Study On The Sensitization Effects Of Noble Metals On ZnO Based Gas Sensors

Gas is the companion of coal mining,its main component is the explosive gas methane(CH₄),and asphyxiating gas carbon monoxide(CO)is the trace component.In the process of coal mining,CH₄explosion and CO poisoning are two common gas disasters,which severely restrict the safety production of coal mines.Therefore,it is particularly important to accurately identify and detect the concentration of gas in the mines to prevent the occurrence of gas accident.The semiconductor gas sensor has become a research hotspot in the sensor field in recent years with its advantages of fast response rate,high sensitivity and convenient use.Zinc oxide(ZnO),a typical wide band gap n-type semiconducting metal oxide,possesses superior electrical properties and excellent chemical stability,and thus it is considered as the most promising candidate for metal oxide semiconductor gas sensors.However,the pure ZnO suffer from the drawbacks of low response,high working temperature and poor selectivity.In order to improve the gas sensing properties of ZnO,we use the noble metal to surface modified the ZnO and prepare the corresponding gas sensors in this paper.This paper mainly includes the following three parts:(1)Palladium(Pd)modified ZnO nanosheets have been successfully prepared via a facile hydrothermal route.Meanwhile,a contrastive study based on the CH₄gas sensing properties of bare ZnO and Pd/ZnO was carried out.According to the gas sensing experimental results,the loading of Pd remarkably upgraded the sensing capability toward CH₄with respect to response and selectivity.Moreover,the optimal working temperature of ZnO was reduced from 240^oC to 200^oC after loading Pd.Among these sensors of Pd modified ZnO,1.0 at.%Pd/ZnO exhibited a high response value of 19.20 toward 5000 ppm CH₄and a low detection limit of 1.10 ppm,which was better than the reported CH₄gas sensing performances of ZnO based gas sensors.The improvement of CH₄sensing performance by Pd nanoparticles could be attributed to the synergy effect with the spill-over effect and the electronic coupling of metal oxide and noble metal.(2)Platinum(Pt)modified ZnO nanosheets have been successfully prepared by a simple one-pot hydrothermal method,and the CO gas sensing properties were studied.The experimental results showed that the ZnO nanosheets loaded with appropriate concentration of Pt have more excellent CO gas sensing performances,especially the0.5 at.%Pt/ZnO demonstrated the best sensing ability,such as an obvious response value of 3.57 toward 50 ppm CO and fast response/recovery time(6/19 s).Besides,the detection limit was as low as 100 ppb and the optimal working temperature was reduced from 210^oC to 180^oC.Therefore,the 0.50 at.%Pt/ZnO is an efficient sensor material for rapidly detecting low-concentration CO.The enhanced CO sensing performance by Pt nanoparticles could be attributed to the combination of chemical sensitization and electronic sensitization.(3)Silver(Ag)modified ZnO flower-like microspheres have been prepared by hydrothermal method and impregnation method,and their gas sensing performances were investigated systematically.The gas sensing measurement results demonstrated that 1.5 at.%Ag/ZnO gas sensor exhibited temperature-modulated dual selectivity to CO at 130 ^oC and CH₄at 200 ^oC.Moreover,the sensor possessed excellent repeatability and long-term stability,which provides the possibility for the dual detection of CO and CH₄.The dual selectivity detecting for CH₄and CO of 1.5 at.%Ag/ZnO through control the working temperature can be attributed to the catalytic discrepancy between sensing materials and target gases.The gas sensing mechanism of Ag/ZnO microspheres were discussed based on the unique hierarchical porous structure,the chemical sensitization of Ag,the formation of Schottky barrier between ZnO and Ag as well as the p-n heterojunction between ZnO and Ag₂O.