Preparation Of CeO2 Porous Hollow Spherical Nanomaterials And Study Of Their Gas Sensing Propertie

Abnormal VOCs concentration will not only cause great harm to human health but also pose a serious threat to the sustainable development of the ecological environment.Therefore,developing an excellent sensitivity and reliability sensor for VOCs gas detection is urgent and significant.Semiconductor gas sensor has been developed into one of the best candidates to gas detector due to its simple structure,small size,cheap price,low power consumption,high sensitivity,and shorter response/recovery times.Ce O₂ has gradually become a research hotspot in the field of gas detector because of with unique Ce 4f electronic structure,large surface oxygen vacancy and reversible conversion between Ce³⁺and Ce⁴⁺.In this paper,a template hydrothermal-etching method was used to synthesize the pure hollow-shell Ce O₂,doping system,and heterojunction composite nanoparticles.To enhance the sensing performance by designing a good micro-nano structure,doping regulation and heterogeneous structure construction.The gas-sensitive characteristics were tested and analyzed in detail,using typical VOCs such as acetone,ethanol,formaldehyde,methanol,and xylene and so on.Furthermore,the sensing mechanism of Ce O₂ nanomaterials to VOCs was systematically analyzed based on first principles.The main research contents of this paper are as follows:（1）Using SiO₂ nanoparticles as template to prepare the intermediate product SiO₂@Ce O₂ by hydrothermal method,and then the hollow-shell Ce O₂ nanoparticles were obtained by etching the SiO₂ template with sodium hydroxide solution.It was found that hollow-shell Ce O₂ nanoparticles with different morphology structure could be obtained at different etching temperatures.When the etching temperature was 10℃,the obtained nanoparticles have porous hollow-shell structure with sponge-like morphology.Furthermore,the spherical structure occurred collapse and nonuniformity due to the etching speed is fast,when the etching temperature was set 50℃.The hollow-shell Ce O₂ nanoparticles with sponge-like morphology had the best gas sensing characteristics toward acetone vapor,the response value to 100 ppm acetone vapor achieved about 9.2 at the 260℃operating temperature,and the response and recovery time were 6 s and 11 s,respectively.Moreover,the sensor has low detection limit,good long-term stability,and excellent resistance to humidity interference.Analyzing the calculated results of first-principles,the band gap was changed from 2.74 to 1.91 e V,and 0.66 e was transferred from the Ce O₂ body to the acetone molecule,when an acetone molecule was adsorbed on the crystal surface of Ce O₂（111）.It will change the conductive characteristics of the Ce O₂ semiconductor.The sensing mechanism of pure Ce O₂ sensor towards acetone vapor was explained systematically by combining with DFT calculated results and previous research results.（2）Ni and rare earth element Eu,Y doped CeO₂ nanoparticles with good uniformity were prepared based on the template hydrothermal and etching method.The obtained Ni doped Ce O₂ nanoparticles have relatively high response sensitivity,and lower operating temperature compare with pure hollow-shell Ce O₂ among the three doping nanoparticles.By doping the Ni and rare earth Eu,Y elements,the optimal detection gas of the sensor was changed from acetone vapor to ethanol vapor,indicating that the selectivity of the doping sensor was obviously changed.The 0.8%,3%and 2%were the optimal doping concentrations of Ni and rare earth element Eu,Y doped Ce O₂sensor,and the obtained responses toward 100 ppm ethanol vapor were 13.2,10.8 and7.2,respectively.Illustrating the doping Ce O₂ sensor have a great improvement compared with the pure Ce O₂.Furthermore,the doping sensors have shorter response/recovery times（18/10 s,14/14 s and 20/8 s）,lower detection limit,good long-term stability,and remarkable resistance to humidity interference.Analyzing the calculated results of first-principles,the band gap of Ni-doped Ce O₂ was reduced to1.26 e V.Meanwhile,the location of Ni-doping was the best adsorption site for ethanol molecule,and three most likely adsorption structures can be formed.Calculating band gaps of the three adsorption structures were 0.79,1.1 and 1.05 e V,and having 0.15,0.39 and 0.43 e were transferred from the Ni doped Ce O₂ body to ethanol molecule for the three absorbing structures.The change of band gap and the transfer of charge will increase the conductive characteristics of Ni doped Ce O₂.Combining with the DFT calculated results and previous research results,the detection mechanism of doping Ce O₂ sensor toward ethanol vapor was analyzed systematically.（3）The CeO₂@MOS（Sn O₂,ZnO,Fe₂O₃）heterojunction nanoparticles were prepared by using hydrothermal synthesis method on basis of the hollow-shell Ce O₂nanoparticles.Comparing with pure hollow-shell Ce O₂,the optimal detection gas of the Ce O₂@MOS heterojunction sensor was changed from acetone vapor to ethanol vapor,indicating that the selectivity of the heterojunction sensor was obviously changed.The Ce O₂@Sn O₂ heterojunction sensor has the best gas sensing performance among all the Ce O₂@MOS heterojunctions.The response achieved 172 when the Ce O₂@Sn O₂heterojunction sensor detected 100 ppm ethanol vapor,and elevated 27 times compared with the pure hollow-shell Ce O₂.Meanwhile,the optimal operating temperature was reduced from 260℃to 190℃,the biggest benefit is that lower operating temperature can effectively improve the safety of combustible gas detector.Remarkable,the Ce O₂@Sn O₂,Ce O₂@Zn O and Ce O₂@Fe₂O₃ heterojunction sensors have good response/recovery characteristics,lower detection limit,good long-term stability,and excellent resistance to humidity interference according to the sensing results.In addition,the electronic structure and density of states were calculated by DFT for Ce O₂@Sn O₂ heterojunction sensor.The computational data revealed that the gap between the O and Ce electrons decreased from 1.26 to 1.07 e V when an ethanol molecule was adsorbed on the molecular model of the as-constructed（111）Ce O₂@（110）Sn O₂ heterojunction.The results also indicated that 0.34 e per molecule was transferred to ethanol,thus inducing a change in the HOMO/LOMO energy gap of the core-shell Ce O₂@Sn O₂ heterojunction system.These results elucidate why the ethanol vapor sensor had a higher sensitivity and lower operating temperature than the previous reports.The detection mechanism of Ce O₂@MOS heterojunctions sensor toward ethanol vapor was analyzed systematically.