Abstract
In this research, we proposed a novel method for the synthesis of zinc oxide (ZnO) nanostructures under supercritical organic solvent. This thesis is categorized into three topics: (1) synthesis and shape control of one-dimensional zinc oxide nanostructures. (2) synthesis of porous zinc oxide nanowires. (3) plug-flow synthesis of novel zinc oxide nanostructures. Zinc acetate-oleylamine (Zn(Ac)2–OLA) complex was chosen as the precursor, and the precursor was then thermal decomposed in the supercritical hexane upon injection. (1) Synthesis and shape control of one-dimensional zinc oxide nanostructures. In a semi-batch reactor, the aspect ratio of zinc oxide nanostructures was successfully tuned by kinetically control of the reaction temperature and injection rate. The shape changed from sphere-like nanocrystals with low aspect ratio to high aspect ratio nanowires when the reaction conditions was changed from low reaction temperature with high injection rate to high reaction temperature with low injection rate. In this study, we used the kinetic approach to describe the growth behavior of zinc oxide nanocrystal. X-ray diffraction (XRD) characterization was applied for further identification of the zinc oxide nanostructure. High-resolution transmission electron microscope (HR-TEM) imaging was used for better understanding of the crystal structure of zinc oxide. The mechanism of the chemical reaction was indentified using Fourier-transform infrared (FTIR) and nuclear magnetic resonance (H-NMR) spectroscope by comparing the spectra of the residue solution before and after the reaction. Result showed that the mechanism is the same with the literature at low temperature reaction. Finally, we discussed the optical properties of zinc oxide products with different aspect ratio. Blue shift of the excitation wavelength peak in the UV region was observed correspond to the increase of aspect ratio. (2) Synthesis of porous zinc oxide nanowires. The precursor, i.e. Zn(Ac)2 – OLA complex was first thermally decomposed under supercritical hexane. The reactor was then allowed to cool to room temperature and kept still for reaction. The high quality and ultra-long porous zinc oxide nanowires were obtained. From HR-TEM images, we realized that these nanowires were seemed to be formed by the assembly of nanosized-ZnO particles. We found that when the reaction time was lengthened, ultra-long nanowires were formed, with length exceeding 80μm. SEM images of the product under different reaction time were used to discuss the growth of the nanowire. Zinc and oxygen atoms distribution in a porous nanowire was characterized by energy dispersive spectroscopy (EDS). (3) Plug-flow synthesis of novel zinc oxide nanostructures. Plug-flow reactor was used as a basis for synthesis of more novel types of zinc oxide nanostructures. SEM images showed that plug-flow reactor have synthesis potential.