Abstract
The surface conductivity changes of 2 at.% Au/MnOx, MnOx and 2 at.% Pt/SnO2 have been measured during cyclical exposures to CO and dry air at 130°C. At each stage of the gas exposures, the corresponding surface compositions were determined using x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The results have been used to gain insight into the surface processes that are responsible for the observed conductivity behavior. Upon CO exposure, the extent of conductivity increase is different for each specimen and a synergistic interaction between Au and MnOx is demonstrated. Whereas exposure to dry air (or oxygen) causes the conductivity of Pt/SnO2 to decrease, the Au/MnOx and MnOx specimens continue to exhibit positive changes in surface conductivity. This is believed to be due to sufficiently high partial pressures of oxygen that transform the MnOx-based materials from n-type semiconductors to p-type. The surface analysis data implicate the importance of hydroxyl groups and/or adsorbed water to the CO gas sensing mechanism on all three materials, but there is evidence suggesting that their role is different with respect to Au/MnOx and Pt/SnO2. The potential advantages of combining Au/MnOx and Pt/SnO2 into a single gas sensor are also considered. © 1998 John Wiley & Sons, Ltd.
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