Research On Gas Sensing Properties And Mechanism Of Mesoporous C-doped WO₃Fibers Activated By Different Fields

In recent years, the pollution of indoor air becomes much worse. People’s health isseverely damaged by kinds of indoor organic volatile gases and harmful gases, such asformaldehyde and toluene. Therefore, more and more researchers have begun to payattention to the detection and purification of the indoor air. For the detection of the indoorair, recently the gas sensing material, which has been widely used, is metal oxidesemiconductor, such as ZnO, SnO₂, WO₃. In the thesis, mesoporous C-doped WO₃fibers（C-WO₃） is used as the object of study, and investigated from the view of activating fieldand gas sensing mechanism, which are synthesized using the cotton fibers as the templates.Its gas sensing properties towards the indoor air pollutant is systematically studied,including sensitivity, selectivity, stability and the effect of humidity.Through the study about the gas sensing properties of C-WO₃activated by thermalfield, it is found that the response of C-WO₃sensor to toluene （50ppm） and xylene （50ppm） reach91and199, respectively, and is a linear function of the concentration of testgases. The enhancement is attributed to its large surface area, special structure ofmesopores, and narrow band gap. By comparing the response of C-doped WO₃sensor toother indoor air pollutant （formaldehyde, ethanol, methane and benzene）, distinctiveselectivity to toluene and xylene is found. In addition, C-doped WO₃exhibits relativehygro-stability, differing from traditional gas sensor material. Furthemore, the selectivityto toluene and xylene has been explained from the view of thermodynamics and dynamics,which is resulted from the adsorption characteristic of test gas and the enthalpy change ofthe dehydrogenation.Activated by light field （visible light）, the gas sensing characteristic of the C-WO₃atroom temperature has been investigated, and the responses to formaldehyde, toluene,actone and ethanol have been measured. In the thesis, the response is redefined, which isdifferent from the traditional definition and is related to the gas sensing reactions. It isfound that the response of C-WO₃activated by white light is better blue light, and is alinear function of the light intensity, and high humidity significantly decreases the gas sensing properties of C-WO₃under visible light. In the selectivity measurement, C-WO₃exhibits high response to formaldehyde, toluene and actone, but little response to ethanol.Moreover, the gas sensing mechanism activated by visible light is explained: under thelight irradiation, the photogenerated oxygen ions [O₂^-（hv）] are produced on the gas sensingmaterial, which oxidize the organic test gases and release the electrons, resulting in theresponse. Finally, using the C-WO₃as the photocatalyst, the photodegradation offormaldehyde is carried out and the results confirm the explanation of gas sensingmechanism. Basing on this research, the experiments results indicate that WO₃is good gassensing material activated by visible light, and thermal field can be completely substitutedwith light field, which solves the problems resulted from the thermal field.