Sensing Mechanism of SnO2 (110) Surface to NO2: Density Functional Theory Calculations

It is necessary to develop NO2 gas sensors as NO2 is a pollutant. While, different from the reducing gases, oxidizing gas NO2 will put up a complicated sensing process. Density functional theory (DFT) calculations are necessary to be performed to understand NO2-sensing mechanisms at the atomic level. In this study we introduce NO2 to SnO2 (110) surface with oxygen species pre-adsorbed. The results show that NO2 sensing mechanism of SnO2 surface strongly depends on the concentration of oxygen in the ambient atmosphere (usually, no effects of temperature and pressure are considered). The direct interactions between NO2 molecule and SnO2 sub-reduced surface (with two rows of fold-coordinated bridging oxygens removed) for very low oxygen concentrations show that, NO2 gas molecules interact directly with Sn instead of reacting with oxygen species, resulting in an increase in resistance of SnO2. We investigate gas-sensing processes of interaction between NO2 molecule and SnO2 surface with pre-adsorbed oxygen species for the case of considerable high oxygen concentrations. Adsorbed molecular oxygen ions compete with adsorbing NO2 molecules for available surface sites and electrons from the SnO2. As the availability of oxygen ions on the SnO2 surface increasing, the interaction between NO2 and adsorbed oxygen species give rise to a reducing interaction, which brings a decrease in resistance of SnO2.