(Invited) Design of Highly Sensitive and Selective Diode-Type H2 Sensors
Authors: Hyodo Takeo , Sakata Wataru , Kamada Kai , Ueda Taro , Shimizu Yasuhiro 
Published: February 2020

Numerous efforts have been directed to developing highly sensitive and selective H2 sensors, because H2, which is the most promising alternative to fossil fuels, is highly flammable and explosive in a wide concentration range (4–75% in air). However, general gas sensors (e.g., semiconductor-type and catalytic combustion-type sensors) cannot operate under O2-free atmosphere, because the effective reaction of H2 with negatively-charged oxygen adsorbates on the oxide surface is essential for the large H2 response. We have thus far demonstrated that diode-type gas sensors using noble-metal (N) sensing electrodes and an anodized titania film (N/TiO2 sensors) show quite large H2 response under oxygen-free atmosphere as well as relatively excellent H2 selectivity to other inflammable gases, in comparison with other gas sensors. Use of Pd as the sensing-electrode material especially resulted in quite large H2 response1
, 2
), and alloying of Pd with Pt (Pd-Pt) effectively improved the H2 response and the long-term stability3). However, the H2 response of these sensors abruptly decreased with an increase in oxygen concentration in target gas. On the other hand, a Pt/TiO2 sensor showed lower H2 response than the Pd/TiO2 and Pd-Pt/TiO2 sensors, but the coating of the Pt-sensing electrode with a slight amount of Au drastically enhanced the magnitude of H2 response in air, probably due to a decrease in the amount of oxygen adsorbates on the Pt electrode4,5). Therefore, effects of the Au coating of the Pt-sensing electrode on the H2-sensing properties have been investigated in this study.
A polished Ti plate (10 mm × 5 mm × 0.5 mm) was annealed at 600°C for 1 h in air, and the half part was anodized at 20°C for 30 min at a current density of 50 mA cm-2 in 0.5 M H2SO4 aqueous solution. A pair of Pt electrodes (3 mm × 3 mm) was fabricated on the surface of both the TiO2 thin film and the Ti plate by magnetron sputtering for 6 min at 300 W in Ar. Subsequently, Au was coated on the Pt electrodes by magnetron sputtering at 40 W in Ar (sputtering time (n): 10–120 s) and Au wires were attached on both the electrodes by using Au paste. The sensing properties of the obtained sensors (Au(n)/Pt/TiO2 sensors, after annealing at 600°C for 1 h in air) to 5–8000 ppm H2 balanced with air or N2 under dry or wet atmosphere were measured at 250°C, while applying a dc voltage of 100 mV to the sensors under forward bias condition (Au(n)/Pt(+)–TiO2–Ti(−)).
The response of a Pt/TiO2 sensor to 8000 ppm H2 in dry air was three orders of magnitude smaller than that in dry N2, but Au coating on the Pt electrode drastically enhanced the forward current of the Pt/TiO2 sensor in air. The Au(20)/Pt/TiO2 sensor showed the largest H2 response in dry air among all Au(n)/Pt/TiO2 sensors, but the magnitude of H2 response in dry air was still smaller than that in dry N2. The addition of moisture to the atmosphere further enhanced the H2 response of the Au(20)/Pt/TiO2 sensor in air, while it decreased the H2 response in N2. Consequently, the response of the Au(20)/Pt/TiO2 sensor to 8000 ppm H2 in air was comparable to that in N2 under the most humidified atmosphere (AH: 12.8 g m-3). The Au(20)/Pt/TiO2 sensor showed linear relationship between the H2 response and the logarithmic H2 concentration, and the sensor could respond to 50 ppm H2 in dry and wet air, while the sensor easily responded to even 5 ppm H2 in dry and wet N2. In addition, the Au(20)/Pt/TiO2 sensor showed quite excellent H2 selectivity against propane and propene. These results indicate that the optimal Au coating on the Pt electrode was quite effective in improving both the H2 response and H2 selectivity in air as well as reducing oxygen-concentration dependence of the H2response, especially under wet atmosphere.
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