Interaction of Ni/Al interfaces with oxygen

The interaction of oxygen with nickel deposited on polycrystalline aluminum surfaces has been investigated using Auger electron spectroscopy, X-ray photoelectron spectroscopy, and angle-resolved X-ray photoelectron spectroscopy. The growth of the nickel on the aluminum surfaces occurs in two stages: formation of NiAlx (x approximate to 0.45) islands 10 monolayers (ML) thick up to a coverage of theta(NiAlx) approximate to 0.7, followed by the formation of metallic nickel islands 8 ML thick that grow over the intermetallic islands previously formed. For surfaces containing NiAlx islands alone, the chemical information obtained by the analytical techniques shows that oxygen exposure causes the formation of aluminum intermediate oxidation states Al+ and Al2+, in addition to Al3+, which are attributed to the formation of Al-O-Ni cross-linking bonds at the interface. The analysis of the Ni 2p peak shape shows that no nickel oxide is formed, the small changes observed in this band being attributed to Ni atoms in an aluminum-depleted layer at the interface. In contrast, nickel oxide is formed during the oxidation of surfaces containing nickel islands. In such a case, the oxide film is composed of a mixture of intermediate aluminum oxidation states that grow over a NiO oxide layer. At the interface between Al and Ni oxides, a NiAl2O4-like mixed oxide or Ni3+ defects are formed.