X-ray photoelectron spectroscopic study of non-stoichiometric nickel and nickel-copper spinel manganites

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 determined from the analysis of the M 2p(3/2) core levels (M=Ni, Cu, Mn). In particular both Cu2+ and Cu+ were evidenced in the determined from the analysis of the M 2p(3/2) core levels (M=Ni, Cu, Mn). In particular, both Cu structure whereas only bivalent nickel was observed. The partial substitution of manganese by copper led to a chemical shift towards lower binding energy in the Ni 2p(3/2) region, which was explained by the displacement of some Ni2+ cations from tetrahedral to octahedral sites of the spinel structure. Finally, the surface atomic ratios Ni/Mn for nickel manganites, Ni/(Mn+Cu) and Cu/(Mn+Ni) 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/(Mn+Ni) was found to be lower than the corresponding bulk value. (C) 2000 Elsevier Science Ltd. All rights reserved.