Study On Construction Of High-performance In₂O₃ Complex Materials For NO₂ Gas Sensing Applications

The preparation of metal oxide semiconductor with high performance for gas sensing has been a research hotspot in the fields of environmental detection and gas sensors.The urbanization acceleration has led to a sharp increase in NO₂ in the atmosphere.The NO₂ gas sensor based on metal oxide is one of the effective ways to achieve low-concentration and high-response detection.However,there are still some problems that need to be solved urgently in the reported works,such as the high operation temperature,the unclear mechanism of the humidity effect on gas sensing properties,the poor humidity resistance,and the unclear gas sensing mechanism.In order to solve these problems,this paper is focused on In₂O₃,and systematically investigates the preparation,sensing performance and sensing mechanism of NO₂ gas sensors based on In₂O₃ by employing different kinds of synthesis strategies and characterization testing methods.The research results are listed below:1.This part mainly contains the preparation and room-temperature NO₂ sensing performance of Pt-In₂O₃ nanofibers.By adding foaming agent in spinning solution,both dense and porous In₂O₃ structures were prepared by electrospinning.The specific surface areas of these two samples are 19.9 m²/g and 86.9 m²/g,respectively.In addition,Pt nanoparticles were respectively loaded on these two samples by reduction method,and Pt/In₂O₃ samples with agglomeration and good dispersion of Pt particles were obtained.The NO₂ gas sensing results suggested that Pt loading could remarkably decrease the operation temperature of sensing materials.When the operation temperatures were the same,the response of sample with porous structure was higher than that of sample with dense structure.Furthermore,the loading of Pt nanoparticles can enhance the sensor response of the samples,and the optimized sample can be used to detect 10 ppb NO₂ with a response of 2.8.In addition,by investigating the structure-function relationships of the samples,a mechanism for one-dimensional porous sensing materials was proposed.The reasons for enhancement of sensing performance can be concluded as follows:（1）the mesoporous structure can promote the gas diffusion and accessibility in the materials and increase the contact area between gases and sensing materials;（2）the loading of highly dispersed Pt nanoparticles can facilitate the NO₂ adsorption process by chemical sensitization effect and decrease the operation temperature.This work provides a feasible experimental scheme for decreasing the operation temperature for NO₂ gas sensor based on In₂O₃.2.In this part,In₂O₃ columnar structures were prepared,and their near-room temperature NO2 sensing properties were studied.By using metal organic framework MIL-68 as template,porous In₂O₃ columnar structures were prepared.Furthermore,Pt/In₂O₃ samples with different heterojunction contact states were synthesized by loading Pt before or after the calcination process.The NO₂ sensing results indicated that only when the compact heterojunction was formed,the operation temperature of gas sensors decreased（from 100°C to 40°C）.The introduction of humidity can promote the response and recovery process of gas sensors,but the sensor response will decrease.Compared with pure In₂O₃ samples,the sensor response of Pt-loaded samples was less influenced by humidity.This is because Pt nanoparticles can facilitate the reaction between water molecules and adsorbed oxygen species and provide more adsorption sites for NO₂.This could reduce the negative effect on sensing performance to some extent.In addition,a gas mixing and testing system was built up for accurately and flexibly controlling the testing atmosphere.This work also providing some experimental data for understanding the humidity effect on gas sensing properties of metal oxides.3.By combining eletrospinning technology and solvothermal method,In₂O₃/ZIF-8 with different ZIF-8 loadings were prepared and used as gas sensing materials for NO₂ detection.ZIF-8 particles were formed by conversion of ZnO in the complex fiber matrix.It suggested that the introduction of ZIF-8 remarkably increased the specific area and pore volume.Furthermore,due to the conversion from ZnO to ZIF-8,many pores were formed inside the matrix,and this can increase the contact area for interactions between gas molecules and the sensing materials.In addition,the introduction of ZIF-8 also significantly decreased the humidity effect on gas sensing performance.This work provided a flexible strategy for preparing metal organic framework/metal oxide one-dimensional composite structures by simply adjusting the kinds of inorganic salts in the spinning solution.This work also provided a design idea for developing NO₂ gas sensors based on metal oxides with humidity-resistance property.4.On the basis of the research results of Pt-loaded In₂O₃-based NO₂ gas sensors,the Fermi level pinning mechanism in noble metal-loaded metal oxides was studied by cooperating with Dr.Barsan’s group（Tubingen University,Germany）,and combining the research foundation and advantages of German in-situ testing technology and WO₃-based gas sensors.The results of Pt/In₂O₃ gas sensors showed that the loading of noble metal could improve the gas sensor response of samples at lower operating temperature by chemical sensitization effect.However,at higher operating temperatures（e.g.300°C）,noble metal-loaded samples exhibited lower sensor response.This can be attributed to the fact that the sensing properties of gas-sensing materials are mainly controlled by Fermi pinning mechanism,i.e.the electronic coupling between oxidized PtO_x and metal oxide reduces the sensor response.Combined with the research results of Pt-loaded In₂O₃,this phenomenon is more obvious for oxidizing gas,such as NO₂,which results in the decrease of sensor response for noble metal-loaded metal oxide at higher operating temperature.In addition,the dispersion of noble metal nanoparticles in the matrix materials also affects the gas sensing performance.The results of this work and the previous work of Pt-loaded In₂O₃ are mutually validated,which provides a theoretical supplement for the gas sensing mechanism.