A route toward more selective and less humidity sensitive screen-printed SnO2 and WO3 gas sensitive layers

We have studied the effect of adhesion promoters (bismuth and copper oxides) on the gas sensing properties of screen-printed tin and tungsten oxide sensors. The gas-sensitive pastes were prepared by mixing either tin or tungsten oxide powders with an organic vehicle based on therpineol. Two different additives to enhance the adhesion of the active films to the substrate (Bi2O3 or Bi2O3+Cu2O) were included in the paste composition. The films that resulted after printing and firing have high porosity and excellent adherence. Morphology studies have shown that the films have well-defined microstructure and gas sensitivity studies have shown that additives, not only help in promoting film adhesion, but also modify the response of the sensors. Their sensitivity at different operating temperatures was evaluated for ammonia, nitrogen dioxide, ethanol, benzene, carbon monoxide, methane and water vapour. It was found that WO3 sensors containing Bi2O3 and Cu2O were always more sensitive to ammonia and, that WO3 sensors containing Bi2O3 only were more sensitive to nitrogen dioxide when operated at 250 °C. The reaction paths that explain the sensitivity changes observed by the inclusion of Cu2O in tungsten oxide sensors are introduced and discussed.