10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Exploring the Room Temperature Sensing Mechanism of SnO2 Nano-Columns Synthesized by Aerosol Routes towards Volatile Organic Compounds: Theoretical Calculations Compared to Experimental Results
AHMED A. ABOKIFA, Kelsey Haddad, John Fortner, Pratim Biswas, Washington University in St Louis
Abstract Number: 262 Working Group: Aerosol Physics
Abstract SnO2 is a wide bandgap semiconducting metal oxide that has been broadly employed as the active sensing material in chemiresitive gas sensors. Recent studies demonstrated the capability of bare-SnO2 sensors to detect various gases, including volatile organic compounds (VOCs), at room temperature. To explore the sensing mechanism, the adsorption of ethanol and acetone on the (110) and (101) surface facets of rutile SnO2 is investigated via density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations. The role of surface oxygen defects, polarity of the VOC molecule, and pre-adsorbed oxygen species from the ambient atmosphere is explored. Theoretical calculations are supplemented by sensing measurements for ethanol using SnO2 nanostructured thin film sensors fabricated by an aerosol chemical vapor deposition (ACVD) technique.
DFT results show that the direct adsorption of both ethanol and acetone on the (110) and the (101) surface facets of SnO2 is energetically favorable. The adsorption of both molecules is accompanied by the release of charge from the adsorbate gas to the surface, which promotes the sensing response. Binding strength of both molecules on the stoichiometric (110) and (101) surfaces is greater than that on the oxygen defective surfaces. Ethanol adsorption is generally stronger than acetone due to the bipolar nature of the hydroxyl (OH) group that interacts with the surface via two distinct charge transfer modes. To date, the most cited model regarding the sensing mechanism of metal oxide chemiresistive sensors is through the interaction with the ionosorbed (O-) species that possess high activity towards oxidizing target gas molecules. However, at room temperature, the less active superoxide molecules (O2-) constitute the majority of the pre-adsorbed oxygen species on the SnO2 surface. DFT results show that minimal interaction takes place between the pre-adsorbed oxygen species (O2-) and the studied polar VOCs upon their adsorption on the reduced surfaces. Taken together, these results suggest that the sensing mechanism of SnO2 towards polar VOCs at room temperature can be explained by their direct adsorption on the surface rather than through their oxidation by means of ionosorbed oxygen species.