Preparation Of In₂O₃-based Composite Nanofibers By Electrospinning And Their Gas-sensing Properties

Gas sensor,a device that can effectively detect harmful and toxic gases in air has attracted many researchers’ attention.They have been widely applied in environmental monitoring,industrial production,medical diagnostics and other fields due to their low cost,small bulk and high sensitivity.During the process of fabricating the gas sensor,the choice of the sensing material plays an important role in sensing performance.Among the many of the sensing materials,In₂O₃ is one of the most traditional and representative materials,which has appealed many researchers’interest because of its’ good selectivity and high response.However,the pure material usually faces the difficulty of getting a high sensing performance due to the single component,limited catalytic capacity and weak redox activity.In order to solve this problem,doping is an effective way which can change the components and microstructure of the material,and can also create some new abilities.Considering this,we have prepared pure In₂O₃ nanofibers by electrospinning method at first.Then,we added the p-type semiconductor NiO to the n-type In₂O₃ to obtain NiO-In₂O₃ composite nanofibers.We further investigated their sensing properties to ethanol.It is expected that the introducing of NiO will lead the formation of p-n heterojunction between NiO and In₂O₃,which will enhance the resistance modulation ability of the material to get a better sensing performance.In addition,we have also prepared reduced graphene oxide（rGO）-In₂O₃ composite nanofibers with different amounts of rGO（1.1 wt%,2.2 wt%,3.6 wt%）,which were further used to investigate their sensing performance to NO2 at low temperatures.The details are as follows.Pure In₂O₃ and NiO-In₂O₃ composite nanofibers with different molar ratios（Ni/In=2.5%,5%and 7.5 mol%）were fabricated by the electrospinning and calcination method.The gas sensing properties of ethanol based on the produced nanofibers were investigated.The results revealed that all of the composite nanofibers were demonstrated with higher sensing performance as compared with the pure In₂O₃ nanofibers.Particularly,the sensor constructed by 5 mol%composite nanofibers displayed the highest response,showing an excellent response value of 78 under 100 ppm ethanol at 300 ℃.Additionally,the sensor fabricated by 5 mol%NiO-In₂O₃ composite nanofibers also exhibited good selectivity and stability to ethanol.From our perspective,the improved sensing performance can mainly be attributed to the formation of p-n junctions between NiO and In₂O₃,high capability of absorbed oxygen species and increased BET specific surface areas of the 5 mol%NiO-In₂O₃ composite nanofibers.The resistance measurements between four samples showed that in comparison with the pure In₂O₃,all of the composite NiO-In₂O₃ composite nanofibers had a much bigger resistance.Especially,the resistances in air for the 5 mol%NiO-In₂O₃ sensor and pure In₂O₃ sensor were 21.0 MΩ and 3.8 KΩ,respectively,showing a resistance increase of about 5526 times.This extremely large resistance increase indicated that p-n heterojunctions were successfully formed in the NiO-In₂O₃ composite nanofibers,which is the main reason for the improved sensing properties.In the third part,we first prepared GO according to the modified Hammer’s method,and the rGO was further prepared by thermal treatment of GO under Ar.Pure In₂O₃ and reduced graphene oxide（rGO）-In₂O₃ composite nanofibers were then prepared by a facile electrospinning technique.The XRD results indicated the high purity of the samples.The TEM results suggested that the rGO nanosheets were adhered to the In₂O₃ interface.The XPS results showed that the prepared rGO had the high reduction degree.The BET results implied that the introducing of rGO improved the specific surface areas of material.Low-temperature gas-sensing properties to NO2 of the produced nanofibers were evaluated.The results indicated that in comparison with pure In₂O₃ nanofibers,the rGO-In₂O₃ heterojunction nanofibers displayed much better sensing properties in response,selectivity and detection limit to NO2.Moreover,the weight ratios of rGO to In₂O₃ were used as a parameter to estimate the best gas-sensing properties of.rGO-In₂O₃ nanofibers.Consequently,the heterojunction nanofibers with an optimized amount of rGO（2.2 wt%）exhibited the highest response of 42 to 5 ppm NO2 at the low operating temperature of 50 ℃,which is 4.4 times higher than that of pristine In₂O₃.From our perspective,the enhanced sensing properties of the composite nanofibers can be mainly attributed to the formation of p-n heterojunctions between rGO and In₂O₃,and ultrahigh specific surface area as well as strong gas adsorption capacity of rGO nanosheets.These excellent gas-sensing properties make the rGO-In₂O₃ heterojunction nanofibers attractive to application for low-temperature NO2 gas sensors.