Au@SnO2 and SnO2@Au Hollow Spheres for Gas Sensors with Different Methylbenzene Selectivity

Abstract

Introduction Hollow or yolk-shell microspheres decorated with catalytic materials have been suggested as the platforms to design high performance gas sensors. For instance, SnO2 and Co3O4 yolk-shell spheres uniformly loaded with Pd catalysts exhibited high selectivity toward methylbenzene [1,2], whereas Pd-loaded SnO2 and Co3O4 nanoparticles in the literature generally did not. This intriguing results can be understood in relation to the micro-reactor role of catalyst-loaded yolk-shell spheres to promote the reforming of less reactive methylbenzene into more reactive species. In order to elucidate the sensing mechanism underlying the improvement of gas selectivity, the effect of catalyst location as well as catalyst material on the gas sensing characteristics of micro-reactor-shaped sensing materials should be investigated. For this, in this contribution, we prepared two different model sensing materials: 1) SnO2 hollow spheres whose inner parts of shells are decorated with 0.3 at% Au (denoted as ‘Au-in-SnO2’) and 2) SnO2 hollow spheres whose outer parts of shells are decorated with 0.3 at% Au (denoted as ‘Au-out-SnO2’) and their gas sensing characteristics were compared and discussed. The Au-in-SnO2 and Au-out-SnO2 sensors showed significantly different gas selectivity and response, suggesting that the location of Au catalysts in micro-reactor-shaped sensing materials is a key factor to determine gas sensing characteristics. Method 0.15 g of sulfonated polystyrene (SPS) beads (diameter: ~ 200 nm) were dispersed in 30 mL ethanol, and 0.075 g of poly-vinylpyrrolidone (PVP, MW = 40,000) was dissolved in 30 mL distilled water. The former slurry and the latter solution were mixed and homogenized by stirring. Then 30 mL of SnSO4 aqueous solution (0.07 M) were added drop by drop to the mixed slurry solution. The slurry solution was stirred at room temperature for 12 h, and subsequently washed and centrifuged five times with distilled water and then ethanol. The precursor powders (SPS@Sn) were converted into SnO2 hollow spheres via a heat treatment at 550 °C for 2 h in air. 0.0021 g of HAuCl4∙3H2O (99.999%) was dissolved in slurry solution containing 30 mL of ethanol and 0.15 g of SPS. After stirring at room temperature for 2 h, 10 mL of NaBH4 (99.99%) aqueous solution (0.0026 M) was added drop by drop to Au containing 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@Sn was formed by coating SPS@Au with Sn precursor according to the same procedure to prepare SPS@Sn. To load the Au nanoparticles outside the SnO2 hollow shell, SPS@Sn@Au was prepared by loading Au on SPS@Sn using the previous procedure to prepare SPS@Au. The Au-in-SnO2 and Au-out-SnO2 hollow spheres were prepared by thermal annealing of SPS@Au@Sn and SPS@Sn@Au at 550 °C for 2 h in air. The [Au]/[Sn] ratios in both samples were fixed to 0.3 at%. Results and Conclusions The sensor using pure SnO2 hollow spheres exhibited the highest response to ethanol and the responses to other 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 sensors 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 the overall sensing characteristics are similar to those of pure SnO2 sensor although the maximum gas responses are 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 selectivity to xylene and relatively high responses to less reactive benzene as well as toluene can be attributed to the reforming of less reactive 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 air pollutants in a highly selective and sensitive manner.