Study On The Modification And Gas Sensing Properties Of Metal Oxide Semiconductor Micro- And Nanostructures

In recent years,gas sensors based on semiconductor metal oxides have become an interested research direction in the field of gas sensors owing to their excellent gas sensing performances,facile fabrication process and low cost.As the sensing materials,semiconductor metal oxide micro-and nanostructures are the base of high-performance gas sensors.In order to make high-quality metal oxide gas sensors for more effective detection of poisonous,harmful,flammable and explosive gases,researchers have devoted to designing and manufacturing sensing materials which have novel structures,simple preparation method,outstanding performance,low cost and friendliness to the environment.In this study,we aim to fabricate gas sensors with excellent performances.The sensing materials based on metal oxides were designed and prepared through structure optimization,doping modification and composites building.The structure characterization and gas sensing properties were carried out.Furthermore,the theory of gas sensitive characteristic and principle of performance enhancements were also studied.Firstly,Zn O hierarchical structures were fabricated by combining electrospinning and hydrothermal methods,and the influence of morphology and structure on the sensing properties was discussed.Secondly,the heterojunction structures of MoO₃/WO₃ composite structure and Pd@Co₃O₄-ZnO nanofibers affecting the gas sensing performance toward VOCs?volatile organic chemicals?were researched.Finally,the impact of rare earth element Er and Ce doping on the gas sensing performances of ZnO nanofibers and In₂O₃ hollow nanospheres was studied.The details are as follows:?1?The influence of hiereachical structures on the gas sensing performances of ZnO nanomaterials was investigated.ZnO nanofibers consisting of nanoparticles were fabricated using electrospinning method,and then ZnO nanowires were in-situ grown on the surface of nanofibers by hydrothermal method to form the hierarchical structures.The ZnO nanowires attach vertically to the sidewall of nanofibers,and present a well symmetrical structure.The results show that the gas sensor based on ZnO hierarchical structures exhibits higher responses to toluene and CO,which are 3.3 and 1.25times of ZnO nanofibers.Moreover,the Zn O hierarchical structures show low limit of detection of 1 ppm towards toluene and short response and recovery time.The superior gas sensing properties of Zn O hierarchical structures can be attributed to the loose structure and high special surfacic area.Besides,ZnO nanowires are generally assembled with high periodicity,thus there is no impediment in gas diffusion towards entire sensing surface,resulting in short response and recovery time.?2?The effects of heterostructures on the gas sensing performances of WO₃and ZnO nanomaterials were investigated.Two kinds of heterojunction of MoO₃/WO₃ composite nanostructures and Pd@Co₃O₄-ZnO nanofibers were synthesized through hydrothermal and electrospinning method.Their gas sensing performances were also studied.1)Different contents of MoO₃/WO₃ composite nanostructures were synthesized by hydrothermal method.It was found that the morphologies of WO₃ nanostructures were significantly influenced from micro-flowers consisting of nanosheets to small size nanoparticles by gradually increasing the amount of Mo.The gas sensing results show that the MoO₃/WO₃ composite nanostructures present lower optimal working temperature.The gas sensor based on WM4 exhibits the best gas sensing properties,whose responses to ethanol and acetone are 2.3 and 1.7 times higher than those of pure WO₃.Furthermore,the response and recovery time and selectivity to ethanol of WM4gas sensor are good.The enhanced sensing properties originate from the synergetic effect and the heterojunction of WO₃ and MoO₃.2)ZnO nanofibers,Co₃O₄-ZnO nanofibers,Pd modified ZnO nanofibers?Pd@ZnO?and Pd modified Co₃O₄-ZnO nanofibers?Pd@Co₃O₄-ZnO?were fabricated by a facile approach of electrospinning for high sensitivity detection of VOCs.The measured results show that the Pd modified Co₃O₄-ZnO nanofibers exhibit the highest response and excellent selectivity to ethanol.The response order of the four gas sensors is:R_Pd@Co3O4-ZnO>R_Pd@ZnO>R_Co3O4-ZnO>R_ZnO.And the response order towards four kinds of VOCs of all sensors is:R_ethanol>R_acetone>R_isopropanol>R_formaldehyde.In addition,the Pd modified Co₃O₄-ZnO nanofibers present quick response and recovery speed,good selectivity and stability.The intensified gas sensing performance is associated to the coefficient of two sensing improvement methods,that is,the electronic sensitization and chemical sensitization of Pd and the p-n junction formed in the interface of the two metal oxides.?2?The impacts of doping with rare earth elements on the gas sensing performances of Zn O and In₂O₃ nanomaterials were investigated.Er-doped ZnO nanofibers and Ce-doped In₂O₃ hollow nanospheres were synthesized through electrospinning and hydrothermal method.Their gas sensing performances were also studied.1)The ZnO nanofibers were doped with rare earth element Er,and the effect of doping content on the gas sensing of ZnO nanofibers were investigated.Four different content Er-doped ZnO nanofibers was fabricated using electrospinning by controling the amount of Er?NO₃?₃ in precursor solution.The diameters of Er-doped ZnO nanofibers decreased with the increase of Er,which is probably due to the increase of the conductivity of the precursor solution caused by the increase of salts.The gas sensing properties were messured on pure and Er-doped ZnO nanofibers and the results clearly show that the sensing performances of Er-doped ZnO nanofibers are much better than those of pure ZnO.Especially,the 1 at%Er-doped ZnO nanofibers present the highest response value towards ethanol,which is about 3.7 times higher than that of pure one.2)Pure and different contents Ce-doped In₂O₃ hollow spheres were synthesized through hydrothermal method.CeO₂ nanoparticles were found attached on the surface of In₂O₃ and In ion might be partial substituted by Ce ion.The formation of heterojunction between In₂O₃ and CeO₂ and the conversion between Ce⁴⁺and Ce³⁺which could enhance the adsorption of oxygen make the Ce-doped In₂O₃ exhibit excellent sensing performance for trace H₂ detection.2 at%Ce-doped In₂O₃ hollow spheres present highest response of 20.66,which is 3 times higher than that of pure In₂O₃.And the limit of detection is very low to 10 ppb.Besides,all the fabricated sensors show fast response and recovery speed because of their special hollow structures.Among them,the response and recovery times of 2 at%Ce-doped In₂O₃ hollow spheres are 1 s and 9 s,respectively.