Morphology and Surface Analysis of Pure and Doped Cuboidal Ceria

Article pubs.acs.org/JPCC

Morphology and Surface Analysis of Pure and Doped Cuboidal Ceria Nanoparticles Umananda M. Bhatta,† David Reid,# Tamilselvan Sakthivel,# Thi X. T. Sayle,§ Dean Sayle,∥ Marco Molinari,⊥ Stephen C. Parker,⊥ Ian M. Ross,‡ Sudipta Seal,# and Günter Möbus*,† †

Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K. Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K. § School of Physical Sciences, University of Kent, Canterbury CT2 7NZ, U.K. ∥ School of Physical Sciences, University of Kent, Canterbury CT2 7NZ, U.K. ⊥ Department of Chemistry, University of Bath, Claverton Down, Bath, Avon BA2 7AY, U.K. # Advanced Materials Processing Analysis Center (AMPAC) and Nanoscience Technology Center (NSTC), Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816, United States ‡

S Supporting Information *

ABSTRACT: Cuboidal nanoparticles of ceria are examined by high resolution imaging and analysis to explore their local morphology of faces, edges, and corners. Synthesized with and without Sm doping using a hydrothermal process, we find a high fraction of particles enclosed by {100} facets, which are normally energy-penalized compared to octahedral {111} facets. Electron tomography conducted at high magnification with lattice resolved imaging is combined with electron energy loss spectroscopy revealing oxidation states of Ce ions. It is found that extended {100} faces exist predominantly without local nanofaceting, except for {111} corner caps and subfacets on {110} edges. Reduced Ce is found on all {100} surfaces, while Sm doping does not lower the reduced Ce concentration. Molecular dynamics simulations are used to complement the microscopy, including the formation of {111} subfacets on {110} edges, formation of a {111} corner facet, and also the fact that reduced Ce ions prefer low coordinated positions like steps and corners along with more active {100} faces.

1. INTRODUCTION Cerium oxide (CeO2−x) is exploited in many applications spanning a wide variety of fields, including automobile exhaust catalysis1 and ion conductors in solid oxide fuel cells.2 Within the past decade, ceria is being increasingly explored in the biomedical field,3−9 as a potential antioxidant agent, as a NO radical scavenger, for controlling retinal degeneration, etc. The reason for its success is its redox chemistryCe ions are readily oxidized (+4) or reduced (+3). Ceria conforms to a fluorite structure in which each Ce4+ cation is surrounded by eight O2− nearest anions forming corners of a cube, and each O2− anion is surrounded by four Ce4+ cations (tetrahedron). This particular co-ordination results in the formation of oxide defects in CeO2−x with x varying between 0 and 0.5.10 Consequently, ceria can capture, store, release, and transport oxygen ions under variable redox conditions; charge neutrality in the unit cell is maintained by adjusting oxygen vacancies and reduced Ce ions. Initially, progress in ceria applications has been driven by maximizing the surface−volume ratio, which is achieved by reducing particle size to the nanoscale. More recently, research has shifted to enhance surface catalytic activity and oxygen storage and release capacity via fabrication of ceria nanostructures with specific morphologies which will have a higher fraction of highly active surfaces being exposed.11−14 It is well established that the binding energy of the © 2013 American Chemical Society

surface atoms on the low index surfaces follows the order {111} > {110} > {100},15,16 which means that maximizing {100} surfaces would increase surface activity proportional to the A{100}/A{111} area ratio of the powder. Enhanced activity can also be achieved by doping with trivalent transition or rare earth elements, which trigger the evolution of oxygen vacancies to maintain charge neutrality. The importance for studying cuboid nanoparticles is in establishing how their expected high oxygen extraction activity correlates with their high energy large {100} faces and how these are realized on the atomic scale. Flatness of faces and edges versus subfacet into zigzag directions of lower-energy indices are the main options, as well as modifications to corners (capping or bulging) or development of defects. Furthermore, it is worthwhile to study the postsynthesis stability of cuboids with respect to temperature and irradiation. Ceria nanoparticle characterization via TEM ultimately involves application of two advanced techniques: (i) Nanoscale spatially resolved electron energy loss spectroscopy (EELS), combined with spectrum imaging (SI), has shown enhanced Received: June 17, 2013 Revised: October 11, 2013 Published: October 11, 2013 24561

