Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)
Toma ́ s ̌ Duchoň , Filip Dvor ̌ a ́ k, Marie Aulicka ́ , Vitalii Stetsovych, Mykhailo Vorokhta, Daniel Mazur,
Kater ̌ ina Veltruska ́ , Toma ́ s ̌ Ska ́ la, Josef Myslivec ̌ ek,* Iva Matolínova ́ , and Vladimír Matolín
Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University in Prague, V Holes ̌ ovič ka ́ ch 2,
18000 Praha 8, Czech Republic
ABSTRACT: 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.
■
INTRODUCTION
Reducible 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 long-
range 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 hex-
Pr
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 c-Ce
2
O
3
phase
has been reported also in the latest study on hex-Pr
2
O
3
(0001)/
Si(111).
31
Here we present a detailed investigation of the properties of
the ceria layers on Cu(111) reduced by the interface reaction
with Ce. Using a stepwise titration of ceria layers with Ce in the
interface reaction, we continuously change the stoichiometry of
the ceria layers on Cu(111) from CeO
2
32
to Ce
2
O
3
.
30
We
characterize the concentration and the depth profile of the
oxygen vacancies in the reduced ceria layers by photoelectron
spectroscopy (XPS) and resonance photoelectron spectroscopy
(RPES), the surface reconstruction of the layers by low-energy
electron diffraction (LEED), the morphology of the layers by
scanning tunneling microscopy (STM), and the surface
composition of the layers by ion scattering spectroscopy
(ISS). We are identifying two surface reconstructions of
ordered reduced ceria on Cu(111)the (√7 × √7)R19.1°
reconstruction corresponding to bulk ι-Ce
7
O
12
29
and the (3 ×
3) reconstruction corresponding to bulk CeO
1.67
. We show that
the process of reduction of ceria via the interface reaction with
metallic Ce is fully reversible upon oxidation by O
2
. This allows
us to interpret the properties of the reduced ceria films in our
experiment in terms of creation, annihilation, and equilibration
of oxygen vacancies in the fluorite lattice of cubic ceria.
Reduction of ceria thin films by metallic Ce complemented by
oxidation by O
2
allows a precise experimental control over both
the concentration and the coordination of oxygen vacancies in
thin ceria films. The coordination of oxygen vacancies
represents a new degree of freedom that can be controlled
Received: September 15, 2013
Revised: December 5, 2013
Published: December 9, 2013
Article
pubs.acs.org/JPCC
© 2013 American Chemical Society 357 dx.doi.org/10.1021/jp409220p | J. Phys. Chem. C 2014, 118, 357-365