Stabilized switchable “black state” in Mg2NiH4∕Ti∕Pd thin films for optical hydrogen sensing

A triple layer thin film (30nm Mg2Ni∕100nm Ti∕10nm Pd sputtered on glass) switches reversibly from a shiny metallic to a “black” state upon exposure to moderate hydrogen pressure (≈5.103Pa). This black state resembles that obtained in thick Mg2NiHx layers and has the great advantage of being stable and easily controlled. Both the reversible high optical contrast (Rreflective∕Rblack≈10 in the red wavelength range) and the fast kinetics of hydrogen absorption and desorption make this material interesting for applications as optical hydrogen sensors.