Pd-coated Au core-shell nanorod metamaterial for optical hydrogen sensing

Hydrogen is widely regarded as an ideal clean-energy carrier, and its safe, efficient utilization is critical to the transition of modern energy systems. However, hydrogen's high diffusivity and flammability make leak monitoring an urgent safety imperative. Here, we propose and fabricate an optical hydrogen sensor consisting of gold core-palladium shell nanorod arrays embedded in porous anodic aluminum oxide (Au@Pd NRAs/AAO). The sensor harnesses near-field coupling between the nanorods' transverse localized surface plasmon resonance (LSPR) and a vertical Fabry-Pérot (F-P) cavity formed by the array, yielding a hybrid LSPR-F-P resonance that amplifies the response to refractive-index perturbations induced by palladium hydride formation in the shell. By combining simulations and experiments, we systematically investigate how geometric parameters govern performance and elucidate the enhancement mechanism. Experimentally, within 0-2 vol% H₂, the sensor exhibits a sensitivity of 11.33 nm/% with excellent linearity. At 2 vol% H₂, the response and recovery times are <20 s and <50 s, respectively, and the device shows outstanding repeatability and stability with no appreciable hysteresis or baseline drift. Numerical simulations further indicate that integrating silver nanodiscs (AgNDs) on the array surface opens a vertical plasmonic-coupling channel, improving the sensor response by ≈ 20.6% under the same hydrogenation conditions. Leveraging the mechanically robust, process-compatible anodic aluminum oxide (AAO) template, the device is compact and inherently immune to electromagnetic interference, making it promising for industrial safety monitoring and new-energy applications.