Mechanisms of tin oxide gas sensor response

Tin oxide gas sensors are widely used for the detection of combustible gases in oxygen-rich atmospheres. Adsorbed oxygen species withdraw electron density from the surface of the Sn02 , increasing its electrical resistance. At elevated temperatures, around 400 °C, combustible analyte gases displace or react with adsorbed oxygen, increasing Sn02 surface electron density and thus decreasing its electrical resistance. Sensor resistance has been found to vary non-linearly with combustible gas concentration in a manner that has not been satisfactorily explained despite thirty years of research into the sensing mechanism. The operating temperature of tin oxide gas sensors is critical information in studies of their response mechanism, yet has seldom been reported accurately in the literature. In the current work, a new method of determining sensor temperature radiometrically has been developed and used to determine the surface temperature of two types of Figaro tin oxide sensors. The operating voltagetemperature relationships for these sensors were found to be pseudo-linear and are reported as T = 103 V+214±3 K for the Figaro TGS813 sensor with its base removed, T = 101 V+224±5 K for the TGS813 with its base attached, and T = 106 V+238±5 K for the Figaro TG52611 sensor. These results indicate that sensor temperatures are significantly higher than most previously reported estimates. Investigations of TGS2611 sensor response reveal that oxygen exhibits nearly ideal (Langmuir) adsorption behaviour on these 5n0 2-based gas sensors. An equation for the response of these devices to oxygen has been developed from a combination of accepted adsorption and electrical conduction theories. Fits of this equation to low and high sensor temperature oxygen response curves confirm previous findings regarding the speciation of adsorbed oxygen, ie at temperatures below — 170 °C, oxygen adsorbs non-dissociatively, while above this temperature it adsorbs dissociatively. From the temperature-dependent response of a TGS2611 sensor operating in air, enthalpies of adsorption have been calculated for non-dissociative (AH = -35.4 kJ mol -1 ) and dissociative (AH = -126.7 kJ mold ) oxygen adsorption. These values are characteristic of physisorption and chemisorption of oxygen to the surface respectively. A combination of infrared studies and measurements of sensor resistance for a TGS2611 sensor have shown that n-alkanes adsorb competitively with oxygen onto the sensor surface. A competitive adsorption (Hinshelwood) mechanism is thus proposed for the response to combustible gases, using the previously developed oxygen response equation as a basis. The sets of equations representing this model are too difficult to solve implicitly, so their validity has been demonstrated using Monte Carlo-type computer simulations of sensor response to single n-alkanes and binary mixtures of these gases. Detailed information has been acquired about the adsorption and kinetic behaviour of oxygen and n-alkanes on tin oxide sensors, including the influence of alkane chain length on static and dynamic temperature responses. The research in this thesis represents the first satisfactory explanation, in terms of heterogeneous adsorption and catalysis theory, of many aspects of sensor response, including the influence of oxygen on sensor resistance, the non-linear analyte response behaviour, the characteristic analyte resistance/temperature profiles, and the complex response of analyte mixtures