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Photoluminescence and Photocatalysis of the Tungstates Nanocrystals

Author: ZhangCuiLing
Tutor: HuChenGuo
School: Chongqing University
Course: Condensed Matter Physics
Keywords: Tungstate nanocrystals barium manganite photoluminescence photocatalytic activity absorption
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Type: PhD thesis
Year: 2013
Downloads: 331
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Abstract


In order to effectively utilize the solar light to reduce the environment pollution,exploration of photoluminescence and photocatalysis of materials is interesting andchallenge work. Several recent studies have indicated that metal tungstates and bariummanganite are important inorganic material and possess chemicalstability andthermostability. In this paper, metal tungstates and barium manganate nano materialswith tunable morphology are synthesized, and the light absorption, photoluminescenceand photocatalytic activity are investigated. We abstract the main content of thisdissertation as following:(1) The length controllable perovskite BaMnO3nanorods are prepared by using acomposite-hydroxide-mediated method and the absorption characteristics areinvestigated based on the band structure, surface defects and electron transition. Twoabsorption peaks are observed in ultraviolet–visible spectra measured on the BaMnO3nanorods at room temperature. The peak centred at402nm is assigned to a transition ofO2P↓to Mn t2g↓states, while the peak centred at301nm is corresponding to the2P↑toMn eg↑states. The increasing of ratio of surface area to volume brings the more oxygensurface layer areas and oxygen vacancies and leads to the red shift of emission andweakening of the absorption peak centred at402nm.(2) Single crystalline wolframite-type monoclinic structure cobalt tungstate(CoWO4) nanowires are obtained by a solvothermal method at180°C for24h with widthof20nm and length of200to400nm, and the photoluminescence properties are studied.Besides the strong blue-green light emission at10-250K, we found much stronger andbroader near infrared emission ranging from700-1000nm at300K under excitationwavelength of325nm. The blue-green emission495nm and530nm are ascribed toelectronic transition from two close3T1ulevels to1A1gand the spin-forbidden transitionsfrom T1gto1A1gallowed partly from spin–orbit coupling, respectively. Co vacancies near(WO6)6-octahedron raise the population of lower energy level T1gproduced by J-Tdeformation and increase the emission intensity on530nm. The much stronger andbroader near infrared emission ranging from700-1000nm at room temperature couldmainly be ascribed to the discrete photoluminescence center Co3+ions nearby the Cocationic vacancy. The temperature dependence (TD) of emission intensity depends on thecombined action of carriers transfer quenching produced by the exciton thermal disintegration and thermostimulated luminescence. This near-infrared emission materialmight have potential applications in infrared detection or stealthy technology.(3) ZnWO4nanowires and nanorods are synthesized by a hydrothermal methodwith different amounts of reactants and pH. ZnWO4with surface modification by AgOnanoparticals are prepared by light irradiation method and ZnWO4/ZnO compositenanomaterial (ZnO nanoparticles (50-80nm) coved by ZnWO4nanoparticles (3-5nm)) isobtained by a two-step process. Two band gaps of ZnWO4nanorods is observed about3.12eV and3.78eV, while the band gap of ZnWO4nanowires is about3.85eV. Themorphology and band structure of photocatalyts and surface modification have effectson the photocatalytic performance which can improved greatly. The mechanisms isdiscussed based on the band structure, surface defects and electron-hole separation.However, the photosensitivity performance on degradation rhodamine B is weak.The studie on photoluminescence properties of ZnWO4nanowires indicates thatbesides the strong ultra violet-visible light emission peaks at380nm,436nm,493nmat10-300K, we found a broader near infrared emission about wavelength longer than650nm under excitation wavelength of325nm. Similar to CoWO4, the mechanisms ofemission and variation with temperature are investigated. The emission peaks ofZnWO4/ZnO composite nanomaterial are located on380nm,436nm,493nm and753nm, and the emission band peaked at380nm comes from ZnO, and the emission bandpeaked at493nm and436nm come from (WO6)6-.(4) Single crystalline wolframite-type monoclinic structure cadmium tungstate(CdWO4) nanorods are obtained by a hydrothermal method at180°C for24h with widthof50nm and length of2m,and the photoluminescence properties is compared withCoWO4and ZnWO4to obtain the contribution of cation to photoluminescenceproperties. The thermostability is discussed by TG-DSC curve, and the results revealthat CdWO4thermally decomposes slowly when temperature rises above850℃。

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