Surface composition of Mn xCo 1− xO solid solutions by X-ray photoelectron and Auger spectroscopies

Abstract

Auger and X-ray photoelectron spectroscopies have been used to study the surface composition of Mn x Co 1− x O (0≤ x≤1) solid solutions. The polycrystalline materials, which are bulk homogeneous, present clear signs of deviation in surface composition at x bulk≈0.2–0.4 to become surface-enriched in manganese at the expense of the cobalt. Whether on stoichiometric or cobalt-depleted surfaces, cobalt retains the characteristic XPS 2p satellite structure and binding energies of the rock salt monoxide (CoO or Mn x Co 1− x O) environment. Manganese 2p XP spectra are appropriately monoxide-like for the stoichiometric surfaces but broaden and give evidence for a second manganese species with Mn 4+ character for the bulk-enriched samples. Oxygen O 1s spectra also show two different surface species, one the characteristic rock salt lattice oxide at 529.4 eV and a second at 531.3 eV. However, the two peaks are observed for all samples, regardless of whether the surface and bulk compositions were found to differ. The second O 1s peak cannot be unequivocally identified, but is most likely defect-oxide or hydroxyl-like in origin. The 531.3-eV O 1s species forms at the expense of the lattice oxide, and the net oxide-surface concentration is approximately that of the stoichiometric surface.