Controllable Preparation Of Tungsten Oxide Micro/nano-structures And Their Performance Study

The micro/nano structure of transition metal oxide has emerged as an effective approach to unlock the potential of many functional materials and offers them special properties different from those of their bulk counterparts. They have many valence electron configurations, valence states and possess unique optical, electrical, magnetic and mechanical properties. Therefore, they have attracted great research interests for their wide applications in photovoltaic, catalysis, sensor and so forth. Due to the directly electron transfer performance, as well as special geometric morphology,one-dimensional (ID) transition metal oxides are expected to become one of the design units of micro/nano-devices. In addition, most of hierarchical structures with a large void space, based on the self-assembly of one-dimensional (1D) nanostructures, possess high accessible surface area and a huge amount of available active sites. Hence, the structure-activity relationship (SAR) research of transition metal oxides shows the important significance.In this thesis, various tungsten trioxides with different crystal structure and morphologies have been controllably synthesized hydrothermally by optimizing the reaction conditions and choosing appropriate reaction system. These micro/nano structures are formed by the self-assembly of one-dimensional nanowire/nanorods.Relevant performance studies of as-prepared samples have been carried out. By choosing appropriate reaction system, these tungsten oxide hierarchical architectures were used as the organic catalyst in the catalytic oxidation of cyclohexanol and cyclohexene, as well as electrode materials of supercapacitor.The main points of the thesis are summarized as follows:(1) One-dimensional single-crystalline hexagonal WO3 nanorods have been synthesized in large-scale by a hydrothermal process using the Na2WO4·2H2O, HNO3 as synthetic recipes and citric acid, sodium sulfate as dispersant and structure-directing agent. More interestingly, each rod is assembled by closely packed and highly aligned thin nanowires growing along the [001] direction. Without using any organic solvent and harmful phase-transer catalyst, the h-WO3 nanorods are used as the catalyst in the selective oxidation of cyclohexanol to cyclohexanone by aqueous hydrogen peroxide.The effective catalysis by h-WO3 nanorods remarkably increases the yield of cyclohexanone from 3.1% to 78.6% under mild conditions (80 ?, ambient pressure),much higher than the 43.0% for commercial WO3 powder. The catalyst also shows high catalytic stability, implying that the as-prepared WO3 nanorods have potential applications in green chemistry for the production of cyclohexanone from cylohexanol.(2) We present a facile hydrothermal method to synthesize WO3 and its hydrates with different morphologies by simply adjusting the pH of the precursor solution. The synthetic recipes only require the two reactants without using any template or surfactant.The tungsten trioxide nanostructures with rod-like, disk-like and sphere-like architectures can be obtained based on the self-assembly of one-dimensional nanowire/nanorods. The influence of pH values on the morphologies and crystal structures of tungsten oxide is systematically studied. Furthermore, these tungsten oxide nanostructures were used as the catalyst for the oxidation of cyclohexene by hydrogen peroxide to produce adipic acid. The results demonstrate that these catalysts have excellent potential applications in green chemistry for the synthesis of adipic acid under mild conditions (90?, ambient pressure).(3) h-WO3·0.33H2O hierarchical architecture have been synthesized hydrothermally using the Na2WO4·2H2O as tungsten source. The hierarchical architecture is formed by the self-assembly of one-dimensional nanorods. From crystal structure, h-WO3 ·O.33H2O provides one-dimensional channels along the hexagonal tunnels on the (001)surface that can be piled up water molecules connecting into water chains, which can serve as effective proton-conducting wires, enabling effective proton conduction throughout the crystal. In addition, such superior capacitances are attributed to the synergy of the 1D WO3 nanorod in supplying fast electron transport pathways. Thus, the specific capacitance of h-WO3 · 0.33H2O electrode could achieve 391 F g-1 when the current density is 0.5 A g-1 and it also delievered a high specific capacitance (298 F g-1)after 2000 cycles at a high current density of 10 A g-1.