Tin dioxide gas sensors. Part 1.—Aspects of the surface chemistry revealed by electrical conductance variations

Abstract

Detailed measurements are reported of the dependence of the conductance of porous, sintered pellets of tin dioxide, on temperature, moisture and oxygen partial pressure. Comparison is made with the behaviour of thin layers prepared by radio-frequency sputtering. The effect of introducing a non-equilibrium mixture of a combustible gas in air, on the conductance and conductance activation energy is described. The theoretical basis for current interpretations of the behaviour of this material is reviewed. The results are discussed using a model in which the conductance of the porous pellets is controlled by different sorts of intergrain contact, represented as necks, necks depleted of conduction electrons, and Schottky barriers. The results are best rationalised by postulating that the conductance is in fact controlled by Schottky barriers separating domains or agglomerates, each comprising a rather large number of crystallites. The effects of temperature, moisture, oxygen partial pressure and combustible gases are discussed in terms of their effect on the Schottky barrier height by way of altrations in the area density, charge and occupancy of surface states (chemisorbed oxygen species). The existence of a surface state level located ≳ 1.1 eV below the conduction band edge is deduced. Adsorbed water strongly affects both the conductance and the response to combustible gases. Loss of water from the surface over the temperature range 280–450 °C results in a sigmoidal conductance-temperature relationship in moist air. The effect is to lower the resistance at temperatures below this range to no more than one-tenth of the values observed for dried pellets. A surface transformation O2–↔ OH– is inferred. In dried air, with dried pellets, inflexions in the conductance-temperature behaviour at temperatures below 230 °C are tentatively attributed to the effect of O2–↔ O–↔ O–2 surface transformations. The onset of a conductance response to the presence of a combustible gas coincides with the onset of a surface-catalysed combustion. At higher temperatures an effect of combustible gases is to lower the conductance activation energy for the porous pellets, but not for the sputtered layers. The effect of moisture on the response to CO was to extend the response to lower temperature; on dried pellets in dried air the response disappeared abruptly when the temperature fell below 350 °C.