Simulation of oxygen exchange of SnO2 surface

Oxygen chemistry at the SnO₂ surface is sensitive to the surrounding gas atmosphere, a reason why it is used in gas sensor applications. The operation principle of these sensors is usually based on the reactions of gas molecules with O₂ ^- and O^- ions adsorbed onto the surface. These surface reactions are observed as a measurable change in the electrical conductance of the sensor material. Here, we have studied the temperature dependency of the equilibrium coverages of O₂ ^-, O^- and surface/lattice oxygen content with lattice gas simulations. Earlier, we have studied oxygen reactions at the SnO₂ surface using rate equation simulations. These models have been used as a basis of our simulations for adsorption, dissociation and desorption steps. Now, the physisorption step has been refined and the oxygen exchange between the adsorbed form and lattice has been added. Simulations have been carried out using an equilibrium lattice gas model. The conductance of the SnO₂ material as a function of temperature is modeled in terms of the different oxygen components. We find that at low temperatures O₂ ^- dominates at the SnO₂ surface, while in the higher temperature region oxygen is in the form of O^-. The transition temperature from O₂ ^- dominating coverage to O^- coverage is about 450 K. The importance of this finding lies in the fact that O^- is the most reactive component in reducing the adsorbed gases, and therefore, it explains the temperature dependence of the sensitivity of SnO₂ based sensors. The exchange process is slow at lower temperatures and accounts for the hysteresis like behavior at low temperatures. The simulated conductance dependence is in good agreement with the experimental measurements of Lantto et al.