Hysteresis-Free Near-Infrared Optical Hydrogen Sensor Based on Ti/Pd Bilayer Thin Films

Palladium (Pd) and titanium (Ti) exhibit opposite dielectric responses upon hydrogenation, with stronger effects observed in the near-infrared (NIR) region. Leveraging this contrast, we investigate Ti/Pd bilayer thin films as a platform for NIR hydrogen sensing—particularly at telecommunication-relevant wavelengths, where such devices remain largely unexplored. Ti/Pd bilayers, coated with Teflon AF (TAF) and fabricated via sequential electron-beam evaporation, were characterized using optical transmission measurements under repeated hydrogenation cycles. The Ti (5 nm) / Pd (x = 2.5 nm) / TAF (30 nm) architecture shows a 2.7 times enhancement in hydrogen-induced optical contrast at 1550 nm compared to Pd/TAF reference films, attributed to hydrogen ion exchange between the Ti and Pd layers. The optimized structure, with Pd thickness of x = 1.9 nm, exhibits hysteresis-free sensing behavior, a rapid response time (t90 < 0.35 s at 4% H2), and a detection limit below 10 ppm. It also demonstrates excellent selectivity with negligible cross-sensitivity to CO2, CH4, and CO, as well as high durability, showing less than 6% signal degradation over 135 hydrogenation cycles. These findings establish a scalable, room-temperature NIR hydrogen sensing platform with strong potential for deployment in automotive, environmental, and industrial applications.