Fundamental Research On CO Gas Sensitivity Of Synergistically Modified SnO₂-based Materials Via Doping And Constructing Heterojunction

Gas sensor is one of the main tools to monitor toxic and harmful gases in the environment.Metal oxide semiconductor(MOS)gas sensors have certain advantages in detecting reducing and combustible gases.However,in the presence of interference H2 gas with similar chemical properties,SnO2 performs poor selectivity and response to CO gas.Single methods such as doping or constructing heterostructure can effectively improve the gas sensing performance.However,the phenomenon of solid solution substitution between metal cations with similar radius always exists when preparing heterojunction materials via liquid-phase method,making it difficult to determine the ion solid solution amount of the doped phase in the heterostructure.There are few reports on efficiently tuning the proportion relationship between doped phase and pristine phase in the heterojunction via combining sol-gel and grinding-annealing methods.The gas-sensing enhancement mechanism of improving CO gas-sensing performance of SnO2 via achieving the synergistic effect with further doping modification of heterostructure to regulate the Fermi level and raise the grain boundary barrier still needs to be discussed in-depth.The main innovative research results of this article are summarized as follows:The influences of In or Co doping on the structure and energy band parameters of SnO2 were studied.The results showed that element doping significantly reduced the grain size of SnO2 and increased the concentration of oxygen defects.When the doping content of In or Co was 8 mol%(SI0.08O)and 3mol%(SC0.03O),the Fermi level(EF)moved down-shift towards the valance band(Ev)with the maximum amplitude of 0.33 eV and 1.08 eV,respectively.Meanwhile,compared with the band gap of SnO2(Eg=3.51 eV),the Eg of SI0.08O and SC0.03O decreased by 0.14 eV and 1.15 eV,respectively.Additionally,the work function(WF)and electron affinity(Ea)of SC0.03O were 0.14 eV and 0.13 eV higher than that of SI0.08O,respectively,indicating that the carrier separation in SC0.03O-In2O3 heterojunction was more efficient.The reason for the difference in gas sensing performance of In or Co doped SnO2-In2O3 heterojunction materials was investigated from the perspective of changing the potential barrier height via EF tuning.The results showed that the introduction of oxygen defects via element doping significantly improved the gas sensing properties.Among them,the gas response of Sn0.92In0.08O2-20mol%In2O3(40ISI0.08O)and Sn0.97Co0.03O2-20mol%In2O3(40ISC0.03O)to 1000 ppm CO reached 9.2 and 12.1,respectively,and the CO selectivity were 1.1 and 1.9,respectively.The response time of 40ISC0.03O to 1000ppm CO was 5 s,which was 22 s and 10 s shorter than that of SnO2-20mol%In2O3(40ISO)and 40ISI0.08O,respectively.Compared with SI0.08O-In2O3,the interface potential barrier height of nanograins in SC0.03O-In2O3 heterojunction increased by 0.13 eV,which explained the reason that the baseline resistance and CO gas sensing response of 40ISC0.03O were higher than those of 40ISC0.08O at the same operating temperature.The synergistic sensitization mechanism of doping and constructing heterojunction was studied by regulating the In doping content in Sn1-xInxO2(SIxO)and the ratio of In2O3 in SI0.08O-In2O3(SI0.08O-xINO).The results showed that the response value of SI0.08O to 1000ppm CO was the highest.When the ratio of In2O3 reached 20 mol%,SI0.08O-40INO showed the highest response to 3000 ppm CO(15.2),which was 1.5 times that of SI0.08O.The response time was 38 s shorter than that of SI0.08O,and the CO selectivity was 1.7 times higher than that of SI0.08O.In this way,the respective contributions of In doping and In2O3 ratio tuning on improving CO gas sensing performance were distinguished.Au/Co bimetal modified SnO2-In2O3 heterojunction materials were prepared by impregnation method,and the effect of Au/Co modification on the gas sensitivity of heterojunction materials was studied.The results showed that the optimum operating temperature decreased to 400℃,the response toward CO increased first and then decreased with increasing Au loading content.The response value of 1Au-40ISC0.03O to 1000ppm CO reached 14.01,which was 2.3 times higher than that of 40ISO.The response time of 1Au-40ISC0.03O to 1000ppm CO was only 4 s,which was 12 times higher than that of pristine SnO2.Comparing the CO gas sensing properties of Pd/In or Au/Co co-modified SnO2-In2O3 heterojunction materials,it was found that the baseline resistance in air of 1Pd-40ISI0.08O was higher than that of 1Au-40ISC0.03O with the same loading ratio.Comparing the height of interface potential barrier,it was found that the formation of Schottky barrier between metal and semiconductor was the main factor to further improve the gas sensing performance compared with the contact barrier between semiconductor materials.