Preparation And Gas Sensing Properties Of In₂O₃-based Nanofibers

Semiconductor gas sensor has important applications in industrial production,environmental monitoring,medical health and other fields.Metal oxide semiconductor usually has good gas sensing properties,how to further improve its gas sensing performance is one of the hot topic in the research field of gas sensing materials.Many strategies have been proposed to improve the gas sensing properties of gas sensing materials,for example,preparing nano-materials,element doping,surface modification of noble metal or metal oxides.Metal oxide nanofibers prepared by electrospinning have attracted much attentions due to their unique characteristics,such as high specific surface area,extremely large ratio of length to diameter,and three-dimensional porous structure,all of which are favorable for electron transfer and transportation in the process of gas response.In this thesis,n-ZnO/n-In₂O₃ and p-CuO/n-In₂O₃ nano-heterojunction were constructed by solvothermal method using one-dimensional electrospun In₂O₃ nanofibers as templates,and the impact of heterogeneous structure on the gas sensing properties were also explored.The main research contents are as follows:?1?In₂O₃ nanofibers were prepared by electrospinning technique,and n-ZnO/n-In₂O₃ heterojunction nanofibers were obtained by solvothermal method.The samples were characterized by simultaneous thermal analyzer?TGA/DSC?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?,Raman spectroscopy and so on.Pure In₂O₃ nanofibers prepared by electrospinning have relatively smooth surface,uniform fiber diameter which is of about 100 nm;after solvothermal reactions,ZnO microspheres with a diameter of about500 nm were embedded in In₂O₃ nanofibers,and no other impurities were generated.The gas sensing properties of the samples were studied by thermal type gas sensor.Side heated gas sensors were prepared based on In₂O₃ nanofibers and n-ZnO/n-In₂O₃ heterojunction nanofibers.It is found that sensitivity of n-ZnO/n-In₂O₃ heterojunction nanofibers increases more than 5 times compared with In₂O₃ nanofibers.The sensitivity is 9.2 with a working temperature of 200 oC when the formaldehyde concentration is 100 ppm.The optimum temperature of n-ZnO/n-In₂O₃ heterojunction nanofibers decreases of 50 oC,and the response time decreases from 145 s to 94 s while recovery time changes very little,and the selectivity to formaldehyde is very good.The improvement of the gas sensing properties may be due to the the synergistic effect of large specific surface area,the confined electron transportion of one-dimensional nanostructures,and the modulation on the depletion layer.?2?Furthermore,traditional p type metal oxide CuO were loaded on In₂O₃ nanofibersand p-CuO/n-In₂O₃ heterojunction nanofibers were obtained by solvothermal method using electrospun In₂O₃ nanofibers as templates.The surface morphologies and microstructures of the composite nanofibers were characterized by field emission scanning electron microscopy,transmission electron microscopy,X-ray diffraction spectrometer and X-ray photoelectron spectroscopy.It is found that the In₂O₃ nanofibers were uniformly decorated with CuO nanoparticles,and the surface densities of CuO nanoparticles with diameters about 200 nm could be controlled by varying the precursor concentration of cupric acetate.Side-heated gas sensors were prepared based on these composite nanofiers.The p-CuO/n-In₂O₃ heterojunction nanofibers exhibit high gas responses and low operating temperature for detecting H2 S gas compared with pure In₂O₃ nanofibers.With increasing the salt concentration in the reaction,the sensitivity to H2 S of the obtained samples increases first and then decreases.The sensitivity of sample S2 could reach 183 at working temperature of 100 degrees and the concentration of H2 S about 100 ppm,which is about 120 times of that for pure In₂O₃ nanofibers.Moreover,the device exhibits P type behavior below 10 ppm,while n type behavior above 10 ppm.The characteristics of p-n conversion might be used to monitor the leakage of H2 S,but the mechanism remains to be further studied.