Characterization of the nickel cobaltite, NiCo2O4 prepared by several methods: An XRD, XANES, EXAFS, and XPS study

The bulk structural properties of the nickel cobaltite, NiCo2O4, prepared b y the thermal decomposition of carbonates, sol-gel methods, and the decompo sition of hydroxides, have been examined by X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (EXAFS and XANES). The results indicate, as expected, that Ni occupies the octahedral sites of the spinel structure while Co occupies both tetrahedral and octahedral sites. The XANES and EXAF S data are compatible with the existence of Ni2+ and Ni3+. The structural d ata indicate that Co is present as Co2+ and Co3+. The bulk cation distribut ion appears to be dependent on the synthesis procedure. The results of the surface examination by X-ray photoelectron spectroscopy (XPS) show that the surface composition is different from the nominal bulk composition and als o varies with the preparative method. All the XP spectra indicate the prese nce of Ni2+ and Ni3+, their relative concentrations being noticeably depend ent upon the synthesis method used. In contrast, the Co XP spectra recorded from all samples are very similar and indicate the presence of both Co2+ a nd Co3+. (C) 2000 Academic Press.