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

Paper SS1-WeM5
Oxidation of Pt(111) by Gas-Phase Oxygen Atoms

Wednesday, November 17, 2004, 9:40 am, Room 210B

Session: Metal Oxides and Clusters I: Formation and Structure
Presenter: J.F. Weaver, University of Florida
Authors: J.F. Weaver, University of Florida
A.L. Gerrard, University of Florida
J.J. Chen, University of Florida
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The oxidation of Pt(111) by an atomic oxygen beam has been investigated as a function of surface temperature using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (ELS). For oxygen coverages below about 0.50 ML (where 1 ML = 1.5 x 10@super 15@cm@super -2@), the rate of atomic oxygen adsorption is found to be insensitive to surface temperature over the range from 150 to 450 K. After 0.5 ML of atomic oxygen is adsorbed, the oxidation rate decreases with decreasing temperature, and the oxygen coverage saturates at values that also decrease with decreasing temperature. At the highest temperature investigated (450 K), the oxygen coverage at saturation is about 2 ML, whereas only 0.25 ML can be generated by dissociatively adsorbing O@sub 2@ on Pt(111) in ultrahigh vacuum. The mechanism for Pt(111) oxidation involves the initial formation of a chemisorbed layer, within which the oxygen atoms experience significant repulsive interactions after the coverage is increased above 0.25 ML. Long-range order in the high-coverage adsorbed layer could not be observed with LEED. At each surface temperature investigated, islands of platinum oxide begin to form after the oxygen coverage exceeds about 0.5 ML. TPD and ELS data suggest that the oxide islands grow three-dimensionally at surface temperatures ranging from 150 to 450 K, and that their formation is accompanied by the regeneration of metallic regions on which additional oxygen adsorbs and subsequently incorporates into growing islands. The kinetics of the oxidation process and the reactivity of the surface oxygen phases will also be discussed.