Synthesis, Characterization, And Application Of Zinc Oxide And Metal Sulfide Micro-/nano-structures

Physical and chemical properties of micro-/nano-materials are a function of their composition, morphology, and size. At present, controllable synthesis, characterization and application of micro-/nano-materials is of great interest to material scientists, physicists, and chemists. Semiconductor materials have attracted much attention because of their unique properties and potential applications. The theme of this doctoral dissertation focused on searching new methods to synthesize semiconductor micro-/nano-materials and preparing new nanostructures; characterizing the obtained products and exploring the possible formation mechanisms. From application point of view, some primarily research on photoelectric conversion property of the obtained zinc oxide (ZnO) nanostructures has been performed.The hollow spherical structures have many potential applications because of their specific structures and unique properties. They can be used in drug controlled release due to their hollow structures. Because of the porous structure and high specific surface area, the semiconductor hollow spheres usually show improved performance for applications ranging from catalysts, gas sensors, and lithium-ion batteries. However, the hollow spherical structures are often prepared by the template synthesis method at present. This template-dependent method usually faces disadvantages related to extremely complicated synthetic procedures and high cost. Zinc oxide, an important wide band gap semiconductor, has the prospect of wide application in many fields because of its outstanding optical and electronic properties. Thus, we first chose ZnO hollow spheres as research object. However, the direct fabrication of ZnO hollow spheres at low-temperature remains a significant challenge. The difficulty lies in the fact that the spherical ZnO structure is not easily obtained without the help of a spherical template owing to different growth rates of ZnO crystal in various directions. We successfully use a high concentration of trisodium citrate to control the nucleation and growth rate of ZnO crystal for producing solid spheres with core/shell structures and then remove subsequently the cores by dissolving in an alkaline solution. Compared with the conventional methods for preparing ZnO hollow spheres, the present synthetic procedure has the advantages of simplicity, low growth temperature, and template-free. During the experimental process,a series of controlled experiments have been carried out to better understand the formation mechanism of ZnO hollow spheres and the function of reactants used in the experiments. Both room-temperature Raman and photoluminescence (PL) spectroscopy has been carried out to explore the optical properties of the annealed ZnO hollow spheres. Raman results demonstrate that the obtained hollow spheres are of a good crystalline wurtzite structure. The room-temperature PL spectrum of the yielded hollow spheres shows a strong UV emission.We have developed a template-free method to prepare ZnO hollow spheres. However, this template-free method is usually only suitable for a certain specific material. Therefore, we have further developed a versatile method to prepare various metal sulfide hollow spheres. We have successfully realized the synthesis of ZnS, Ag2S, PbS, CuS, Cu2S, Bi2S3, and Sb2S3 hollow spheres. The key point of the method is to utilize the large difference in solubility between ZnS and the other metal sulfides for the effective transformation. The optical properties of the metal sulfide hollow spheres have been systematically investigated by absorption, micro-Raman, and photoluminescence spectroscopy.Although we have realize the synthesis of ZnO and metal sulfide hollow spheres. However, the compositions of previously prepared hollow spheres are simple. We use transitional metal element Mn to partly substitute nonmagnetic element in ZnO for realizing the synthesis of diluted magnetic semiconductors hollow spheres. A series of experiments have been carried out to demonstrate that Mn2+ ions are successfully incorporated into the ZnO host lattice. We also investigate the magnetic properties of the Mn doped ZnO hollow spherical structures. The result shows all the prepared samples exhibit room temperature ferromagnetism. Therefore, we realize the combination of the merit of semiconductor and magnetism property in hollow spherical structures. We have further expected that the present research can pave the way for the realization of doped ZnO hollow sphere in applications and further theoretical researches.ZnO has two polar surfaces and three fast growth directions. These unique structural properties have benefited the realization of many kinds of novel ZnO nanostructures, such as nanowires, nanotubes, nanobelts, nanorings, and so on. The varieties of morphology broaden the ZnO nanostructure applications. In spite of these successes, more complex composition and higher integrated nanostructures are required in some device applications. As for synthesizing ZnO nanostructures with complex composition, we have successfully realized the synthesis of various ZnO-based core/shell structures (such as ZnO/ZnS, ZnO/Ag2S, and ZnO/CuS).As for synthesizing ZnO nanostructures with complex morphology, two methods have been used: the chemical route and the thermal evaporation process. We have successfully realized the synthesis of ZnO nanoflowers by adding ammonia into the solution for preparing ZnO nanowires. Although this chemical method has the advantage of simplicity and low temperature, the obtained ZnO nanoflowers are not highly integrated. Therefore, we have also successfully realized the integration of single nanostructures and prepared a variety of hierarchical ZnO nanostructures with the controllable morphology through the two-step thermal evaporation process. During the experiments, we have firstly synthesized the Zn nanowires with little oxidation on Si substrate, and then prepared the hierarchical ZnO nanostructures successfully by heating Zn source and the step-one prepared samples. During the whole experiment, only Zn powder has been used, which avoids the contamination of impurities on the nanostructure. Room-temperature photoluminescence measurements demonstrate that the optical properties of ZnO nanostructures can be modulated by the controlled morphologies of the yielded hierarchical nanostructures.Finally, the application of ZnO nanostructures in solar cell has been studied. In order to obtain high performance solar cells, we used high ordered ZnO nanowires with better electron transport property as photoelectrodes. During the experimental process, we found there are large interstices between the ZnO nanowires. Therefore, ZnO nanospheres have been grown between and on the top of the nanowires for increasing the surface area of the photoelectrodes to further improve photoelectric conversion efficiency. ZnO nanospheres in photoelectrodes play two important roles: First, they can increase the surface area of the photoelectrodes. Second, they can act as light scattering center to improve light harvesting efficiency.