Investigation On ZnO Nanomaterials: Synthesis And Photocatalytic Properties

In recent years,industrial wastes,including waste water,waste gas,and other waste materials,increase rapidly with the rapid development of industry.As a result,water pollution becomes one of the global problems.Many researchers have been focusing on projects of the disposal of water pollution.ZnO,a wide direct bandgap semiconductor material,is regarded as one of the potential photocatalysts due to its high electrochemical stability,thermal stability and excellent absorption of ultraviolet light.However,the degradation efficiency of generally used ZnO photocatalysts is limited by their relatively small specific surface area and high carrier recombination rate.Furthermore,the recycle process of ZnO powder photocatalyst is not only complicated and time-consuming(for examples,it is difficult to separate the photocatalyst from the aqueous solution),but also the photocatalyst remaining in the aqueous solution might lead to the secondary pollution.In this thesis,the synthesis and properties of ZnO photocatalysts with particular morphologies,ZnO hollow spheres thin film and Na-doped ZnO nanowires,have been studied.The relationship among the morphological,structural and optical properties of ZnO nanomaterials has been disscussed in detail.The photocatalytic activity of degradation on methyl orange(MO)aqueous solution has been studied and the mechanism of photocatalytic reaction has been proposed.Those are summarized as following:(1)ZnO hollow sphere films with different size were synthesized by using PS microspheres templates,where the PS microspheres templates were prepared by dip-coating method.The hollow nature of ZnO nanospheres is proved by SEM and TEM analysis.All ZnO hollow sphere films show a strong absorption in the ultraviolet range.The enhancement in photocatalytic efficiency of ZnO hollow sphere films in comparison to ZnO thin film is ascribed to the hollow nature of the spheres,the features of the high specific surface area of hollow sphere films and the light trapping inside the hollow sphere under the irradiation of UV.Furthermore,the photocatalytic efficiency on degrading MO aqueous solution of 400 nm ZnO hollow spheres is the largest among all samples.(2)Au/ZnO hollow sphere composite thin films were fabricated by sputtering Au nanoparticles on the surface of ZnO hollow spheres.The photocatalytic activity on degrading MO aqueous solution of ZnO hollow sphere and Au/ZnO hollow sphere composite thin films under UV irradiation were discussed.The deposition of Au nanoparticles improved the photocatalytic rate of ZnO hollow sphere films significantly because of the Schottky barrier at the Au/ZnO heterojunction interface.The Schottky barrier resulted in the preferential migration of the photogenerated electrons from the ZnO conduction band to the surface of the Au nanoparticles,which suppressed the recombination of the electron-hole pairs and enhanced the photocatalytic activity of the ZnO hollow sphere films effectively.In addition,the main species in the photocatalytic degradation of MO aqueous solution were ·OH and ·O2-.(3)ZnO nanowires were successfully prepared by CVD method using Au nanoparticles as catalyst.The influences of the pH value of the MO aqueous solution on photocatalytic efficiency of ZnO nanowires was discussed.The p H value was adjusted by adding appropriate acetic acid or ammonia water.The results showed that the photocatalytic efficiency is the largest when the pH value is 2.7.(4)The Na-doped ZnO nanowire was successfully prepared by adding a certain amount of sodium pyrophosphate in the zinc source material.The photocatalytic activity of ZnO nanowires was improved due to abundant surface oxygen vacancy was formed after the doping of Na.By adding different capture agents,it was found that the main active species of Na-doped ZnO nanowires were ·OH and ·O2-during photocatalytic reaction,while the main active species of undoped ZnO nanowires was ·OH.The photocatalytic mechanism of Na-doped ZnO nanowires is ascribed to the surface oxygen vacancy,which inhibits the recombination of electron-hole pairs effectively,leading to a considerable improvement in the photocatalytic efficiency.Furthermore,it has been proved that the photocatalyst material curing on substrates is benifical for the recycling.