Discerning specific gas sensing at room temperature by ultrathin SnO2 films using impedance approach

Room temperature gas sensing of ultrathin SnO2 sensors prepared by Langmuir Blodgett technique has been investigated under various chemically polluting ambiances (NH3, H2S, SO2, CO, H-2, NO2 and CH4) using impedance spectroscopy. There was no change in impedance response when the sensors were exposed to CO, H-2 and CH4. On exposure to H2S and SO2, the response was an increase in ac conductivity. The sensors showed a rapid decrease in conductivity only in presence of NH3 and NO2. 90% recovery for NH3 took place within 30 min whereas recovery for NO2 was slow. Further, transient impedance response for NO2 was distinctly different from that of NH3. New insights into sensing mechanisms for NO2 and NH3 have been proposed. Multifrequency approach indicated the absence of grain boundary barrier effects between the nanocrystalline grains on interaction with NH3, explaining the recovery at room temperature. For NO2 interaction, impedance analysis suggested formation of NO as the short lived transient species followed by a stable chemisorbed species. Single frequency study demonstrated real time detection of NH3, H2S and NO2 - each with specific transient response, using single sensor at room temperature in presence of other interfering gases, with sensitivity required for air quality monitoring. (C) 2012 Elsevier B.V. All rights reserved.