Surface potentials of ceria and their influence on the surface exchange of oxygen

Cerium oxide is an important material for modern three-way catalysts for automotive exhaust emission control and an interesting candidate for the use in solid oxide fuel cells as electrode and electrolyte. For these application, the surface exchange of oxygen plays an important role as it is assumed, especially at lower temperatures, to be the limiting factor for the device performance. The transfer of electrons from the oxide to adsorbed oxygen molecules is often discussed to be rate determining for the oxygen incorporation. Based on this assumption, this thesis will investigate a possible direct relation between the surface exchange coefficient k and the electrochemical potential of electrons, i.e. the Fermi level, at the CeO2 surface. The absolute position of the Fermi level in reference to the vacuum energy is given by the work function of the material. Thus, following the hypothesis of this thesis, the work function of ceria should have a direct effect on the surface exchange coefficient. This thesis constitutes a systematic investigation of the Fermi level, ionization potential and work function in dependence of surface orientation for acceptor-, donor- and undoped ceria thin films. The films, prepared by magnetron sputtering from ceramic targets, were analyzed using photoelectron spectroscopy. To obtain information about the crystal orientation of the films, X-ray diffraction was performed. In addition, oxygen tracer exchange experiments combined with secondary ion mass spectrometry and conductivity relaxation measurements were executed in collaboration with the IPC at RWTH Aachen for the purpose of determining the exchange and diffusion coefficients. From conductivity measurements at different temperatures and oxygen partial pressures the defect structure of CeO2 was investigated by comparing the measured conductivities with calculations from a point defect model. While a direct correlation of the work function with oxygen exchange coefficient could not be shown with ceria as a model system, this thesis provides new insights to CeO2 surface potentials. Furthermore, it provides the first systematic study of these properties as a reference for other studies, where surface potentials of ceria are of importance