Study On Nano-WO₃Gas Sensing Materials And NO₂Gas Sensor

NO₂gas, a kind of toxic and harmful gas, is harmful to humanrespiratory system and is the main matter of forming acid rain, too. It is veryimportant for the protection of both environment and human health tomonitor and detect the NO₂gas. Therefore, researches on NO₂gas sensorsare not only of academic research value but also of practical applicationsignificance. As a kind of semiconductor functioning material developed fornearly10years, WO₃is found to be sensitive to NO₂gas and has wider andwider application in fields of gas sensors and photochromism.In this study, three different methods of preparing nano-WO₃gassensing materials were carried out, respectively. The microstructure, surfacemorphology, and material composition of the nano-WO₃materials wererespectively analyzed by XRD, SEM and TEM. Then, the NO₂gas sensorswere developed using these nano-WO₃materials, and their sensitiveperformances were detected. Moreover, two new potential types ofsemiconductor gas sensor were proposed and designed.Firstly, nano-WO₃gas sensing material has been successfully preparedby thermal decomposition of APT and doped silica by sol–gel. The grainsize of the WO₃material is8.5nm by XRD analysis, and its mean dispersedparticle size is only3nm-4nm by TEM observation. It is a breakthrough inthe technology of preparing nano-WO₃materials, which is the first time inthe country reports to find less than10nm for WO₃particle size.Furthermore, sensitivity of the NO₂gas sensor made by this nano-WO₃material is up to191for1×10-6NO₂, which is believed to be the highestsensitivity to the NO₂about WO₃materials made by solid phase reactionmethod in China so far.Secondly, the mean dispersed particle size of WO₃materials made bysol-gel process is17nm. It shows a special layered structure. The sensitivityto100×10-9NO₂gas is60, therefore, it can be realized to detect ppb level with low density NO₂gas.Thirdly, a new method of making WO₃is proposed, using methods ofself-burning of electrified tungsten. By this method, unique morphology ofnano-WO₃can be obtained， and then, a gas sensor is developed. Theequipment of this method is very simple, and further, some specialnano-WO₃materials can be obtained by controlling reaction atmosphere oradjusting voltage to control the growing process of materials afterimprovement.On the basis of the thermo-electromotive effect and carrier theory, athermo-electromotive force of semi-conductor is deduced, and then the gassensing mechanism of thermo-electromotive force is proposed. Based onabove theory, a new thermo-electromotive force gas sensor is designed andits gas sensing function is successfully realized. It is favorable to improvethe stability of gas sensors by the thermo-electromotive force functioningmechanism.Moreover, on the basis of the Hall-effect and carrier theory, a Hallpotential of semi-conductor is deduced, and then the gas sensing mechanismof Hall potential is also proposed. Based on above theory, another new Hallpotential gas sensor is designed and its gas sensing function is successfullyrealized, too.The relationship between sensitivity and operating temperature of gassensing components, and the effect of WO₃material preparation by silicondioxide doped are explained by physics chemistry adsorption theory, thesemiconductor band theory and the absorption effect theory. Finally, a newhypothesis of tungsten trioxide on the nitrogen dioxide gas sensingmechanism is proposed according to semiconductor defect theory based onthe original traditional tungsten trioxide theory of sensing mechanisms.