Optical,Magnetic And Photocatalytic Properties Of ZnO Based Nanostrucures Prepared By Microwave-assisted Hydrothermal Method

As a wide band gap oxide with good properties of photoelectricity,piezoelectricity,ferroelectricity,thermoelectricity,etc.,ZnO has been intensively studied.With the development of nanotechnology,most of the studies focus on the nanostructured ZnO.Owing to the size effect of nanomaterials,to well control the size of the prepared ZnO nanomaterials is one of the mot important problems that need to be solved all the time.On the other hand,element doping is an effective way to modify the semiconducting,magnetic,and optical properties of ZnO based materials.By introducing magnetic elements into ZnO semiconductors,it is possible to obtain dilute magnetic semiconductor materials with room-temperature ferromagnetism,which has shown wide potential applications in the new spin-electronic devices.ZnO can also be employed as a catalyst for the degradation of organic pollutants due to its wide band gap,but its application under sunlight has been limted due to the facts that the proportion of ultraviolet light in the natural is very small and the photo-generated electron hole pairs are easy to recombine.To improve its photocatalytic properties,forming nanostructures and element doping are the effective approaches.In this thesis work,the novel microwave-assisted hydrothermal method was used to prepare ZnO based nanostructures with different morphologies.The controllable size and properties were realized by modifying the process parameters.In the mean time,the effects of morphologies and rare earth doping on the photocatalytic properties of ZnO were investigated.The optical and magnetic properties of Co-doped and Cu-doped ZnO nanomaterials were studied.The main research results are summarized as follows.Firstly,ZnO nanorod arrays were synthesized on Si substrates by a combination of physical vapor deposition and microwave hydrothermal method.The effects of reaction conditions including reaction concentration,reaction time and reaction temperature on the structure and luminescence properties were systematically studied.It was found that the size and photoluminescence properties of the nanorods could be well controlled by adjusting the process parameters.Secondly,different processes were used to synthesize ZnO nanostructures with various morphologies including nanoparticles,nanosheets,nanorods and nanoflowers.The effects of reaction conditions including reactant ratios,heating temperatures,heating time and catalysts on morphologies were studied systematically.The Zn O morphology has effect on the photocatalytic degradation of methylene blue and ZnO nanoparticles had the best photocatalytic activity.In addition,the effects of the input of ZnO and the initial concentration of methylene blue on the photocatalytic degradation of methylene blue were also investigated.Thirdly,Co-doped ZnO nanostructures were synthesized by microwave hydrothermal method based on ZnO nanostructures with different morphologies including nanoflowers,nanoparticles and nanorods.The effects of Co doping concentration on the morphologies,structures,optical and magnetic properties were studied.In Co-doped Zn O nanorods,second phase Co3O4 was found,but no second phase was found in Co-doped ZnO nanoflowers and nanoparticles.The Co has an important effect on the morphologies of ZnO nanostrutures.After Co ions were doped into nanoflowes,no flower was observed.The nanoparticles doped with Co ions showed smaller size than that without doping.After doping with Co,the surface of the nanrods became no longer smooth.In addition,with the doping of Co,the photoluminescence intensity of all samples decreased.Pure ZnO nanoflowers and Co-doped ZnO nanoflowers showed room-temperature ferromagnetism.With the increase of doping concentration,the saturation magnetization increased continuously.The pure ZnO nanoparticles showed diamagnetism.When the Co doping concentration increased from 0 to 3%,the samples exhibited ferromagnetic behavior.When the doping concentration increased to 4%,the sample was paramagnetic.The pure ZnO nanorods mainly showed diamagnetism.After Co doping,the samples exhibited ferromagnetic behavior.With the increase of doping concentration,the saturation magnetization increased and then decreased,which was related to the precipitation of second phase Co3O4 with antiferromagnetic properties.In addition,the optical and magnetic properties of Cu-doped ZnO nanoparticles were investigated.Cu doping did not lead to second phase but slightly reduced particle size.The valence state of Cu in Zn O was +2.With the doping of Cu,the photoluminescence intensity of the samples decreased.Cu-doped ZnO nanoparticles showed ferromagnetic behaviors because Cu2+ can produce magnetic moment.The saturation magnetization was enhanced with the increasing doping concentration.In the mean time,there was diamagnetism in the samples which was probably due to the heterogeneity of the doping.Finally,Ce-doped ZnO nanoparticles were synthesized by microwave hydrothermal method.The effects of Ce concentration on morphology,structure,optical and magnetic properties were systematically studied.With the Ce doping,some smaller nanoparticles of the second phase CeO2 were observed in the samples.With the increase of doping concentration,the UV absorption edge of the samples had a red shift,the band gap was decreased and the response to visible light was enhanced.In addition,the effects of Ce doping concentration on the degradation of methylene blue using Ce-doped ZnO nanoparticles under ultraviolet and visible light were investigated.With the increase of Ce doping concentration,the photocatalytic degradation rate of the samples increased and then decreased under UV and visible light.The present work is of great significance to understand the basic characteristics of ZnO-based nanomaterials and to develop their new properties for potential applications.