Role of Pt aggregates in Pt/SnO2 thin films used as gas sensors - Investigations of the catalytic effect

Fine dispersions of metal aggregates such as Pt, Pd, or Cu in metal oxides are well known to improve the electrical sensitivity and selectivity of sol id-state gas sensors put in contact with reducing gases (CO and H2S, for in stance) of solid-state gas sensors. The improvement originates in the catal ytic role played by the aggregates in regard to gas oxidation/reduction. Th e details of the solid-gas interactions are nevertheless not fully understo od. To further investigate the role of Pr aggregates in the sensing mechani sms results of in situ X-ray absorption and Anger electron spectroscopy fro m Pt/SnO2 films have been combined. Films with a Pt concentration in the ra nge 0.6-6 atom % have been prepared using a modified chemical vapor codepos ition method. Results show that the sensing mechanism in mainly driven by t he CO oxidation catalyzed by the aggregates: first a spillover effect occur s between CO, chemisorbsd oxygen, and PtOy aggregates, and in a second step , after a period of time depending on temperature, the aggregates are reduc ed/oxidized provided that they are small enough. The correlations between P t particle behavior and film electric properties are discussed. (C) 2000 Th e Electrochemical Society. S0013-4651(00)01-035-1. All rights reserved.