Thin Film TaFe,\nTaCo, and TaNi as Potential Optical\nHydrogen Sensing Materials

This paper studies the structural and optical properties\nof tantalum–iron-,\ntantalum–cobalt-, and tantalum–nickel-sputtered thin\nfilms both ex situ and while being exposed to various hydrogen pressures/concentrations,\nwith a focus on optical hydrogen sensing applications. Optical hydrogen\nsensors require sensing materials that absorb hydrogen when exposed\nto a hydrogen-containing environment. In turn, the absorption of hydrogen\ncauses a change in the optical properties that can be used to create\na sensor. Here, we take tantalum as a starting material and alloy\nit with Fe, Co, or Ni with the aim to tune the optical hydrogen sensing\nproperties. The rationale is that alloying with a smaller element\nwould compress the unit cell, reduce the amount of hydrogen absorbed,\nand shift the pressure composition isotherm to higher pressures. X-ray\ndiffraction shows that no lattice compression is realized for the\ncrystalline Ta body-centered cubic phase when Ta is alloyed with Fe,\nCo, or Ni, but that phase segregation occurs where the crystalline\nbody-centered cubic phase coexists with another phase, as for example\nan X-ray amorphous one or fine-grained intermetallic compounds. The\nfraction of this phase increases with increasing alloyant concentration\nup until the point that no more body-centered cubic phase is observed\nfor 20% alloyant concentration. Neutron reflectometry indicates only\na limited reduction of the hydrogen content with alloying. As such,\nthe ability to tune the sensing performance of these materials by\nalloying with Fe, Co, and/or Ni is relatively small and less effective\nthan substitution with previously studied Pd or Ru, which do allow\nfor a tuning of the size of the unit cell, and consequently tunable\nhydrogen sensing properties. Despite this, optical transmission measurements\nshow that a reversible, stable, and hysteresis-free optical response\nto hydrogen is achieved over a wide range of hydrogen pressures/concentrations\nfor Ta–Fe, Ta–Co, or Ta–Ni alloys which would\nallow them to be used in optical hydrogen sensors.