pubs.acs.org/NanoLett
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 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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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 4669
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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 4670
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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). REFERENCES AND NOTES (1) (2) (3) (4) (5)
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