Impacts of Surface Modification of Pt-Sensing Electrodes with Au on Hydrogen-Sensing Properties and Mechanism of Diode-Type Gas Sensors Based on Anodized Titania

The impacts of the surface modification of Pt-sensing electrodes with Au on the H2-sensing properties and mechanism of diode-type gas sensors based on anodized titania (TiO2) were discussed in this study. The sensors using Pt electrodes modified with and without Au (Au(n)/Pt/TiO2 (n: sputtering time (s)) and Pt/TiO2 sensors, respectively) were fabricated by employing an anodized TiO2 film on a Ti plate. The surface modification of the Pt electrodes with Au(20) having a thickness of ca. 10 nm was the most drastically enhanced H2 response of the Pt/TiO2 sensor especially in air. The oxidation activity of H2 over the Pt and typical Au(n)/Pt electrodes was investigated to clarify the H2-sensing mechanism, together with analyses of crystal structure and chemical state of these electrodes by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The oxidation activity of H2 over the Pt electrode decreased with an increase in the amount of the surface-modified Au. Besides, the addition of moisture into the gaseous atmosphere reduced the oxidation activity of H2 in air. The alloying of Pt with Au was confirmed after annealing of the Au(n)/Pt electrodes at 600 °C in air, and the number of oxygen adsorbates on the surface increased with an increase in the amount of the surface-modified Au. On the basis of these results, we can suggest that the large H2 response of the Au(n)/Pt/TiO2 sensors arises from both a decrease in the number of highly active oxygen adsorbates and an increase in dissociatively adsorbed hydrogen species on the surface. The water molecules and/or hydroxy groups adsorbed on the surface by the addition of moisture into the gaseous atmosphere seem to have a crucial role in increasing the dissociatively adsorbed hydrogen species on the surface, to enhance the H2 response.