| The requirements for the quality of production and living environments is increasing with the progress of modern society.Real-time online detection of toxic,flammable or explosive gases in different scenarios is becoming more and more urgent.The development of low-cost and high-performance gas sensors has become a trend.Metal oxide semiconductor gas sensor has been widely studied and applied in recent years because of its low production cost,good stability and high sensitivity.How to further improve the sensitivity and selectivity of the sensor and reduce its power consumption has become the focus of current research.In addition,due to the lack of advanced research methods,there is still no unambiguous theory to explain the sensing mechanism.How to realize the in-situ characterization of the gas sensor in the real-time working condition is of great significance to the in-depth study of the gas sensing mechanism.In this thesis,a variety of gas sensing materials with different nanostructures were prepared by means of morphology control,noble metal modification,metal ion doping and defect control by converting metal-organic frame(MOF)materials.Their gas sensing performance to ethanol and ammonia were studied.In addition,the gas sensing process of the sensor under working conditions was studied by near ambient pressure X-ray photoelectron spectroscopy(NAPXPS),and the related gas sensing mechanisms were explained.The main research contents are as follows:(1)Aiming at the problem that nanoparticles are easy to aggregate,which reduces the performance of gas sensor,a porous nanomaterial via the metal-organic framework(MOF)as a template is proposed for gas sensor.Nano-granular Co3O4 and polyhedral Co3O4(Co3O4 HP)with porous structure were prepared by different calcination methods.Compared with nano-granular Co3O4,porous Co3O4 HP shows higher ethanol response performance.In addition,due to the lack of direct experimental evidence,the gas sensing mechanism of p-type semiconductors for volatile organic compounds(VOCs)is still controversial.In this work,NAPXPS technique was used to study the surface chemistry of Co3O4 HP sensor in ethanol sensing process.The results show that the gas sensing process of the sensor conforms to the interpretation of oxygen adsorption model.Moreover,NAPXPS results also suggest that Co3O4 HP sensor has the resistance to humidity,no obvious carbon deposition,good stability and repeatability.Finally,the relationship between binding energy of oxygen gaseous peak and energy band structure of the sensor is constructed,which further reflects the change of resistance.In this work,an advanced studying approach of gas sensing mechanism is preliminarily established using a simple p-type semiconductor gas sensor as a model.(2)The sensitivity and selectivity of the sensors can usually be improved by the spillover effect of noble metals.However,due to the lack of powerful experimental results,the illustration on the spillover effect driven by metals in gas sensors is still not clear.In this part of work,it is proposed to introduce Pd element into the prepared MOF,and then Pd modified porous Co3O4 nanoparticles were prepared by calcination.SEM and TEM results show that ultra-small Pd nanoparticles were successfully loaded on porous Co3O4.XPS results show that Pd mainly present metallic state.Because of its high specific surface area and catalytic sensitization effect of Pd,the sensor exhibited the advantages of high sensitivity and rapid response in the ethanol detection.NAPXPS was used to study the surface chemistry of Pd/Co3O4 sensor in gas sensing process.The redox process of Pd supports the theory of metal driven spillover effect.(3)Compared with single metal oxide semiconductors,mixed metal oxide semiconductors with two metal elements have been gradually applied to the highperformance gas sensors because of their unique electrical and chemical features.In this work,ZnCo2O4 hollow polyhedrons were prepared by a simple template-assisted method and assembled as an ethanol sensor.SEM and TEM results show that it is a polyhedral superstructure of about 200 nm composed of ultra-small ZnCo2O4 nanoparticles.Benefiting from its special structure and composition,ZnCo2O4 HP sensor shows good sensitivity(14/20 ppm)and high selectivity for ethanol vapor at 200℃.The detection limit is even as low as 1 ppm.The sensing mechanism was further studied by NAPXPS.The results show that oxygen is firstly adsorbed on the defect site of ZnCo2O4 and formed as reactive oxygen.When the sensor is exposed to ethanol vapor,ethanol molecules can react with reactive oxygen species and be decomposed into carboxylates.These new evidences show that the sensing mechanism of ZnCo2O4 HP sensor for ethanol conforms to the oxygen adsorption theory.This work provides a strategy for the rational design of AB2O4 materials and an advanced method for the study of its sensing mechanism.(4)Previous studies have shown that the oxygen vacancy concentration of metal oxides has an important impact on the gas sensing performance.In this part of work,the sensing performance of indium oxide with rich oxygen vacancies to ammonia was explored at a room temperature.Firstly,rod-like indium oxide nanomaterials were successfully prepared by calcining rod-like MOF(In)precursors under the protection of argon.SEM and TEM results show that it is a hollow rod structure composed of small nanoparticles,and these small nanoparticles are attached to the rod-shaped carbon skeleton.XPS results show that the sample has rich oxygen vacancies and N element.The gas sensing performance test results show that N-doped In2O3-x has a short response time(1 s)and recovery time(0.6 s)for NH3 at room temperature.Through heat treatment under H2,the oxygen vacancy concentrations were further increased.And it was found that the sensitivity of the sensor was about 1.8 times higher than that before heat treatment.In addition,the sensor also exhibited the characteristics of good humidity resistance. |
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