Highly Stable and Ultrafast Hydrogen Gas Sensor Based on 15 nm Nanogaps Switching in a Palladium–Gold Nanoribbons Array

Abstract

Hydrogen gas (H2) is ecofriendly and abundant and can be used as a renewable resource in hydrogen-powered energy systems, such as fuel cells. Despite these positive features, H2 is rapidly diffused in air and is undetectable by human senses, making it a significant explosion hazard. In fact, at H2 concentrations greater than 4% in air,[1] H2 is highly flammable and explosive. Developing a fast and stable H2 sensor with an extremely low limit of detection (LOD) is therefore necessary for early hazard prevention before the hydrogen-powered and/or related systems can be widely exploited. In the pertinent literature, diverse types of H2 sensors based on electrochemical,[2,3] optical,[4–7] and chemiresistive[8–11] principles have been reported. In particular, chemiresistive sensors have potential in the Internetof-Things (IoT) era because they can be miniaturized and integrated into smart electronic devices requiring low power. In this regard, conventional metal oxides, such as SnO2 and TiO2, are unsuitable materials for chemiresistive sensors due to their high operating temperature and large volume. Thus, alternative materials capable of detecting H2 at low or room temperatures[12–14] have been the focus of many recent investigations. Among the proposed alternatives, palladium (Pd) is considered the most promising material because it can selectively absorb H2 at room temperature.[15] In general, the absorbed hydrogen atoms form palladium hydride (PdH) and disturb the electron flow,[16] thereby decreasing electrical conductivity.[17] Most Pd film-based H2 sensors proposed to date rely on this sensing principle; however, they unavoidably suffer from durability degradation due to repetitive changes in the PdH volume.[18] Consequently, PdHx volume changes (where x denotes hydrogen to metal ratio, = H/Pd) have been proposed as a new sensing principle in a so-called Pd nanogap sensor. In this approach, nanogaps or cracks existing on discontinuous Pd structures[19–22] close or open depending on the volume change induced by H2 absorption, which leads to a conductivity change corresponding to an exposed H2 concentration. In a previous Palladium (Pd) nanogap hydrogen gas (H2) sensors based on the large volume expansion of β phase palladium hydride (β-PdH) are highly promising, owing to their fast and accurate sensing capability at room temperature in air. However, such sensors do not work well at H2 concentrations below 1%. At such low H2 concentrations, Pd exists as α-PdH, which has a slow and insufficient volume expansion and cannot completely close nanogaps. Furthermore, the lattice strains induced from the phase transition (α-PdH → β-PdH) behavior degrade the stable and repeatable long-term sensing capability. Here, these issues are resolved by fabricating an array of periodically aligned alloyed palladium–gold nanoribbons (PdAu NRB) with uniform 15 nm nanogaps. The PdAu NRB sensor enables highly stable and ultrafast H2 sensing at the full detection range of H2 concentrations from 0.005% to 10% along with the excellent limit of detection (≈0.0027%), which is sufficiently maintained even after seven months of storage in ambient atmosphere. These breakthrough results will pave the way for developing a practical high-performance H2 sensor chip in the future hydrogen era.