Splitting of Ni 3d states at the surface of NiO nanostructures

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 Ni2+ ion performed in both octahedral (bulk) and pyramidal (surface) symmetries confirm this interpretation. This new interpretation of the surface electronic structure of NiO suggests a revision of the accepted concepts concerning this system.