Oxidation of clean and Pt-covered Ni(111)

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Clean and Pt-covered Ni(111) surfaces have been exposed to oxygen at ambient and elevated temperatures. The platinum coverage is determined with Rutherford backscattering spectrometry. The extent and rate of oxidation of the surfaces is monitored in situ using ellipsometry. The absolute oxygen coverages have been calibrated using the 18O(p, α) 15N nuclear reaction. After oxidation, concentration profiles have been obtained by means of Auger depth profiling. On the clean nickel surface a closed oxide layer is formed at room temperature. At 250 and 400°C, oxygen penetrates into the bulk of the crystal. Small amounts of platinum are capable of decreasing drastically the extent of oxidation of the nickel crystal at all temperatures explored. At room temperature, no oxidation occurs above platinum coverages in excess of about one monolayer. At 250 ° C, a nickel oxide layer is formed on top of the platinum overlayer, but no oxygen penetrates through the platinum layer into the bulk. At 400 ° C, finally, a nickel oxide layer is formed at the surface on top of the platinum layer, while also a small fraction of the oxygen diffuses through the platinum layer into the bulk of the crystal. An explanation of the observed phenomena, also based on other deposition and annealing measurements on this system, is presented. Furthermore, a comparison is made with previous work on the oxidation of a bulk Pt 0.5Ni 0.5(111) surface.