Highly sensitive hydrogen gas sensor based on a capacitor-like Pt/TiO2/Pt structure with large-scale nanoporous top electrode

Abstract

Metal-oxide semiconductor gas sensors with capacitor-like Pt/TiO₂/Pt electrode arrangement and very narrow (∼100 nm wide) top electrodes were shown to exhibit extremely high responses (R_air/R_H2 = ∼10⁷ at 10 000 ppm H₂) to hydrogen even at room temperature. It is thanks to the very high electric field intensity and the narrow top electrode, which allows for hydrogen diffusion under the electrode. Moreover, they are capable of resistive switching within the same sensing structure, which allows them to operate as gasistors, i.e. gas sensors with intrinsic memristive memory. Such sensors however suffer from two major drawbacks: the very high baseline resistance (>1 TΩ at 0 ppm H₂) which renders response to low H₂ concentrations unmeasurable, and the necessity to use e-beam lithography in the fabrication process. Here we present a sensor which eliminates both of the problems. It is based on a similar capacitor-like Pt/TiO₂/Pt structure, but with a large-scale nanoporous top Pt electrode. It is shown that the sensor can detect 3 ppm of H₂ gas in dry air even at room temperature, while providing extremely high response to higher H₂ concentrations (∼10⁹ at 10000 ppm H₂). Humidity decreases the sensor response moderately, particularly at room temperature. We also demonstrate that the sensor is capable of resistive switching, although the device resistance after the electroforming process becomes too low to be effectively used as gasistor.