Fluorescence Tunability And Photocatalytic Activity Of ZnO Nanostructures

With the progress of technology and the development of human society, the energy are running out, the environmental pollution is more and more increasingly to seriously threat to the survival of human beings. Therefore, the developments of clean energy and efficient catalysts would be inevitable to solve energy and environmental problems by photocatalytic technology. This paper aims to develop the simple, economical and green methods to prepare nano-ZnO and nano-ZnO based composites, study their fluorescence tunability and photocatalytic activity, and explore the fluorescent materials used in full-color display and the efficient photocatalysts, which would provide the necessary raw materials to solve the energy and environmental pollution problems in today’s global attention.Firstly, ZnO nanoparticles and nanoleaves were synthesized by typical solution route and ultrasound irradiation method, respectively. Effects of thermal annealing temperature on the morphologic and fluorescent properties of ZnO nanoparticles were studied. The thermal annealing in air and nitrogen induced the crystal growth, and enhanced the green and orange emissions of ZnO nanoparticles. Using Monte Carlo software, the fractal growth process of ZnO nanoleaves was simulated. The fern-shaped ZnO nanoleaves were self-assembled from zero-dimensional ZnO nanoparticles.Secondly, using metal Zn as the only raw material, Zn/ZnO composite structures were prepared by ultrasonic irradiation, boiling-water method and vapor condensation method, respectively. The metal-semiconductor junctions were formed at the interface of Zn/ZnO core-shell structures prepared by ultrasonic irradiation and boiling-water method, it suggusted that the fluorescence efficiency of the ZnO shell is lowered. ZnO nanostructures with the controllable morphology and fluorescence were obtained by adjusting the temperature and time of zinc vapor condensation in an air-filled box furnace. With increased the growth time from1to7hours, the sizes and density of the one-dimensional ZnO nanostructures grown from Zn foil at500℃in air were increased, and the green emissions were gradually enhanced. When the time of Zn vapor condensation is fixed to be10hours, the morphology of ZnO nanostructures grown from Zn foil with increased the temperature from500to900 ℃could be modified from one-dimensional nanorods, two-dimensional nanoplates to three-dimensional tetrapods. Furthermore, their room-temperature green emissions is also gradually enhanced with increased the growth temperature.Thirdly, using the sponge-like porous silicon (PS) films as two-dimensional porous matrixes, the alkaline sol-gel solutions of zinc acetate as the original materials of ZnO nanocrystals, we prepared ZnO/PS nanocomposite films by the sol-gel spin coating technology, and investigated the effects of Zn2+concentration in sol-gels and annealing temperature on their fluorescence properties. It is found that the fluorescence of the ZnO/PS nanocomposites could be tuned from red to green-blue by increasing the concentration of zinc cations in the sol-gels from7.5to240mM. Post thermal annealing of the ZnO/PS nanocomposite films can shift the fluorescence further into violet-blue.Finally, using methyl orange (MO) solution as dye waste water, we studied the photocatalytic activities of the synthesized nano-ZnO and Zn/ZnO core-shell microspheres. Under UV light irradiation of20W high-pressure mercury lamp for1380minutes, the as-prepared ZnO nanoparticles could degrade73%MO, the ZnO nanotetrapods were up to85%while the fern-like ZnO nanoleaves were only27.9%. However, Zn/ZnO core-shell microspheres prepared by ultrasonic irradiation and boiling-water method were only irradiated for120minutes, the degradation rate could reach94%and75%, respectively. Obviously, the Zn/ZnO core-shell structures exhibited higher photocatalytic activities than that of ZnO nanostructures in degrading MO. The enhanced photocatalytic activity could be attributed to the metal-semiconductor junction formed at the Zn-ZnO interface.In a word, this paper developed the simple, economical and green synthesized methods: ultrasonic irradiation, boiling-water method, vapor condensation method and sol-gel spin coating method to prepare the nano-ZnO, Zn/ZnO composites and ZnO/PS nanocomposite films, effectively achieved the UV-visible fluorescence tunabilities and the enhanced photocatalytic activities of the nano-ZnO, which provided the reliable key raw materials to realize the full-color display and to solve the global environmental problems.