Electronic structure of nanocrystalline and polycrystalline hydrogen storage materials

To optimise the choice of the compounds for a selected application, a better understanding of the role of each alloy constituent on the electronic properties of the material is crucial. In this work, we study experimentally the electronic properties of nanocrystalline and polycrystalline (Mg1−xMx)2Ni, (Mg1−xMx)2Cu, La(Ni1−xMx)5, and Ti(Ni1−xM′x) (MMn,Al; M′Fe,Mg,Zr) alloys. The nanocrystalline and polycrystalline samples were prepared by mechanical alloying (MA) followed by annealing and arc melting method, respectively. All X-ray photoelectron spectroscopy (XPS) spectra were measured immediately after cleaning of the sample surface in a vacuum of 8×10−11mbar. Furthermore, we have measured XPS spectra of in situ prepared nanocrystalline and polycrystalline LaNi5, TiNi, and Mg2Ni thin films and compared with those obtained for ex situ prepared bulk materials. The substitution of Mg in Mg2Ni and Mg2Cu, Ni in LaNi5 and TiNi by transition metals leads to significant modifications of the shape and width of the valence band of the nanocrystalline as well as polycrystalline samples. Especially, the valence bands of the MA nanocrystalline alloys are considerably broader compared to those measured for the polycrystalline samples. Results also showed that the strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence on their hydrogenation properties.