Macroscopic and microscopic investigations of the effect of gas exposure on nanocrystalline SnO2 at elevated temperature

Gas sensors based on polycrystalline SnO2 offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime, and high selectivity and sensitivity. We have investigated the effect of O2 and reducing gases (CH4 and CO) exposure on nanocrystalline SnO2 in vacuum and at elevated temperatures (120DGC) using three different techniques: X-ray photoelectron spectroscopy, in vacuum resistance measurements and scanning tunnelling microscopy and spectroscopy. XPS and resistance measurements showed that O2 chemisorbtion causes an upward surface band bending of 0.2 eV and a resistance increase of 50 MQ while CH4 exposure resulted in a 0.1 eV downward band bending and a 20 MQ drop in resistance. Stable STM imaging at 120DGC was achieved and clearly resolved the 8 nm particles. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O2 exposure.