A rate equation approach to the gas sensitivity of thin film metal oxide materials

Thin film metal oxide materials exhibit a bell-shaped variation of the gas sensitivity with sensor operation temperature. With respect to the temperature TM at which a sensitivity maximum occurs, the distribution of the gas sensitivity is asymmetric exhibiting a relatively steep increase below TM and a more moderate drop-off above TM. In this paper a rate equation approach is described, which successfully reproduces temperature- and gas-concentration dependent sensitivity distributions S(T, cgas) experimentally determined for a number of reducing analyte gas molecules. We show that such distributions are determined by two energetic parameters, which are specific for the special adsorbate/adsorbent system involved. These are (i) the strength of adsorption of neutral analyte gas molecules Eads and (ii) the kinetic barrier Ea that needs to be overcome to induce a surface combustion event involving an adsorbed analyte gas molecule and a surface oxygen ion.