NOx gas sensing characteristics of WO3 thin films activated by noble metals (Pd, Pt, Au) layers

WO3-based gas sensor devices have been fabricated by reactive rf sputtering on glass substrates. Palladium (Pd), platinum (Pt), gold (Au) noble metals (100-600 Angstrom) were evaporated as activator layers onto WO, thin films and Al layers (1500 Angstrom) were sputtered on top of them as upper electrodes for sensor output. The described sensing element was found to possess excellent sensitivity towards NO and NO2 gases, at low temperatures (100-300 degrees C). The optimum operating temperature was 150 and 200 degrees C for Pt, Pd and Au, respectively. The resistance of the thin-film gas sensor increases reversibly in the presence of low concentrations of NO (0-440 ppm) and NO2 (0-10 ppm) gases in air. The activator layers have an promotional effect on the speed of response to NOx at low temperatures and on selectivity enhanced with respect to other reducing gases (CO, CH4, H-2, SO2, H2S, NH3). The temperature dependence of response and recovery times has been examined. The influence of the thickness of the noble metals on NOx gases sensitivity has been investigated. The NO and NO2 gases calibration curves have been recorded for the optimum performances of each fabricated sensor-device. (C) 1998 Elsevier Science S.A. All rights reserved.