OXIDATION AND DIFFUSION-PROCESSES IN NICKEL-TITANIUM OXIDE SYSTEMS

The oxidation of nickel in Ti/Ni (titanium evaporated on Ni), Ni/TiO2 (nickel evaporated on TiO2) and Ni/TiO(x) (nickel evaporated on TiO2 previously submitted to Ar+ bombardment) systems has been studied by XPS and ISS. The deposition of Ti on Ni, as followed by ISS occurs in a two-dimensional way up to approximately half a monolayer, when it starts to agglomerate in three-dimensional islands. XPS has shown that while for clean nickel oxidation up to 80% of the topmost layers occurs after exposure to 1800 L of O2 at 573 K, only partial oxidation (i.e. approximately 31%) takes place when nickel is covered with half a monolayer of Ti. No oxidation occurs for a coverage of 2.5 monolayers (i.e. full coverage). These results show that besides a physical protection by coating, oxidation of nickel can be prevented by a modification of its oxidation pathways due to TiO2 deposits. Oxidation of Ni evaporated on TiO2 and TiO(x) (x = 1.5) was followed by XPS. In Ni/TiO(x), some diffusion of Ni into the defective oxide occurs at 298 K during deposition of the metal, while small differences in the degree of oxidation of nickel with respect to Ni/TiO2 can be ascribed to a Ni2+ + Ti(n) + (n < 4) --> Ni0 + Ti4+ reaction which occurs at 573 K. Heating the Ni/TiO(x) system in vacuum at T > 773 K enhances the diffusion of Ni into the TiO(x) defective layer as indicated by the loss of the Ni peak in the ISS spectrum and a drastic decrease in the Ni/Ti XPS peak ratio. Under these conditions, exposure to O2 (1800 L) at 298 and 573 K only produces a further decrease in the Ni/Ti peak ratio while TiO(x) is oxidized at the surface. These results suggest that diffusion of Ni0 in defective TiO(x) can be a very important process during the oxidation of Ni-Ti systems.