| With the development of industry,the pollution of odorous gas is becoming more and more serious.Monitoring the concentration of odorous gas in the surrounding environment is very important for pollution prevention and control,industrial safety.Moreover,on-site as well as real-time detection/monitoring is the most effective method.At present,the existing detection methods are still based on expensive and large-scale complex equipment,such as gas chromatography/mass spectrometer and laser spectrometer.Considering the high-cost and complexity of large-scale equipment,it is difficult to achieve the real-time as well as on-site detection,so it is necessary to develop new detection technology to achieve real-time monitoring of odor gases.Semiconductor metal oxide gas sensors are widely used in environmental monitoring,public safety and national defense aviation because of their low cost,convenience,easy integration and effective integration with Internet of Things technology.However,long-term research have found that the essence of semiconductor-based gas sensors,that is,the resistance changes caused by surface oxidation-reduction reaction,often leads to poor selectivity and high working temperature,which will greatly hinder their application in practical monitoring.Through an in-depth study and analysis of the semiconductor sensing process,it is found that the sensing process mainly includes gas identification process,signal conversion process and utilization process.It is also found that the key to improve the selectivity of sensors for these three processes is how to boost the selective catalytic adsorption on the surface of sensitive materials.The loading of noble metal nanocatalyst can enhance the catalytic activity,change the kinetic path of surface reaction,and promote the selectivity of semiconductor gas sensing.Therefore,functional nanocomposites were constructed to detect odor gas,and the selectivity of semiconductor sensors was significantly improved by doping specific noble metal nano-catalysts and exciting ultraviolet light.Importantly,in-situ XPS and density functional theory(DFT)calculations are used to explore the possible sensing mechanism in the sensing process.The main research results and innovations of this paper are as follows:1.SnO2/a-Fe2O3 heterojunction nanocomposites and their enhanced gas sensing property.SnO2/a-Fe2O3 heterojunction nanocomposites were prepared by combining two-step hydrothermal method with annealing treatment.By high resolution transmission electron microscopy(HRTEM),it was found that SnO2/a-Fe2O3 heterojunction nanocomposites was rooted in the epitaxial growth of ultrathin(3-FeOOH nanosheets on the surface of hollow SnO2 spheres.The prepared SnO2/a-Fe2O3 heterojunction was used as sensing material,exhibiting excellent sensing performances for dimethyl disulfide(DMDS)gas with the detection limit of 200 ppb.Importantly,in terms of sensitivity and selectivity,SnO2/?-Fe2O3 sensors show enhanced DMDS sensing performance compared with pure Sn02 sensors.Meanwhile,by the mearsurements of H2-TPR,CO-TPD and NH3-TPD for the samples,it is found that the improved sensing performances of SnO2/?-Fe2O3 sensor toward DMDS may be attributed to two aspects:on the one hand,the enhancement of oxidation ability caused by alkaline changes on the surface of nanocomposites.On the other hand,the formation of"electronic accumulation layer"on the heterojunction between SnO2 and a-Fe2O3.2.Pt nanoparticles modified SnO2/a-Fe2O3 nanoheterojunction composites and their enhanced gas sensing performance to styrene.Ultrafine Pt nanoparticles modified SnO2/a-Fe2O3 nanoheterojunction composites were successfully prepared by integrating hydrothermal method with in-situ reduction method.In particular,the composite material was used as sensing layer to show excellent sensing properties for styrene gas.The detection limit of SnO2/a-Fe2O3/Pt composites can be down to 50 ppb,and the response/recovery time is 3/15 s.Moreover,compared with SnO2 and SnO2/a-Fe2O3 materials,SnO2/a-Fe2O3/Pt sensing layer exhibits higher gas response value and enhanced selectivity to styrene under the same condition.Through careful analysis,it is found that the enhanced sensing mechanism of the sensing layer for styrene is mainly attributed to the electronic sensitization effect caused by n-n nanoheterojunction and Schottky junction,and the catalytic sensitization effect of Pt nanoparticles.3.Pd nanoparticles decorated yolk-shell In2O3 composites and their gas sensing properties and mechanism toward CS2.Pd nanoparticles decorated In2O3 composites,featured with yolk/shell structure,were successfully achieved by combining MOF-templated method with subsequent annealing treatment.It is found that the prepared Pd/In2O3 composite displayed excellent sensing performances for carbon disulfide(CS2)gas,and the detection limit could reach to 1 ppm,which is much lower than the threshold of 6.7 ppm.Particularly,the Pd nanoparticles supported on the In2O3 composite greatly improved the selectivity and gas response value for CS2 in comparison with the pure In2O3 material.Importantly,the density functional theory(DFT)calculation shows that the intermediate S,produced by Pd-catalyzed desulfurization reaction,is the key to achieve high CS2 gas response and ultra-selectivity during the CS2 sensing process.Meanwhile,quasi-in-situ XPS measurement further demonstrates that a large number of S existed on the surface of Pd/In2O3 during the CS2 sensing process.Based on the results of DFT calculation and XPS analysis,a new sensing mechanism for CS2 sensing on Pd/In2O3 surface is proposed.4.Enhanced gas sensing property and mechanism study of SnO2/AuPd composite to DMDS.SnO2 hollow nanospheres with uniform size were primarily prepared by the hydrothermal method.Then ultra-small Au/Pd alloy particles supported on SnO2 were acquired by in-situ co-reduction method.Further research shows that the SnO2/AuPd composite has excellent sensing properties for DMDS gas.Moreover,in contrast with Sn02,Au/SnO2 and Pd/SnO2 sensors,the prepared SnO2/AuPd composite exhibited higher DMDS gas response value and ultra-selectivity.Additionally,DFT calculations and in situ XPS measurements showed that the decoration of Au/Pd alloy nanoparticles significantly enhanced the adsorption energy of Sn02 for DMDS in comparison with other interfering gases(NH3,H2S,methyl sulfide,methyl mercaptan,TMA,styrene,CS2,ethanol and acetone),and the adsorption energy for other interfering gases was very small,which may be the reason for the ultra-selectivity of the composite to DMDS.In addition,the S,produced during the sensing process,possibly caused the enhancement of the response value of SnO2/AuPd toward DMDS.5.SnO2 monolayer array films and their enhanced gas sensing performance to NO2 under ultraviolet light irradiation.Close-packed SnO2 monolayer arrays were obtained by combining hydrothermal method with air/water interface self-assembly method.Furthermore,the SnO2 monolayer array as a sensing film shows an excellent room temperature sensing performance for NO2 gas with a striking selectivity under ultraviolet(UV)-light irradiation.The detection limit is as low as 100 ppb and the response/recovery speed is very fast(7/25 seconds).Further research exhibits that the thickness of SnO2 film has a great influence on the gas response value,response/recovery time and selectivity toward NO2 under the UV-light irradiation.Moreover,it reveals the essential relationship between the thickness of SnO2 film and the sensing performance.In particular,the possible sensing mechanism is discussed in detail. |
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