Tantalum and Tantalum–Palladium Coated FBGs for Low-Concentration Hydrogen Sensing

Hydrogen is a cornerstone of the emerging net-zero carbon economy, and its widespread deployment demands sensitive, stable, and scalable detection technologies. In this study, we present a comparative performance analysis of Fibre Bragg Grating (FBG) sensors coated with nanometre-thick metal hydride-forming layers—tantalum (Ta), tantalum-palladium alloy (Ta <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.88}$</tex-math></inline-formula> Pd<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.12}$</tex-math></inline-formula>), palladium (Pd), and palladium-gold alloy (Pd <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.6}$</tex-math></inline-formula> Au<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.4}$</tex-math></inline-formula>)—for optical hydrogen sensing. The integration of Ta and Ta <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.88}$</tex-math></inline-formula> Pd<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.12}$</tex-math></inline-formula>, two tantalum-based metal hydrides, with FBG sensors is introduced here for the first time, offering a promising alternative to conventional Pd-based materials. All coatings were deposited via magnetron sputtering and tested under controlled hydrogen exposure across concentrations ranging from 0.001% to 100% H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>. The Ta-based FBGs exhibited outstanding performance, showing a remarkably linear relative wavelength shift over the full tested range (0.001% to 100% H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>), with sensitivity detectable down to 10 ppm—the lowest concentration achievable in the current setup. Both Ta and Ta <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.88}$</tex-math></inline-formula> Pd<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.12}$</tex-math></inline-formula> sensors exhibited fully reversible and hysteresis-free response characteristics, with rapid response and recovery. Among them, the Ta <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.88}$</tex-math></inline-formula> Pd<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.12}$</tex-math></inline-formula> sensor with a 100 nm coating demonstrated the highest logarithmic sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>9 pm/decade(%H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>), corresponding to a 9 pm wavelength shift for every tenfold increase in hydrogen concentration between 0.001% and 100% H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>. In contrast, Pd and Pd <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.6}$</tex-math></inline-formula> Au<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.4}$</tex-math></inline-formula> sensors showed degraded performance at low concentrations and greater signal hysteresis. These results underscore the potential of Ta and Ta <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.88}$</tex-math></inline-formula> Pd<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.12}$</tex-math></inline-formula> coatings as robust and high-performance alternatives to conventional Pd-based materials for next-generation distributed fibre-optic hydrogen sensing systems.