Study On Synthesis And Properties Of ZnO Micro/Nanostructures And Their Composites

In this paper, a large amount of ZnO products were synthesized through hydrothermal, solvothermal and electrospinning methods, including rod-like ZnO, flower-like ZnO, ZnO rhombic sheets, ZnO nanorod arrays, and one-dimensional ZnO. The composites of ZnO rhombic sheets towards Ag2 O and nanorod arrays towards Ag have also been fabricated. The influencing factors on morphologies and formation mechanisms of these micro/nanostructures were carefully studied. Among these products, Ag2O/ZnO composite and one-dimensional ZnO show excellent photocatalytic activity for degrading pollutants. The specific research contents are as follows:1. Through a facile solvothermal method, rod-like, flower-like, double-columned and double-hemisphered ZnO microstructures were successfully fabricated in glycerol/water system with no surfactants or templates, and the possible growth mechanisms of these ZnO products were carefully discussed. Under alkaline condition, a large amount of growth units exsited in the solution resulted in rapid nucleation and growth of ZnO, which led rod-like and flower-like ZnO were obtained. Without NaOH, ZnO crystallites were generated mainly by esterification between Zn(CH3COO)2·2H2O and glycerol. In this condition, double-columned and double-hemisphered can be achieved by adjusting the proportion of glycerol and water. This study will provide a feasible route for fabricating other semiconductor materials with novel morphologies.2. ZnO rhombic sheets consisted of small ZnO particles with diameters range of 150-250 nm were successfully fabricatrd by calcining zinc glycerolate precursor. According to previous literature and the study on calcination process, the growth mechanisms of zinc glycerolate and ZnO rhombic sheet were proposed. Photocatalytic tests indicate that the crystallinity of the small particle shows significant effect on highly efficient photocatalysis of ZnO sheets. Through a precipitation method, monodispersed Ag2 O nanoparticles were assembled uniformly on the surface of the rhombic sheet. It was found that Ag2O/ZnO composites show excellent photocatalytic activity under ultraviolet and visible light irradiation for degradation of methyl orange(MO) and 2, 4-dichlorophenol(DCP), which can be ascribed to the efficient separation of photogenerated electrons and holes as well as the increase of light response range. Further investigations demonstrated the ZnO rhombic sheets with a molar ratio of Ag2O/ZnO of 1/6 had the best activity. Besides, the special morphology of this composite is favorable for the separation of the photocatalyst, which will facilitate its practical utilization.3. ZnO nanorod arrays decorated with Ag nanoparticles(NPs) have been successfully fabricated by a facile two-step solution method on indium tin oxides(ITO) glass at relative low temperature. It can be observed from the SEM images that Ag NPs with a diameter range of 40-50 nm are deposited uniformly on the side facets of ZnO nanorods. The concentrations of silver nitrate and polyvinylpyrrolidone as well as ice bath treatment were significant to achieve Ag/ZnO composite with uniform morphology. Besides, the possible growth mechanism of Ag/ZnO composite had been discussed. PL results manifested that the deposition of Ag NPs can strengthen charge separation effect, indirectly confirming the formation of Ag/ZnO heterostructure. This method can be further developed to perpare other ZnO based noble metal or semiconductor composites.4. ZnO nanorod arrays have been synthesized on glass substrates by an electrospinning assisted hydrothermal method at relative low temperature. It’s the first time that electrospinning technique is used to prepare a seed layer for the synthesis of ZnO nanorod arrays. When coating time is short, novel ZnO bundles consisting of well crystallized nanorods with diameters of 100-130 nm are obtained. X-ray diffraction patterns show the ZnO products have a typical hexagonal wurtzite structure. The density of the ZnO seeds keeps increasing with the coating time prolonging, and eventually densely well-oriented ZnO nanorod arrays are obtained. The large-scale fabrication of ZnO nanorod arrays can be much conveniently achieved via the electrospinning coating, and this method can also be used for the growth of other semiconductors on substrates.5. By electrospinning technique, a lot of one-dimensional ZnO structures, such as nanotubes, microbelts, nanochains, and solid fibers, have been successfully fabricated. Experimental results show that both the viscosity of spinning solution and calcination process have important influences on the morphology of ZnO samples. When more zinc salt was added to increase the viscosity of spinning solution, ZnO microbelts were obtained. Meanwhile, ZnO nanochains and solid fibers will be prepared if sectional calcination was not applied in the process of thermal treatment. Based on these results, the formation mechanisms of ZnO nanotube and other one-dimensional structures were proposed. Besides, due to relatively large surface area and high crystallinity, ZnO microbelts showed the best photocatalytic performance among the ZnO samples.