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

## TL;DR

A new hydrogen sensor using Ti/Pd thin films detects hydrogen quickly and accurately in the near-infrared range, with potential for use in various industries.

## Contribution

A Ti/Pd bilayer thin film is introduced for hysteresis-free, high-speed NIR hydrogen sensing with enhanced optical contrast and durability.

## Key findings

- Ti/Pd bilayers showed 2.7x higher hydrogen-induced optical contrast at 1550 nm compared to Pd/TAF films.
- The optimized sensor had a detection limit below 10 ppm and a rapid response time (t90 < 0.35 s at 4% H2).
- The sensor exhibited high durability with less than 6% signal degradation over 135 cycles.

## Abstract

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 investigated Ti/Pd bilayer thin films as a platform for NIR hydrogen sensing—particularly at telecommunication-relevant wavelengths, where such devices have remained largely unexplored. Ti/Pd bilayers coated with Teflon AF (TAF) and fabricated via sequential electron-beam and thermal evaporation were characterized using optical transmission measurements under repeated hydrogenation cycles. The Ti (5 nm)/Pd (x = 2.5 nm)/TAF (30 nm) architecture showed a 2.7-fold enhancement in the hydrogen-induced optical contrast at 1550 nm compared to Pd/TAF reference films, attributed to the hydrogen ion exchange between the Ti and Pd layers. The optimized structure, with a Pd thickness of x = 1.9 nm, exhibited hysteresis-free sensing behavior, a rapid response time (t90 < 0.35 s at 4% H2), and a detection limit below 10 ppm. It also demonstrated 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.