Molecular adsorption on oxide surfaces: Electronic structure and orientation of NO on NiO(100)/Ni(100) and on NiO(100) as determined from electron spectroscopies and ab initio cluster calculations

We have investigated the adsorption of NO on a thin NiO(100) film of several layers thickness grown on top of a Ni(100) surface in comparison with data of an in vacuo cleaved NiO(100) single crystal. The layer exhibits a high defect density. We demonstrate via application of several surface-sensitive electron-spectroscopic techniques [i.e., x-ray photoelectron spectroscopy (XPS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), near-edge x-ray-absorption fine structure (NEXAFS), and high-resolution electron-energy-loss spectroscopy (HREELS)] that this layer has similar occupied (ARUPS) and unoccupied (NEXAFS) states as a bulk NiO(100) sample. In spite of its limited thickness, the band structure of the film exhibits dispersions perpendicular to the surface compatible with bulk NiO(100). It is shown that the electronic structure of the oxygen sublattice can be described in a band-structure picture while for the Ni sublattice electron localization effects lead to a breakdown of the band-structure picture. NO on NiO desorbs at 220 K. This indicates weak chemisorption. The NO coverage is close to 0.2 relative to the number of Ni surface atoms as determined by XPS.

HREELS reveals that there is only one species on the surface documented by the observation of only one bond-stretching frequency. NEXAFS data on the system and a comparison with previous data on the system NO/Ni(100) indicate that the molecular axis of adsorbed NO is tilted by an angle of approximately 45° relative to the surface normal. The N 1s XP spectra of the weakly chemisorbed species show giant satellites similar to the previously observed cases for weak chemisorption on metal surfaces. This is the first observation of an intense satellite structure for an adsorbate on an insulator surface, which shows that there must be sufficient screening channels even on an insulating surface. A theoretical assignment of the peaks is discussed. We compare the spectroscopic properties of the NO species on the thin-film oxide surface, which is likely to contain a certain number of defects, with NO adsorbed on a basically defect-free bulk oxide surface by thermal-desorption (TDS) and XP spectra. TDS and XP spectra of the bulk system are basically identical as compared with the oxide film, indicating that the majority of species adsorbed on the film is not adsorbed on defects but rather on regular NiO sites. Results of ab initio oxide cluster calculations are used to explain the bonding geometry of NO on regular NiO sites.