Surface potentials of (111), (110) and (100) oriented CeO_2−x thin films

Highlights: • Fermi level, work function and ionization potential of CeO₂ thin films determined. • The state of the surface is varied by different deposition conditions and post-deposition treatments. • The ionization potential varies more than 2 eV. This is much higher than for other oxide surfaces. • The Fermi level position varies only slightly upon surface oxidation and reduction. • A Ce³⁺ concentration of >10% remains on the most strongly oxidized surfaces. - Abstract: Differently oriented CeO₂ thin films were prepared by radio frequency magnetron sputter deposition from a nominally undoped CeO₂ target. (111), (110) and (100) oriented films were achieved by deposition onto Al₂O₃(0001)/Pt(111), MgO(110)/Pt(110) and SrTiO₃:Nb(100) substrates, respectively. Epitaxial growth is verified using X-ray diffraction analysis. The films were analyzed by in situ photoelectron spectroscopy to determine the ionization potential, work function, Fermi level position and Ce³⁺ concentration at the surface in dependence of crystal orientation, deposition conditions and post-deposition treatment in reducing and oxidizing atmosphere. We observed a very high variation of the work function and ionization potential of more than 2 eV for all surface orientations, while the Fermi level varies by only 0.3 eV within the energy gap. The work function generally decreases with increasing Ce³⁺ surface concentration but comparatively high Ce³⁺ concentrations remain even after strongly oxidizing treatments. This is related to the presence of subsurface oxygen vacancies.