Effects of surface modification of noble-metal sensing electrodes with Au on the hydrogen-sensing properties of diode-type gas sensors employing an anodized titania film

摘要

Diode-type gas sensors employing an anodized TiO2 film and noble-metal (Pt, Pd and Pd-Pt) sensing electrodes modified with and without a small amount of Au (Au/M/TiO2 and M/TiO2, respectively, M = Pt, Pd, Pd-Pt (Pd:Pt = 58:42 in weight)) have been fabricated, and the effects of surface modification of the electrodes with Au on their H2-sensing properties have been investigated at 250 °C in both air and N2 under dry and wet atmospheres. H2 response of all the M/TiO2 sensors in N2 was much larger than that in air under both dry and wet atmospheres. The Pt/TiO2 sensor showed the smallest H2 response in air among them, but the surface modification of the Pt electrode with Au was quite effective in enhancing the H2 response of the Pt/TiO2 sensor in air. Therefore, the response of the Au/Pt/TiO2 sensor to 8000 ppm H2 in air was comparable to that in N2, especially under wet atmosphere, and thus the response of the Au/Pt/TiO2 sensor to 8000 ppm H2 was almost independent of oxygen concentration under wet atmosphere. In addition, the annealing of the sensor at 600°C did not affect the response of the Au/Pt/TiO2 sensor to 8000 ppm H2. The homogeneous mixing of Au into the Pt electrode also improved the H2 response of the Pt/TiO2 sensor, but the effectiveness was much lower than that of the surface modification of the Pt electrode with Au. On the other hand, the response of the as-fabricated Au/Pd/TiO2 and Au/Pd-Pt/TiO2 sensors to 8000 ppm H2 in air was much larger than that of the Au/Pd/TiO2 and Au/Pd-Pt/TiO2 sensors annealed at 600°C, especially under wet atmosphere. The large H2 response observed with the Au/Pt/TiO2 sensor and the as-fabricated Au/Pd/TiO2 and Au/Pd-Pt/TiO2 sensors can be explained by large suppression of H2 oxidation by Au components remained at the surface of their sensing electrodes and then increased amount of dissociatively adsorbed hydrogen species at the sensing electrode surface.