Stability of the Ce3+ valence state in cerium oxide nanoparticle layers.

A high concentration of Ce(3+) ions, above 80%, was created in 5 nm thick cerium oxide nanoparticle (nanoceria) layers deposited on a polymer substrate. The reduction from Ce(4+) to Ce(3+) was achieved by irradiating the nanoceria layers with Ar ions, which resulted in the formation of oxygen vacancies at the surface. The samples were exposed to ambient air and compared with nanoceria pellets. Ce retained the 3+ valence state in the particle layers, but not in the pellets, even after 47 days of exposure to humid air. Thus, the irradiated nanoceria particle layers contain a high level of Ce(3+) ions and possess an outstanding stability in air.