Role of the Au oxidation state in the CO sensing mechanism of Au/iron oxide-based gas sensors

A study on the CO sensing mechanism of sensors based on Au-doped/iron oxide thick films is reported. Thick films were prepared from coprecipitated powders of Au/Fe2O3 calcined at temperatures between 100 and 400 degreesC. A detailed microstructural characterization by XRD, TEM and XPS has shown that nanometer sized gold particles with gold in a positive oxidation state are predominant after calcination of the powders at 100 degreesC. The mean size and the metallic character of the gold particles increase with increasing the calcination temperature. Electrical tests have shown an anomalous response, i.e. a conductivity decrease, to CO over the thick film calcined at 100 degreesC. Over films calcined at the temperature of 200 degreesC or above a normal response was found. It is suggested that different CO sensing mechanisms operate over these sensors. The anomalous response observed over the film annealed at the lowest temperature has been related to the participation of Au(III) ions in the CO sensing mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.