Study Of Oxide Nanomaterials: Preparation, Characterization And Photocatalytic Application

ZnO, Zn₂SnO₄ and TiO₂ nanomaterials are wideband semiconductors. Thesematerials have the advantages of abundant, cheap, non-toxic, and stable. They can beused in many areas, including photocatalysis of pollutants and dye-sensitized solarcells. Although tremendous efforts have been paid in studying such kinds ofnanomaterials and remarkable progresses have been achieved in the past, there stillremain great challenges in fabricating the nanomaterials with controllablecompositions and microstructures, exploring new physical phenomena andapplications. In the present work, we focused on the controllable fabrication andcharacterization of ZnO, Zn₂SnO₄ and TiO₂ nanomaterials. The photocatalyticactivities, targeting on the degradation of pollutants and dye-sensitized solar cells,were also studied. Based on systematic experiments and analysis, we have thefollowing results:1. Synthesis and photocatalytic properties of ZnO and Zn₂SnO₄ nanowires(1) One dimensional Zn₂SnO₄ nanowires, ZnO nanowires and ZnO-Zn₂SnO₄mixed nanowires were prepared through thermal evaporation method. Theresults demonstrated that average diameters of the nanowires can beadjusted in the range of 150～500 nm by controlling the experimentconditions.(2) The Zn₂SnO₄ nanowires have single-crystalline spinel structure and [111]growth direction, and there are many Zn and O vacancies in these nanowires.The ZnO nanowires have single-crystalline wurzite structure and [01(?)0]growth direction. The synthesized ZnO-Zn₂SnO₄ nanowires are a mixture ofZnO and Zn₂SnO₄ nanowires. All of the nanowires are straight and have auniform diameter along its entire length.(3) The photocatalytic activities of the ZnO-Zn₂SnO₄ nanowires were evaluatedby observing the photodegradation of MO aqueous solution under UV light.The ZnO-Zn₂SnO₄ mixed nanowires have best photocatalytic activity.Moreover, the photocatalytic activity can be enhanced by reducing theaverage diameter of the nanowires. The photodegradation constant k reaches the highest value of 0.146 h^-1, when mixed ratio of ZnO:Zn₂SnO₄ is 5:1, andthe concentration of the mixed nanowires was 0.5 g/L.(4) The photocatalytic mechanisms were also discussed. Our study suggeststhat the improved photocatalytic activity of the ZnO-Zn₂SnO₄ mixednanowires may be attributed to the hetero-junction effect, which canefficiently separate the electron-hole pairs. Reducing the diameter of thenanowires can also promote the separation of electron-hole pairs, andenhance the photocatalytic efficiency.2. Novel three-dimensional dandelion-like TiO₂ structure with high photocatalyticactivity(1) Pure rutile phased TiO₂ powders with novel three dimensional (3D)dandelion-liked structures were synthesized by using a facile hydrothermalmethod with TiCl₃ as the main starting material with a yield of～90 mol%.The 3D dandelion structure has an average diameter of 1.5 to 2μm, and ispacked radially by nanorods with a [001] preference growth direction. Eachindividual nanorod is hundreds of nanometers in length, and tens ofnanometers in diameter.(2) The formation mechanisms of the dandelion-like structure were alsodiscussed. A new oxidation process of Ti(â…¢) to Ti(â…£) was suggested:The Ti(â…¢) oxidizes to TiO₂ by oxygen inside the autoclave and [H⁺] cationsin solution.(3) The obtained 3D dandelion-like TiO₂ powder has a high photocatalyticactivity, which is equivalent to the commercial available P25 titania powder.We attribute the improve photocatalytic activity of the 3D dandelion-likedTiO₂ to the hierarchical structures. The hierarchical structures have largespecific surface area, and provide diffusion channels for the molecules ofpollutants.3. Preparation and Characterization of the Electrodes for Dye-sensized Solar Cells(1) Electrodes of dye-sensitized solar cells were prepared by using the TiO₂nanoparticles obtained by hydrothermal method. The obtained electrode hasan equivalent efficiency with that prepared by the P25 TiO₂ powder. The influence of the film thickness on the performance of solar cells was alsodiscussed. There exists an optimum thickness of TiO₂ film.(2) The TiO₂ films prepared with the 3D dandelion-like TiO₂ and 1D TiO₂nanowires have comparable photovoltaic properties to that prepared by theTiO₂ nanoparticles. We think the 3D dandelion-liked hierarchical structurescan improve the photocatalytic activities of the solar cells. The hierarchicalstructure does not only have large specific surface area, but also provideschannels for the transportation of electrons.(3) We propose a new approach for the construction of the solar cell electrodes.By using novel nanomaterials, the photovoltaic performance of solar cellsmay be improved.