Surface stabilises ceria in unexpected stoichiometry

R. OLBRICH; G. E. MURGIDA; V. FERRARI; C. BARTH; A. M. LLOIS; M. REICHLING; M. V. GANDUGLIA PIROVANO

Szeged

Conferencia; 33º European Conference on Surface Science; 2017

By annealing a 180 nm thick ceria film [1-3] in an ultra-high vacuum (UHV) environment at various temperatures up to 1100K, we stabilize periodic structures representing reduction stages ranging from CeO2 to Ce2O 3. These surface reconstructions are revealed by direct imaging with a non-contact atomic force microscope (NC-AFM). An accurate understanding of the surface reconstructions is achieved by combining high-resolution NC-AFM imaging with extensive spin-polarized DFT+U calculations [4]. Starting from (1 × 1) CeO2 we identify four reconstructions namely (√7×√7)R19.1° Ce7O12., (√7 × 3)R19.1° Ce3O 5, (√3 × √3)R30° Ce3O 5, and (1 × 1) Ce2O 3.The (√7 × 3)R19.1° reconstruction is most interesting as it is the only phase that has an oblique rather than a hexagonal structure and theoretical modeling shows that this phase appears at the surface but cannot be stabilized in a bulk structure. We also observe that some phases coexist on the same terraces [5].The sequence of occurrence of the observed phases at evaluated temperature is explained with DFT and thermodynamic modeling whereby also the coexistence of phases and the missing observation of the (3 × 3) phase that has been observed in other experiments [6-8] can be understood.These results help understanding the surface defect structure which is of paramount importance in catalytic and sensor applications exploiting the high oxygen storage capacity of ceria [5]ACKNOWLEDGMENTSSupport from the COST Action CM1104 is gratefully acknowledged. 1.Barth, C., et al., A perfectly stoichiometric and flat CeO2(111) surface on a bulk-like ceria film. Scientific Reports, 2016. 6: p. 21165.2.Olbrich, R., et al., A well-structured metastable ceria surface. Applied Physics Letters, 2014. 104: p. 081910.3.Zoellner, M.H., et al., Stacking behavior of twin-free type-B oriented CeO2(111) films on hexagonal Pr2O3(0001)/Si(111) systems. Physical Review B, 2012. 85(85): p. 035302.4.Murgida, G.E. and M.V. Ganduglia-Pirovano, Evidence for subsurface ordering of oxygen vacancies on the reduced CeO2(111) surface using density-functional and statistical calculations. Physical Review Letters, 2013. 110(24): p. 246101.5.Olbrich, R., et al., Surface Stabilizes Ceria in Unexpected Stoichiometry. The Journal of Physical Chemistry C, 2017. 121(12): p. 6844-6851.6.Duchoň, T., et al., Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111). Journal of Physical Chemistry C, 2014. 118(1): p. 357-365.7.Höcker, J., et al., Unraveling the Dynamic Nanoscale Reducibility (Ce4+ -> Ce3+) of CeOX-Ru in Hydrogen Activation. Advanced Materials Interfaces, 2015. 2(18): p. 1500314.8.Luches, P., F. Pagliuca, and S. Valeri, Structural and morphological modifications of thermally reduced cerium oxide ultrathin epitaxial films on Pt(111). Physical Chemistry Chemical Physics, 2014. 16(35): p. 18848-18857.