The Hydrothermal Synthesis And Properties Of Ruthenium Oxides

Mixed-metal oxides are key components for many technological applications, and these functional materials often contain one or more transition-metal elements whose oxidation states are crucial for determining electronic, magnetic or redox properties. Important contemporary uses include the areas of electronics (capacitors, magnetic devices), communication (microwave resonators for mobile telephones), photocatalysis (materials for organic waste destruction or for splitting of water), oxidation catalysis (ammonia or methane oxidation, for example), high-temperature solid-oxide fuel cells (as oxide ion conductors in methanol combustion) and as recyclable battery materials (hosts for reversible lithium uptake). Although the crystal chemistry (i.e., composition and atomic structure) of an oxide phase is vital in dictating its underlying properties, in terms of practical applications control of crystal form (morphology, particle size and shape, textural properties, or porosity) is also vital for allowing the fabrication of useful devices based on any material. A good example is the need to deposit thin layers of various metal oxides with precise orientation and thickness for the fabrication of an electronic device.These materials, with dense structures associated with ceramics, have traditionally been prepared using high-temperature solid-solid reactions, where separate oxide precursors containing each metal are mixed and fired at temperatures often in excess of 1000℃for periods of days, or even weeks, with intermittent regrinding of the mixture to ensure homogeneity. These reactions rely simply upon solid-state diffusion and require the brute force of high temperatures to bring about the reaction, often combined with pressure or controlled gas atmospheres when particular oxidation states of a transition metal are desired. The products formed by this'shake and bake' synthesis are highly crystalline but there is little scope for control of crystal form, and only the most thermodynamically stable phases are produced. Understandably, solid-state chemists have long sought milder reaction conditions using'soft chemistry', in particular to overcome the difficulties of achieving homogeneous mixing of solid reagents and to investigate the possibility of control over crystal form (particle size and shape). The idea of being able to isolate new, metastable phases, not seen at high temperature and pressure also adds to the appeal of being able to explore a wide variety of synthetic conditions for solid state materials. Soft chemical methods have been well documented, and these include methods such as sol-gel or co-precipitation where elemental mixing is achieved in a first step before a firing step is performed to burn off complexing ligands and to induce crystallinity; molten-salt methods that use a low melting alkali-metal salt as a reaction medium, which itself may act as a reagent; and topochemical reactions where a solid precursor host (which itself has to be synthesised) is treated in such a way as to insert (or remove) atoms or ions in a stoichiometric manner.Hydrothermal reaction provides a simple method to synthesis a variety of complex materials in one step. In hydrothermal method, solid and/or liquid reagents are sealed in containers, heated close to or higher than the boiling point. This method provides a step in the synthesis of composite materials approach. Hydrothermal reaction is very commonly used inorganic solid, such as in the preparation of open framework structure, silicates and phosphates and other inorganic materials, it is also used in the synthesis of inorganic-organic hybrid materials. Historically, in the evolution of hydrothermal method, it was previously used in the preparation of a number of large crystals for industrial use, especially quartz. Recently, more and more attentions are paid to the hydrothermal preparation of complex oxides. Even if when no template or structure directing agent is added, densed oxides are crystallized directly out of the solution. The advantage of solution synthesis is the control over crystal form to get well-defined morphology including a number of important perovskite materials, e.g. BaTiO3, PbZr1-xTixO3. and Na1-xKxNbO3. The synthesis of these materials is usually associated with high temperature which is used in solid-state chemistry. The hydrothermal synthesis of these materials can allow the deposition of thin films, the controlled over the formation of nanocrystalline powders is in the aspect of shapes varying from spherical to anisotropic forms, such as plates or rods. Traditionally, samples of ruthenates have been synthesized by a solid state reaction between the appropriate rare-earth oxide and ruthenium dioxide. In such reactions, a high temperature anneal (1100℃) is necessary to facilitate the crystallization. Other synthesis methods also have been tried, eg spray pyrolysis and coprecipitation and hydrothermal synthesize. Historically, certain ruthenates, such as binary Ru-Sn oxides was prepared by mild hydrothermal condition, lithium, calcium and barium ruthenates were prepared using high-temperature (>600℃) and high-pressure(>150MPa) hydrothermal synthesis. Recently, a pyrochlore oxide (Ce0.67Na0.33)2Ru2O7 was synthesized under subcritical hydrothermal conditions. Anyway, no systematic study of the hydrothermal chemistry of ruthenates has been undertaken.Our group has explored the hydrothermal synthesis of complex oxides for over 10 years, including the synthesis of manganites with perovskite structure and pyrochlore structure. Since the absence of reports on the hydrothermal synthesis of the ruthenates, in this paper, we present the successful synthesis of several different ruthenates under mild hydrothermal conditions.In Chapter 1, a review over the field of ruthenates is presented.In Chapter 2, the hydrothermal syntheses of two ruthenium containing perovskite are reported including 9R BaRuO3 and 8H Ba4Ru3NaO12. The crystal of 9R BaRuO3 could be grown in one step from aqueous solutions of metal salts and sodium hydroxide at temperatures 240℃. Samples were characterized by powder X-ray diffraction, scanning electron microscopy and variable temperature DC magnetic susceptibility. When oxidant is added to this system,8H Ba4Ru3NaO12 is formed. The characterization over Ba4Ru3NaO12 showed anti-ferromagnetic transition at 265K and a Curie tail at low temperatures. Electronic conductivity measurement indicated that 8H Ba4Ru3NaO12 is semiconductor.In Chapter 3, a series of pyrochlore ruthenate oxides, R2Ru2O7(R=Pr3+, Sm3+-Ho3+), were synthesized crystalline phases from aqueous solutions of Ru3+. R3+ and sodium hydroxide under mild hydrothermal conditions for the first time. All the samples are single-phase with uniform morphology. The yield of products depends on the type and the precise amount of alkalinity, the agent RuCl3 also has critical influence on the formation of products. Powder X-ray diffraction, scanning electron microscopy, and variable temperature dc magnetic susceptibility (SQUIDS) were performed to characterize the samples. Nano- sized Pb2Ru2O6.5 and Bi2Ru2O7 are also formed in similar conditions.