Stabilization of the ceria ι-phase (Ce7O12) surface on Si(111)

Wilkens, H.; Schuckmann, O.; Oelke, R.; Gevers, S.; Schaëfer, Andreas; Bäumer, Marcus; Zoellner, M. H.; Schroeder, Thomas; Wollschläger, J.

doi:10.1063/1.4795867

Cited by 30 publications

(49 citation statements)

References 19 publications

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“…Figure 6(a) displays the reference (111) plane of CeO 2 (bulk model ICSD #169029). Figure 6(b) displays the corresponding cut through the bulk ι-Ce 7 O 12 phase (PCD #1805255) according to Wilkens et al 29 and Figure 6(d) the cut through the bulk cCe 2 O 3 phase (ICSD #96202) according to Stetsovych et al 30 In the figures, the surface unit cell corresponding to the LEED patterns of (1 × 1) (Figure 6(a)), (√7 × √7)R19.1°( Figure 6 According to RPES measurements ( Figure 4) the stoichiometry of the (3 × 3) reconstruction is close to the (√7 × √7)R19.1°( CeO 1.71 ) reconstruction. Therefore, we propose that the (3 × 3) surface unit cell includes three oxygen vacancies corresponding to the model stoichiometry CeO 1.67 .…”

Section: ■ Experimental Resultsmentioning

confidence: 99%

“…The (√7 × √7)R19.1°LEED pattern on a layer of CeO x was associated with a bulk termination of the ι-Ce 7 O 12 phase. 26,29 The (4 × 4) LEED pattern on a layer of Ce 2 O 3 was associated with a bulk termination of the c-Ce 2 O 3+δ phase 28,30 that was realized for δ = 0 as a thin film of c-Ce 2 O 3 on Cu(111). Both (√7 × √7)R19.1°and (4 × 4) reconstructions are realized in the present experiments as well.…”

Section: ■ Experimental Resultsmentioning

confidence: 99%

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Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)

et al. 2013

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Changes of stoichiometry in reducible oxides are inevitably accompanied by changes of the oxide structure. We study the relationship between the stoichiometry and the structure in thin epitaxial films of reduced ceria, CeO x , 1.5 ≤ x ≤ 2, prepared via an interface reaction between a thin ceria film on Cu(111) and a Ce metal deposit. We show that the transition between the limiting stoichiometries CeO 2 and Ce 2 O 3 is realized by equilibration of mobile oxygen vacancies near the surface of the film, while the fluorite lattice of cerium atoms remains unchanged during the process. We identify two surface reconstructions representing distinct oxygen vacancy ordering during the transition, a (√7 × √7)R19.1°reconstruction representing a bulk termination of the ι-Ce 7 O 12 and a (3 × 3) reconstruction representing a bulk termination of CeO 1.67 . Due to the special property to yield ordered phases of reduced ceria the interface reaction between Ce and thin film ceria represents a unique tool for oxygen vacancy engineering. The perspective applications include advanced model catalyst studies with both the concentration and the coordination of oxygen vacancies precisely under control. ■ INTRODUCTIONReducible oxides play an important role in heterogeneous catalysis. 1−7 Due to their ability to store or release oxygen, reducible oxides usually act as an oxygen supply or a reducing agent during catalytic reactions. 8,9 Reactions over reducible oxides are typically accompanied by changes in the oxide stoichiometry that are often realized on complex phase diagrams 10−17 and may influence the catalytic activity through changes in local coordination, surface termination, and longrange ordering in the oxide. 18−21 Model studies isolating the changes of the oxide stoichiometry are of the utmost importance for understanding the role of stoichiometry in the reaction mechanisms over reducible oxides and for improving and developing new catalysts.The reactivity of cerium oxide-based catalysts is greatly influenced by the presence of oxygen vacancies in ceria. 22,23 The ability to adjust the concentration and the distribution of oxygen vacancies allows for the control over the reactivity and the selectivity of ceria-based catalysts. 24,25 For this reason, having experimental access to ordered phases of cerium oxide with different concentration and coordination of oxygen vacancies greatly enhances the possibilities of model catalytic studies. Several phases of ordered reduced ceria have been prepared in the past in the form of powder or single-crystal samples, 26−28 but only recently ordered reduced phases of ceria have been realized in the form of thin films on single crystalline supports. The thin film of the ι-Ce 7 O 12 phase on hexPr 2 O 3 (0001)/Si(111) substrate was obtained by Wilkens et al. via heating of the CeO 2 layer in vacuum. 29 A thin film of the c-Ce 2 O 3 phase on Cu(111) was obtained by our group via an alternative method of reducing the CeO 2 layer in an interface reaction with metallic Ce. 30 The thin film of the ...

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Section: ■ Experimental Resultsmentioning

confidence: 99%

Section: ■ Experimental Resultsmentioning

confidence: 99%

Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)

et al. 2013

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“…Several intermediate bulk phases exist in the phase diagrams of ceria and praseodymia 22 which can be stabilized in pure ceria thin film systems. 23,24 Hence, we assume that this structure can be attributed to a reduced crystalline phase of the mixed oxide with low order. The samples with higher praseodymium content do not show this structure.…”

Section: Resultsmentioning

confidence: 99%

Post deposition annealing of epitaxial Ce_1−xPr_xO_2−δfilms grown on Si(111)

Wilkens

Spieß

Zoellner

et al. 2015

Phys. Chem. Chem. Phys.

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In this work the structural and morphological changes of Ce 1Àx Pr x O 2Àd (x = 0.20, 0.35 and 0.75) films grown on Si(111) due to post deposition annealing are investigated by low energy electron diffraction combined with a spot profile analysis. The surface of the oxide films exhibit mosaics with large terraces separated by monoatomic steps. It is shown that the Ce/Pr ratio and post deposition annealing temperature can be used to tune the mosaic spread, terrace size and step height of the grains. The morphological changes are accompanied by a phase transition from a fluorite type lattice to a bixbyite structure. Furthermore, at high PDA temperatures a silicate formation via a polycrystalline intermediate state is observed.

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“…However, oxygen vacancies formed at higher temperature at the interfaces [25], and Si atom is readily oxidized as a consequence of the competitions among several reactions mentioned above. The appearance of Si2p peaks at 99.9 eV in the CeO 2 /Si interface shows the reactions between them.…”

Section: Surface Characterization Of Ceo 2 Thin Films During Vacuum Amentioning

confidence: 98%