Atom-resolved images and dynamic behaviors of surface oxygen atoms and adsorbed molecules on CeO_2 (111) surface studied by noncontact atomic force microscopy and scanning tunneling microscopy

摘要

Cerium oxide (CeO_2) can work as an oxygen buffer to control oxygen concentrations in catalytic systems. We have studied surface structures of CeO_2 (111) with nearly stoichiometric compositions by NC-AFM and STM. When density of surface oxygen vacancies increased up to 1.3 percent, multiple defects began to appear. Multiple defects constituted a kind of local reconstruction structure or a metastable structure without reconstruction. Reoxidation processes at oxygen deficient sites on a slightly reduction CeO_2 (111) surface was also imaged. From these results, we concluded that the CeO_2 (111) surface is an oxygen-terminated surface. Successive NC-AFM observations revealed that surface oxygen atoms on the CeO_2 (111) surface with a multiple defect structure without reconstruction were mobile even at RT. It was a thermally activated process and was accelerated at higher temperature. To elucidate the chemical reactivity of CeO_2, we used methanol as probe, which was imaged by NC-AFM and STM. Successive NC-AFM observations under methanol atmosphere indicated a high reactivity of the CeO_2 (Hi) surface even at RT.