Research On The Field Emission And Photocatalysis Of Low Dimensional ZnO And ZnO Composite

Zinc oxide?ZnO?with high n type doping concentration and wide application is always one of the most influential functional materials of present age.Particularly one-dimensional?1D?ZnO,which are equally important as carbon nanotubes and silicon nanowires?NWs?,is one of the three hottest materials for the scientific research.Due to the excellent electronics property,ZnO is also considered to be the most potential material for field emission?FE?and photocatalysis.However,there is still a big challenge due to the high work function and wide band gap of ZnO.In this dissertation,on the FE performance of ZnO,based on low dimensional n-type ZnO,1D radial core-shell heterojunctions were synthesized and the influential factors of turn-on field were investigated,and zero-dimensional?0D?,1D and two-dimensional?2D?hybrid structure with a low turn-on field was designed and synthesized.On the photocatalysis performance of ZnO,sandwich structure and porous 1D structure were synthesized to increase the absorption of sunlight,and the method of using indirect band gap semiconductor materials to reduce the recombination rate of photogenerated electrons and holes was discussed.Some significant results are listed as follows:Excessive amount of Ag₂S forms clusters on the 1D NW arrays?NWAs?and covers the NWs tips,therefore,as the content of Ag₂S increases,the FE properties of ZnO-Ag₂S core-shell sturcture first improve and then deteriorate.The ZnO NWAs with the pyramid-like ZnS shell have more nanotips and a much lower turn-on field?3.0 V/?m?than the ZnO NWAs with smooth surface of ZnS shell.By comparing the FE properties of ZnO-Ag₂S,ZnO-ZnS and ZnO-CdS,the conclusion is drawn that the number of nanotips?emission site?is the main influential factor of turn-on field.It is also found that 1D ZnO decorated with low-cost GO nanosheets inserted with trace amount of noble metal?Ag/GO/ZnO composite emitter?has exhibited efficient FE properties with a low turn-on field of 1.4 V/?m and a high field enhancement factor of 7018.The excellent FE properties of Ag/GO/ZnO composite can be attributed to the tunneling effect of GO which increases current density and the fact that GO nanosheets sink in the space between NWs without affecting the advantage of 1D nanostructure.In this dissertation,tow methods were used to improve the absorption of sunlight.The first is to reduce reflectivity.ZnO/Ag/ZnO?dielectric-metal-dielectric?sandwich films with larger roughness were synthesized and confirmed in effectively decreasing the reflectivity in the near-infrared?NIR?region by 30%⁵0%.The second is to increase absorption.Porous ZnO NWAs which are perpendicular to the substrates of Si or conductive glass and evenly distributed were synthesized using hydrothermal process approaches.The surface of ZnO NWs was increased,the absorption of ZnO was clearly enhanced in the visible light region,and the optical band-gap of ZnO became narrower,which result in a better photocatalytic activity.A new method of using indirect band gap semiconductor materials in which the electron transition needs the assistance of phonons to reduce the recombination rate of photogenerated electrons and holes in photocatalysis is also presented.The photocatalytic activity of the synthesized ZnO-Ag₂S-Si increased0.69 times more than ZnO-Ag₂S.This method has the merit that there is low requirement whether bands of materials can match a stair-step band structure.