A Study On Synthesis, Characterization And Property Of Tin Oxide Nanostructures

Owing to their unique optical, electric, photoelectric transition, and photocatalytic properties, semiconductor nanomaterials have drawn wide research attentions. Designing reasonable synthetic routes of nanostructures, controlling their sizes, morphologies, dimensions, components, and crystal structures as well as studying the relationship between nanostructures and their properties are the important sections of nanoscience and nanotechnology. They are also of great significance in realizing the manipulated synthesis of functional materials to our will. In this dissertation, semiconductor SnO₂ was chosen as the target and several kinds of chemistry methods were conducted to prepare new morphologies and nanostructures. The main strategies were involved in using ordinary tin salts as the precursors to synthesize SnO₂ nanocrystals in solution-based reaction system by introducing polymers, doping Zn in the SnO₂ nanocrystals, tuning the components of solvents, and using natural cotton as templates. The obtained products were characterized by XRD, FESEM, TEM, HRTEM, FT-IR, UV-Vis, Raman spectra, PL, XPS, et al. The formation mechanisms of the obtained nanostructures were discussed and their properties were also investigated. The main work is summarized as follows:1. Sevaral kinds of SnO₂ nanostructures were prepared assisted by polymers Poly(acrylic acid) (PAA), polyethlene glycol (PEG) 200, and polyvinyl alcohol (PVA) 1799: Firstly, single-crystalline SnO₂ nanocones with average 1.0μm in length and 100-500nm in root size and their self-assembly morphologies of hollow spheres were obtained through a solvothermal process assisted by PAA. The influence of crystal growth parameters such as alkalinity of the solution, components of the solvents, dosage of PAA on the morphologies and sizes were investigated. The crystal growth process experienced a self-assembly of nanoparticles directed by PAA and oriented attachment mechanism to form the single-crystalline nanocones. With nanometer-sized tips and rugged surfaces, the nanocones have high surface free energy and further assemble into hollowspheres. The Raman spectrum of the nanocones shows new group of shifts related to the crystal microstructures; Secondly, single-crystalline SnO₂ nanorods were obtained in hot PEG 200. PEG 200 was crucial in this anisotropic growth process and believed to act both as a solvent and a surfactant. The preferred growth direction of the nanorods was along [11-2] which has seldom reported by now. In this reaction system, the growth parameters such as temperature, time, alkalinity of the solution, and concentration of initial ion were all important to the size of nanorods. The growth process experienced an oriented attachment process. The photoluminescence spectrum displayed strong excitonic PL and defects PL; Finally, PVA 1799 was used as the substrate and PVA-SnO₂ nanocomposite was firstly obtained by fluxing at high temperature and oxidation process. When this nanocomposite was subjected to calcination, the polymer was removed and the pure SnO₂ crystals were obtained. Nanostructures such as SnO₂ hollow spheres composed of nanoparticles were obtained.2. The morphologies and sizes were well controlled by Zn doped into SnO₂ crystals. Branched Zn-doped SnO₂ nanorods clusters were prepared through a facile solvothermal process in ethanol-water media. The introduction of a small quantity of Zn2+ was used as dopant and the obtained products exhibited pure SnO₂ rutile structures. With the content of Zn in the products, the diameters of the nanorods were decreased while length increased. The growth parameters such as component of solvent, alkalinity of the solution, molar ratio [Zn²⁺]:[Sn⁴⁺], and the introduction of polymer PAA can greatly influence the crystal phase and morphologies. The possible growth mechanism has been proposed by investigating the transition of crystal phase and formation of the nanorods during the growth process. These Zn-doped SnO₂ nanorods exhibited unique Raman spectra in contrast with the undoped SnO₂ nanostructures and some new shifts were observed. The gas sensing properties of these nanorods to ethanol gas, acetone gas, and benzene gas were also investigated.3. Controlled growth of SnO₂ crystals was investigated in homogenous solutions composed of two or several kinds of solvents through sovlothermal processes: At first, SnO₂ nanostructrues such as flower-like nanorod clusters, nanorod microrings, and individual nanorods were obtained via a solvothermal approach in a mixed solution of ethylenediamine and distilled water. By just simply adjusting the volume ratio of the above two solvents, alkalinity of the solution, and temperature, the size and hierarchical structures of SnO₂ nanocrystals could be easily varied. The preferential growth direction of the nanorods was [11- 2] and there were high concentration of oxygen vacancy and lattice defects in the nanorods. The formation mechanism of the nanorods and their hierarchical structures were also discussed. The optical spectra including FT-IR spectra, UV-vis absorption spectra, and photoluminescence spectra of the samples prepared in different volume ratios of ethylenediamine and distilled water all showed changes of intensities and shifts. On the base of the above results, other solvent component such as ethylenediamine:ethanol:distilled water(1:1:1), diethenetriamine: distilled water (1:1), et al. were also used as reaction media to control the sizes and morphologies of SnO₂ nanocrystals. A conclusion can be drawn that the inherent properties and functional groups of the solvent molecules are vital for the crystal growth during the solvothermal process.4. A novel chemistry strategy was conducted to prepare cotton-like SnO₂ using natural cotton as templates. The obtained fiber-like SnO₂ were composed of nanoparticles with the average size of 14nm which interconnected each other and formed porous microstructures. The original morphology of cotton fibers was found to be replicated by the SnO₂ crystals from nanometer to micrometer regimes. However, the size of the obtained cotton-like SnO₂ became smaller. Because the porous microstructures were in favor of the diffusion of the target gases, these fiber-like SnO₂ showed high sensitivities to ethanol gas and were good candidates to gas sensing materials.Finally, the main experimental results and innovations in this dissertation were summarized and the following expectations were proposed.