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atomic structure of CeO2 GBs

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Atomic Structure of a CeO2 Grain Boundary:
The Role of Oxygen Vacancies
Hajime Hojo,† Teruyasu Mizoguchi,‡ Hiromichi Ohta,§,| Scott D. Findlay,† Naoya Shibata,†,|
Takahisa Yamamoto,†,⊥ and Yuichi Ikuhara*,†,⊥,#
Institute of Engineering Innovation, School of Engineering and ‡ Institute of Industrial Science, The University of
Tokyo, Tokyo 113-8656, Japan, § Graduate School of Engneering, Nagoya University, Nagoya 464-8603, Japan,
|
PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan, ⊥ Nanostructures Research
Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan, and # WPI advanced Institute for Materials
Research, Tohoku University, Sendai 980-8577, Japan
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†
ABSTRACT Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides.
Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the
Ce and oxygen sublattices of a (210)Σ5 CeO2 grain boundary were determined. Oxygen vacancies are shown to play a crucial role in
the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the
atomic scale determination of atom and defect locations.
KEYWORDS CeO2, grain boundary structure, TEM, HAADF, ABF, oxygen nonstoichiometry
A
t grain boundaries in oxides, the structural discontinuity results in specific grain boundary structures
and nonstoichiometry, which often strongly affect
the macroscopic electrical and mechanical properties of the
material. Such effects are expected to become more significant in nanocrystalline materials. Therefore, it is essential
to clarify the nature of the grain boundaries at the atomic
scale to understand and control the properties arising from
the grain boundaries. Recent developments in scanning
transmission electron microscopy (STEM) have enabled us
to investigate materials with subangstrom resolution,1 and
this technique has been applied to study the atomic structure
of internal defects, including grain boundaries, and to
demonstrate the presence of nonstoichiometry at grain
boundaries2-4 and dislocation cores5 in several oxides.
However, knowledge of the oxygen atoms and nonstoichiometry is rather limited given their impact on various
physical properties of oxides. Especially, atomic scale evidence of oxygen nonstoichiometry and its role at grain
boundaries are still under controversy. This is partly due to
the difficulty of directly observing oxygen atoms, requiring
rather dedicated techniques such as high-voltage electron
microscopy,6,7 aberration corrected TEM with negative spherical aberration,8 and exit surface wave function reconstruction.9
Among the oxides, fluorite-structured ceria (CeO2) and
ceria-based compounds are attractive materials for electrolytes in solid oxide fuel cells and catalysis because of their
unique redox and transport properties. It is well-known that
both oxygen ionic and electronic conductivities are strongly
affected by impurity segregation at the grain boundaries10
and by grain size,11-15 indicating that grain boundaries play
an important role in the transport properties of this system.
Moreover, since the cubic fluorite structure of CeO2 is stable
without the addition of dopants (unlike zirconia, the other
widely studied fluorite-structured oxide), it is an ideal material in which to study intrinsic grain boundary structures
of fluorite-structured materials. For example, in previous
grain boundary studies using bicrystals of yttria-stabilized
zirconia (YSZ),3,4,16,17 detailed discussions on what factors
determine the stable grain boundary structures were difficult
because both yttrium and the resultant oxygen vacancies are
present in real YSZ.
Here, the atomic structure of a (210)Σ5 CeO2 grain
boundary was studied using STEM with high-angle annular
dark-field (HAADF) and annular bright-field (ABF) detectors,
together with electron energy-loss spectroscopy (EELS) and
theoretical calculations. We selected a (210)Σ5 grain boundary with a common tilt axis of [001] as a model grain
boundary. The aim of this study is to determine both the
cation and oxygen sublattices of this CeO2 grain boundary,
including the oxygen nonstoichiometry so as to reveal the
role of the oxygen vacancies on the grain boundary atomic
structure. The contrast of HAADF images is known to be
sensitive to the atomic number,18 allowing direct image
interpretation. However, light elements such as oxygen are
barely visible, especially when heavy elements are present.
On the other hand, we can directly image oxygen atoms in
crystals using state-of-the-art ABF imaging.19,20 Two contrast
formation mechanisms contribute to the absorptive form of
ABF images. On heavy atom columns, thermal scattering is
* Corresponding author, [email protected]
Received for review: 08/19/2010
Published on Web: 10/26/2010
© 2010 American Chemical Society
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DOI: 10.1021/nl1029336 | Nano Lett. 2010, 10, 4668–4672
the dominant mechanism attenuating the signal. On light
atom columns, channeling focuses the electron intensity into
the forward direction and so also attenuates the signal in the
outer area of the bright field region in which the ABF detector
is placed. Both mechanisms produce an effect of similar
magnitude, making the two types of columns visible simultaneously: the oxygen sublattice should be resolvable here
despite the presence of the heavy cation sublattice.
