Surface composition and structure of Co3O4(110) and the effect of impurity segregation

Abstract

The Co 3 O 4 (110) single crystal surface has been characterized by low energy electron diffraction ͑LEED͒, Auger electron spectroscopy, and x-ray photoelectron spectroscopy ͑XPS͒. LEED analysis of the clean Co 3 O 4 (110) spinel surface shows a well-ordered pattern with sharp diffraction features. The XPS spectra are consistent with stoichiometric Co 3 O 4 as determined by the concentration ratio of oxygen to cobalt (C O /C Co ) and spectral peak shape. In particular, the cobalt 2p XPS spectra are characteristic of the spinel structure with Co 3ϩ occupying octahedral sites and Co 2ϩ in tetrahedral sites within the lattice. During prolonged heating at 630 K, bulk impurities of K, Ca, Na, and Cu segregated to the surface. Sodium desorbed from the surface as NaOH at 825 K, potassium and calcium were only removed by sputtering since no desorption from the surface was detected for temperatures up to 1000 K. Copper also disappeared upon heating above 700 K, most likely by desorbing although the possibility of diffusion back into the bulk could not be eliminated. The appearance of copper impurities correlated with Co 3 O 4 (110) surface reduction to CoO, and the surface could not be fully reoxidized even upon extended oxygen annealing as long as the copper impurity remained on the surface. Upon removal of the Cu from the near-surface region, the surface was easily reoxidized to Co 3 O 4 by O 2 .

Optical and Physical Properties of Cobalt Oxide Films Electrogenerated in Bicarbonate Aqueous Media

“…Similar XP spectra for wellidentified CoO and Co 3 O 4 standards were reported in the literature. 34,35 As demonstrated in Figure 4, Ar + sputtering of standard Co 3 O 4 leads to the formation of CoO. Therefore, XPS investigation of the underneath layer of electrogenerated films is not appropriate for the identification of the passive film inner layer.…”

Cobalt oxide surface chemistry: The interaction of CoO(100), Co3O4(110) and Co3O4(111) with oxygen and water

“…These impurities include calcium, potassium, sodium, and copper, and the presence of copper, in particular, appears to prevent reoxidation of the substrate by the formation of a copper oxide overlayer. The segregation and reoxidation process is described in greater detail elsewhere [8,31].…”

Electronic states and magnetic structure at the Co3O4(110) surface: A first-principles study

“…9 (also see Fig. 2 14,15 augmented with an on-site Coulomb repulsion U term in the 3d shell of the cobalt ions. The GGA+U approach reduces significantly the delocalization error arising from the incomplete cancellation of the Coulomb self-interaction in pure GGA calculations, 21 and gives a value of the band gap for bulk Co 3 O 4 (1.96 eV) in reasonable agreement with experiment (~1.6 eV).…”