Liquid-phase Synthesis, Characterization And Properties Studies Of Zinc Oxide, Zinc Tungstate And Lead Tungstate Nanomaterials

The study of synthesis methods of inorgnic nanomaterials and nanostructures is a hot area of nanoscience, and is also the prerequisite of preparation of complex advanced microdevices. Due to so many advantages of the liquid-phase syntheses, it will be one of the important methods to assemble the basal nanoparticles in the future. Up to now, it is still an important task in the field of material that how to develop new methods for preparing nanomaterials. Although there are many liquid-phase methods reported for preparing materials, it is still difficult to obtain materials with controllable morphologies and sizes. Therefore, it is attracting a great deal of attention of the chemists and materials researchers to explore new liquid-phase methods for obtaining low-cost mild reaction.Our group has rich experience in the technology of preparation of inorganic nanoparticles in liquid-phase system. On the base of our group's works in the past, this paper chooses ZnO, ZnWO₄, PbWO₄ as study objects, using liquid chemical synthesis technology's advantages in controlling the materials microstructures, morphologies and size. We explore new methods and control conditions for liquid chemical synthesis of nanomaterials, and find some rules in synthesis and morphology control in nanomaterials. This study both enriches the liquid-phase methods of nanomaterials, and also provides a new thought for preparing complex materials with special nano-structures. The detailed information of the dissertation is listed as follows.1. A singularity flower-like ZnO nanostructure was prepared on a large scale through a very simple solution method at room temperature and under ambient pressure in a very short time. The flower-like ZnO nanostructures had a diameter of about 1.5μm, and were self-assembled by thin and uniform nanosheets, with a thickness of around 5 nm. Citrate played an important roal in the growth of flower structure. Firstly, citrate could coordinate with Zn²⁺, so the sedimentation rate was slowed down, resulting in better crystalline. Secondly, citrate could also control the ZnO crystal as it absorbed to the (002) surface and forced the crystal to grow into plates. Based on the time-dependent experiment, the possible growth mechanism was discussed, and it was the self-assemble process. At first, ZnO nanosheets were formed, and then they self-assembled in flower-like structures in order to reduce their high surface energy. UV-vis spectra of the ZnO nanostructures showed a strong exciton peak at 357nm, which has a blue shift compared to that of bulk ZnO. This should be mainly due to the quantum confinement of the ZnO nanosheets, since the man thickness of the sheets is comparable to the Bohr radius of ZnO. Hence, the products had advanced absorption of ultraviolet radiation; this may have extensive applications in optics devices.2. ZnO flower-like micro-structures were sucsessfully synthesized through a simple hydrothermal route, using potassium citrate as shape modifier. Based on the experament result, the pH value of the system played an important role on the morphology of the products. By changing amount of NaOH introduced into the system, many different morphologies of ZnO were obtained, such as solid spheres, hexahedrons, flowers, flower-clusters and aggregates made up of nanorods. Preliminary view was that different complex ways betweent Zn²⁺ and OH^-, that is different precursors, resulted in different morphologies.3. We synthesized two kinds of hollow twinning ZnO microstructures through a simple hydrothermal method without templates. Dumbbell-like and shuttle-like ZnO microstructures with hollows were obtained by changing the materials source. Experment results showed that different precursors resulted in different morphologies. That is Zn(OH)₄²⁺ prefered dumbbell-like products, and Zn(NH3)₄²⁺ resulted in shuttle-like products. Based on the time-dependent experiments, we investigated the growth process of these two hollow twinning structures and found the "Ostwald-ripening process" played an important role. The interesting part of this growth process was that the interface of the two twinning structure performed as the activate center where the Ostwald-ripening process carried out. We also investigated the luminescent properties of the as-obtained products by photoluminescence (PL) spectroscopy, and found that these two hollow structures both showed strong visible emission in the 400-500 nm regions, which is much stronger than bulk ZnO.4. ZnWO₄ nanowires/nanobelts were synthesized through a hydrothermal method. ZnWO₄ nanobelts were synthesized through a poly (vinylpyrrolidone) (PVP) assisted hydrothermal process. PVP molecules absorbed on some surface of the ZnWO₄ crystals could significantly decrease their growth rates and lead to highly anisotropic growth, resulting in nanobelts. We investigated the growth process of the products and found that the "Ostwald-ripening process" played an important role. Photo-decomposition experiments indicated that the morphology and crystallinity of ZnWO₄ photocatalyst had a significant influence on the photocatalytic activity for aqueous Rhodamine B, and ZnWO₄ nanobelts showed a much higher photocatalytic activity than nanowires. There were reasons for ZnWO₄ nanobelts's high photocatalytic activity: firstly, nanobelts had higher surface, which provided large surface area for absorbing substrate; secondly, nanobelts had more bare planes containing W and O atoms, thus enhance the photocatalytic activity of nanobelts. Besides, ZnWO₄ nanobelts also exhibited a much stronger luminescence property than nanowires.5. ZnWO₄ hollow spheres made up of nanorods were successfully prepared through a tri-potassium citrate assisted hydrothermal process at 180℃. This is the first time that ZnWO₄ hollow structures are obtained in aqueous system. The hollow spheres' diameter was about 400nm, and these spheres were made up of nanorods with a diameter of about 10 nm and a length of about 50nm. Based on experiments, the growth of these hollow spheres followed an aggregation-Ostwald ripening process. The photocatalytic activities for aqueous Rhodamine B of samples were investigated, and it was amazing that ZnWO₄ hollow spheres exhibited a strong photocatalytic activity, which was caused by hollow spheres' high surface.6. Hierarchical PbWO₄ spheres assembled by nanoparticles were successfully synthesized through a tri-potassium citrate assisted hydrothermal process. It was found that citrate played a key role on the morphology of PbWO₄ products. By adjusting citrate's concentration, PbWO₄ octahedrons, hierarchical spheres, hierarchical ellipses could be obtained. Based on time-dependent experiments, we found the growth of the hierarchical spheres followed a self-assembly process. The most interesting part was that the hierarchical spheres/ellipses showed a blue emission peak at 440nm, which differs from the typical green one at 500nm as reported. We believed that the PbWO₄ aggregates made up of nanorods exhibit high light-collection efficiency and enhanced luminescence performance due to their large surface area. The greatly enhanced luminescence performance is exciting and may have significant technological applications in the inorganic scintillating field.7. By introducing Zn²⁺ as shape modifier into the synthesis of PbWO₄ nanostructures under hydrothermal route, a series of various PbWO₄ nanostructures were successfully prepared, including rod-like and flower-like aggregates, most of them had not been reported. It is interesting that the products showed emission peaks at 460nm and 550nm, which was different from that of bulk PbWO₄, and this had not been reported.