Design And Modification Of Tungsten Oxide Nanostructures For Gas-Sensing And Photocatalysis

With the rapid economic growth and industrialization, the emission of toxic and hazardous gases increases dramatically. Air pollution has been becoming more serious for human health and safety. To effectively detect the hazardous gases, significant efforts have been focused on exploring novel gas-sensing materials and developing gas sensors with improved performance. Tungsten oxide (WO3), as a typical non-stoichiometric, acid-resisting and thermostable semiconductor, is very prospective for gas-sensing. In this thesis, WO3nanomaterials with different shapes and sizes were prepared and their morphology-dependent gas-sensing property is investigated. Heteroatom doping and organic/inorganic hybridization were also applied to futher enhance the response and selectivity of WO3. Apart from the desirable gas-sensing properties, WO3also has potential in photocatalysis due to its anti-photocorrosion and suited bandgap. As its main drawback, the conduction band level of WO3is more positive than the reduction potentials of O2to O2-, impeding its real application. Consequently, two composite photocatalysts of Fe2O3@WO3and RGO/WO3·H2O were designed and prepared to improve the photocatalytic activity of WO3under visible light.(1) WO3nanorods with aspect ratio of～50were synthesized by hydrothermal reaction at100°C with the assistance of oxalic acid and potassium sulfate. The nanorods exhibit high response of209to10ppm at operating temperature of200"C, accompanying high selectivity or19and30toward CO and CH4, respectively. A possible adsorption and eaction model of the WO3nanorod sensor was proposed by impendence neasurement to illustrate the gas-sensing mechanism.(2) Nanosheet-assembled WO3microspheres were synthesized using rapid sonochemistry followed by thermal treatment. Assembly mechanism of ODâ†'2Dâ†'3D is proposed based on oriented attachment and reconstruction. The obtained samples demonstrated fast sensor response (response time is lower than10s) during the detection range of10-100ppm, which is attributed to the fast diffusion of gas molecules in the hierarchically porous nano/microstructures.(3) Morphology-controlled synthesis of WO3nanostructures was achieved by a facile carboxyl-assisted hydrothermal process. The amount of carboxyl groups is found crucial for the morphological evolution.3D hierarchical spheres exhibited higher NO2sensitivity and faster response as compared to OD nanoparticles,1D nanorods and2D nanosheets. It is demonstrated that the change of defect feature in crystalline WO3is responsible for the morphology-dependent gas-sensing properties.(4) The electronic structure and density of states of monoclinic WO3as well as the adsorption behavior of NO2on WO3(200) surface were investigated by first-principles calculation. The upper part of valence band of WO3is mainly composed of O2p, while the lower part of conduction band is mainly composed of W5d states. NO2molecule most possibly adsorbed at terminal OIc site of WO3leads to introduction of new surface states, which is responsible for the intrinsic NO2-sensing properties.(5) By Sb doping, the gas-sensing property of WO3was improved remarkably. The Sb was substitutionally introduced in the form of Sb6O13with a Sb3+/Sb5+ratio of1/2. New Sb5p state appeared in the conduction band of WO3. The response of5%Sb-doped WO3reached3.2to1ppm NO2, which is about3times of pure WO3. On the other hand, a novel hybrid sensing material of polythiophene/WO3was prepared through in-situ chemical oxidative polymerization. The obtained organic/inorganic hybrid was developed for the first time to detect ppm-level H2S, whose response is2times of WO3and11times of polythiophene at low operating temperature of70°C.(6) Based on energy band engineering of semiconductor, OD Fe2O3nanoparticles were homogeneously decorated on3D WO3framework to constitute a multiple-dimensional and hetero-structured Fe2O3@WO3photocatalyst. The1%Fe2O3@WO3sample improved the photocatalytic activity of pure WO3by68%, which is thoroughly discussed in relation to the structural feature, light absorption and band configuration of the photocatalyst. In addition, RGO/WO3H2O composite with enhanced charge transfer was prepared through in-situ photocatalytic reduction. The rate constant of5%RGO/WO3·H2O for visible-light-driven photocatalytic RhB degradation is approximately17times of WO3·H2O.