Effects of surface silver additives on tin oxide thin film gas sensors

Abstract

The metal oxide semiconductor SnO2 is widely used as a gas sensor material to detect reducing gases [1-3]. The addition of small amounts of noble metals, such as palladium and platinum [4, 5], to SnO2 sensors is known to enhance the sensor performance (sensitivity, recovery and selectivity). Two sensitization mechanisms, chemical and electronic, for interpreting the promotional role of these additives have been proposed [6]. However, a full understanding of how selectivity is controlled by these additives is still lacking. SnO2 gas sensors can be prepared from bulk sintered powder [7], or from deposited thin or thick films [5, 8]. In this letter, we present thin film SnO2 sensors prepared by pyrolytic spray deposition. We observed the clusters grown on the film surface after silver addition to SnO> The clusters act as catalytic activators and the gas sensing characteristics of Ag-SnO2 thin film sensors were investigated. SnO2 films were prepared on silica or alumina substrates by aerosol spray pyrolysis [9] from aqueous solution containing 1 M SnC14.5H20 and 0.2 M SnF> The film thickness and crystallinity were controlled by spraying time and substrate temperature. Films prepared at temperatures between 250 and 450 °C had sheet resistances in the range of 1 k~ to 1 M~ per square. The film thickness was 0.5 to 1/~m. Silver dag, a colloidal solution of fine silver powder in a methyl isobutyl ketone solvent, was applied to the tin oxide film surface as diffusion sources as well as the electrical contacts. The two contacts, in the shape of either a circle or a strip, were about 1.5 cm apart. After the silver dag had dried, the sample was placed in a small open-air oven. X-ray diffraction (XRD) indicated that the silver oxidized (Ag20) on the surface layer at temperatures above 200 °C. At temperatures above 300 °C silver metal or silver ions decomposed from the silver oxide, diffused over the film and aggregated at nucleation points. The stable clusters were observed after the temperature decreased below 300 °C. Experimental details have been reported elsewhere [10]. Fig. 1 shows some typical clusters that appeared in an area away from the silver source. In a previous publication [11], we elaborated that these clusters have remarkable scaling and universal properties, and the fractal dimension for these two-dimensional clusters is in excellent agreement with the theoretical value of 1.70 predicted by a diffusion-limited aggregation model. We also found that these clusters when grown on the surface of SnO 2 films were formed by silver metal with a layer of silver oxide on the metal surface [12]. Figure I Micrograph showing clusters grown on the SnO2 film in the region away from the silver source.