X-ray photoelectron spectroscopic study of non-stoichiometric nickel and nickel-copper spinel manganites
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چکیده
The surface of non-stoichiometric nickel and nickel–copper spinel manganites has been investigated by X-ray Photoelectron Spectroscopy (XPS). The oxidation states of the nickel, copper and manganese cations present on the surface of the samples were 21 1 determined from the analysis of the M 2p core levels (M5Ni, Cu, Mn). In particular, both Cu and Cu were evidenced in the 3 / 2 structure whereas only bivalent nickel was observed. The partial substitution of manganese by copper led to a chemical shift towards 21 lower binding energy in the Ni 2p region, which was explained by the displacement of some Ni cations from tetrahedral to 3 / 2 octahedral sites of the spinel structure. Finally, the surface atomic ratios Ni /Mn for nickel manganites, Ni /(Mn1Cu) and Cu/(Mn1Ni) for nickel–copper manganites, determined from XPS data, were compared to the ratios corresponding to the bulk composition. This study shows in all cases a nickel enrichment at the surface which is not affected by the copper content of the oxide. On the contrary, the ratio Cu/(Mn1Ni)! was! found! to! be! lower! than! the! corresponding! bulk! value.
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