AVS 51st International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM3
Surface and Subsurface Oxidation of Ni(100) and Ni(111) Studied by Medium Energy Ion Scattering

Wednesday, November 17, 2004, 9:00 am, Room 210B

Session: Metal Oxides and Clusters I: Formation and Structure
Presenter: M.A. Muñoz-Márquez, University of Warwick, UK
Authors: M.A. Muñoz-Márquez, University of Warwick, UK
R.E. Tanner, University of Warwick, UK
D.P. Woodruff, University of Warwick, UK
Correspondent: Click to Email
Using medium energy ion scattering (MEIS) and specifically 100 keV H@super +@ incident ions, the oxidation of the Ni(100) and Ni(111) surfaces have been investigated at temperatures around 200-300°C and with oxygen exposures up to 6000 L. This treatment range corresponds to that typically used to produce well-defined NiO(100) surfaces on Ni(100) and involves significantly thicker oxide formation than has been investigated by conventional surface science techniques in the past. The potential of MEIS to probe the deeper subsurface is highly relevant in these studies, as oxide formation >100 Å is seen on Ni(111). While it is well-established that the earliest stages of oxidation occur at lower exposures on Ni(111) than Ni(100), attributed to site blocking in the different chemisorbed oxygen phases on the two surfaces, our results show that this greater reactivity of Ni(111) persists to rather deep oxidation. Throughout these oxide films the stoichiometry of NiO is retained. Through the use of 'double-alignment' scattering geometries and qualitative LEED, the crystallography of these oxide layers has been investigated. LEED shows that on Ni(111) two (of the possible three) orientational domains of NiO(100) form, attributed to a slight misorientation of the surface. In general, the average crystalline registry with the bulk is poor, although at intermediate exposures a surprising result is that the outermost layers appear better-ordered than the near sub-surface. This is attributed to the probable influence of thick islands of limited lateral extent which causes the more penetrating ions to pass through differently oriented oxide islands. On Ni(111) simulations using the VEGAS code confirm that the data are consistent with this rationale. On Ni(100), the apparent poor order can also be reproduced in simulations assuming the NiO(100) crystallites are tilted on the surface as previously proposed on the basis of SPA-LEED investigations.