Study Of ZnO Semiconductors: Control Of Morphology, Microstructure Modulation And Property Characterization

In recent years, semiconductor nanomaterials exhibiting lots of terrific properties, for example, novel unique quantum size effect, surface effect, macroscopic quantum tunnel effect, have attracted more and more attention. Synthesis of semiconductor nanomaterials goes deeper with the understanding of their unique properties. As a kind of new materials in the information age, semiconductor nanomaterials can be applied in many fields such as catalysis, medicine, optics and functional materials. The properties of nanomaterials are associated with the nanostructures, which will be greatly influenced by the technology and method of their synthesis. At present, the development tendency of nanomaterials science and technology focuses on the study of controllable process, including materials'shape, dimension, surface and microstructure, so that their unique properties could find their corresponding applications in the industry.Zinc Oxide, as a new functional semiconductor materials, is an important kind of materials for its high efficiency, non-toxic nature and low cost, so it has been paid much attention in the degradation and complete mineralization of environmental pollutants. The morphologies of inorganic materials are demonstrated to have great effects on their widely varying properties and corresponding potential applications. It is well known that ZnO exhibits the richest range of morphologies among the wide band gap semiconductors. Therefore, the fabrication of ZnO nanomaterials with special morphologies could effectively improve their chemical and physical properties. In this thesis, we chose zinc oxide as the objective of our studies. By changing the kinds of additive, the concentration of precursor and the reaction time, we fabricated a series of ZnO particles with various morphologies and structures, and further we studied the relationship between ZnO morphologies and its properties. Besides, ZnO/NiO and ZnO/ZnS heterojunction have been successfully fabricated, which are important to expand the better properties of ZnO. The systematically investigation of the relationship on microstructures and properties of ZnO has provided a theoretical foundation for the future studies and applications.In chapter one, we briefly introduced the development and categories of semiconductor materials, and the properties and preparation methods of nanomaterials. Then we discussed the crystal structure and properties of ZnO semiconductor nanomaterials. The important part is that the progress of studies on ZnO was presented from three aspects.In chapter two, ZnO twin-prisms were prepared by a simple solvothermal process with glutamic acid as surface modification agent in ethanol. Compared with the twin-cones that obtained when glutamic acid was absent, the ZnO twin-prisms have optimized crystallinity with high symmetry, sharp crystal edges and few defects. Probable reaction mechanism between ZnO and glutamic acid was proposed based on our experimental analysis. Then it was verified by a series of comparison experiments by employing glycine, acetic acid and ethylamine as surface modification agents. Our results indicate that both alkaline and acidic groups play an important role on the formation of ZnO twin-prisms, and when either of them was absent, twin-prism structures can not be obtained. The photoluminescence properties of as-prepared ZnO twin-prisms and twin-cones were also studied.In chapter three, a series of homogeneous ZnO microstructures were synthesized via a facile solvothermal method with tartaric acid as additive. Absolute ethanol was not only the solvent but also a reactant that provided the original OH" for ZnO growth units. The products were characterized by XRD, SEM, TEM and HRTEM. A dissolution-recrystallization process was explored as reaction time went on. The ionization of tartaric acid, which could be controlled by temperature, influenced the evolution of ZnO. The etching of tartaric acid and decomposition of zinc tartaric complex were considered to be the main causes of dissolution and recrystallization. The PL spectra reflected that the obtained particles had favorable fluorescent properties and the highlight was the rare red emission of ZnO pineapples derived from the interfacial emission. Morever, the variational quantities of tartaric acid controlled the ZnO morphology and shape as well as the self-assembly of ZnO crystals into complex architectures. Possible growth mechanisms of obtained ZnO with different morphologies were proposed. The influence of tartaric acid to ZnO growth was further indicated by FT-IR spectra. In addition, the photoluminescence (PL) properties of these ZnO samples were investigated at room