Study On Preparation And Mechanism Of 1-D ZnO Whiskers

Zinc oxide (ZnO) whiskers, one of the new versatile functional ultrafine materials, has attracted increasing attention due to its distinguished performance in electronics, optics and photonics. Furthermore, the remarkable high strength, hardness and tenacity of ZnO whiskers make it suitable for preparation of many reinforced composite materials. In recent years, one-dimensional(1-D) ZnO nanowires with its direct wide band gap (3.37 eV) and large exciton binding energy (60 meV) has been regarded as potential materials in manufacturing electronic and optoelectronic devices, especially for ultraviolet(UV) laser device.In this paper, we report a novel solution route for preparation of 1-D ZnO whiskers. Using ZnCl₂ and NaOH as raw materials, 1-D ZnO whiskers were successfully synthesized at an attractive low temperature(85 °C) in the presence of anionic surfactant(SDS or SDBS). It is proved to be a very simple method with mild reaction conditions.Based on the preparation technology of 1-D ZnO whiskers, we had a deep insight into the transformation from Zn(OH)₂ to ZnO in the presence of surfactant. The results revealed a detail formation mechanism of 1-D ZnO whiskers, and provided a theoretical assistance for controlled preparation of 1-D ZnO.The main contents of the thesis are summarized as followed:(1) In order to establish the optimum way to prepare ZnO whiskers, the effects of different experimental conditions on morphology of the products were investigated in the system of ZnCl₂-NaOH. Results show that the key factors for the preparation of well-defined 1-D ZnO whiskers involve proper molar ratio of ZnCl₂ to NaOH, the process of pre-stirring at room temperature for the start reaction solution, as well as anionic surfactants. It was also found that adding some organic solvents in the solution can obviously reduce the diameter of 1-D ZnO whiskers.(2) To further understand the formation mechanism of ZnO crystal in solution, we monitored the reaction processes with different experimental conditions through concentration measurements of Zn(II) remaining in the solution, scanning electron microscopy and X-ray diffraction analyses of the solid products, and revealed the general laws of nucleation and growth of ZnO in aqueous solution. The results indicate that the formation of 1-D ZnO whiskers is a phase transformation from 89-0138 Zn(OH)2 to ZnO under given conditions, and follows the dissolution-reprecipitation mechanism. Its needle-like morphology ascribes to the different growth velocity of ZnO22" units at the positive and negative planes. Controlling molar ratio of ZnC^ to NaOH can regulate Zn(II) concentration in solution and the crystal quality of precursors, then regulate the nucleation and growth rate of ZnO crystal, and as a result, give ZnO crystals different morphologies. Pre-stirring at room temperature plays an important role in getting perfect 89-0138 Zn(OH)2 precursor, which is a crucial factor for synthesizing 1-D ZnO whiskers.(3) The roles of surfactant in the transformation from Zn(OH)2 to ZnO were investigated in detail. By studying the fluctuation of Zn(II) titrating curves, phase transformation rates, as well as the diameters and aspect ratios of obtained 1-D ZnO crystal with different surfactants, we proposed a model that surfactant micelles were used as "micro-reactors" or templates to regulate the formation of 1-D ZnO whiskers. The different influencing principles of ionic surfactants on the formation was also studied.