Study On Synthesis And Properties Of Micro-/Nano-Materials Of Tungstate

In this thesis, the Synthesis, Mechanism and Properties of Micro-/Nano-Materials of Tungstate were investigated. Three-dimensional (3D) hierarchical Bi2WO6microsphere and octahedral Bi2WO6have been synthesized by a facile hydrothermal method using KNO3solution and distill water as solvent, respectively. A possible mechanism on the formation of Bi2WO6microsphere was proposed; A novel coupled system of ZnWO4-Cu and ZnWO4-Cu2O were successfully synthesized by the hydrothermal method. The effects of the amount of Cu on the photocatalytic performance of ZnWO4-Cu were investigated in detail. The cubic ZrW2O8was successfully synthesized by hydrothermal method with mild conditions, and the influence of synthetic conditions on the outcome was discussed. ZrMoxW2-xO8this had the cubic β-ZrW2O8structure at room temperature was successfully synthesized by doping a certain proportion of Mo.The details are summarized as follows:1. Controlled Synthesis of Three-Dimensional Hierarchical Bi2WO6Microspheres with Optimum Photocatalytic ActivityThree-dimensional (3D) hierarchical Bi2WO6microsphere and octahedral Bi2WO6have been synthesized by a facile hydrothermal method using KNO3solution and distill water as solvent, respectively. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2adsorption/desorption (BET), and UV-vis diffuse reflectance spectroscopy (DRS) in detail. The concentrated KNO3played a key role for the formation of3D Bi2WO6hierarchical microspheres. By changing the concentration of KNO3solution, we deduced that3D hierarchical Bi2WO6microsphere was produced through crystal plane-selective growth starting from cores, not through aggregation of rectangular platelets which were produced independently. The photocatalytic activity of the as-synthesized products was evaluated by monitoring the degradation of MB solution. The decoloration ratio of MB was33.56%and99.17%respectively for the octahedral Bi2WO6and Bi2WO6microsphere, which was attributed to the larger surface area and special hierarchical structure of Bi2WO6microsphere. For large surface area, the ratio of the surface charge carrier transfer rate to the electron-hole recombination rate could be greatly improved. The hierarchical architecture with large surface area and pore system not only improved the molecular transport of reactants, but also allowed more efficient light harvesting. By pyridine adsorption, the surface acidity of catalyst Bi2WO6was measured using infrared spectroscopy. The relationship between surface acidity of Bi2WO6and photocatalytic degradation of RhB was explored. It was found that the more surface acid sites of Bi2WO6, the stronger adsorption between with RhB and Bi2WO6. Consequently, the photogenerated electrons on the dye could more easily transfer to the photocatalyst, leading to the photosensitization and photocatalysis.2. ZnWO4-Cu System with Enhanced Photocatalytic Activity by Photo-Fenton-Like Synergistic ReactionA novel coupled system of ZnWO4-Cu was successfully synthesized by the hydrothermal method. The synthesis was simple and green. The as-synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity was evaluated by monitoring the degradation of MB solution under UV light irradiation. The catalytic activity of ZnWO4-Cu was much higher than that of bare ZnWO4and the photocatalytic activity of the ZC-0.2was best, which was attributed to the synergistic effect between photocatalysis and Fenton-like process. Cu played a key role in the process of photocatalytic degradation of MB. The copper reagent not only served as a heterogeneous catalyst for the decomposition of H2O2so as to generate hydroxyl radicals to attack the contaminants, but also acted as an electron shuttle that promoted the separation of the photogenerated carriers. Furthermore, the effects of the amount of Cu on the photocatalytic performance of ZnWO4-Cu were investigated in detail. The photocatalytic degradation ratio of MB decreased when too much Cu was introduced which was attributed to the passivation layer on the Cu surface could not be reduced timely by the photogenerated electron. The redundant Cu+/Cu2+may react with both photogenerated electrons and holes as recombination center, thereby make the photocatalytic activity of ZnWO4decreased.The possible mechanism of the synergistic system was proposed. A coupled system of ZnWO4-Cu2O was synthesized by the hydrothermal method. The photocatalytic activity was evaluated by monitoring the degradation of MB solution under sunlight irradiation. The degradation ratio of MB was46.54%、78.5%、88.34%and76.53%respectively for ZnC-0, ZnC-0.1, ZC-0.2and ZC-0.3. The effects of the amount of Cu2O on the photocatalytic performance of ZnWO4-Cu2O were investigated.3. Hydrothermal synthesis of negative thermal expansion ZrW2O8and ZrMoxW2-xO8As an excellent kind of isotropic negative thermal expansion materials, cubic ZrW2O8is coming to be people’s concern. But it is difficult to be synthesized because it is metastable and has narrow thermodynamic stability range.The cubic ZrW2O8was successfully synthesized by hydrothermal method with mild conditions. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectrometer (FT-IR). The influence of synthetic conditions on the outcome was discussed and made optimization in this passage. ZrMoxW2-xO8this had the cubic β-ZrW2O8structure at room temperature was successfully synthesized by doping a certain proportion of Mo.