AVS 49th International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM8
Interface Mediated Defect Formation in Ultrathin Cerium Oxide Films

Monday, November 4, 2002, 10:40 am, Room C-112C

Session: Oxide Structure and Surface Chemistry
Presenter: C. Castellarin-Cudia, Karl-Franzens-Universität Graz, Austria
Authors: C. Castellarin-Cudia, Karl-Franzens-Universität Graz, Austria
S. Surnev, Karl-Franzens-Universität Graz, Austria
S. Eck, Karl-Franzens-Universität Graz, Austria
M.G. Ramsey, Karl-Franzens-Universität Graz, Austria
F.P. Netzer, Karl-Franzens-Universität Graz, Austria
Correspondent: Click to Email
The oxygen storage and release capabilities of cerium oxide are important factors for its use as an additive in the three-way catalyst for automotive emission control. The ceria can act as an oxygen pump, which under fuel-rich conditions gives up oxygen, whereas under fuel-lean conditions it can take in oxygen. A key step in this process is the formation and destruction of oxygen vacancy defects at the ceria surface. Here we report the observation of defect formation, at the atomic level using STM, in ultrathin cerium oxide layers on a Rh(111) substrate surface. The ceria overlayers have been fabricated in situ by reactive evaporation of Ce metal onto the heated substrate (250°C) in an oxygen atmosphere, and reducing ambient conditions have been simulated by annealing in vacuum. Ceria overlayers on Rh(111) grow in a CeO@sub 2@(111)-type structure and annealing of submonolayer coverages to ~500°C produces thin, well-ordered Ce-oxide island nanostructures. The island surfaces display atomic resolution in the STM and reveal a Moiré superstructure, as a result of the lattice mismatch between the oxide overlayer and the Rh substrate. Further annealing to ~600°C leads to partial reduction and an ordered array of oxygen vacancies, which form a defect superlattice of the same dimensions as the Moiré structure. Several defect signatures can be distinguished in the atomically resolved STM images. We propose that the defect superlattice is mediated by the metal-oxide interface, via a lattice-mismatch induced strain effect.This creates catalytically active sites for the preferential reduction of the ceria, i.e. the formation of oxygen vacancy defects, and constitutes a novel mechanism for the formation of interface-stimulated active centers. @super *@Supported by the Austrian Science Foundation.