We report experimental and theoretical evidence of surface effects in the Ni 2p x-ray photoemission spectra ͑XPS͒ of NiO. The Ni 2p 3/2 surface-enhanced XPS of a NiO sample show a relative enhancement of the intensity of the known satellite at 1.5 eV higher binding energy from the main line, indicating a considerable surface contribution of this satellite. The results are discussed in terms of bulk-octahedral and surfacepyramidal Ni symmetries. Other contributions, like nonlocal screening effects, cannot be neglected.

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We report the Ni 2p x-ray photoelectron spectra of NiO thin films grown on different oxide substrates, namely, SiO 2 , Al 2 O 3 , and MgO. The main line of the Ni 2p spectra is attributed to the bulk component, and the shoulder at 1.5 eV higher binding energies to the surface component. The spectra of the NiO thin films show strong differences with respect to that of bulk NiO. The energy separation between the main peak and the shoulder increases with the substrate covalence. This indicates the strong covalent interactions between the NiO thin films and the oxide substrates, and reflects changes in the bonding at the interface from a more ionic to a more covalent interaction. These conclusions are supported by cluster model calculations with a reduced O 2p-Ni 3d hybridization.

The electronic structure of NiO nanometric planar islands on highly oriented pyrolitic graphite has been studied by means of the Ni 2p and O 1s x-ray absorption spectra. The O 1s spectrum of the early stages of growth shows a double peak at threshold which is attributed to a splitting of the unoccupied Ni e g states. This spectrum is compared to previous results for 3 nm NiO nanoparticles which also show a rather similar splitting of the Ni e g states. This splitting observed in the O 1s spectra of the NiO nanostructures is caused by the reduced symmetry of the Ni ions at the surface. Cluster model calculations for a high-spin Ni 2+ ion performed in both octahedral ͑bulk͒ and pyramidal ͑surface͒ symmetries confirm this interpretation. This new interpretation of the surface electronic structure of NiO suggests a revision of the accepted concepts concerning this system.