Interplay between O2 and SnO2:: Oxygen ionosorption and spectroscopic evidence for adsorbed oxygen

Tin dioxide is the most commonly used material in commercial gas sensors based on semiconducting metal oxides. Despite intensive efforts, the mechanism responsible for gas-sensing effects on SnO2 is not fully understood. The key step is the understanding of the electronic response of SnO2 in the presence of background oxygen. For a long time, oxygen interaction with SnO2 has been treated within the framework of the "ionosorption theory". The adsorbed oxygen species have been regarded as free oxygen ions electrostatically stabilized on the surface (with no local chemical bond formation). A contradiction, however, arises when connecting this scenario to spectroscopic findings. Despite trying for a long time, there has not been any convincing spectroscopic evidence for "ionosorbed" oxygen species. Neither superoxide ions O-2(-), nor charged atomic oxygen O,(-) nor peroxide ions O-2(2-) have been observed on SnO2 under the real working conditions of sensors. Moreover, several findings show that the superoxide ion does not undergo transformations into charged atomic oxygen at the surface, and represents a dead-end form of low-temperature oxygen adsorption on reduced metal oxide.