Optical hydrogen sensing beyond palladium: Hafnium and tantalum as effective sensing materials

•Palladium-capped hafnium and tantalum thin films can optically track hydrogen.•These thin films offer a hysteresis-free optical response to hydrogen that is stable over time.•The sensing range of hafnium and tantalum extends over a much larger hydrogen pressure region than palladium-based hydrogen sensing materials, making these materials especially advantageous in detecting low hydrogen pressures.•The hydrogen content in both hafnium and tantalum scales linearly with the optical signal.•The applicability of hafnium and tantalum illustrate that palladium-capped transition metals provide ample opportunities to design optical hydrogen sensors with desirable properties. The detection of hydrogen in a fast, efficient and accurate way is vital for its safe handling in industrial processes and in its use as an energy carrier. Thin film metal hydrides are able to probe the hydrogen pressure, often relying on a change of the optical properties of a sensing layer comprised of palladium and alloys thereof. Apart from hysteresis, these palladium-based thin films suffer from a limited operating range. Here, we study the optical and structural properties of palladium capped hafnium and tantalum thin films. Both tantalum and hafnium thin films offer a stable and hysteresis-free optical response to hydrogen over a much larger pressure range than palladium-based films. Remarkably, the hydrogen content in both cases proves to be linear with the optical signal. In a wider perspective, these results illustrate that palladium-capped transition metals provide ample opportunities to design optical hydrogen sensors with desired properties.