Precise control of the size and interfaces of Pd grains is very important for designing a high-performance H2 sensing channel because the transition of the Pd phase from α to β occurs through units of single grains. However, unfortunately, the grain controllability of previous approaches has been limited to grains exceeding 10 nm in size and simple macroscopic channel structures have only shown monotonic response behavior for a wide concentration range of H2. In this work, for the first time, we found that Pd channels that are precisely grain-controlled show very different H2 sensing behavior. They display dual-switching response behavior with simultaneous variation of the positive and negative response direction within single sensor. The Pd nanopattern channel having smallest grain size/interface among previous works could be fabricated via unique lithographic approaches involving low-energy plasma (Ar+) bombardment. The ultrasmall grain size (5 nm) and narrow interface gap (<2 nm) controlled by Ar+ plasma bombardment enabled both the hydrogen-induced lattice expansion (HILE) (Δ RH2 < 0) and surface electron scattering (Δ RH2 > 0) mechanisms to be simultaneously applied to the single Pd channel, thereby inducing dual-switching response according to the H2 concentration range. In addition, the unique high-aspect-ratio high-resolution morphological characteristics made it possible to achieve highly sensitive H2 detecting performance (limit of detection: 2.5 ppm) without any hysteresis and irreversible performance degradation. These noteworthy new insights are attributed to high-resolution control of the grain size and the interfaces with the Pd nanostructure channel.
Keywords: gas sensor; hydrogen; palladium; switching response; α to β transition.