Preparation And Microstructure Characeterization Of ZnO Nano-structures

ZnO is a wide bandgap semiconductor material with high exciton binding energy. Because of its excellent optical, electrical, magnetic and mechanical performance, it is widely used in optoelectronic devices, sensors, liquid crystal displays, solar cells, and surface acoustic wave devices. Due to its unique polar crystal structure, ZnO owns crystallographic anisotropy which provides a possibility to grow rich variety of ZnO microstructures. In recent years, the research about growth of ZnO nanostructures and their applications has received wide attention at home and abroad.In this paper, we have prepared ZnO nano-structures, by the hydrothermal reaction in the mixed ethanol/water solvent system which used the organic zinc salt as the starting material, to study their growth mechanism, aiming at finding the key experimental parameters that impacts the changes in morphology of grown nanostructures which would provide theoretical and experimental basis to achieve controllable growth morphology in the future. We used X-ray diffraction, SEM, TEM and AFM to study the crystal structure and morphological characteristics and FT-IR and TG-DTA to analyze chemical composition and phase transformation process of the grown products, respectively. Through a comprehensive study of the growth patterns of the precursor layered basic zinc acetate (LBZA) and of ZnO growth process, we obtained the following main results.(1). There formation mechanism of LBZA is different in mixed ethanol/water solvent and aqueous solution systems. In the mixed solvent system, the alcohol play an important role to change the solvation effects of zinc ion, which not only make the combination of zinc ions and acid ions more stable but also reduce collision probability between the zinc ions. Therefore, it can promote hydrolysis of zinc acetate to LBZA with small amount of water. However, in the aqueous solution system under acidic conditions, with the addition of a small amount of hydroxyl ions, the zinc ion, extra hydroxyl ions and acetate ions form LBZA which has the lower solubility and lower free energy.(2). The chemical formula of LBZA is Zn(OH)1.6(AC)0.4, which can own different water of crystallization. There are three major phase transitions under the changes in temperature; at about100℃, LBZA experience removal of water of crystallization; when the temperature reaches about130℃, the dehydration of the hydroxyl group in the LBZA structure occurs resulting in the formation of ZnO crystal while the acetate still exists between ZnO particles; while the temperature continue to rise up to about300℃, the aeetate of the LBZA decomposes and ZnO crystallinity improves.(3). From the results of LBZA growth patterns, it was inferred that the surfactant C’l’AB has limited effect on the stacking crystal growth of LBZA along the c-axis direction, meanwhile its a-axis growtli was increased. Whereas. MKA restricted the LBZA growth in all directions making the grain size smaller. In addition, configurational ion of IB/A can continue to grow along non c-axis direction into a belt structure by sharing hydroxide radical under the low temperature hydrothermal preparation. And the belt shape of LBZA tends to be wider with the addition of CTAB while more long and thin with the addition of MKA. Moreover, these belts shaped LBZA, after the calcinations and thermal decomposition reaction retained their morphology and formed ZnO nano-belts.(4). Among all kinds of ZnO morphologies, the rod-like ZnO grows by the homogeneous nuelcation and stacking of the growth units ([Zn(OH)4]2-) formed from a supersaturated solution according to the original ZnO crystal habit, and the growth of dumbbell-shaped and double coned ZnO is based on the original precipitation through heterogeneous nuclcation and growth. Furthermore, we found that acetate ion can slow down the growth rale of ZnO along the e-axis direction via adsorption effects which helps to produce relatively Hat hexagonal planes.(5). Through orthogonal test analysis, two most important experimental parameters affecting the morphology of ZnO crystals in the hydrothermal preparation method were summarized which includes the concentration of zinc acetate and the volume of MKA. The role of MKA has two aspects: First, it can provide an alkaline environment for the system to reduce the critical transition temperature; second, it plays the role of surface modifier and inhibition of growth via adsorption phenomenon at the growth process. The change in the concentration of zinc acetate can mainly change the degree of supersaturation of the solution at the critical reaction temperature. In addition, we found that the increase in the reaction temperature can also increase the degree of supersaturation. It is suggested that we should simultaneously consider the degree of supersaturation of solution and the role of MKA while analyzing the growth mechanism of ZnO nanostruetures under the hydrothermal reaction. According to their different impact conditions, the dissolution-recrystallization process is also different which can affect the nuclcation and growth mode of ZnO crystal.