dx.doi.org/10.1021/jp405993v | J. Phys. Chem. C 2013, 117, 24561−24569

The Journal of Physical Chemistry C

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concentrations of Ce3+ within the first few monolayers on many kinds of ceria nanoparticle;10,17,18 however, no data is available for hydrothermally prepared cuboidal particles12 or particles with redox active doping. (ii) Electron tomography is a technique initially used to reconstruct the 3D morphology of mainly noncrystalline carbon-based biomaterials from their 2D projections,19−21 which has recently been successfully extended to reveal the 3D structures of inorganic and crystalline nanomaterials, including heavier elements22−24 such as nanoceria.25,26 These techniques also provide valuable data to an increasing number of atomistic simulation studies enabling the models to be tested and fine-tuned and in return contributing to the interpretation. Here, we extend earlier studies on ceria nanoparticles with octahedral morphologies to ceria nanoparticles with cuboidal geometries, prepared using a modified hydrothermal method. We also analyze the role of redox changes in doped and undoped systems using EELS.

amorphous specimens, this excludes crystalline objects unless diffraction contrast is suppressed (e.g., by EFTEM or HAADFSTEM), and also causes an upper thickness limit of validity.22,23,35−38 HRTEM lattice fringes also fail this requirement. An alternative way to retrieve the 3D shape of nanostructures is to simply rely on the peripheral projection/ shadow of the image at different angles. This is one of the groups of methods called “geometric tomography”39 and consists of the signal and the background being binarized to black and white or vice versa. For the general case, an oversized object, called “convex hull”, is reconstructed, but for axially convex and homogeneous objects, the 3D shape is retrieved correctly and identical to standard computed axial tomography.40 Tilt series of lattice resolved HRTEM images can thus be reconstructed by splitting the information content into the binarized image support for geometric tomography, and the lattice fringe pattern for later superimposing of crystallographic orientation.38 For electron energy loss spectra (EELS) of lanthanide rare earth metals (but also for X-ray absorption fine structure), the Medge which indicates transitions from 3d levels to the conduction band is preceded by a sharp double line (M5 at ∼886 eV and M4 at ∼903 eV), due to only partially filled upper levels of 4f type. Due to transition rules for energy sublevels and the deviating occupancy of 4f levels in oxidized and reduced cerium ions, the height of the M4 and M5 lines becomes a fingerprint for cerium valence. Fortner et al.27 showed that second derivative spectra facilitate replacing background adjustment and provide welldefined intervals for M4, M5 peak area measurements (i.e., above zero crossings). Empirically, a linear relationship between valence and the integrated intensity line ratio (M5/M4) of the second derivative signal of the original spectrum has been confirmed for Ce and Pr.27 2.3. Atomistic Modeling. At first, to explore the distribution of Ce3+, a nanoparticle of CeO2, comprising 15 972 atoms, was generated and amorphized. Then, 532 Ce4+ ions (about 10%) were substituted by Ce3+; charge neutrality was facilitated by creating 266 oxygen vacancies (about 5%) giving a nanoparticle with a stoichiometry of