XPS Valence band and segregation effect in nanocrystalline -type materials

X-ray photoelectron spectroscopy studies showed that the surface segregation of Mg atoms and valence band width in the nanocrystalline Mg 2 Ni alloy are greater compared to those observed in polycrystalline Mg 2 Ni thin films. Especially, a strong surface segregation of Mg atoms was observed for the Mg 2 Ni / Pd composites. In that case, Mg atoms strongly segregate to the surface and form a Mg-based oxide layer under atmospheric conditions. The lower lying Ni and Pd atoms form a metallic subsurface layer and could be responsible for the observed relatively high hydrogenation rate. Furthermore, the valence band broadening observed in the nanocrystalline Mg 2 Ni alloys and Mg 2 Ni / Pd composites could also significantly influence their hydrogenation properties. Results show, that the shape and broadening of the valence band of the Mg 2 Ni / Pd composites strongly depends on the preparation conditions.