temperature, which indicated that the ZnO morphology could influence its optical property. On the whole, the fluorescence was violet emission and orange emission primarily.In chapter four, ZnO mushrooms composed of tower-like petals have been prepared by a simple solvothermal method with ethanol as solvent. The key strategy is that we employed an organic acid to be active agent, and the whole experiments were carried out in a sour environment. The main crystalline phase of the as-prepared products measured by X-ray diffraction was proven to be wurtzite-type ZnO. Raman spectroscopy was employed to characterize the crystalline structure and perfection. Scanning Electron Microscope results revealed that the ZnO mushrooms had average size of 12μm and the top pileus was made up of hexagonal towers packed with hexagonal slices. Possible growth process has been proposed to be charges adsorption among the polar ZnO slices. In addition, the results of the degradation of organic dye indicated that the prepared ZnO mushrooms showed good photocatalytic activity and it could be considered as a promising photocatalyst for dyes wastewater treatment. Besides, by changing the Zn2+ concentration, we obtained two other ZnO morphologies:bouquet ZnO and spherical flower ZnO. For the overflow of organic molecules, the obtained ZnO have different morphologies. Because of the small size and big specific surface area, the bouquet ZnO has high photocatalytic activity.In chapter five, a series of ZnO particles with different morphologies have been successfully fabricated by a facile solvothermal reaction with acetic acid as an additive. The whole reactions were taken in a strong corrosive environment and the samples were tested by XRD proving to be wurtzite ZnO. The difference of the obtained ZnO morphologies demonstrated that controlling the addition of acetic acid could easily control the morphologies of obtained ZnO. Morever, the strong corrosion of acetic acid could etch the ZnO surface and make the (001) planes exposed in varying degrees. The exposure of active planes improved the reactive activities of ZnO, which made the ZnO application in photoelectric field in the future.In chapter six, regular octahedron NiO was successfully obtained by a facile thermal degradation. Changing the annealing temperatures, the NiO crystals had different crystal edges. And with the annealing temperatures higher, the regular octahedron NiO gradually converted to be cubic structure, which had the adhesion to each other. What's more, with the solvothermal reaction, we have obtained the NiO flowers made up of plenty of slices. Because the NiO and the former ZnO have similar growing environment, the ZnO/NiO heterojunction was fabricated through a solvothermal reaction, and the heterojunction have the same flower shape. Observed by HRTEM, the heterojunction was obvious and compact, belonging to type II heterojunction. The investigation based ZnO was the basis for its application in optical field.In chapter seven, using hexagonal ZnO as template, ZnO/ZnS heterojunction has been obtained by replacing partial oxygen atoms with sulfur atoms. Its structure and surface morphology were analyzed by XRD and FESEM.The result showed that the prepared ZnS is wurtzite structure because of the hexagonal template and the ZnO/ZnS partical size is nano-scale. The PL spectrum showed that the as-prepared ZnO/ZnS heterojunction had a high rate of blue emission, which make it possible that ZnO/ZnS heterojunction would be applied in optical devices. The photocatalytic activity of the heterojunction in different conditions was tested by decomposing methyl orange. The results indicated that the obtained heterojunction had the best photocatalytic activity when the preparation tempreture is 160℃, which can decompose 70%of methyl orange under UV irradiation for 2 hours.In chapter eight, we summarized our work and discussed the problems remained to be solved. At last, we made a plan for the future work and looked forward to the futurity.The studies on semiconductor nanomaterials is very important for the both the development of human civilization and the exploration of science. ZnO semiconductor nanomaterials have special excellent properties, which makes the ZnO progress more important. In this thesis, by controlling the experiment conditions, we investigated the relationship of microstructuresand properties of ZnO, and modulated the microstructures to improve their properties. Besides, we have successfully obtained the heterojunctions based ZnO, which have excellent properties as well. Our studies on the ZnO microstructure modulation and properties are proven to be an important and efficient way to improve the properties. And it is also important for the practical applications of ZnO in the future.