Synthesis And Characterization Of Oxides Nanomaterials By Hydrothermal Method

Since the discovery of carbon nanotubes in early 1990's, quasi one-dimensional nanomaterials have received intensive interests due to their novel physical, chemical, and biological properties as well as the potential applications in nanodevices. Large-scale, cost-effective, simple and practical synthesis and assembly of one-dimensional nanomaterials is of importance for the fundamental research and application.In the dissertation, we developed a simple approach to hectogram-scale synthesizing straight and thin ZnO nanorods at low temperature (95℃) by a seed-assisted chemical reaction. The following factors played the key role for the growth of ZnO nanostructures: (1) the molar ratio of ZnO seed and zinc source controled the growth of ZnO nanorods. At low molar ratio of ZnO seed and zinc source (0.003), the javelin-like ZnO nanorods consisting of the thin ZnO nanorods with the diameter of 100 run and the thick ZnO nanorods with the diameter of 200 nm were obtained. In contrast, the straight ZnO nanorods with the diameter of about 20 nm were prepared. (2) Nearly neutral reactants including zinc nitrate hydrate and diethylenetriamine could not dissolve the ZnO seeds before the chemical reaction. (3) The dispersant such as poly(vinyl alcohol) (PVA) acted as a spatial obstructor to control the length of ZnO nanorods. Moreover, the ZnO nanostructures were tunably grown by the heterogeneous nucleation (CdS or MnO₂ nanoparticles) assisted chemical reaction at low temperature (95℃) and umbrella-like ZnO nanostructures were achieved.A surfactant assisted hydrothermal method was employed to prepare MnOOH nanostructures. Moreover, the MnO₂ nanostructures were obtained by the thermal conversion of MnOOH nanostructures. It was indicated that MnOOH nanorods with the diameter of about 10-50 nm were obtained by cetyltrimethylammonium bromide (CTAB) assisted hydrothermal process and MnO₂ nanorods were achieved from the thermal conversion of MnOOH nanorods. It was revealed that the formation mechanism of MnOOH nanorods was due to the rolling process of the flake-like complex. Moreover, the disc-like MnO₂ nanostructures with the diameter of about 200nm and the thickness of about 10 nm were prepared by citric acid (CA) assisted hydrothermal process and subsequently calcination. Further investigation indicated that the growth of MnOOH nanostructures along c axis was limited due to the preferential absorption.Finally, Fe₂O₃ nanostructures were prepared by the ethylene glycol (EG) assisted hydrothermal process. The effects of the molar ratio of Fe³⁺ and OH^-, the amount of EG, the concentration and annealing on the morphology and composition were investigated. Moreover, the effects of Fe₂O₃ nanostructures with different morphology and size on the magnetic properties were also investigated. The results indicated that only hexagonal -Fe₂O₃ nanoparticles can be obtained when the molar ratio of Fe³⁺ and OH" was less than 1:4 with or without the addition of EG; when the Fe³⁺ and OH" was more than 1:4, the orthorhombic airplane-like FeOOH nanostructures could be achieved with the assistance of EG; while the Fe³⁺ and OH^- is more than 1:4, the orthorhombic FeOOH nanorods could be achieved without the assistance of EG. The airplane-like FeOOH nanostructures and FeOOH nanorods were transformed into the porous airplane-like Fe₂O₃ nanostructures and Fe₂O₃ nanorods by caicination. The different size and morphology of Fe₂O₃ had great influence on their magnetic properties.