Atom-Resolved Noncontact Atomic Force Microscopic Observations of CeO₂(111) Surfaces with Different Oxidation States:  Surface Structure and Behavior of Surface Oxygen Atoms

Hexagonally arranged surface oxygen atoms, oxygen point defects, and multiple oxygen defects at oxygen-terminated CeO2(111) surfaces in different oxidation states were visualized by noncontact atomic force microscopy (NC-AFM). The multiple defects such as line defects and triangular defects were stabilized by a local reconstruction, where edge oxygen atoms surrounding the multiple defects were displaced and gave enhanced brightness due to a geometric reason. Successive NC-AFM measurements of the same area of a slightly reduced CeO2(111) surface revealed that hopping of surface oxygen atoms faced to metastable multiple defects was thermally activated even at room temperature. In contrast, no hopping was observed either at a point oxygen vacancy or a line defect that is stabilized by local reconstruction. It was also confirmed from atom-resolved NC-AFM observations that the surface oxygen defects were easily healed by exposure to O2 gas at room temperature.