Adsorption of CO and NO on NiO and CoO: a comparison

Polar (111) and non-polar (100) thin epitaxial NiO and CoO films were prepared on suitable Ni and Co single-crystal metal surfaces. On these oxide films, CO and NO were adsorbed to probe the local electronic and geometric structure of the substrates using electron spectroscopic methods, especially HREELS and NEXAFS. On all oxide surfaces, the N-O stretching frequencies exhibit a red shift due to the chemical bonding to the surface, whereas the C-O stretching frequencies all lie in the vicinity of the C-O gas phase value with a tendency for a blue shift because of the purely electrostatic interaction with the substrates. An evaluation of these data together with NEXAFS data show that the NiO(111) film, which undergoes octopolar reconstruction upon heating, exhibits microfacets with fourfold sites tilted about 55° away from the surface normal even at room temperature. The situation on CoO(111), which cannot be heated sufficiently to prepare a reconstructed surface, seems to be somewhat different; a model of the possible structure of the unreconstructed CoO(111) surface is proposed