Abstract : A possible way to improve the selectivity of tin dioxide sensors is to deposit a membrane with specific properties above the sensing element. The purpose of this paper is to investigate the effect of a platinum membrane on the sensing properties of thin (chemical vapour deposition) and thick (screen-printing technology) SnO2 films under gases (air, diluted pollutant gases CO, CH4 or C2H5OH).
The measurement of the catalytic activity of Pt has been investigated. At above 500 °C, this metal is very efficient for the total oxidation of CO and C2H5OH, but CH4 does not react. As a consequence, it should be possible to develop a selective sensor to CH4 in presence of C2H5OH and CO as interfering gases. Firstly, we tried to correlate this catalytic activity to the sensing detection properties of the thin SnO2 film+Pt structure. The membrane strongly decreases the sensor response under alcohol, and does not affect the CH4 one, which is in agreement with catalytic tests. In CO's case, at high temperature (500 °C), although Pt is very efficient with regard to CO oxidation, it does not strongly decrease the sensor response. To explain this result, a mechanism based on the contact between Pt particles and thin SnO2 film is proposed. Hence, in order to decrease the CO's response, Pt membrane has to be insulated from SnO2 film. A new structure, thick SnO2 film+thick insulator coating (SiO2)+Pt, has been tested. A good correlation between catalytic tests and the effects on the sensor response is obtained. However, contrary to thin film device, a good correlation is also obtained in the thick SnO2 film+Pt structure when a contact between Pt and SnO2 is materialised. In thick-film structures, no insulator coating seems to be necessary.