Due to the difficulty in obtaining CeO2 bicrystals, a CeO2
film was epitaxially grown on a YSZ bicrystal substrate by
pulsed laser deposition (PLD). A YSZ bicrystal containing a
[100](210)Σ5 grain boundary was fabricated by diffusion
bonding of two single crystals at 1600 °C for 15 h in air,
and processed into substrates. The YSZ bicrystal substrate
(as polished) was placed on the substrate holder (Inconel)
and annealed at 900 °C in the deposition chamber under
an oxygen atmosphere of 3.0 × 10-3 Pa for 20 min. Then,
a CeO2 film was deposited on the substrate by irradiating
focused KrF excimer laser pulses (pulse width ∼20 ns,
repetition rate 10 Hz, fluence ∼1 J cm-2 pulse-1) on a dense
CeO2 ceramic target for 30 min. During the film deposition,
the oxygen pressure was kept at 3.0 × 10-3 Pa. The distance
between the substrate and the target was 4 cm. The deposition rate was ∼3 nm min-1, as estimated by X-ray reflection
measurements (Rigaku ATX-G, data not shown) on the
resultant film. After the film deposition, pure oxygen gas was
additionally introduced into the deposition chamber (20 Pa)
and the film was then cooled down to room temperature.
Out-of-plane and in-plane X-ray diffraction patterns confirmed that the CeO2 thin film was grown on the YSZ
substrate with a cube-on-cube type epitaxial relationship.
Specimens for STEM observations were prepared by back
thinning from the substrate side, including mechanical
polishing to a thickness of 60 µm and further dimple gliding
down to about 15 µm. Finally, to achieve electron transparency, Ar-ion thinning was done using a precision ion milling
system (model 691, Gatan Co. Ltd.) with a gun voltage of
1.5-3.5 kV and a milling angle of 8°. These specimens were
observed using a JEM-2100F TEM/STEM microscope (JEOL
CO. Ltd.) equipped with a Cs-corrector (CEOS CO. Ltd.). The
probe-forming semiangle was around 25 mrad. HAADF and
ABF images were taken with 73-194 mrad and 10-25
mrad detectors, respectively. EELS spectra were acquired in
STEM mode by an Enfina spectrometer (Gatan Inc.) in JEM2100F using a ∼0.5 nm × 5 nm box scan. Image simulations
were carried out assuming an aberration-free probe, 65 nm
thick specimens, and accounting for a finite effective source
size characterized by a Gaussian half-width at half-maximum
of 0.04 nm.
A combination of static lattice and first-principles calculations were performed to determine the atomic structure of
CeO2 grain boundaries. The supercell geometry is discussed
later. For static lattice calculations, Buckingham-type twobody ionic potentials were employed with the potential
parameters reported by Minervini et al.21 First-principles
© 2010 American Chemical Society
FIGURE 1. (a) HAADF and (b) ABF images of a [001](210)Σ5 grain
boundary in a CeO2 thin film. (c) Simulated HAADF and (d) ABF
images of the nonstoichiometric grain boundary model structure.
(e) Simulated HAADF and (f) ABF images of the stoichiometric grain
boundary model structure. The structural units of each boundary
are indicated by polygons. The contrast in (c) and (e) has been
aligned a little to fit to the experimental image. A noise-reduction
procedure was applied to the ABF image by a background subtraction filter.37
density functional calculations were performed using projector augmented wave (PAW) potentials as implemented in
the VASP code22-25 with a 1 × 1 × 3 Monkhorst-Pack
k-point grid and a 330 eV plane-wave cutoff energy. We used
the local-spin density approximation (LSDA)+U formalism
to account for the strong on-site Coulomb repulsion among
the localized Ce 4f electrons.26 The value of Ueff was set to 6
eV to reproduce the experimental lattice constant of CeO2.
