Study On Controllable Synthesis And Photocatalytic Properties Of Nano ZnO And Its Composities

Nano-ZnO is a new type multifunction inorganic material. Due to its unique physical and chemical properties, nano-ZnO has wide application prospect in many fields, especially in environment purification which is closely correlated with the survival and health of human beings. Apparently, the synthesis, modification and photocatalytic performance of ZnO have been and continue to be an area of active research. Firstly, the mechanism and influencing factors of photocatalytic reaction are introduced in brief. In particular, we discuss the research progress of ZnO in photocatalysis in details. Subsequently, nano-ZnO powders and Ag/ZnO nanocomposites were synthesized by microwave method or chemical precipitation method, ZnO arrays were synthesized by hydrothermal method, and the photocatalytic performances of as-prepared powders and arrays were investigated. This work mainly includes the following aspects:Nano-ZnO powders were prepared by microwave method. The effects of microwave irradiation time, the concentration of reactants and microwave power on the morphology and size of nano-ZnO powders were investigated. When the microwave irradiation time is relatively short (5min), large amounts of ZnO short rods form in the solution. An increased microwave irradiation time to10min leads to the synthesis of ZnO bipods with uniform distribution of sizes. The formation process of ZnO bipods includes three stages: nucleation stage, crystal growth stage and link-age stage. When the microwave irradiation time is prolonged to30min, the top of ZnO bipods begin to dissolve, and small mounts of X-shaped ZnO rods and ZnO microflowers have been observed. When the concentration of reactants is as low as0.02M, these ZnO bipods have sharp tips with smaller diameter than that of the middle. When the microwave power is as low as120W, due to the energy of the reaction system is low, the growth rate of ZnO is relatively slower, which leads to the formation of ZnO bipods with small diameter and large aspect ratio. Methylene blue (MB) is selected as models to investigate the photocatalytic performance of ZnO bipods with different aspect ratios. The photocatalytic activity of ZnO bipods increases with the aspect ratio increased.Ag/ZnO nanocomposites were prepared via one-step microwave method at a low microwave power (120W). The Ag/ZnO nanocomposites are composed of ZnO nanorods and metallic Ag nanoparticles attached on the surface of ZnO nanorods. The success of the one-step strategy should be mainly attributed to the important role of the HMTA. HMTA can be converted into ammonia (NH3) and formaldehyde (HCHO). For one thing, Zn(NH3)42+and Ag(NH3)2+complexes can be formed respectively in the presence of NH3; for another, Ag2O can be reduced by HCHO. The mechanism of interaction between ZnO and Ag in Ag/ZnO Nanocomposites was discussed in detail by XPS analysis and energy band theory. Since Fermi energy level of Ag is higher than that of ZnO, when Ag and ZnO contacts with each other, electrons will transfer from Fermi energy level of Ag to that of ZnO to get equilibrium, and then a new Fermi energy will be formed. The Ag/ZnO nanocomposites showed enhanced photocatalytic activity compared with pure ZnO. Specifically, the photocatalytic activity of Ag/ZnO nanocomposites increased with increasing Ag content from0.5%to1.5%. However, further increasing Ag content to2.0%would lead to the decrease of photocatalytic activity.Ag/ZnO nanocomposites were prepared by one-step chemical precipitation method using glucose as a reducing agent. The Ag/ZnO nanocomposites are composed of ZnO nanorods and metallic Ag nanoparticles attached on the surface of ZnO nanorods. At the initial stage of the reaction, under alkali condition, Zn(OH)42-and Ag(OH)2-complexes were formed respectively. Then under alkali thermal conditions (70℃), intermolecular dehydrolysis between Zn(OH)4-and Ag(OH)2-may occur, resulting in the formation of Ag2O/ZnO nuclei through a Zn-O-Ag bond. Ag2O was further reduced by glucose under alkali condition. At the growth stage, the ZnO crystals grew very fast along [0001] direction (c-axis), enhancing the formation of nanorods, and Ag nanoparticles were formed on the surface of ZnO nanorods. The photocatalytic performance of the Ag/ZnO nanocomposites with different Ag contents was investigated. With the evolution of the Ag content from0.5%to1.5%, the photocatalytic activity of the Ag/ZnO nanocomposites increases. While the evolution of the Ag content from1.5%to8.0%, the photocatalytic activity of the Ag/ZnO nanocomposites decreases. The recycled experiments for the photodegradation of MB under UV irradiation were performed to evaluate the photostabilities of the Ag/ZnO nanocomposites with1.5%Ag content and pure ZnO powders. Compared to pure ZnO powders, the Ag/ZnO photocatalyst exhibits a good durability and its photocatalytic activity has no significant change even after recycled for five times.Large-scale ZnO arrays with a series of morphologies, including nest-like, tower-like, and flower-like samples, were synthesized by hydrothermal method. The morphologies of the obtained ZnO arrays can be conveniently tailored by changing seeding conditions. The room temperature photoluminescence spectra of ZnO arrays with three different morphologies similarly show three emission bands:the UV emission at around390-400nm, the green emission at about470nm and the yellow emission at about590nm. The nest-like ZnO arrays showed higher photocatalytic activity than tower-like and flower-like ZnO arrays.The unique bouquet-like ZnO arrays were successfully synthesized via a two-step route. First, flower-like ZnO arrays were prepared by hydrothermal method. Then the bouquet-like structure was obtained by the selective etching of the ZnO nano-flowers in NaCl aqueous solution. Each ZnO bouquet was composed of many tube-like petals radiating from the center outwards. Hierarchical rods-in-tube nanostructures could be observed on the top of petals. A possible mechanism involving face-selective etching and defect-selective etching was proposed for the formation of bouquet-like ZnO arrays. Compared to tube-like and flower-like ZnO arrays, bouquet-like ZnO arrays showed higher photocatalytic activity.