Hydrogen (H2) is highly flammable and explosive in a wide concentration range (4–75% in air), while H2 is the most promising alternative to fossil fuels. Fuel cell vehicles (e.g., Toyota MIRAI and Honda CLARITY) and household fuel cell cogeneration systems (e.g., ENE FARM manufactured by Panasonic, Kyocera, Aishin, etc.) have already been marketed with some infrastructures for hydrogen production, purification, transportation, and supply (hydrogen stations) in Japan. Therefore, numerous efforts have been actively directed to developing highly sensitive and selective H2 sensors. We have so far demonstrated that diode-type gas sensors using noble-metal (N) sensing electrodes and an anodized titania film (N/TiO2 sensors) show a quite large H2 response even under oxygen-free atmosphere as well as relatively excellent H2 selectivity to other inflammable gases, in comparison with other gas sensors1–3).
The sensors are fabricated as follows. After half part of a polished Ti plate is anodized at 20°C for 30 min at a current density of 50 mA cm-2 in 0.5 M H2SO4 aqueous solution, two Pt and/or Pd films are fabricated as sensing and counter electrodes on the surface of both the TiO2 thin film and the Ti plate, respectively, by magnetron sputtering for 6 min at 300 W in Ar. Subsequently, an appropriate amount of Au is modified onto the electrodes by magnetron sputtering at 40 W in Ar (sputtering time (n): 10–120 s) and Au wires are attached on both the electrodes by using Au paste. In our group, the sensing properties of the obtained sensors (Au(n)/Pt/TiO2 sensors, after annealing at 600°C for 1 h in air) to H2 balanced with air or N2 under dry or wet atmosphere are generally measured at 250°C under forward bias (100 mV).
The magnitude of H2 response of all N/TiO2 sensors decreases with an increase in the partial pressure of oxygen in the target gas. Among them, the Pt/TiO2 sensor shows a relatively small H2 response in comparison with those of other N/TiO2 sensors. However, the surface modification of the Pt-sensing electrode with the optimum amount of Au (n: 20 (s)) maximally enhances the magnitude of H2 response, especially in air4–6). In this presentation, the mechanism for enhancing the H2-sensing properties by the surface modification of the Pt-sensing electrode with Au will be mainly discussed, on the basis of the catalytic combustion behavior of H2 over the Au(n)/Pt/TiO2 films and the compositional and microstructural characterization investigated mainly by X-ray photoelectron microscopy.

1) Y. Shimizu, N. Kuwano, T. Hyodo, M. Egashira, Sens. Actuators B 83 (2002) 195.
2) Y. Shimizu, K. Sakamoto, M. Nakaoka, T. Hyodo, M. Egashira, Adv. Mater. Res. 47-50 (2008) 1510.
3) T. Hyodo, M. Nakaoka, Y. Shimizu, M. Egashira, Sens. Lett. 9 (2011) 641.
4) T. Hyodo, T. Yamashita, Y. Shimizu, ECS Transactions 50(12) (2012) 171.
5) T. Hyodo, T. Yamashita, Y. Shimizu, Sens. Actuators B 207 (2015) 105.
6) T. Hyodo, W. Sakata, K. Kamada, T. Ueda, Y. Shimizu, ECS Trans. 75(16) (2016) 115.