title: Cerium Oxide Nanoclusters on Graphene/Ru(0001): Intercalation of Oxygen via Spillover.
abstract: Cerium oxide is an important catalytic material known for its ability to store and release oxygen, and as such, it has been used in a range of applications, both as an active catalyst and as a catalyst support. Using scanning tunneling microscopy and Auger electron spectroscopy, we investigated oxygen interactions with CeOx nanoclusters on a complete graphene monolayer-covered Ru(0001) surface at elevated temperatures (600-725 K). Under oxidizing conditions (PO2 = 1 × 10(-7) Torr), oxygen intercalation under the graphene layer is observed. Time dependent studies demonstrate that the intercalation proceeds via spillover of oxygen from CeOx nanoclusters through the graphene (Gr) layer onto the Ru(0001) substrate and extends until the Gr layer is completely intercalated. Atomically resolved images further show that oxygen forms a p(2 × 1) structure underneath the Gr monolayer. Temperature dependent studies yield an apparent kinetic barrier for the intercalation of 1.21 eV. This value correlates well with the theoretically determined value for the reduction of small CeO2 clusters reported previously. At higher temperatures, the intercalation is followed by a slower etching of the intercalated graphene (apparent barrier of 1.60 eV). Vacuum annealing of the intercalated Gr leads to the formation of carbon monoxide, causing etching of the graphene film, demonstrating that the spillover of oxygen is not reversible. In agreement with previous studies, no intercalation is observed on a complete graphene monolayer without CeOx clusters, even in the presence of a large number of point defects. These studies demonstrate that the easily reducible CeOx clusters act as intercalation gateways capable of efficiently delivering oxygen underneath the graphene layer.
authors.affiliation: Fundamental and Computational Sciences Directorate and Institute for Interfacial Catalysis, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Zbynek
authors.initials: Z
authors.lastname: Novotny
authors.affiliation: Surface and Interface Physics, Institute of Physics, Karl-Franzens University , A-8010 Graz, Austria.
authors.forename: Falko P
authors.initials: FP
authors.lastname: Netzer
authors.affiliation: Fundamental and Computational Sciences Directorate and Institute for Interfacial Catalysis, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Zdenek
authors.initials: Z
authors.lastname: Dohnálek
keywords: Auger electron spectroscopy; ceria; graphene etching; graphene on Ru; oxide cluster; oxygen intercalation; scanning tunneling microscopy; spillover