We found that the CeO2 grain boundary structure is little
affected by the value of Ueff, unlike electronic structures.27-30
All atoms in the supercell were optimized until the residual
forces were less than 0.05 eV/Å. The relative stability of the
stoichiometric and nonstoichiometric grain boundaries was
evaluated as a function of oxygen chemical potential from
µO ) µO2(gas)/2 in the oxidation limit to µO ) (µCeO2-µCe(bulk))/2
in the reduction limit. µO2(gas) and µCe(bulk) were obtained from
total-energy calculations for an O2 molecule and a face
center cubic nonmagnetic R-Ce metal.30
Panels a and b of Figure 1 display typical HAADF and ABF
images for the (210)Σ5 CeO2 grain boundary viewed along
[001]. In the HAADF image, the bright spots correspond to
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DOI: 10.1021/nl1029336 | Nano Lett. 2010, 10, 4668-–4672
the Ce column locations, while the O columns surrounded
by four Ce columns are not evident because the atomic
number Z for oxygen is too small. On the other hand, oxygen
columns can be directly identified in the ABF image, in which
the columns appear with dark contrast, as indicated by the
arrows in panels b and d of Figure 1. It is found that the
atomic columns of Ce are observed even in the grain
boundary core region and the grain boundary is made of
repeating structural units, which are marked by quadrilaterals in panels a and b of Figure 1. We generally observe such
structural units repeating over stretches of interface of
10-20 nm in length with steps in between these stretches.
In this region, the atomic structure is reproducible, and we
characterized this structure for further calculations. Panels
c and e of Figure 1 show the simulated HAADF images of
the nonstoichiometric and stoichiometric grain boundaries,
respectively. Panels d and f of Figure 1 show the corresponding ABF images. We will discuss the simulations in
detail after introducing the model structures.
In order to determine the atomic structure of the observed CeO2 grain boundary, the grain boundary structure
was first modeled using a static lattice calculation with the
GULP program code.31 We used a rectangular supercell that
contained 240 atoms and two equivalent grain boundaries,
as shown in Figure 2a. It should be noted that the supercell
contains one Ce atom and two O atoms along the projected
direction. Here, two kinds of grain boundaries were considered: one is stoichiometric and the other is nonstoichiometric. In the nonstoichiometric case, oxygen vacancies were
assumed since CeO2 is well-known for nonstoichiometry in
the oxygen content.32 To induce the oxygen vacancies, one
of the four oxygen atoms facing the grain boundary plane,
which are marked with dotted rectangles in Figure 2a, was
systematically removed, under the assumption that the
vacancy formation energies at the grain boundary region are
lower.11 There are eight ways of removing oxygen atoms to
make two identical grain boundaries in the supercell. To find
the stable structure, three-dimensional rigid body translations were fully considered in all models. The resultant stable
structures were again optimized using a first-principles PAW
method. The stable grain boundary structures thus obtained
are shown in parts b and c of Figure 2 for the stoichiometric
and nonstoichiometric cases, respectively. Due to the presence of oxygen vacancies, the electrostatic repulsion between oxygen atoms changes and different translation states
are stabilized. The structural units are designated by polygons in each case. It is found that nonstoichiometric grain
boundaries have nearly mirror symmetric cation arrangements with respect to the grain boundary plane, which gives
better agreement with the experimental image. In addition,
theoretical calculations of the grain boundary energy revealed that the nonstoichiometric grain boundary is stable
under reducing conditions of µO < -2.5 eV whereas the
stoichiometric grain boundary is stable under higher µO, that
is to say oxidizing conditions. This means that the nonsto© 2010 American Chemical Society
FIGURE 2. (a) An initial supercell with two equivalent grain boundaries: GB1 and GB2. In the nonstoichiometric case, one of the four
oxygen atoms marked with small dotted rectangles was systematically removed to introduce oxygen vacancies. (b) Stoichiometric and
(c) nonstoichiometric stable grain boundary structures. The structural units of each boundary are indicated by polygons.
ichiometric grain boundary is preferentially formed under
the reducing atmosphere.
