Gas-Sensors Based On ZnO And In₂O₃ Nanofibers

Gas sensors are a type of devices that have the capability of detecting the composition and concentration of target gases.They play an important role in contemporary life and are widely used in environmental safety,medical health,industrial production,and national defense.As a key factor affecting the performance of sensors,the preparation of sensitive materials and the optimization of their performance are the research hotspots.Oxide semiconductors are considered to be the materials of choice for gas sensors due to their high sensing response,long life and low manufacturing cost.However,due to the limitation of semiconductor characteristics and sensing mechanism,oxide semiconductor-based gas sensors generally have some problems,such as high operating temperature,high detection limit,long response and recovery time,and poor selectivity.Therefore,to meet the demanding requirements of practical applications,the sensing performance of oxide semiconductors needs to be further improved.One-dimensional?1D?nanostructures are the preferred structures for sensitive materials because of their high specific surface area,effective carrier transport channels,good stability.Thus,this thesis aims to improve the gas sensing response and lowering work temperature of gas sensors based on ZnO and In₂O₃ nanofibers,and explore the sensing mechanism.The research works mainly focuse on increasing the carrier concentration,the initial resistance and surface active sites,and promoting the adsorption/desorption processes of gases by element doping,constructing core-shell heterostructure,and decorating noble metal,with the electrospinning as the main synthesis method.This thesis involves the following sections:?1?Increasing specific surface area and doping modificationBy an uniaxial electrospinning method,1D ZnO nanofibers and ZnO nanotubes were prepared.The test results show that these two 1D ZnO nanostructures exhibited a high response to ethanol at 275°C and a high response to acetone gas at 300°C,respectively.Furthermore,owning to the increase of specific surface area,the response of ZnO nanotubes is higher than that of ZnO nanofibers.In order to further improve the performance of ZnO nanotube-based sensors,indium-doped ZnO?IZO?nanotubes with various indium contents?In,1-20 mol%?were synthesized via the electrospinning method.The research results revealed that impurity indium has an important influence on the morphologies,electrical properties and sensing properties of ZnO nanotubes.The IZO-0.01 nanotubes?1 mol%In?not only shows a high response?Ra/Rg=81.7?,which is twice of the undoped ZnO nanotubes?40.0?toward 100 ppm ethanol at an operating temperature of 275°C,but also has a short response?2-6 s?and recovery?56-63 s?time.The sensing enhancement of IZO-0.01 is mainly related to the adjustment of carrier concentration caused by donor doping of indium.?2?Constructing core-shell heterostructureWe have prepared 1D ZnO@In₂O₃ core-shell nanofibers by a one-step coaxial electrospinning.The research results show that the 1D ZnO@In₂O₃ core-shell nanofibers exhibit a high response?31.87?to 100 ppm ethanol gas at the optimum operating temperature?225°C?.Compared with ZnO nanofibers and In₂O₃ nanofibers,the response value?31.87?to 100 ppm ethanol gas is increased by 2.3 and 1.5 times,respectively.Except lowering the operating temperature and improving the sensing response,the ZnO@In₂O₃ core-shell nanofibers are also capable of excellent selectivity,repeatability and long-term stability.Unlike the traditional two-step method,the ZnO@In₂O₃ core-shell nanostructures prepared by the coaxial electrospinning can not only adjust the conductive channel width,but also adjust the transmission of carriers between the adjacent nanofibers by the height of the interface barriers.?3?Decorating noble metal?i?In₂O₃ nanofibers modified by Au nanoparticle?IO-Au?have been successfully prepared by a coaxial electrospinning.Compared with In₂O₃ nanofibers,the IO-Au-0.42 sample can not only reduce the optimal operating temperature to ethanol gas from225°C to 175°C,but also increase the sensing response to ethanol by 6 times at lower operating temperatures?175°C?.At the same time,IO-Au-0.42 nanofibers also have lower detection limit?1 ppm?,excellent selectivity,good long-term stability,and shorter response and recovery time?2 and 152 s,to 100 ppm ethanol?at 175°C.Furthermore,the IO-Au-0.42 nanofibers can detect ethanol gas at room temperature.The response to100 ppm ethanol gas is 11.12,much higher than that of the In₂O₃ nanofibers?2.05?.?ii?Considering the influence of the size of noble metal nanoparticles on its catalytic activity,Au NPs?3-8 nm?decorated In₂O₃ nanofibers?Au/IO?have been prepared by a photo-assisted deposition method,to further study the effect of Au nanoparticle content and size on their sensing performance.The test results show that the sample?Au/IO-C40-300?obtained under appropriate conditions can realize dual-function sensing of ethanol gas at a low temperature region?50-150°C?and acetone gas at a high temperature region?above 200°C?.The Au/IO-C40-300 nanofibers have a response of 775.6 to 100 ppm ethanol at 150°C,36.8 times of the highest response of the pure In₂O₃ nanofibers.Moreover,they also exhibit a high response?14.3?to 100ppm ethanol gas at near room temperature?50°C?.For acetone gas,Au/IO-C40-300shows a higher response?588.4/100 ppm?at 200°C,while the response of pure In₂O₃nanofibers is 16.5.In addition,using the same method,we have also prepared Au NPs modified ZnO nanofibers?Au/ZO-C40-300?.The results reveal that the introduction of Au NPs can directly reduce the optimal operating temperature of ZnO nanofibers to ethanol gas from 275 to 125°C and increase the sensing response at low temperature?125°C?to 556.8.Compared with pure ZnO nanofibers,the response value is approximately 24 times higher.According to our studies,the enhanced sensing enhancement of In₂O₃ and ZnO nanofibers decorated by Au NPs is mainly attributed to the formation of Schottky junctions,and the catalysis and spillover effect of Au nanoparticles.