Atom-resolved noncontact atomic force microscopic and scanning tunneling microscopic observations of the structure and dynamic behavior of CeO2(1 1 1) surfaces

Atomic-scale structures and dynamic behaviors of CeO₂(1 1 1) surfaces were imaged by noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). Hexagonally arranged oxygen atoms, oxygen point vacancies, multiple oxygen vacancies, and hydrogen adatoms at the surfaces were visualized by atom-resolved NC-AFM observations. Multiple defects were stabilized by displacement of the surrounding oxygen atoms around the multiple defects, which gave enhanced brightness in the NC-AFM image due to a geometric reason. Multiple defects without reconstruction of the surrounding oxygen atoms were reactive and were healed by exposure to O₂ gas and methanol at RT. Successive NC-AFM and STM measurements of slightly reduced CeO₂(1 1 1) surfaces revealed that hopping of surface oxygen atoms faced to the metastable multiple defects was thermally activated even at room temperature (RT) and more promoted at higher temperatures. Heterogeneous feature of the reactivity of surface oxygen atoms with methanol was imaged by successive NC-AFM observations. These observations gave a new insight for understanding the surface structures and behavior of CeO_2−x with the facile oxygen reservoir and oxidation–reduction properties related to the unique catalysis.