Studies On The Controlled Synthesis And Mechanism Of Zinc Oxide Micro/Nanostructures

Oxide nanomaterials have attracted worldwide attention and laid some foundation for the nanodevices due to their unusual optical, electronic, magantic, mechanical and catalytic properties. It is well known that the properties of nanomaterials are sensitively dependent on both the shape and size. Therefore, the most important problem faced by the scientists is how to fabricate nanostructures with the controllable shape and size. In comparison with other nanostructures, ZnO hollow nanostructures exhibit stronger or novel functionalities due to their higher surface area and their capability of forming composite structures by embedding specific particles in their interiors, and they thus may find potential applications in a wide range of areas, including catalysis, drug delivery, storage and release systems, bioencapsulation, nanoreactors, and templates for functional architectural composite materials. Although, many hollow Oxide nanomaterials have been fabricated, the synthesis of ZnO hollow structures still remain a big challenging subject to chemists and material scientists. Therefore, it is highly necessary to explore simple and general accesses to preparing oxide nanomaterials with desired dimensions and sizes.In this dissertation, valuable explorations have been carried out to prepare hollow ZnO microspheres using thermal evaporation by Zn powder precursor-template, and the effect of the experimental conditions including growth temperature, air pressure, flow rate of Ar and air on surface morphology of ZnO hollow spheres have been scrutinized in detail. ZnO dendritic nanostructure has been synthesized on silicon (100) substrates by thermal evaporation of metal Zn powder at relative low temperature, as well as their formation mechanism and the novel properties of the as-obtained nanostructures. The main points can be summarized as follows:We have realized the synthesis of uniform zinc oxide (ZnO) hollow spherical structure consisting of highly radial and oriented ZnO nanorods, which results from a facile precursor-templated through thermal evaporation at a relative low temperature without any catalyst. Two main growth steps may occur in the process:the first step is formation of hollow ZnO shell when air was introduced into the tube at 400℃, Zn atoms on the surface of Zn microsphere reacted with O atoms to form thin ZnO shell layer on the surface of Zn microsphere. The second step is the growth of the ZnO nanorods on the surface of the microsphere. The nanocrystalline protrusions on the surface of ZnO sphere shell are very beneficial to the preferred aligned growth of ZnO nanorods. In this approach, pre-deposited Zn powder particles on Si substrates act as temporary templates to form hollow ZnO sphere shells, while additional Zn powder acts as a Zn source to grow single crystal nanorods/nanowires on the surfaces of spherical shells. Lacking the need for the additional template removal is an important advantage of this approach over others and it therefore can be used to prepare hollow ZnO nano/microsphere shells and hollow ZnO microspheres with nanorods/nanowires at low cost and at large scale. These kinds of special high surface area hollow spherical structures may find potential applications in photocatalysis, light-weight composite fillers, acoustic insulation, UV nano/micro-optoemission devices and photoanodes of dye-sensitized solar cells.By optimizing the experimental parameters, the obtained results demonstrate that the morphology of ZnO is very sensitive to growth temperature, air pressure, flow rate of Ar and air and Zn powder precursor. Detailed structural analyses confirm that this microstructure exhibits a wurtzite hexagonal phase, and there arenanorods, nanowires, tower-like nanorode, particle et.al growing on the shells of ZnO hollow spheres. The formation mechanism of the microstructure is proposed to be a vapor-solid process. This method may provide a simple and versatile approach to large-scale production of hollow spherical ZnO structure and other hollow nanomaterials.Intriguing ZnO three-dimensional (3D) dendritic nanorods on silicon substrates have been successfully synthesised by thermal evaporation of pure zinc powder at a relative low temperature of 478℃without any metal catalyst. ZnO dendritic nanostructure exhibits unique shape and it is composed of stems and nanorod branches. It is found that the nanorods are single crystalline wurtzite structures, and each nanorod grows along the [0001] direction. At different growth temperatures, the shapes of ZnO nanostructures can be altered. System analysis reveals that the formation and morphology of ZnO dendritic nanostructures are sensitive to the growthtemperature. Finally, room temperature photoluminescence spectrum is also investigated,revealing that the ZnO dendritic nanostructure could find application in UV optoelectronic devices; the nanostructure implies some potential applications for nanoscale functional devices.