido em ultrassom. A sulfidização alternativa do óxido de tungstênio, por sua vez, não foi efetiva, uma vez que não produz sulfeto de tungstênio, mas apenas a redução parcial do óxido. Porém, nanoestruturas unidimensionais de W18O49 são formadas quando este óxido é submetido a tratamento térmico sob atmosfera inerte. Este óxido mostrou-se bastante interessante para as reações de sulfidização em H2S, uma vez que também pode ser utilizado para a preparação de nanoestruturas cilíndricas de WS2 / Abstract: The main of this Dissertation is the preparation of nanostructured WS2 and MoS2 from a template reaction with the respective nanorods oxides. Molybdenum oxide nanorods with diameters around 100 nm and microscale lengths were prepared from MoO3·2H2O via a one step solvothermal reaction. The formation of MoO3 rods proceeds in acidic media at 180 °C. The oxide was converted in quantitative yield to MoS2 nanorods by H2S in a reducing atmosphere. TEM and SEM analysis reveals that the rod-like morphology of the oxide precursor is preserved during the H2S treatment. Monoclinic W18O49 nanorods of 5¿15 nm in diameter have been synthesised by a low temperature hydrothermal route using sodium sulfate as structural and morphological modifier. The important role of Na2SO4 salt in the synthesis has been demonstrated. These nanorods were found suitable as a precursor for the synthesis of nanostructured WS2 by reducing them with H2S at 800 °C for 30 min. This reaction can work out without a reducing atmosphere. The morphological and structural features of WS2 nanotubes, generated from WO3 nanorods, by an in situ heating process, have been studied. The nanorods were prepared by a simple annealing method of a low-crystalline tungsten oxide (from a sol-gel reaction between WCl6 and methanol) at 600-1000 °C. Finally, an alternative route to molybdenum and tungsten sulfide by solution chemical reactions was also explored. The reaction was carried out from the respective oxides and sulfurization reagents such as S, CH3CSNH2 and CSN2H4 through hydrothermal method. MoS2 nanostructures including flower-like particles have been synthesized. The hydrothermal reaction with tungsten has not produced WS2 but W18O49 nanorods after thermal annealing. These oxides were converted in nanostructured WS2 by solid-gas reaction in H2S atmosphere at 800°C / Mestrado / Quimica Inorganica / Mestre em Química

The thesis consists of eight chapters of which the first chapter presents a brief overview of inorganic nanostructures. Synthesis and magnetic properties of MnO and NiO nanocrystals are described in Chapter 2, with emphasis on the low-temperature ferromagnetic interactions in these antiferromagnetic oxides. Chapter 3 deals with the synthesis and characterizations of nanocrystals of ReO3, RuO2 and IrO2 which are oxides with metallic properties. Pressure-induced phase transitions of ReO3 nanocrystals and the use of the nanocrystals for carrying out surface-enhanced Raman spectroscopy of the molecules form Chapter 4. Use of ionic liquids to synthesize different nanostructures of semiconducting metal sulfides and selenides is described in Chapter 5. Synthesis of Mn-doped GaN nanocrystals and their magnetic properties are described in Chapter 6. A detailed investigation has been carried out on the growth kinetics of nanostructures of a few inorganic materials by using small-angle X-ray scattering and other techniques (Chapter 7). The study includes the growth kinetics of nanocrystals of Au, CdS and CdSe as well as of nanorods of ZnO. Results of a synchrotron X-ray study of the formation of nanocrystalline gold films at the organic-aqueous interface are also included in this chapter. Chapter 8 discuses the use of the organic-aqueous interface to generate Janus nanocrystalline films of inorganic materials where one side of the film is hydrophobic and other side is hydrophilic. This chapter also includes the formation of nanostructured peptide fibrils at the organic-aqueous interface and their use as templates to prepare inorganic nanotubes.