Au@SnO2 and SnO2@Au Hollow Spheres for Gas Sensor Applications: Selectivity Control

Abstract

Introduction Metal oxide semiconductor (MOS) based gas sensors have attracted growing attention due to their excellent gas sensing characteristics, reasonable price, and facile integration. However, MOS based gas sensors show similar gas responses to a number of gases, which means that selective detection to the gases remains challenging. It has been frequently reported that the selectivity of MOS gas sensors can be controlled by doping or loading noble metal catalysts. In this research, we suggest that not only the type but also the location of catalysts in the nanostructure may cause a change in gas selectivity. We fabricated Au@SnO2 and SnO2@Au sensors and measured their sensing characteristics. : SnO2 hollow nanospheres whose inner part or outer part of shell are decorated with 0.3 at% Au. Method 0.15 g of sulfonated polysrtrene (SPS) beads were dispersed in 30 mL ethanol, and 0.075 g of poly-vinylpyrrolidone (PVP, MW = 40,000) was dissolved in 30 mL distilled water. Two solutions were mixed and homogenized by magnetic stirring. 30 mL of SnSO4, (95%) aqueous solution (0.07 M) were added drop by drop to the mixed solution. The solution was stirred at room temperature for 12 h, and subsequently washed and centrifuged five times with distilled water and ethanol. Finally, the precursor powders (SPS@Sn) were converted into SnO2 hollow spheres via a heat treatment at 550 °C for 2 h in air (2 °C min-1).0.0021 g of HAuCl4∙3H2O (99.999%) was dissolved in 30 mL ethanol containing 0.15 g of SPS. After magnetic stirring at room temperature for 2 h, 10 mL of NaBH4 (99.99%) aqueous solution (0.0026 M) was injected drop by drop into Au contained SPS suspension and stirred at 60 °C for 2 h. After washing and centrifuging five times with distilled water and ethanol, the suspensions were dried at 70 °C for 6 h to obtain SPS@Au. To confine the Au nanoparticles inside SnO2 hollow shell, SPS@Au were coated with Sn precursor in the same way as the synthesis of SPS@Sn.To load the Au nanoparticles outside the SnO2 hollow shell, Au was loaded on the as-prepared SPS@Sn, followed by SPS@Sn@Au. Loading Au on the surface of SnO2 was followed the previous way of preparing SPS@Au. The ratio between Au, and Sn ([Au] / [Sn]) were fixed to 0.3 at%. Results and Conclusions The pure SnO2 sensor exhibited the highest response to ethanol and the responses to other analyte gases (HCHO, p-xylene, toluene, benzene, and CO) were low at all sensing temperatures (300 – 375 oC). Both Au-out-SnO2 and Au-in-SnO2 showed significantly enhanced responses to all the analyte gases. However, they showed completely different gas selectivity depending on the location of Au catalysts. The Au-out-SnO2 sensor showed the highest gas response to 5 ppm ethanol over the entire range of sensing temperatures and this sensing characteristics are similar to those of pure SnO2 sensor although the maximum gas response levels were different from each other. In contrast, the Au-in-SnO2 sensor showed the highest response to p-xylene at 300 - 350 oC. Moreover, the responses to toluene and benzene were higher than those of ethanol at 350 and 375 oC. High response to xylene and relatively high responses to less reactive benzene and toluene can be attributed to the reforming of aromatic compounds into more reactive species for gas sensing. The catalyst-loaded micro reactor is a good model system to understand the gas sensing mechanism and provide a promising platform to detect the aromatic indoor pollutants in a highly sensitive and selective manner.