OXIDATION-INDUCED SEGREGATION AT THE PT0.5NI0.5(111) SURFACE STUDIED BY MEDIUM-ENERGY ION-SCATTERING

The kinetics of oxygen uptake at 400°C at low O2 pressures on a Pt0.5Ni0.5(111) surface is investigated using ellipsometry. The oxidation process exhibits a gradually decreasing oxidation rate. It saturates above 1500 langmuir of O2 exposure. Medium-energy ion scattering combined with shadowing and blocking is used to investigate the surface composition before and after exposure to 350 langmuir of O2 at 400°C. This exposure results in an oxygen coverage of (2.3 ± 0.3) X 1015 atoms/cm2, about half the coverage of the saturated surface. Whereas the first atomic layer of the alloy surface before exposure to oxygen contains mainly Pt atoms, after exposure to O2 the top layer is disordered and contains only Ni and O atoms. Underneath this disordered top layer a crystalline layer of metal atoms is present containing (65 ± 5)% of Pt. The third atomic layer (the second crystalline layer) contains (63 ± 8)% of Pt. The total number of Ni atoms present in the disordered toplayer and the first crystalline layer is (2.1 ± 0.3) X 1015 atoms/cm2. The first two crystalline interlayer distances are found to be expanded by (2.4 ± 0.5)% and (2.3 ± 0.5)% respectively, both with respect to the bulk value. Upon O2 exposure at 400°C, the initial oxidation rate of a bulk Pt0.5Ni0.5(111) surface is equal to that on a Ni(111) surface covered with 4.7 monolayers of platinum. The similarity is explained with previously observed surface alloy formation on a Pt-covered Ni(111) upon annealing. The surface region of this alloy is very similar to the Pt0.5Ni0.5(111) surface. © 1990.