Are modulations of the Schottky surface barrier the only explanation for the gas-sensing effects in sintered SnO2?

Conductance transients of thick, sintered SnO2 films at step isothermal changes in oxygen partial pressure show exponential decays with time constants which cannot be explained in the frame of the usually accepted model of Schottky barriers at intergranular contacts dominating the conductance behavior. An alternative model, based on a linear relationship between conductance and the surface coverage by chemisorbed oxygen, accounts well for the experimental results. This model proposes the existence of a highly resistive sub-surface layer, isolating the surface from the bulk; it identifies the oxygen chemisorption sites with surface oxygen vacancies. The model is also supported by the temperature-variation of conductance in “ramp” experiments with increasing and decreasing temperature, and by the effects of prolonged heat treatments in oxygen upon the conductance and gas sensitivity.