Defects in oxide overlayers at nickel single-crystal surfaces

Abstract

The chemisorption of oxygen at Ni(100) and Ni(210) surfaces has been studied by X -ray photoelectron spectroscopy in the temperature range 77-300 K. O(1s) spectra distinguished between the formation of O - and O 2- species while Ni(2p) spectra revealed the formation of Ni 2+ and Ni 3+ states. The binding energies of the latter are 854.7 and 856 eV respectively. These values were established by the generation of difference spectra in the Ni(2p) spectral region and assignments confirmed by studies of bulk nickel oxides of different stoichiometry. There were two distinct régimes of oxygen interaction; for low oxygen coverages, θ < 0.6, there was no evidence for the higher oxidation states (Ni 2+ or Ni 3+ ). For θ > 0.6 the Ni(2p) difference spectra indicated that both states were formed, their relative proportions being dependent on temperature and coverage. It is suggested that the initial stage of oxygen chemisorption involves only the Ni(4s4p) band while the Ni(3d) electrons participate in oxide formation, which is reflected by the emergence of Ni 2+ and Ni 3+ states. Ni (L 2, 3 M 4, 5 M 4, 5 ) Auger spectra and also recent theoretical work support this conclusion. The mixed valence states, Ni 2+ and Ni 3+ , are a characteristic feature not only of oxygen interaction with Ni(210) and Ni(100) at low temperature but also of the surface of bulk nickel oxides. The latter unexpectedly exhibit a high relative proportion of Ni 3+ states. It is suggested that Ni 3+ is formed in a surface redox reaction from Ni 2+ with the simultaneous generation of O - . Angular dependent studies confirm that the Ni 3+ and O - species are at the surface relative to Ni 2+ and O 2- .