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 H-sensing properties and mechanism of diode-type gas sensors based on anodized titania (TiO) were discussed in this study. The sensors using Pt electrodes modified with and without Au (Au()/Pt/TiO (: sputtering time (s)) and Pt/TiO sensors, respectively) were fabricated by employing an anodized TiO 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 H response of the Pt/TiO sensor especially in air. The oxidation activity of H over the Pt and typical Au()/Pt electrodes was investigated to clarify the H-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 H 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 H in air. The alloying of Pt with Au was confirmed after annealing of the Au()/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 H response of the Au()/Pt/TiO 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 H response.