Structure Design And Gas-sensitive Properties Of Metal Oxide Semiconductor Based Heterojunctional Nanofibers

Gas sensors play an increasingly important role in many fields.Especially,the gas sensors based on metal oxide semiconductor（MOS）materials have been widely studied because of their unique advantages of high response,low cost,simple structure and easy integration.The working principle of these sensors is mainly based on the reversible resistance change of the electrode material caused by the adsorption/desorption of the target gas on the surface.However,the sensors based on single MOS material often suffer from their high operating temperature,poor selectivity,slow response/recovery and high limit of detection（LOD）,which further make them difficultly meet the needs of practical work.Thus,heterojunctions were often introduced in to MOS gas sensors,to synergistically enhance their gas sensing performance of the sensor via integrating the unique sensing properties of two components.So far,heterojunctions with various configurations（such as surface-modified,core-shell,and laminated types）have been successfully constructed to improve the performances of gas sensors.However,the discontinuity of the surface-modification-type heterojunctions often blocks to the lateral transmission of gas-sensitive electrons;the core-shell type heterojunctions have a problem that the core material is covered by the shell one and cannot be completely depleted;and the laminated type heterojunction often show a great decrease in specific surface area and porosity,which further hinders the adsorption and reaction of gas molecules.Therefore,in view of the drawbacks of the above configuration of heterojunctions,this dissertation presented the designs of heterojunctions with sandwich and side-by-side configurations,the bicomponent of heterojunction can effectively deplete and directly participate in the gas-sensing reaction,effectively promote the synergistic enhancement effect and alleviate the influence of inappropriate heterostructures on the reaction process and further improve the total performances of the sensors.The main research contents of this dissertation were summarized as following:（1）Design and gas sensing properties of sandwich-configuration heterojunction.First,sandwich-structured In₂O₃/In₂S₃/In₂O₃（IOSO）and In₂S₃/In₂O₃/In₂S₃（ISOS）hollow nanofibers（HNFs）were prepared by electrospinning combined with one-step vulcanization/oxidation methods.Via tuning the sulfide/oxide ratio,the performances and mechanisms of the sensors were investigated and analyzed in details.As for IOSO HNFs,the sensors assembled using the samples oxidized at 495°C for 2 h（IOSO-2 HNFs）exhibited the best gas sensing performance.Towards 100 ppm ethanol at 220°C,the IOSO-2 HNF sensors showed a response value of 22.0,which is about 3 times that of the pure In₂S₃ ones.And the recovery time of IOSO-2 HNF sensor is only 24 s,which is much shorter than that of the In₂S₃ ones.Furthermore,the sensor still output a response value of 4.4 towards 10 ppm ethanol,showing a large detectable concentration range.In addition,the sensor based on IOSO HNFs also showed a good selectivity towards ethanol.As for ISOS HNFs,the sensors assembled using the sample oxidized at 500°C for40 min（ISOS-40 HNFs）exhibited the best total performance.Towards 100 ppm ethanol at 200°C,the ISOS-40 HNF sensors showed a response value of 23.0,which is respectively about 1.8 and 3.3 times those of the pure In₂O₃ and In₂S₃ ones.More interestingly,the ISOS-40 HNF sensor can complete response and recovery within 1 s/25 s,showing much faster response and recovery speed than those of the In₂O₃,In₂S₃and In₂S₃/In₂O₃ core-shell solid nanofiber（ISO-40）ones.In addition,the sensor has a large detectable concentration range and a good selectivity towards ethanol.In summary,the IOSO and ISOS HNF sensors have high response value,fast recovery speed and good selectivity.These improvements in the sensing performances are mainly attributed to the design of sandwich-configuration heterojunction.At the same time,the increased content of defect sites and the formation of solid solution also contribute to the enhancement in gas sensing performances.（2）Design and gas sensing properties of side-by-side configuration heterojunction.Although the sensor based on sandwich-configuration heterojunction exhibited good gas sensing performance,but the interlayer material is still difficult to directly contact and react with O₂ and target gas.Thus,in this chapter,In₂S₃/In₂O₃（ISO）and Sn O₂/Ti O₂（STO）side-by-side bicomponent heterojunctional nanofibers（SBHNFs）were prepared by self-made“single source double field”electrospinning technique.The gas sensing properties of the sensors towards ethanol and H₂ were systematically studied,the gas sensing mechanism of this configuration was analyzed in details.As for ISO SBHNFs,at the optimal operating temperature of 220°C,their response towards 100 ppm ethanol reached 41.06,which is much higher than those of the pure In₂O₃ and In₂S₃.Even towards 10 ppm ethanol,their response still kept as high as 7.3.Moreover,benefitting from the rapid charge transfer through the heterojunction,the ISO SBHNF sensors can complete the response and recovery processes within 1 s and 13 s,and also showed an greatly improved selectivity towards ethanol.On the other hand,the ISO SBHNF sensors also show a good response towards H₂,outputting a response of 5.85 towards 10 ppm H₂ at 260°C,which is about 2.1 and 3.4 times those of the pure In₂O₃ and In₂S₃ ones.Also,the sensors still has a significant response towards 500 ppb H₂ with a response value of 1.45.More interestingly,the response/recovery times of the sensor are within 1 s/3 s,showing extremely fast response and recovery speeds.As for STO SBHNFs,at the composite ratio（Ti/（Ti+Sn））of 12 mol%（i.e.STO-12 SBHNFs）,the assembled sensors showed a response of 40.7 towards 10 ppm H₂at280°C,which is respectively about 2.8 and 31.3 times those of the pure Sn O₂ and Ti O₂ones.And the STO-12 SBHNF sensor also exhibited a response of 23.3 towards 100ppm ethanol at 180°C,with an excellent linear concentration-response linear relationship.Furthermore,this sensor can complete response and recovery processes within 1 s and 25 s towards 10 ppm H₂ at 280°C,showing a much faster recovery than that of the pure MOS ones.More imspiringly,the sensors still exhibited a response of6.05 towards 200 ppb of H₂,indicating a much low LOD.In summary,the ISO and STO SBHNF sensors exhibited an excellent total performance,including high response,fast response/recovery speed,excellent selectivity and low LOD.The improvement in these performances is mainly attributed to the design of the side-by-side configutation to promote the synergistic effect of the heterojunction.In addition,the formation of long-range solid solution provides a unique channel for charge transfer,and the increase in the oxygen vacancy content and the decrease in the grain size also contribute to the improvement of sensing performances.