Investigation Of Hydrothermal Synthesis And Properties Of Oxide Nanostructures

Due to the high specific surface area and quantum size effect, nanomaterials have potential applications in importantant fields such as information, biology and energy. During the past two decades, nanomaterials have become one of the most active research fields in international materials research communities. Up to now, several decades of physical and chemical technologies have been developed to prepare a variety of nanomaterials. Among these technologies, hydrothermal method has emerged as an important one due to its obvious advantages such as simplicity and controllability.In this dissertation, two hydrothermal processes hav been proposed to synthesize oxide nanostructures. One is a novel hydrothermal process using hydrazine hydrate as a mineralizer; the other is a simple citric acid-assisted hydrothermal process. Nanostructures and thin films of some important oxides have been successfully synthesized by the hydrothermal processes, including SnO₂ quantum dots (<2.7nm), indium oxyhydroxide (InOOH) hollow spheres, small-sized zinc stannate(Zn₂SnO₄) nanorods (<4nm), small-sized tetragonal ZrO₂ nanocrystals, ZnO nanorods, Fe₃O₄ nanocrystals,γ-Fe₂O₃ nanocrystals, La(OH)₃:Tb³⁺ nanorods and Fe₃O₄ thin solid films. Normally, hydrazine hydrate has been only used as a reducing agent. The detailed investigation in this dissertation has revealed that hydrazine hydrate itself can act as complexing agent, slow release alkaline mineralizer and reductive protecting agent in this hydrothermal process. In comparision with NaOH and NH₃·H₂O, hydrazine hydrate is a more versatile alkaline mineralizer for hydrothermal method. In addition, the structure, morphology and properties, such as optical, magnetic, electrochemical and electric properties, of the oxide nanostructures and the films have been investigated in detail. The chemical mechanism and growth mechanism for hydrothermal formation of the oxide nanostructures and films have been also proposed. The most significant results achieved in this dissertation are given as follow: (1) Hydrothermal synthesis of ultra-small sized nanostructures is still less successful. In this dissertation, the novel hydrothermal process using hydrazine hydrate as the mineralizer is propsed to prepare ultra-small sized oxide nanostructures. SnO₂ quantum dots (<2.7nm), Zn₂SnO₄ (<4nm) and tetragonal ZrO₂ nanocrystals (<5nm) were successfully synthesized by the hydrothermal process for the first time. Furthermore, the possible chemical mechanism has been proposed, and FTIR spectra confirmed that hydrazine hydrate reacted with the salts (e.g., SnCl₄, ZnCl₂ and ZrOCl₂) to form hybrid complex clusters (e.g., (SnCl₄)_m(N₂H₄)_n). The clusters made critical roles as intermediate and soft template. The hydrothermal transformation from the complex clusters to oxides was successfully controlled at local regions of the clusters. Consequently, ultra-small sized oxide nanostructures were obtained.(2) Fe₃O₄ nanocrystals were readily synthesized by the hydrothermal reaction between hydrazine hydrate and ferrous salts (e.g., FeSO₄). Base on the hydrothermal synthesis of Fe₃O₄ nanocrystals, a new hydrothermal process to prepare dense Fe₃O₄ thin films on carbon steel and nickel substrate has been developed. Furthermore, the factors for the preparation of the dense films (e.g., reactants, substrate, time and hydrothermal temperature) and growth mechanism have been presented in this dissertation. Electrochemical analysis (Tafel and electrochemical impedance spectroscopy) indicated that the anodic dissolution reaction was effectively limited and the corrosion resistance increased by the Fe₃O₄ coating. Growth of Fe₃O₄ films on steel is called "blackening" in industry. The hydrothermal technology for grwoth of Fe₃O₄ films presented here is environmentally friendly and has promising appliations to blackening.(3) Maghemite (γ-Fe₂O₃) is an important magnetic material.α-Fe₂O₃ is the stable phase of Fe₂O₃, whileγ-Fe₂O₃ is metastable. Therefore, hydrothermal synthesis ofγ-Fe₂O₃ nanocrystals is much more difficult than that ofα-Fe₂O₃. Herein, a facile hydrothermal route for the synthesis ofγ-Fe₂O₃ nanocrystals is proposed. The synthesis route included two steps: (i) hydrothermal synthesis of Fe₃O₄ nanocrystals in presence of hydrazine hydrate, and (ii) hydrothermal oxidation of the Fe₃O₄ nanocrystals to theirγ-Fe₂O₃ counterpart in presence of H₂O₂. Consequently, phase-pureγ-Fe₂O₃ nanocrystals were successfully prepared by the two-step hydrothermal process. Their magnetization curves revealed that the Fe₃O₄ andγ-Fe₂O₃ nanocrystals showed ferromagnetic behavior, and theγ-Fe₂O₃ nanocrystals have high saturation magnetization of 68 emu/g at room temperature which is close to the theoretical value of its bulk powder. In situ XRD study revealed theγ-Fe₂O₃ nanocrystals exhibited high thermal stability with enhancedγ→αphase transition temperature (650℃).(4) Many hydrothermal processes have been developed to prepare ZnO one-dimensional nanostructures such as nanorods, but in all these processes surfactants or complexing agents are needed to control its morphology. In this dissertation, ZnO nanorods were successfully synthesized by the hydrothermal process using hydrazine hydrate as the mineralizer without addition of any other additive. The hydrazine hydrate acted as both complexing agent and alkaline mineralizer. FTIR investigation revealed that the complex precipitate formed by the reaction between hydrazine hydrate and ZnCl₂ is stable even after heat treatment at 90℃for 24 h in the solution. In subsequent hydrothermal stage (150℃), the complex precipitate transformed into ZnO nanorods after 4 h hydrothermal treatment.(5) Hydrothermal method has been widely used to prepare rare earth nanophosphors, but no attention has been paid to the oxidation of rare earth ions such as Ce³⁺ or Tb³⁺ during the hydrothermal stage. This dissertation definitely reveals that La(OH):Tb³⁺ nanorods prepared by normal hydrothermal method suffer significant loss of luminescence due to the oxidation of Tb³⁺ to Tb⁴⁺ at hydrothermal stage for the first time. A reductive hydrothermal process using hydrazine hydrate as a protecting agent is proposed to synthesize La(OH):Tb³⁺ nanorods. The oxidation of Tb³⁺ has been effectively prevented by the reductive hydrothermal process. Consequently, the La(OH):Tb³⁺ nanorods exhibited much stronger photoluminescecne than that prepared by the normal hydrothermal method. Furthermore, analysis of two kinds of the La(OH):Tb³⁺ nanorods by FTIR revealed that the Tb⁴⁺ ions did exist in the nanorods prepared by the normal hydrothermal method. The reductive hydrothermal process proposed here is desirable for the synthesis of other efficient Ce³⁺ or Tb³⁺ doped nanophosphers.(6) As an important wide band-gap oxide semiconductor, In₂O₃ and its nanostructures have been widely investigated. In comparision, the prepration and properites of InOOH is not well studied. In this dissertation, a simple citric acid-assisted hydrothermal process is proposed to synthesize InOOH hollow sphere, which is composed of thousands of nanoparticle with the diameter of 20 nm. Analysis of UV-vis diffuse reflectance spectroscopy revealed that the optical band gap, E_g, was estimated to be 3.7 eV, very close to that of In₂O₃. Therfore, InOOH is also a wide band-gap semiconductor. Furthermore, the factors for hydrothermal formation of the hollow spheres and formation process are presented in detail. The hydrothermal formation of the InOOH hollow sphere goes through three stage: 1) formation of chain, 2) formation of ring, 3) formation of hollow sphere.