Study On Room–Temperature Gas Sensing Properties And Mechanisms Of Catalysts–SnO₂ Composite Nanoceramics

As an important gas detection device,gas sensors are widely used in various fields of industrial production and daily life.For example,hydrogen energy is increasingly used in fuel cells and hydrogen–powered vehicles.When hydrogen leakage occurs,the lack of effective detection methods for hydrogen can easily lead to fire or explosion and other safety incidents,or carbon monoxide,which is easily produced in our daily life,will threaten people's lives.As some gases are more and more widely used in the daily life or industrial production,their potential safety hazards are bound to face a more severe situation.Therefore,it is of great significance to develop safe,reliable,cheap,practical and sensitive gas sensors.SnO₂ and other metal oxide semiconductors are important gas sensitive materials,but there is an obvious defect in commercial SnO₂ porous thick film gas sensors,that is,they all have to work at rather high temperatures.On the one hand,more energy consumption occurs,on the other hand,the cross–responses of other reducing gases lead to the reduction of the service time,thus the application of SnO₂ thick film materials is severely limited.Room temperature gas sensors based on low-dimensional nanostructured MOXs?semiconducting metal oxides?have appeared in the laboratory,yet they also have some inherent shortcomings which makes it difficult for them to be applied.In this study,a variety of catalysts–SnO₂ composite nanoceramics have been prepared in the form of catalysts–oxide semiconductor system.They have outstanding response to hydrogen and carbon monoxide at room temperature.These bulk composite ceramics have shown great application prospects,such as high mechanical strength,good preparation consistency,simple process,low cost,sensitive response and so on.Through a large number of experiments,those systems with obvious response to target gases at room temperature were screened out.The room temperature gas sensing properties of some composite porous ceramics with excellent comprehensive properties were comprehensively characterized,including humidity tolerance,long–time stability and so on,then the gas sensing mechanism was deeply analyzed.Detailed discussions are as follows:?1?Through pressing and sintering,many kinds of catalysts–SnO₂ composite nanoceramics have been prepared using SnO₂ nanoparticles,SnO₂ microparticles,SnO₂agglomerate powder separately,the catalysts are Pt and Pd.The experimental results show that those porous composite ceramics with many pores,large specific surface area and high mechanical strength can be prepared under appropriate pressure and sintering temperature.?2?Porous ceramics prepared using SnO₂ nanoparticles and Pd have strong response to carbon monoxide at room temperature.When exposed to 0.04%CO–20%O₂–N₂,the resistance drop can be up to two orders of magnitude.The content of Pd and sintering temperature play an indispensable role in the room temperature gas sensing capability of the porous ceramics.Most composite nanoceramics with Pd contents up to 2 wt%show no responses to CO at room-temperature.Those samples with Pd contents between 0.25wt%and 2 wt%and heat-treated up to 900?exhibit extraordinary room–temperature CO sensing capabilities.The surface morphology,phase structure and valence state of the samples were analyzed.The results show that Pd⁴⁺in the samples could induce the chemical adsorption of CO at room temperature,thus these samples show Extraordinary room-temperature CO sensing capabilities.?3?The composite nanoceramics prepared using SnO₂ agglomerate powder typically show a sensitivity of 800 to 1%H₂–20%O₂–N₂ in air of 50%relative humidity?RH?at room temperature,compared with the previous Pt–SnO₂ composite nanoceramics,the response increased by 50%,and the humidity tolerance has been improved obviously.When the RH reaches 70%,the response is still quite strong,with a sensitivity of 450.These results thus demonstrate a strong and unexpected microstructure dependence of humidity tolerance for room–temperature hydrogen sensitive Pt–SnO₂ composite nanoceramics,the response to hydrogen and the humidity tolerance of the samples prepared by agglomerate particles were improved obviously.?4?The catalysts–SnO₂ composite nanoceramics not only have excellent room temperature gas sensing capability,but also can be preserved for a long time under natural conditions,that is,its long–time stability deserves attention.For a Pd–SnO₂ composite nanoceramic,even after six months of aging,it still has considerably room–temperature sensing capability.Pt–SnO₂ composite nanoceramic?prepared using SnO₂ agglomerate powder?has a good response and selectivity to hydrogen at room temperature,after six months of aging,the sensitivity and response speed are basically consistent with the initial preparation,only the recovery speed has slowed down.Tracking test of the sample after a year shows that the room temperature hydrogen sensing curve of the composite nanoceramics is consistent with that of aging for half a year,which suggests that the room temperature hydrogen sensing capability of this system can be stabilized for a long period of time.These catalysts–SnO₂ composite nanoceramics have good performances in long-time stability and have good application prospects.However,their sensing capability still need to be improved,and more investigations are needed in the future.