Preparation Of Side-by-side Heterojunction Nanofibers Of Metal Oxide Semiconductor And Their Gas Sensing Performances

Metal oxide semiconductor(MOS)based gas sensors,due to their unique advantages such as long working life,simple structure,easiness to integration,and etc,have been widely applied in many fields.In principle,MOS-based gas sensors use the changes in the electrical conductivity of the MOS film caused by the reversible redox reaction between it and the target gas,to detect the gas concentration.However,due to the limitation of MOS'own characteristics,most of MOS based gas sensors still face many deficiencies,such as such as high operating temperature,long response/recovery time,poor selectivity,and so on.To address these problems,heterojunctional nanostructures,due to their unique electrical,physical and chemical properties,are often used to enhance the gas sensing performances of MOS based gas sensors.However,during the construction of heterojunctional nanostructures,the core-shell structure has been first and most frequently applied to the design of sensor electrodes.In a core-shell structure,the core MOS materail due to being completely covered by the shell one,can hardly play its synergistic effect on gas response,thereby limiting the performance improvement of the overall design.In view of this point,in this dissertation,a side-by-side design was used to construct heterojunctional nanomaterials of MOS,to simultaneously exposure of both materials in the heterojunction to the targert gas,and accordingly enhance their synergistic effect on the gas response.Thus,the main contents of this dissertation were summarized as follows:(1)Construction of n-n side-by-side heterojunctional nanofibers and their gas sensing performances.Firstly,two types of n-n side-by-side heterojunctional nanofibers(SHNFs)with different composition ratios were prepared using self-developed“V-channel”electrospinning technique,i.e.In₂O₃/WO₃(IWO)and In₂O₃/Zn O(IZO)SHNFs.This work has systematically studied the effect of composition ratios and composite ratios of heterojunctions on the sensors'response,response/recovery rate and sensing selectivity to ethanol gas.As for IWO SHNFs,at composite ratio(W/(In+W))of 12 mol%,the IWO SHNFs show a response of 25.59 to 100 ppm ethanol at 240?,which is 2 and 4times those of the pure In₂O₃and WO₃NFs,respectively.Moreover,the response/recovery times of IWO SHNFs are 1 s/66 s,respectively,both of which are below those of the In₂O₃NFs.Besides,the IWO SHNFs also show an highly improved response selectivity to ethanol.As for IZO SHNFs,at composite ratio(Zn/(In+Zn))of 55 mol%,the IZO SHNFs show a response of 82.78 to 100 ppm ethanol at 260?,which is 6 and 11.5 times those of the pure In₂O₃and Zn O NFs,respectively.Moreover,this response value is also 1.5 times of that of IZO-55-C NFs.Furthermore,the response time of IZO SHNFs is 3 s,which is a bit longer than that of the pure In₂O₃NFs;while the recovery time of IZO SHNFs is only 27 s,which is much shorter than that of the pure In₂O₃NFs.The increase in resoponse time could be due to the much higher response to ethanol.Thus,conbining with the gas sensing behaviors above,it was found that the higher response,faster response/recovery rate and improved sensing selectivity of IWO and IZO SHNFs could be mainly due to the designed side-by-side heterojunctional structure.At the same time,the snaller grain size and higher concertraion of oxygen vacancies can also contribute to the enhancement in gas sensing response.(2)Construction of side-by-side p-n heterojunctional nanofibers and their gas sensing performances.Compared to n-n junction,the p-n junction will form a built-in electric field in space-charged region with a direction from n to p.Then,through a reasonable design,this built-in electric field will promote the formation of the depletion layer of gas-sensitive material and accelerate the transport of gas-sensitive electrons,and then improve the gas response value and response rate of the materials.Here,we have also designed two types of p-n SHNFs with different composition ratios using self-developed“V-channel”electrospinning technique,i.e.In₂O₃/Ni O(INO)and In₂O_3/Co₃O₄(IZO)SHNFs.As for INO SHNFs,at composite ratio(Ni/(In+Ni))of 10 mol%,the INO SHNFs show a response of 210.44 to 100 ppm ethanol at 260?,which is 32 and 23.9 times those of the pure In₂O₃and INO-10-C NFs,respectively.This response value is far higher than most of the reported ones.Moreover,the response time of INO SHNFs is4 s,which is a bit longer than that of the pure In₂O₃NFs and could be due to the much higher response to ethanol,while the recovery time of INO SHNFs is only 36 s,which is much shorter than that of the pure In₂O₃NFs(94 s).Besides,the INO SHNFs also show a much improved gas selectivity.As for ICO SHNFs,at composite ratio(Ni/(In+Ni))of 11 mol%,the ICO SHNFs show a response of 584.63 to 100 ppm ethanol at 260?,which is 61 and 148 times that of the pure In₂O₃and ICO-11-C NFs.Moreover,the response time of ICO SHNFs reaches 17 s,which is much longer than that of the pure In₂O₃NFs and also could be also due to the much higher response to ethanol,while the recovery time of ICO SHNFs is only 15 s,which is much shorter than that of the pure In₂O₃NFs.Thus,conbining with the gas sensing behaviors above,it was found that the p-n heterojunction significantly improves the gas sensitivity response and response/recovery speed of MOS materials,especially the recovery speed,which is mainly attributed to the acceleration effect form their built-in electric field.Moreover,the high specific surface area of the nanofibers,finer grains,the catalytic properties from the p-type MOS and the side-by-side structure are also responsible for the enhanced sensing properties of the materials.