Progress and challenges of catalytic room-temperature gas sensitive metal oxide ceramics
Due to their simple structures and high stability,gas sensors based on semiconducting metal oxide thick films have found extensive applications.However,the elevated working temperatures(200-400℃)bring an explosive risk in the course of detecting explosive gases.Extensive investigations have been devoted to developing room-temperature gas sensitive metal oxide materials for decades,in most of which metal oxide nanomaterials are synthesized and adopted.It has to be pointed out that those protocol room-temperature gas sensors based on low-dimensional metal oxide nanomaterials are usually of low mechanical strength,slow recovery in air,and are highly susceptible to moisture,which have actually prevented them from many applications.In recent years,a new strategy has emerged for developing room-temperature gas sensitive metal oxide materials,which enables metal oxides to be room-temperature gas sensitive through the adoption of effective catalysts.This strategy is basically different from the one based on the adoption of metal oxide nanomaterials,in which metal oxides are not necessarily of nano-scale dimensions.As a matter of fact,SnO2 powder with as large as 5 μm agglomerate particles and WO3 powder with as large as WO3 grains have been deliberately adopted to prepare room-temperature hydrogen-sensitive Pt-SnO2 and Pt-WO3 composite ceramics,respectively,which both not only have extraordinarily strong room-temperature responses to hydrogen but also have extremely high resistance to moisture.Impressive room-temperature CO-sensitive Pt-SnO2 and Pd-SnO2 composite nanoceramics,and room-temperature NO2-sensitive Pt-Au-SnO2 composite nanoceramics have also been successfully developed in recent years.As for room-temperature gas sensing mechanisms,hydrogen and CO have been found able to be chemisorbed on some metal oxides at room temperature,and H2O molecules in air have been found to play a vital role for NO2 molecules to desorb from SnO2 at room temperature.All these results suggest that room-temperature gas sensing through metal oxides is much more complicated than usually expected.However,all these room-temperature gas sensitive ceramics that have been developed up to date were found to degrade easily in their room-temperature gas sensing capabilities,either after long-terms of aging or being exposed to gases like H2S,which will be a major challenge to their practical applications of these ceramics.Fortunately,a mild heat-treatment like 200℃ for 10 min is able to completely restore those degraded room-temperature gas sensing capabilities.As the influences of H2S exposure and long-term aging are quite similar for room-temperature gas sensitive ceramics,it is proposed that the observed aging has mostly resulted from the adsorption of impurity gases in air on those room-temperature gas sensitive ceramics and their removal through mild heat-treatments is responsible for the recovery.Future research should aim to further improve the room-temperature gas sensing capabilities for those known systems,explore more systems of catalysts-metal oxide semiconductors to find materials room-temperature sensitive to other kinds of gases,and optimize periodic heat-treatment systems for room-temperature ceramic gas sensors.For room-temperature ceramic gas sensors,an optimized periodic heat-treatment should be able to cause little interference to their room-temperature operations,greatly decrease their cost and help them to maintain their room-temperature gas sensing capabilities for long periods of time.
gas sensorsmetal oxidesroom temperatureceramicscatalysts
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