Surface potentials of (111), (110) and (100) oriented CeO2-x thin films

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 A1203(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. (C) 2016 Elsevier B.V. All rights reserved.