Study Of ZnO Nanostructures Growth By Using Hydrothermal Method

Zinc oxide (ZnO) is a semiconductor material with wide band gap of3.37eV and exciton binding energy up to60meV which is much higher than room temperature ionization energy of26meV. Such properties make the ZnO material potential candidates for many applications such as short wavelength light emitting devices, Transparent conducting electrodes, solar cells, field emission devices and so on. Zinc oxide nanostructures have diverse morphologies such as nanotubes, nanorods, nanoneedles, nanoring and nanoflowers etc. The performances of ZnO-derived devices strongly depend on the properties of ZnO nanostrcures. Therefore, exploring the optimal preparation process to realize controllable Growth of ZnO nanostructures is a matter of considerable importance for the development of novel devices.This thesis which makes preparation of ZnO nanostructures as a starting point focuses on what influence the preparation parameters such as growth temperature, cooling method, growth time would have on synthesis of ZnO nanostructures by hydrothermal method. The main conclusions are as follow:(1) We have studied the hydrothermal temperature dependence of ZnO nanostructure morphologies grown on GaN/Al2O3substrates. The field emission scanning electron microscopy images showed that at relatively low temperature, continuous ZnO thin films consisting of hexagonal ZnO grains were obtained. While at high temperature, the morphology grew into well-aligned ZnO nanoswords arrays. XRD results indicated that the nanoswords arrays were highly c-axis oriented and had high crystal quality. By studying the early stage of nanorods growth, we find that the initial nucleation density decrease as the growth temperature increase. Therefore, the nucleation density was very high at low hydrothermal growth temperature, which made those nuclei coalesce with each other to produce continuous films in growth process. In contrast, at high hydrothermal growth temperature, the nucleation density became lower. The lower nucleation densities meant larger nuclei distances, which made the nuclei evolve into nanoswords along the preferred orientation independently each other.(2) We have studied the effect of cooling rate on the tip geometry of ZnO nanorods on GaN/Al2O3substrates. After hydrothermal growth, we made the Teflon lined stainless steel autoclave cool to room temperature11℃with three different conditions. The sealed autoclave of sample I was opened directly without cooling; sample Ⅱ was immersed into the11℃water to cool; sample Ⅲ was cooled in the air naturally. It was found out that the length of sample II and Ⅲ are longer than that of sample I, which proves the ZnO nanorods grow continually during cooling in the water and the air. The nanorods of sample II cooled by water were easier to form the ultra-sharp ZnO nanotips.(3) We have studied the effect of growth time (<1h) on the morphology of ZnO nanorods on Si substrates by hydrothermal method. It was found out that the top of nanorods would become sharper as the growth time is increasing. That is because we put the reaction solution sealed in the autoclave into the preset temperature oven at room temperature before the hydrothermal growth. Solution temperature will not reach the preset temperature of oven and keep in a continuous heating state, until after an enough growth time. If a growth time is not long enough to make solution reach the set temperature, the longer growth time means higher final solution temperature, which in turn increases cooling rate during the same quenching step. As a consequence, sharper tips of nanorods are obtained.