Comment pubs.acs.org/JPCC
Comment on “Ordered Phases of Reduced Ceria as Epitaxial Films on Cu(111)” Tomás ̌ Duchoň, Filip Dvořaḱ , Marie Aulická, Vitalii Stetsovych, Mykhailo Vorokhta, Daniel Mazur, Kateřina Veltruská, Tomás ̌ Skála, Josef Mysliveček,* Iva Matolínová, and Vladimír Matolín Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University in Prague, V Holešovičkách 2, 18000 Praha 8, Czech Republic
J. Phys. Chem. C 2014, 118 (1), 357−365. DOI: 10.1021/jp409220p
I
n our recent contribution1 we have described a method for obtaining ordered layers of reduced ceria by interfacial reaction between a metallic Ce deposit and a CeO2 buffer layer supported on Cu(111). In this Comment, we demonstrate the applicability of this method on ceria layers supported on Ru(0001). Ru(0001) is a widely used support for model ceria catalysts with a large body of studied heterogeneous surface chemical reactions.2−4 Ordered layers of reduced ceria now offer the possibility to control both the concentration and the coordination of oxygen vacancies in ceria and to investigate the surface chemistry of well-defined substoichiometric ceria. With the clean Ru(0001) substrate we exactly reproduce the experimental procedures of ref 1 and prepare ordered layers of reduced ceria with thickness of 3−4 nm. The thickness of the layers is determined from the attenuation of Ru 3d5/2 in XPS. Photoelectron spectra (XPS) and electron diffraction (LEED) of the layers are displayed in Figures 1 and 2, respectively. The growth parameters, the degree of reduction, and the estimate of
Figure 2. LEED patterns of the ordered phases of reduced ceria on Ru(0001) measured at primary electron energy of 40 eV. The surface unit cells of CeO2 (solid line) and the respective reconstructions of reduced ceria (dashed line) are outlined in red.
Table 1. Parameters of the Preparation of the Samples thickness of the CeO2 buffer
reconstruction (1 × 1) (√7 × √7)R19.1° (3 × 3) (4 × 4)
nm nm nm nm
3.2 nm 3.5 nm 4.1 nm
the in-plane lattice constant of the layers are summarized in Tables 1 and 2. Ceria layers exhibit the same characteristic behavior as the layers on Cu(111).1 Particulary, the stoichiometry of the layers can be tuned from CeO2 to Ce2O3 by varying the amount of metallic Ce in the interfacial reaction. During the transition from CeO 2 to Ce 2 O 3 we observe the same surface
Figure 1. XPS spectra of the ordered phases of reduced ceria on Ru(0001). Spectra were measured at 0° (black curves) and 60° offnormal emission (gray curves). The areas of the presented spectra have been normalized to 1, and the curves have been offset for clarity. For better orientation, the positions of visually the most prominent peaks in Ce4+ and Ce3+ spectra are marked by arrows. © 2014 American Chemical Society
2.8 2.8 2.8 2.8
thickness of the reacted layer
Received: December 19, 2013 Revised: February 5, 2014 Published: February 15, 2014 5058
dx.doi.org/10.1021/jp412439b | J. Phys. Chem. C 2014, 118, 5058−5059
The Journal of Physical Chemistry C
Comment
K.; et al. Epitaxial Cubic Ce2O3 Films via CeCeO2 Interfacial Reaction. J. Phys. Chem. Lett. 2013, 4, 866−871.
Table 2. Stochiometries and In-Plane Lattice Constants of the Samples x in CeOx reconstruction
XPS 0°
XPS 60°
(1 × 1) (√7 × √7)R19.1° (3 × 3) (4 × 4)
1.99 1.83 1.72 1.51
1.99 1.76 1.66 1.50
in-plane lattice constant 3.80 3.83 3.87 3.90
± ± ± ±
0.05 0.05 0.05 0.05
Å Å Å Å
reconstructions corresponding to ordered phases of reduced ceria. (1 × 1) reconstruction corresponds to CeO2(111), (√7 × √7)R19.1° to ι-Ce7O12, (3 × 3) to CeO1.67, and (4 × 4) to Ce 2O 3 .1 On the starting CeO 2 buffer with (1 × 1) reconstruction a weak rotational domain characteristic for growth of CeO2 on Ru(0001) is apparent.3,4 As with the Cu(111) substrate, the degree of reduction of the ceria layers is larger when measured with higher surface sensitivity (60° vs 0° off sample normal, Table 2). As the degree of reduction increases, we observe the accompanying relaxation and the effective increase of the in-plane lattice constant of the ceria layers (Table 2).5 Our results show that reduction of ceria layers in the interfacial reaction with metallic Ce is applicable on Ru(0001) and, presumably, on other metal substrates with the same results as on the Cu(111) substrate. The general conclusion of ref 1 that the properties of the thin films of reduced ceria are determined by equilibration of mobile oxygen vacancies in a relatively rigid frame of cubic ceria seems to be independent of the choice of the metal substrate.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: josef.myslivecek@mff.cuni.cz. Phone: +420 221 912 333. Fax: +420 284 685 095. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Czech Science Foundation (GAČ R P204-11-1183, GAČ R 13-10396S) and by the Ministry of Education of the Czech Republic (LG12003, LH11017). T.D., F.D., M.A., and V.S. acknowledge the support of the Grant Agency of the Charles University (GAUK 794313, GAUK 610112, GAUK 320313, GAUK 339311).
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REFERENCES
(1) Duchoň, T.; Dvořaḱ , F.; Aulická, M.; Stetsovych, V.; Vorokhta, M.; Mazur, D.; Veltruská, K.; Skála, T.; Mysliveček, J.; Matolínová, I.; et al. Ordered Phases of Reduced Ceria as Epitaxial Films on Cu(111). J. Phys. Chem. C 2014, 118, 357−365. (2) Mullins, D.; Radulovic, P.; Overbury, S. Ordered Cerium Oxide Thin Films Grown on Ru(0001) and Ni(111). Surf. Sci. 1999, 429, 186−198. (3) Lu, J.-L.; Gao, H.-J.; Shaikhutdinov, S.; Freund, H.-J. Morphology and Defect Structure of the CeO2(111) Films Grown on Ru(0001) as Studied by Scanning Tunneling Microscopy. Surf. Sci. 2006, 600, 5004−5010. (4) Kaemena, B.; Senanayake, S. D.; Meyer, A.; Sadowski, J. T.; Falta, J.; Flege, J. I. Growth and Morphology of Ceria on Ruthenium (0001). J. Phys. Chem. C 2013, 117, 221−232. (5) Stetsovych, V.; Pagliuca, F.; Dvořaḱ , F.; Duchoň, T.; Vorokhta, M.; Aulická, M.; Lachnitt, J.; Schernich, S.; Matolínová, I.; Veltruská, 5059
dx.doi.org/10.1021/jp412439b | J. Phys. Chem. C 2014, 118, 5058−5059
