Room-temperature hydrogen absorption in Mg-based thin films

The growing interest in the study of reversibly hydrogen-absorbing thin films is mainly due to their potential application as hydrogen sensors or switchable mirrors (smart windows) in electronics. From an economic point of view, magnesium would be the most suitable material for such applications. In contrast to bulk magnesium, Mg thin films covered with a palladium layer can absorb hydrogen at room temperature and pressures of up to 1 bar. One important experimental problem that still needs to be solved is the improvement of the too-slow absorption kinetics. This paper presents the results of studies leading to a significant improvement in hydrogen absorption kinetics by depositing an ultrathin Ni (Al or C) layer between the top Pd catalytic layer and the Mg base layer. A significant improvement in the absorption kinetics was also achieved by replacing pure Mg with a Mg₂Ni alloy. In addition, in order to determine the mechanisms responsible for the improvement of absorption kinetics, the effect of atom mixing in the interface region was studied in detail using X-ray photoelectron spectroscopy. The obtained results confirmed the important role of the Ni interlayer in improving hydrogen absorption kinetics in Pd/Ni/Mg trilayers. In Pd/Mg₂Ni bilayers, the Ni interlayer is formed spontaneously due to the segregation of Mg atoms on the surface. In the case of Al and C interlayers, the improvement of absorption kinetics occurs due to the spontaneous formation of small islands in the interface region containing Al atoms and magnesium carbide, respectively, which can form heterogeneous nucleation centres. The optimal thicknesses of Ni, Al and C layers are 3.0, 0.5, and 1.4 nm, respectively. The obtained results can be used to obtain new thin-film metallic nanomaterials with improved functional properties at room temperature.