Ce2x3+Ce1−2x4+O2−x, with x ≈ 0.05. The nanoparticle was then crystallized using MD simulation following methods described in Sayle et al.41 As explained in the Results and Discussion section, energy barriers in the experimental cases of growth and transformation processes prevent cuboids to switch to {111}-dominated octahedra. Thus, we used the METADISE code42 to simulate a 2048 CeO2 unit cube. The cube was annealed under molecular dynamics simulation using the DLPOLY code43 and applying the same potential model as in section 3.5.1. The trajectories were generated in the canonical NVT ensemble by means of the Verlet leapfrog algorithm with a time step of 0.1 ps. The temperature was raised from 300 up to 2500 K and back in a 3 ns run. At each step, the temperature was kept constant by a Nosé−Hoover thermostat. The electrostatic interactions were calculated using the Ewald summation where periodic boundary conditions were applied.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Preparation of Ceria Nanoparticles. Cube-like ceria nanoparticles were prepared by a hydrothermal method. Separate solutions of 0.45 M Ce(NO3)3·6H2O, 0.45 M Sm(NO3)3·6H2O, and 10 M NaOH were prepared in double distilled water. For pure CeO2 nanocubes, 20 mL of Ce3+ solution was added with 20 mL of NaOH solution and stirred for 30 min in room temperature (RT). For 10% Sm3+ doped Ce3+, 2 mL of Sm3+ solution was added with 18 mL of Ce3+ solution. The Ce3+/Sm3+solution (20 mL) was poured into the NaOH solution and stirred for 30 min at RT. In each case, the precursor solutions were then placed in Teflon-lined autoclaves and were subjected to hydrothermal treatment with temperature at 180 °C for 24 h. Powder products in the solutions were separated by centrifugation, washed with deionized water repeatedly, and dried at 60 °C in air overnight. Samples were dispersed on a carbon-coated copper grid. 2.2. TEM Measurements. High-resolution transmission electron microscopy (HRTEM) was conducted on three complementary instruments: For high resolution studies of edges and corners, and their irradiation induced changes, we used a 200 kV JEOL JEM2010F-TEM and an aberration corrected 300 keV JEOL JEM3100-R005 TEM-STEM microscope, equipped with Gatan GIF (GIF US2k) and Digiscan STEMcontrol. STEM-EELS scans were taken using the R005 STEM in condenser corrected mode, with spherical aberration minimized. The general method of extraction of information about oxidation levels for transition elements and rare earth elements, including cerium valence from the Ce M-edge double peak fine structure, has been reported by several groups,27−33 which relies on the transitions for the M5 and M4 subedges being at a characteristic ratio for Ce3+ and Ce4+, in comparison to reference samples, such as bulk CeO2 and bulk CePO4. For the tomographic study, the specimen was loaded in a Fischione single tilt holder specially designed for tomographic studies which allows a wide range of tilt angles. Tomographic tilt series were recorded in a 300 keV JEOL JEM3010 microscope from −65 to +65° in steps of 5°. Post processing and tomographic reconstructions of the tilt series were done using ImageJ (TomoJ)34 software, and the 3D morphology of the nanoparticles was visualized by AMIRA 4.1 software. Electron tomography is only straightforward for specimens complying with the essential requirement of a monotonic relationship between image intensity and an atomic density. Apart from

