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

Abstract

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 were 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 dissociativelyadsorbed hydrogen species at the sensing electrode surface.

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

“…In contrast, the surface of the Pt electrode is not easily oxidized even under heat treatment at 600 • C in air [21]. These results which were obtained by utilizing the XPS analysis have already been demonstrated in our previous papers [19,21]. The electron affinity of PdO (ca.…”

“…On the other hand, the surface of the Pd electrode is easily oxidized to PdO under heat treatment at 600 • C in air, and the PdO is partially reduced in N 2 at elevated temperatures [19,21]. In contrast, the surface of the Pt electrode is not easily oxidized even under heat treatment at 600 • C in air [21]. These results which were obtained by utilizing the XPS analysis have already been demonstrated in our previous papers [19,21].…”

“…Generally, the work function of Pt (5.26-5.69 eV) is larger than that of Pd (4.87-5.25 eV) [24], and thus the Schottky barrier of the Pt/TiO2 interface (1.7-1.8 eV) tends to be larger than that of the Pd/TiO2 interface (1.2-1.3 eV) [25]. On the other hand, the surface of the Pd electrode is easily oxidized to PdO under heat treatment at 600 °C in air, and the PdO is partially reduced in N2 at elevated temperatures [19,21]. In contrast, the surface of the Pt electrode is not easily oxidized even under heat treatment at 600 °C in air [21].…”

“…5.5 eV) is larger than the work function of Pd [26], and various defect and impurity levels are easily produced at the M/TiO2 interface [27,28]. On the other hand, Pd and Pt in the bulk of the Pd-64Pt electrode was alloyed and the alloy Pd-Pt phase in the bulk is likely to be chemically and thermally stable as a metal at elevated temperatures, even in air [19,21]. After the heat treatment at 600 °C in air, however, most of Pd in the vicinity of the surface of the Pd-64Pt film were oxidized to PdO, while Pt just on the surface was only oxidized to PtO [19].…”

“…Namely, the mixing of oxygen into the H2-containing gaseous atmosphere largely reduced the magnitude of current of all the sensors. This indicates that the certain percentage of H2 was oxidized with oxygen species (e.g., oxygen adsorbates) on the surface of M electrodes and thus the effective H2 concentration on the surface reduced in dry air, whereas the H2 concentration on the M electrodes hardly decrease in dry N2 [21]. Furthermore, the Pd/TiO2 and Pd-64Pt/TiO2 sensors showed ohmic-like I-V characteristics in H2 balanced with dry N2, due to the quite small Schottky barrier at the M/TiO2 interface.…”