Catalyst-Free Controlled Synthesis And Growth Mechanism Of One-Dimensional Zinc Oxide Nanostructures

One-dimensional (1-D) zinc oxide (ZnO) nanostructures, including ZnOnanowires, nanorods, nanobelts and nanotubes, have attracted a lot of attentionover the past decade due to their extensive and exciting applications inelectronic, optoelectronic devices, sensors, field emission, solar cells andphotocatalysis. Among the various methods to synthesize1-D ZnOnanostructures, catalyst-free thermal evaporation and vapor-phase transport isan extensively used, simple, and low-cost approach which can provide largequantity and high crystalline quality products without the contamination of themetal catalysts. However, the catalyst-free growth process of the1-D ZnOnanostructures has not been well understood until now. Growth process analysisand growth control are still lacking. In this thesis, we will synthesize1-D ZnOnanostructures by the catalyst-free thermal evaporation method, investigate theeffects of the synthesis parameters and the additives on their growth, study thegrowth mechanisms and search for the means of catalyst-free controlledsynthesis.Firstly, we have obtained well-aligned ultrathin (about11nm in diameter)ZnO nanowire arrays on Si wafers pre-coated with c-oriented ZnO thin films bythe catalyst-free thermal evaporation method. The synthesis oxygen flow ratewas found to have a great effect on the morphology of the products, based onwhich the growth of the ZnO nanowires was attributed to the self-catalyzedvapor-liquid-solid (VLS) mechanism. The growth process of the ZnO nanowireswas analyzed by the classical nucleation theory, and the Zn vaporsupersaturation was proposed to be a key factor to affect the diameter and theareal density of the nanowires. By controlling the oxygen flow rate and hencethe Zn vapor supersaturation, the average diameter of the ZnO nanowires can befinely controlled in the range of12-31nm. X-ray photoelectron spectroscopyand photoluminescence measurements were performed to characterize thestoichiometry and the optical properties of the nanowires. Besides, we have alsoobtained a ZnO nanowire/nanorod mixed array, which was employed toinvestigate the size effect and the surface adsorption effect on the photoluminescence of the ZnO nanowires.Next, by introducing Sb₂O₃additive into the precursor, we found that themorphology of the ZnO nanowires can be controlled from nanowires tonanocones. Based on the scanning electron microscopy observations, theformation mechanism of the nanocones was attributed to the radial growth of thenanowires, the kinetics of which was modified by the adsorption of SbOxspecies on the nanocone surfaces. As predicted, the tapering degree of thenanocones can be controlled by the concentration of the Sb₂O₃additive in theprecursor. By further increasing the Sb₂O₃concentration in the growth vapors,Sb-doped ZnO nanowires with kinking structures were obtained. Transmissionelectron microscopy observations revealed that the kinks of the nanowires arecaused by twinnings, which were proposed to be induced by the addition ofSb₂O₃. The optical properties of the synthesized nanocones and kinkednanowires were investigated by photoluminescence.Finally, porous ZnO nanofibers were obtained by introducing CuCl₂·2H₂Oadditive into the precursor. Based on the comparative experiments, we proposedthat the porous structures of the nanofibers were resulted from the gradualdecomposition of the unstable Cl-containing intermediate products. Adye-sensitized solar cell was fabricated based on the porous ZnO nanofibers andshowed an energy conversion efficiency of0.88%.