Reduced ceria nanofilms from structure prediction

Abstract

Experimentally, Ce2O3 films are used to study cerium oxide in its fully or partially reduced state, as present in many applications. We have explored the space of low energy Ce2O3 nanofilms using structure prediction and density functional calculations, yielding more than 30 distinct nanofilm structures. First, our results help to rationalize the roles of thermodynamics and kinetics in the preparation of reduced ceria nanofilms with different bulk crystalline structures (e.g. A-type or bixbyite) depending on the support used. Second, we predict a novel, as yet experimentally unresolved, nanofilm which has a structure that does not correspond to any previously reported bulk A2B3 phase and which has an energetic stability between that of A-type and bixbyite. To assist identification and fabrication of this new Ce2O3 nanofilm we calculate some observable properties and propose supports for its epitaxial growth.

Electronic properties of epitaxial cerium oxide films during controlled reduction and oxidation studied by resonant inelastic X-ray scattering

“…At specific concentrations of Ce 3+ sites different structural phases are expected to be stable in the bulk material 27,28 . In ultrathin films the situation is even more complex, due to the possible stabilization of low dimensionality substrate-induced metastable phases 12,[29][30][31][32] . When Pt is used as a substrate, previous studies by our group showed the formation of peculiar reconstructions in the LEED patterns of ultrathin films at significant degrees of reduction, indicating important rearrangements of the surface structure 13,14 .…”

Structure, Morphology and Reducibility of Epitaxial Cerium Oxide Ultrathin Films and Nanostructures

“…The different phases stabilized in reduced cerium oxide films at reduced dimensionality have also been addressed by theoretical studies employing the simulated mechanical annealing approach to select the most stable structures, which were subsequently simulated by DFT [50]. The study shows that the relative stability of the C-type and A-type phases in ultrathin films depends on the in-plane lattice parameter [50].…”

“…The study shows that the relative stability of the C-type and A-type phases in ultrathin films depends on the in-plane lattice parameter [50]. Furthermore, it identifies a new structure without a bulk counterpart which is more stable than the A-type and C-type phases at larger in-plane lattice parameters, such as those that might be induced by epitaxy on Re(0001) or Pt(111) [50]. This and related studies provide important input to unravel the origin of the complex phases identified experimentally and to understand their properties.…”

Reducible Oxides as Ultrathin Epitaxial Films

“…The exposed works show that ultrathin cerium oxide films offer new opportunities in view of obtaining modifications of the material properties, and in particular for the stabilization of metastable structural phases. This last issue has been investigated also in a rather recent theoretical investigation aimed at identifying the most stable Ce 2 O 3 structures at the ultrathin limit [56]. The work, using simulated mechanical annealing searches and DFT calculations, shows that different ultrathin film structures are stable at different values of the in plane lattice parameter and it identifies in particular a new structure, which does not correspond to any known bulk crystalline polymorph, which is more stable than the A-type and than the c-type Ce 2 O 3 structures at specific, relatively large, in-plane lattice parameters (figure 5) [56].…”

“…This last issue has been investigated also in a rather recent theoretical investigation aimed at identifying the most stable Ce 2 O 3 structures at the ultrathin limit [56]. The work, using simulated mechanical annealing searches and DFT calculations, shows that different ultrathin film structures are stable at different values of the in plane lattice parameter and it identifies in particular a new structure, which does not correspond to any known bulk crystalline polymorph, which is more stable than the A-type and than the c-type Ce 2 O 3 structures at specific, relatively large, in-plane lattice parameters (figure 5) [56]. As mentioned at the beginning of this section, a few studies have also been focused on the stabilization of ultrathin cerium oxide films exposing surfaces different from the (111) on metal substrates.…”