In order to analyze the experimental images (Figure 1a,b)
in detail, multislice HAADF and ABF image simulations20,33
were carried out. The simulated HAADF and ABF images for
the nonstoichiometric structure are shown in panels c and
d of Figure 1, while those for the stoichiometric structure
are shown in parts e and f of Figure 1. It is confirmed that
the bright spots in the HAADF image correspond to Ce
column locations. Weak contrast inside the structural unit,
the position of which corresponds to that of oxgen in the
nonstoichometric model structure, is also visible in the
HAADF image. In the case of ABF images, it is confirmed
that the black and gray spots correspond to Ce and O column
locations, respectively. Gray contrast due to the presence of
O columns is visible inside the grain boundary structural
units in the simulated ABF image, but the contrast is weaker
than that for the O columns in the bulk. This is to be
expected since the O column density in the grain boundary
area is half that in the bulk region. Gray contrast is likewise
present inside the grain boundary structural units in the
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DOI: 10.1021/nl1029336 | Nano Lett. 2010, 10, 4668-–4672
can be seen that the EELS spectrum from the grain boundary
region is slightly broader than that from the grain interior
region. Since the M5/M4 intensity ratio and the onset energy
is related to the valence state of Ce, as shown in the inset of
Figure 3a,35 it is expected that this broadening feature is
related to the presence of Ce3+. To study the valence state
of Ce more quantitatively, M5/M4 intensity ratios were
calculated using the positive part of second derivative of the
experimental spectra and plotted in Figure 3b. The second
derivative of the reference spectra shown in the inset of
Figure 3a was also numerically calculated and the M5/M4
ratios were determined to be 0.90 and 1.25 for Ce4+ and
Ce3+, respectively. These values are in good agreement with
those reported by Fortner et al.36 From Figure 3b, it is seen
that the M5/M4 ratio in the grain interior is close to 0.90,
whereas that at the grain boundary tends to be larger,
going toward the Ce3+ side. This indicates that the Ce ions
at the grain boundary region tend to be reduced due to
the oxygen vacancies at the grain boundary regions. This
EELS result also supports the nonstoichiometric grain
boundary model.
In summary, the atomic and electronic structures of a
(210)Σ5 grain boundary in CeO2 have been investigated
using STEM, EELS, and theoretical calculations. Through this
study, we have directly determined the Ce and oxygen
sublattices and obtained evidence of oxygen nonstoichiometry at the grain boundary. The presence of oxygen vacancies was also confirmed by EELS measurements. Our results
revealed that oxygen nonstoichiometry plays a crucial role
in the stable grain boundary structure of CeO2. The importance of considering nonstoichiometry in constructing grain
boundary structural models is demonstrated, especially for
systems where a high degree of nonstoichiometry is expected. This finding paves the way for comprehensive
understanding of grain boundaries through atomic scale
determination of atom and defect locations.
FIGURE 3. (a) Typical Ce M4,5-edge EELS spectra taken from the grain
boundary region and from the grain interior region. The inset shows
reference spectra from ref 21 obtained for CeO2 and Ce2O3, corresponding to valence states of Ce4+ and Ce3+, respectively. (b)
Variation of the M5/M4 intensity ratio calculated by the positive part
of second derivative of the experimental spectra at several grain
boundary regions and grain interior regions.
experimental image, but there is insufficient clarity in this
image to directly determine the oxygen locations. There is
also a notable difference in clarity of the O columns in the
upper and low grains in the experimental ABF image, Figure
1b, suggesting a slight tilt/twist between the crystals, which
is not incorporated in our simulations but may further affect
the visibility of the gray contrast at the grain boundary core
region. From these results, that is to say from the structure
model energetic calculations and from the comparison of the
optimized structure with the experimental images, it can be
concluded that the nonstoichiometric grain boundary model
and not the stoichiometric grain boundary model is the most
plausible model for the experimentally observed CeO2 grain
boundary and that oxygen vacancies play an important role
in determining the stable grain boundary structure.
EELS measurements were conducted to confirm the
presence of oxygen vacancies at the grain boundaries. When
a neutral oxygen vacancy is formed, two electrons are left
behind. It is generally accepted that these electrons localize
on the f-state of the nearest Ce atoms,34 which change their
valence state from +4 to +3. In other words, the presence
of Ce3+ could be evidence of oxygen vacancy formation.
Figure 3a shows typical Ce M4,5-edge EELS spectra taken at
the grain boundary region and at the grain interior region.
The weak peak intensity at the grain boundary region
reflects the low density of Ce atoms at the grain boundary
region as is expected from the grain boundary structure. It
© 2010 American Chemical Society
Acknowledgment. H.H. is supported by the Japan Society
for the Promotion of Science (JSPS). A part of this study was
supported by Grant-in-Aid for Scientific Research on Priority
Areas “Nano Materials Science for Atomic Scale Modification
474” and Young Scientists (A) 22686059 from Ministry of
Education, Culture, Sports, Science and Technology (MEXT)
of Japan. This study was partially supported by “MACAN
(Grant No. 233484)” project funded by European Framework
Programme 7 (FP7).
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