An interpretation for the components of 2core level x-ray photoelectron spectra of the cations in some inverse spinel oxides.

In an effort to reconcile the various interpretations for the cation components of the 2observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2core levels of NiFeOthin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles NiCoFeO(= 0.2, 0.5, 0.8, 1) resemble the surface of the NiFeOthin film. Surface-to-bulk core level shifts are also observed in CoFeOand NiCoOthin films but not as significantly as in NiFeOthin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles.