Paper SS+AS-TuM10
Surface Structure of α -Cr₂O₃(0001) by X-ray Photoelectron Diffraction after Activated Oxygen Exposure

Tuesday, October 29, 2013, 11:00 am, Room 201 A

The detailed structure and electronic properties of the surfaces of the corundum-type oxides α -Al₂O₃, α -V₂O₃, α -Cr₂O₃, and α -Fe₂O₃ continue to be of significant interest due to their importance in catalysis, photocatalysis, and environmental chemistry. The surface structure, and thus the properties of the surface, can be dramatically altered with different processing or treatment conditions. Recently, Henderson¹ has shown that the Cr-terminated α -Cr₂O₃(0001)surface can be fully passivated by exposure to activated oxygen from an electron cyclotron resonance (ECR) plasma source, blocking both the dissociative and the molecular H₂O adsorption channels. We use x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) to investigate the surface structure of α -Cr₂O₃(0001) epitaxial thin films before and after exposure to activated oxygen from an ECR plasma source. The films are deposited on α-Al₂O₃(0001) substrates by oxygen-plasma-assisted molecular beam epitaxy. When cooled or annealed in vacuum, strong evidence for a Cr-Cr-O₃- termination is obtained by comparing the Cr³⁺ XPD angular scan to single scattering simulations. However, after plasma exposure, a high binding energy feature is observed in the Cr 2p XPS spectrum that possesses an ordered structure distinct from the underlying Cr³⁺ of Cr₂O₃, which remains Cr-Cr-O₃-like. Investigation of this new surface structure with simulations of various candidate structures tentatively rules out CrO₂-like configurations. The high binding energy feature likely arises from a higher oxidation state of Cr, although a quantitative charge state assignment is not straightforward. One possibility is the oxidation of the surface layer of Cr to Cr^6-δ with a double chromyl structure (O=Cr=O).

1. M. A. Henderson, Surf. Sci.604, 1502 (2010).