X-ray photoelectron spectroscopic studies of nickel-oxygen surfaces using oxygen and argon ion-bombardment

We have studied the surface chemistry of the nickel-oxygen system using both temperature changes and ion bombardment as techniques for elucidating the surface structure. The spectra of metallic Ni, NiO and Ni2O3 were characterized from samples prepared directly in the spectrometer. The Ni2O3 species could be distinguished from an authentic Ni(OH)2 sample from both the X-ray photoelectron lines and the Auger transitions. The oxides of NiO and Ni2O3 could be prepared by bombardment with low energy (400eV) O2+ ions as well as by exposure of Ni to oxygen at reduced pressure (∼ 100 torr). The Ni2O3 was found to be present on most nickel-oxygen surfaces except those prepared by exposing Ni to air for many hours at high temperature (> 600°C), indicating that the stability of Ni2O3 decreased as the temperature increased. Exposure of both NiO and Ni2O3 to 400 eV Ar+ ion bombardment caused reduction to metallic Ni. This observation has also been noted for several other oxides and a prediction of whether or not reduction should be observed is presented by examining the free energy of formation of the molecule.