CO-Sensing Properties of Diode-Type Gas Sensors Employing Anodized Titania and Noble-Metal Electrodes under Hydrogen Atmosphere

CO-sensing properties of diode-type sensors employing an anodized TiO2 film and noble-metal (M) electrodes (M/TiO2 sensor, M: Pd, Pt, and Pd-nPt, n: the amount of Pt (wt %) in the Pd-nPt electrode) were investigated at 50–250 ◦C in dry or wet H2. All the M/TiO2 sensors showed nonlinear I–V characteristics as a diode device in air and N2, but the I–V characteristics of the sensors were actually linear in H2 because of the negligible small height of Schottky barrier at their M/TiO2 interface. The Pd/TiO2 sensor showed no CO response in H2, but the Pt/TiO2 and Pd-nPt/TiO2 sensors responded to CO in H2. Among them, the Pd-64Pt/TiO2 sensor showed the largest CO response at 100 ◦C in H2. The reason why the mixing of Pd with Pt was effective in improving the CO response is probably because of a decrease in the amount of dissolved hydrogen species, an increase in the amount of dissociatively adsorbed hydrogen species, and an increase in the amount of adsorbed CO species in CO balanced with H2 by the mixing of Pt into Pd. The interference from moisture in the target gas on the CO response should be largely improved from a practical application perspective.