Heterogeneous reaction rate based description of the response kinetics in metal oxide gas sensors

The kinetics of the resistance response to a steep change in composition of the atmosphere was analysed in metal oxide thin film gas sensors. A phenomenological model was proposed in which the response kinetics was related to the rate parameters of heterogeneous catalytic reaction divided into the elementary steps such as adsorption, desorption and bimolecular interaction in which two adsorbed species are involved. The model was based on the rate equations (Langmiur type) for the oxygen and the gas coverage (both homogeneous) and on the relationship between the electrical transport over the potential barrier and the surface charge in polycrystalline films of the n-type conductance. Based on the model calculations and the experimental results, an influence of the rate parameters on the response transients was studied. The changes of the transients were characterised by a phenomenological time constant defined by a straightforward relationship to the rates of the elementary steps. The model was used for the interpretation of the experimental responses of SnO2 sensors modified with the catalysts (Pt, Au, Ag) to H2 and CO gases in dry air. The dependencies of the response transients on the sensor temperature and the gas pressure were explained in terms of the process rates. The role of the catalysts in the modification of the sensors was discussed.