Microwave-assisted Solution-based Synthesis Of Transition Metal Oxides And Characeterization

As a special synthesis method, microwave heating technique has the virtues of high heat efficiency, short preparation time, no temperature grads, simple equipment and easily-controlled operating condition, so it has been widely employed during recent years. In this thesis, the metal oxides were successfully prepared and carefully investigated. A domestic microwave oven was modified with refluxing system and used to directly prepare three kinds of metal oxides such as ZnO, Fe₃O₄ and CuO. The as prepared products were characterized by various instruments including X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Selected Area Electron Diffraction (SAED), photoluminescence (PL), vibrating sample magnetometer (VSM) and Infrared Spectrum (IR). Products prepared by the route are of size uniformity, high purity and high crystallinity. The contents are summarized as follows:(1) Flower-like ZnO microcrystals were prepared in large-scale by a facile microwave-assisted solution-based route under mild conditions (1.5 mL NH3·H2O, 1 mmol ZnCl2, power 30% and reaction duration 20 min). These flower-like ZnO microcrystals were made of ZnO microrods and synthesized by the reaction of ZnCl2 and NH3·H2O. It has been found that morphology and size of final products prominently depend on reaction conditions such as concentration of NH3·H2O, microwave power and reaction time. Especially, the amount of ammonia plays a key role in the formation of self-assembly structure of final products. The possible formation mechanism of flower-like ZnO microcrystals has also been proposed based on the experimental results. Room temperature PL emission investigation reveals red shift of final products compared with bulk ZnO.(2) Hexagonal magnetite (Fe₃O₄) nanosheets with length in the range from 50 to 80 nm were successfully prepared in large-scale by a facile microwave-assisted route under conditions of 1 mmol FeSO4, 0.01 g CH3COONa, 0.02 g C6H8O7, 0.05 g NaOH and 1 g NaH2PO4·H2O, power 30% and reaction duration 10 min). It has been found that morphology and size of final products strongly depended on reaction conditions such as reaction time, microwave power, and concentration of NaOH and other additive agent. Citric acid plays a key role in the nucleation and growth process of final products. Magnetic studies revealed that hexagonal Fe₃O₄ nanosheets have low saturation magnetization of 36.4 emu/g, much lower than that of commercial Fe₃O₄ powder (84.5 emu/g).(3) Based on the similar method, CuO flower-like micro-spheres have been successfully synthesized by the reaction of CuSO4 and NH3·H2O (1 mmol CuSO4, 1 mL NH3·H2O, power 30% and reaction duration 10 min). The micro-spheres are made of nanoplates with oriented attachment. With the introduction of the different amount of NH3·H2O, different copper source and NaCl, the morphology transformed from micro-sphere to rose-like, dandelion-like and sea urchin-like microcrystals. The size of these microcrystals can be finely controlled. The possible formation mechanism for the CuO flower-like microspheres has also been proposed.