Catalytic enhancement of SnO2 gas sensors as seen by the moving gas outlet method

The moving gas outlet technique was employed to determine the transient and steady-state response behaviour of nano-crystalline SnO2 gas sensors. We find that the temperature dependence of the response and recovery time constants to reactive gases follow an Arrhenius-type behaviour of the form τ(T) = τ0 exp(Ea/kBT) with the activation energies Ea being lowered by catalytic activation. We further find that the catalyst-induced lowering of Ea is partly compensated by an increase in the prefactor τ0. Using the moving gas outlet technique in a stationary mode we further find that in the normal diffusion-limited mode of sensor operation the gas sensitivity of metal oxide gas sensors is not solely determined by the speed of the detection reactions at the sensor surface but also by the diffusive transport of gas species through a thin stagnant layer of air that forms at the sensor surface as a result of the sensor heating.