Evaluating the power-law response for tin dioxide nanostructured sensors in the presence of oxygen and reducing gases

It is well known that the electrical conductance of gas sensors, based on polycrystalline metal-oxide semi-conductors, obeys a power-law response with a gas pressure as p(-gamma). The value of the exponent gamma, which determines the sensor sensitivity, is regularly derived resorting to the mass action law applied to proposed reactions at the surface of the semiconductor. Conversely, based on the experimentally observed values of gamma for different gases, researchers also determine the possible species present at the surface and the reactions that can occur. In line with this approach, we present experimental results, for oxygen and a typical reducing gas, which confirm that oxygen is adsorbed/desorbed neutral, as predicted by the Wolkenstein theory of chemisorption, and ionosorbs doubly charged.