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 by the thermal decomposition of carbonates, sol-gel methods, and the decomposition 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 EXAFS data are compatible with the existence of Ni2+ and Ni3+. The structural data indicate that Co is present as Co2+ and Co3+.The bulk cation distribution 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 also varies with the preparative method. All the XP spectra indicate the presence of Ni2+ and Ni3+, their relative concentrations being noticeably dependent 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+ and Co3+.