The influence of a platinum membrane on the sensing properties of a tin dioxide thin film

A possible way to improve the selectivity of tin dioxide sensors is to deposit a membrane with specific proper-ties above the sensing element. The purpose of this paper is to investigate the effect of a platinum membrane on the sensing properties of a chemical vapour deposition (CVD) SnO2 thin film under gases (air, pollutant gases CO, CH4 or C,H,OH). Electrical measurements have shown that the platinum membrane thickness has to be limited to avoid a short-circuit of the SnO2. For a small platinum layer thickness (10 nm) the conductance of SnO2 film decreases with increasing platinum thickness. This effect is attributed to additional oxygen species on SnO2 resulting from an enhanced oxygen dissociation by Pt. Concerning the influence on the selectivity, the measurement of the catalytic activity of Pt has been investigated. Above 500 degreesC, this metal is very efficient for the total oxidation of CO and C2H5OH, but CH4 does not react. We tried to correlate this catalytic activity to the sensing detection properties of the SnO2 + Pt structure. Firstly. the membrane strongly decreases the sensor response under alcohol, which is true to catalytic tests. In CO case, on one hand, at high temperature (500 C). although Pt is very efficient in regards of CO oxidation, it does not strongly decrease the sensor response. A mechanism is proposed, based on oxygen exchange between Pt particles and SnO2 film during CO oxidation. On the other hand, at temperature lower than 150 degreesC, Pt membrane induces a strong increase of the conductance under CO. We have shown that this electrical phenomenon only results from Pt particles not from SnO2 thin film. Finally, the CH4 response is slightly decreased which is not true to catalytic properties of Pt. (C) 2002 Published by Elsevier Science B.V.