X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced CeO2(111) Surfaces

Abstract

We have studied the electronic structure of oxidized and reduced CeO2(111) surfaces using x-ray photoelectron spectroscopy (XPS). The 50 nm thick CeO2(111) film was grown on a YSZ(111) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in situ (RHEED) reflection high energy electron diffraction and ex situ x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and Rutherford backscattering spectroscopy (RBS). The lattice mismatch between CeO2(111) and YSZ(111) is less than 5% and yields a flat surface that is comprised of an equivalent number of Ce4+ and O2− ions. Oxidation with O2 at 773 K under UHV conditions was sufficient to fully oxidize the CeO2(111). Surface reduction was carried out by annealing in UHV at 973 K. Ceria is a technologically important metal oxide with many interesting catalytic properties. The most common use of ceria is in the treatement of automobile exhaust gases, primarily due to its oxygen strorage capacity (OSC), which allows reduction of NO as well as oxidation of CO in the catalytic converter. In a reducing atmosphere cerium ions shift from Ce4+ to Ce3+ whereas under oxidizing conditions they shift from Ce3+ to Ce4+, and the charge compensation is facilitated by oxygen vacancies that are produced on the reduced surface. In this study we have have used x-ray photoelectron spectroscopy (XPS) to investigate the electronic states of in situ oxidized and reduced CeO2(111).