Synthesis And Formation Mechanism Of 3-D Flowerlike Zinc Oxide Microcrystals Via A Solution Route

Zinc oxide (ZnO) is a wide band gap semiconductor (Eg=3.37ev) with a large exciton binding energy (60meV). It is widely used in photoelectric application such as transparent conductor, gas sensors, solar cells, ultraviolet protection and ultraviolet laser devices. ZnO is also used in the fields of pigment, rubber additive, catalytic, cosmetic application, medicine carrier, and so on. Controlling the sizes and shapes of inorganic nanocrystals has attracted considerable interest, because these elements represent key elements that determine their electrical and optical properties. Comparing to other semiconductor materials, ZnO has various morphologies including flower-, tower-, dandelion-, urchin-like assembly. These ZnO materials with novel morphologies may be have promising application in the field of micro-electronics and nano-laser devices.In this paper, we developed a novel one-step solution route to prepare three dimensional(3-D) ZnO microcrystals. Well-defined 3-D flowerlike ZnO crystals were obtained from zinc chloride(ZnCl₂)/zinc nitrate(Zn(NO₃)₂) and sodium hydroxide (NaOH) through aging at constant temperature(<100℃) for a few hours without any templates. The formation mechanism of ZnO crystals in solution has also been studied, which provided a theoretical assistance for controlled synthesis of 3-D ZnO microcrystals.The main contents of the thesis are summarized as followed:(1) The nucleation and growth mechanism of 3-D flowerlike ZnO crystals in ZnCl2-NaOH system was investigated through monitoring the crystal structure, morphology, and Zn(II) concentration in reaction solution and at regular intervals. Results show that ZnO microcrystals with 3-D structure were formed through the phase transformation from Zn(OH)₄^2- to ZnO, which followed the precipitation from supersaturated solution mechanism. The 3-D structure of ZnO maybe originate from the nuclei (or primary particles ) aggregation at the early growth stage. Regulating the nucleation and growth rate of ZnO crystals, by changing the solution basicity can determine the particle size and shapes.(2) The effects of different experimental conditions on morphology and size of the obtained crystals was also investigated along with the synthesis in ZnCl2-NaOH system. Results indicate that the influences of reactant concentration, molar ratio of ZnCl2 to NaOH, reaction temperature and solvent media can influence the particle size and shape.(3) We find that 1-D ZnO microcrystals can be prepared in Zn(NO)₃)₂:NaOH=1:6 system via a solid phase transformation from intermediate Zn(OH)₂, which formed in the early reaction stage. Changing the experimental conditions such as reaction temperature and solution basicity can regulate the intermediate stability, which can finally control the morphology (3-D or 1-D) of obtained crystals.