The Study Of Ammonia Gas Sensing Properties Of WO₃ Modified By Surface Metal Elements

As an important component of human exhaled gases,ammonia is an important physiological marker of liver dysfunction,kidney disease,Helicobacter pylori infection,and oral diseases.Especially in kidney disease,the concentration of NH₃ in exhaled gases can reach several ppm.Therefore,a rapid and accurate detection of NH₃ has important significance in human health,and it can provide a new and non-invasive diagnosis method for kidney disease.The preparation process of tungsten trioxide gas sensing material is simple,and it has a good gas responsive performance to various gases.At present,it has been used for gas sensing of NH₃.In this thesis,WO₃ nanoparticles were prepared via an acidification method.Metallic elements?Ru,Pd and Cr?were loaded on the surface of WO₃ nanoparticles by an impregnation method.Through XRD,SEM,TEM,XPS and TPR characterization methods,WO₃ nanoparticles and metal elements modified WO₃ nanoparticles were characterized and analyzed in detail.The results show that the WO₃ nanoparticles have monoclinic phase,and the metal loading does not change the surface morphology and pore structure of the WO₃nanoparticles.The dynamic gas distribution test system was used to test the influence of different metal loadings and different working temperatures on the gas sensing performance of NH₃.The results show that the metal loading significantly improves the gas sensing properties of WO₃ to NH₃,and the gas sensing response of WO₃ to NH₃ first increases and then decreases with the increase of the loading amount.When the metal loading is 1 wt%,the sensor response to NH₃ is best,and can respond to NH₃ as low as 1 ppm.At the same time,the working temperature of the sensor has great influence on the gas sensing response to NH₃.When the operating temperature increases,the gas sensing response of the sensor to NH₃ will be improved due to the enhancement of the oxygen adsorption capacity of the material surface.Ru-WO₃ has the maximum gas response to NH₃ at 350oC.Pd-WO₃ and Cr-WO₃ have the maximum gas response to NH₃ at 400oC.Through the oxygen partial pressure test experiment,the linear relationship between the sensor resistance and oxygen partial pressure was analyzed,the role of oxygen in the NH₃ response process was explored,and the gas sensing response mechanism of NH₃ was explained.The experimental results show that oxygen is adsorbed on the surface of the sensitive materials in the form of O^-.In addition,the presence of metal loading changes the gas sensing response mechanism of WO₃ to NH₃,that is,WO₃ modified without metal loading is prone to produce NO and N₂O in the gas sensing response of NH₃,and NO is easily oxidized in the presence of oxygen.As NO₂ gets electrons adsorbed on the surface of the material at the same time,the gas sensing performance of WO₃ to NH₃ is poor.However,the existence of metal loading will promote WO₃ to produce N₂ in the gas sensing process of NH₃,which can effectively improve the gas sensing performance of WO₃ to NH₃.In this thesis,the investigation of gas sensing properties and mechanism of the metal modified WO₃ nanoparticles to NH₃ have laid the foundation for the development of high performance NH₃ gas sensor.In order to develop an NH₃ sensor that is closer to the actual application of breath analysis,further research should be conducted on improving the sensor's immunity to humidity and the selectivity of NH₃.