Controlled Synthesis And Growth Mechanism Of ZnO Materials

Zinc Oxide (ZnO) is a kind of self-excitation semiconductor with wurtzite structure and a direct wide band gap, which has special electric, optical, magnetic, mechanic, chemical properties. So ZnO is used potentially in micro-electrical, optoelectronic devices. These performances not only depend on materials' compose, structure, scale, but also dimensionality and configuration. Consequently, designing ZnO material' capability and selective assembly are of significant important for various applications. Controlled synthesis of ZnO with controlled appearances is systematically investigated in this paper.Hydrothermal method and vapor-phase approach are respectively used to synthesis ZnO controlled structures. In the hydrothermal method, by controlling the concentration of reagents, temperature, heating speed, reaction time etc, we have synthesized various ZnO structures: hexagonal rods, acicular rods, rod arrays, hexagonal tubes, plate-like rod/tube arrays. In the vapor-phase approach, by controlling raw materials, temperature, gas pressure, substrate, etc, we have synthesized various ZnO structures: dendritic, networks, arrays, comb. The compositions, morphology and microstructures of ZnO have been investigated by means of X-ray Diffraction, Scanning Electron Microscope and Transmission Electron Microscope.Hexagonal rods and tubes have straight and regular configuration with slippery side surfaces. The ZnO rods/tube arrays grow oriented vertically on the substrate and the intersectant ZnO nanosheets stand on the backbones of rods/tubes. The chemical reaction processes of the solution system are investigated. When the reaction temperature is below boiling point, ZnO tends to form theoretic crystal shape, and grows into hexagonal rod. When above boiling point, the system tends to reach its thermodynamic stability state and the polar faces of rods are dissolved, leading to the formation of tubes. Concentrations of reagents determine scale of ZnO crystal configuration. Rising the temperature rapidly, the nanosheets around the ZnO rodsgrow by the secondary nucleation and growth process. The ZnO tubes are formed by the etching of ZnO polar faces.Dendritic-like ZnO structure consists of a hexagonal central trunk, with six side branches emanating from facets of the central core in a certain angle. ZnO belts grow in three directions of 60° value to form a uniform crossed network. A line of regular branches are perpendicularly grown from a main ribbon shore, forming comb-like structure. They grow in Vapor-Solid(VS) mechanism. The key of controlled morphology is controlled pressures of Zn(g) and O2(g). Rising the reaction temperature results in the increasing pressure of Zn(g). Dendritic-like and network structures are formed in higher pressure of Zn(g). Comb-like structure is formed in lower pressure of Zn(g). Changing the dosage of raw materials, pressures of Zn(g) and O2(g) change correspondingly, which directly influences crystal shape. The branches of comb-like structure represent rods, needles or dentation.Based on ZnO hexagon crystal structure and polar growth behaviour, the morphologies of ZnO depend on growth speeds of the group crystal surfaces, and have related to reaction conditions. The key for controlling the shape and size is to control ZnO nucleation and growth. Crystal morphologies result from the supersaturation of ZnO in solution or vapor surroundings, which strongly depends on the processing parameters. So ZnO conformations with various shape and size are obtained by reasonly adjusting these parameters.