AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM1
Oxide Nucleation Kinetics on Al(111): The Role of Short-range and Long-range Interactions between Chemisorbed O Atoms

Thursday, November 6, 2003, 8:20 am, Room 327

Session: Catalysis III: High vs. Low Pressures
Presenter: I. Zori@aa c@, Chalmers University of Technology and Göteborgs University, Sweden
Authors: D.E. Oner, Chalmers University of Technology and Göteborgs University, Sweden
B. Kasemo, Chalmers University of Technology and Göteborgs University, Sweden
I. Zori@aa c@, Chalmers University of Technology and Göteborgs University, Sweden
Correspondent: Click to Email
Experimental work indicates that oxide nucleation on Al(111) takes place in a heterogeneous surface phase, at an average coverage of chemisorbed O atoms of about 0.15ML, specifically at the edges of chemisorbed O islands containing 10-15 O atoms. The former observation was based on spectroscopic evidence while the latter was directly observed in STM experiments. In this contribution we suggest that the rate for oxide nucleation is strongly influenced by a combination of a long-range repulsive (elastic) interaction in competition with a short-range (nearest neighbor) attractive interaction between O adatoms. The model used to describe oxide formation is an extension of an earlier kinetic model that in addition to the oxide nucleation step also includes adsorption/chemisorption step. In the pure chemisorption phase O@sub 2@ dissociative adsorption takes place that leads to an O island size distribution in agreement with the STM observations. Chemisorbed O atoms interact via superimposed short-range nn attractive and long-range repulsive interactions. The oxide nucleation is simulated via a place exchange step. The latter step is the rate-limiting step for oxide nucleation. The short-range nn interaction stabilizes the chemisorbed phase, while the repulsive long-range interaction promotes oxide formation, i.e. it enhances the place exchange jump rate. Our Monte Carlo simulations, based on the transition theory approach, can account for: a) the observed threshold in O island size needed for oxide nucleation to occur, b) the total oxygen uptake kinetics and c) the oxide formation kinetics, reported in recent STM-surface spectroscopic experiments. The two important parameters in the model are the strengths of the long-range and short-range interactions, respectively. Agreement between simulation predictions and experimental data is achieved for a unique and reasonable choice of model parameters.