Structural elements of CeO2(111) surfaces

The atomic structure of the CeO2(111) surface in different states of cleanliness and reduction is studied in an ultra-high vacuum with high resolution dynamic scanning force microscopy operated in the non-contact mode (nc-AFM) and its structural elements are described by their characteristic contrast patterns. From a synopsis of results we develop a self-consistent interpretation of contrast features that is cross-checked by a systematic variation of experimental conditions and a comparison to previously obtained results. The most common deviations from the regular structure of the stoichiometric surface are surface oxygen vacancies, water adsorbed on top of cerium ions and hydroxide substituting surface oxygen. All of the species are found as single defects as well as in the form of structures composed of several similar defects. We find that water readily adsorbs on the surface and forms hydroxide if oxygen vacancies are present, while both the clean and defective surfaces are rather inert against exposure to molecular oxygen.