Low energy Ar-ion bombardment effects on the CeO2 surface

The structure and electronic properties of epitaxial grown CeO2(1 1 1) thin films before and after Ar+ bombardment have been comprehensively studied with synchrotron radiation photoemission spectroscopy (SRPES). Ar+ bombardment of the surface causes a new emission appearing at 1.6 eV above the Fermi edge which is related to the localized Ce 4f1 orbital in the reduced oxidation state Ce3+. Under the condition of the energy of Ar ions being 1 keV and a constant current density of 0.5 μA/cm2, the intensity of the reduced state Ce3+ increases with increasing time of sputtering and reaches a constant value after 15 min sputtering, which corresponds to the surface being exposed to 2.8 × 1015 ions/cm2. The reduction of CeO2 is attributed to a preferential sputtering of oxygen from the surface. As a result, Ar+ bombardment leads to a gradual buildup of an, approximately 0.69 nm thick, sputtering altered layer. Our studies have demonstrated that Ar+ bombardment is an effective method for reducing CeO2 to CeO2−x and the degree of the reduction is related to the energy and amount of Ar ions been exposed to the CeO2 surface.