Nano-crystalline SnO2 gas sensor response to O2 and CH4 at elevated temperature investigated by XPS

The electronic changes occurring at the surface of a nano-crystalline SnO2 sensor upon exposure to O2 and CH4 at elevated temperature have been investigated by X-ray photoelectron spectroscopy (XPS). Gas exposures and subsequent XPS scanning were conducted at 120 and 250 °C to simulate the working conditions of the sensor. Exposure to O2 at 120 °C resulted in a 0.2 eV upward band bending while subsequent exposure to CH4 resulted in a 0.1 eV downward band bending, as expected from oxidising and reducing gases, respectively. Similar changes in surface band bending were observed at 250 °C, although quantitative analysis suggests that oxygen absorption might be enhanced. The results clearly indicate the electronic nature of the gas sensing mechanism when exposed to O2 and CH4 and the very high sensitivity of the sensor.