AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS3-MoA

Paper SS3-MoA6
Oxygen-Induced Morphological Changes in Cu and Ni Islands on TiO@sub2@(110)

Monday, November 3, 2003, 3:40 pm, Room 328

Session: Structure and Reactivity of Metal Clusters
Presenter: D.A. Chen, University of South Carolina
Authors: D.A. Chen, University of South Carolina
J. Zhou, University of South Carolina
Y.C. Kang, Pukyong National University, South Korea
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Metal particles supported on oxide surfaces serve as excellent model systems for developing a better understanding of commercial heterogeneous catalysts. Scanning tunneling microscopy (STM) studies of Cu islands grown on a rutile TiO@sub2@(110)-(1x1) surface demonstrate that these islands disappear from the STM images after exposure to oxygen gas. Based on X-ray photoelectron spectroscopy (XPS) experiments, the disappearance of the Cu islands cannot be explained by the loss of Cu from the surface or by a dramatic change in the electronic properties of the islands. The adsorption of oxygen appears to weaken the Cu-Cu bond, allowing two-dimensional (2D) Cu islands to form on the surface at the expense of the existing three-dimensional (3D) islands. The formation of 2D islands is thermodynamically favorable for Cu based on the lower surface free energy of oxidized compared to that of both Cu and TiO@sub2@. Oxygen-induced disappearance of Ni islands has also been observed, but the rate of disappearance is much slower even though Ni is more easily oxidized than Cu. This effect can also be explained in terms of thermodynamics; oxidation of Ni islands does not reduce the surface free energy of the islands below that of TiO@sub2@, and therefore there is less driving force for the Ni islands to become 2D. As expected, oxidation of Cu islands at 500 K increases the rate of disappearance of the 3D islands. However, oxidation of the Ni islands at 500 K causes significant changes in the surface morphology due to the oxidation of the TiO@sub2@ surface itself. The new titania layers preferentially regrow around the Ni islands, resulting in a much rougher surface.