Dependence of NO₂ gas sensitivity of WO₃ sputtered films on film density

Pure WO₃ films and WO₃ films doped with noble metals such as Au, Pt, and Ru were deposited on quartz substrates by dc reactive magnetron sputtering to investigate the NO₂ gas sensitivity. The temperature of the substrate and the pressure of the discharge gas were changed. The film structures were studied by X-ray diffraction, atomic force microscopy, and density measurements. The WO₃ films had a triclinic structure and seemed to be composed of columns surrounded by voids. The film density decreased as the discharge gas pressure increased and the substrate temperature decreased. Gas sensitivity was measured at an NO₂ gas concentration of 3 ppm. Low film density and Au doping caused a high sensitivity to NO ₂ gas. The highest sensitivity was observed in a film with an Au concentration of 0.25 at.% and a density of 5.0 g/cm³, which was significantly smaller than the bulk density of 7.3 g/cm³.