3. RESULTS AND DISCUSSION The preparation of different nanostructures starts with nucleation from precursor solutions followed by growth of the resulting nuclei. By varying synthesis conditions, both the shape and growth direction of nuclei can be manipulated to obtain nanostructured materials with desirable size, shape, and morphologies. On the basis of this knowledge, synthesis temperatures that have been identified as key in controlling 24562



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Figure 1. Bright field TEM images of typical agglomerates of hydrothermally prepared ceria cube-like structures.

Figure 2. (a) Tomography of a ceria nanocrystal using bright field HRTEM tilt series from −65 to +65° at a 5° angular increment. (b) 3D reconstructed tomogram in surface rendered view showing a cuboid covered by six {100}-type faces.

clearly shows that the ceria particle is a cuboid covered by six {100} faces (Figure 2b). 3.2. Microstructure of Corners and Edges. Most faceted octahedral ceria nanoparticles show tiny capping facets cutting the corners to minimize surface energy. As is evident from some of the above tilt series images, the corners of the cuboid particles appear predominantly well-rounded instead of capped (Figure 3b). This can be further confirmed on images of nearby nanoparticles outside the ones chosen for the tilt series, such as in Figure 3a. On another particle, oriented at 45°, where the image (Figure 3c) is bordered by a {100} face (top of image) and an edge with edge direction and normal direction forming a {110} plane (right of image), a breakdown of the edge into nanofacets of {111} is observed, which here is not an irradiation effect. A similar reconstruction has been reported earlier, however triggered by the electron beam.44 It is well established that atomically flat and smooth surfaces, such as {100} and {110}, can be inherently unstable and prefer to transform into more stable energetically favorable morphologies through, e.g., formation of dislocations, stepped surfaces, etc.45 Oxygen content on the surface plays a crucial role in such transformations.46 Thus, it is evident that in this case the relatively unstable {110} surface transformed into a faceted surface covered by more stable {111} faces, as predicted in refs 15 and 16. Nie et al. also have reported a similar reconstruction, albeit on a larger scale,47 since, below a critical thickness, ceria forms {111} faceted islands when ceria is deposited on sapphire as a thin film. Faceting of these edges is not observed as a general rule on such cuboids. Due to the radiation independence, this faceting can be explored by and compared to molecular dynamics simulations, as detailed in sections 2.3 and 3.5, both experiment and simulations for the undoped case. The finding is that the

nucleation and crystal growth were explored, but results were found relatively insensitive to the ceria precursor concentration. Therefore, as long as the best suitable temperature (180 °C) is carefully maintained, ceria nanocubes of approximately the same size distribution are reproducibly obtained. TEM images showing similar ceria nanocubes were produced at the same temperature but with varying ceria precursor concentrations (0.23 and 0.35 M); see also Figure S4 (Supporting Information). The hydrothermally prepared ceria nanocuboids are found to range from 5 nm to over 200 nm in size (Figure 1) within one sample. Small-range agglomerates have sometimes a rather monodisperse size ratio, such as the group of less than 20 nm particles in Figure 1a and 70−80 nm sized particles imaged in Figure 1b. In order to establish the exact morphology (cube vs cuboid), the orientation of the crystal lattice relative to the morphology, and the structural details of corners and edges, we use electron tomographic tilt series imaging combined with HRTEM. 3.1. Electron Tomography. A 15−20 nm sized noncubic cuboid was selected for the electron tomography tilt series at high magnification (500 000×) with lattice resolution, reproduced as a demagnified overview in Figure 2a, comprising a few representative angles taken from the tilt series. The same image series is used twice, once with lattice fringes for extracting the crystallography and once binarized (Figure S1, Supporting Information) for 3D reconstruction. The latter helps improve the signal-to-noise ratio and suppress artifacts. This method of “geometric tomography” is applicable here because the cuboidal nanostructures are axially convex.40 It also accounts for the use of HRTEM images without a linear intensity−thickness relationship and with dominant Bragg scattering. The 3D reconstruction, in combination with evaluation of zone-axis HRTEM patterns, 24563



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progress of corner growth and cuboid-face growth, atoms will diffuse from corners to faces, and, e.g., nucleate a 2D island, which then continues to grow to cover the face completely. By exchanging atoms, the corner may change shape from round to sharp and vice versa during growth.48 Cubes form due to enhanced [111] growth speed during synthesis conditions, in spite of {100} faces being energetically unfavorable; they can therefore be assumed metastable relative to a morphology built by {111} faces. One way of testing whether energy supply can trigger morphology conversion through surface diffusion is to use electron irradiation at intermediate levels of TEM beam focusing to initiate atomic motion.49,50 For example, Rankin et al. demonstrated that at high temperatures neck formation between two adjacent nanoparticles (MgO,51 ZrO252) can be explained using similar arguments. In the case of MgO particles, they claim the initial neck growth is due to the surface diffusion of material away from the local edges to the growing neck due to differences in surface energies, which makes the edges rounder as the growth of the neck progresses. However, during the rupture of the neck, they suggest the material does not move back to the local edges and instead it moves into adjoining faces until they complete a full monolayer, again owing to the difference in free energies. All of these processes took place in an already completed crystal, with high temperature being the trigger. Similarly, here we aim to see if the impinging electron beam (focused or otherwise) would show similar effects. In Figure 4a,

Figure 3. (a) A typical ceria nanoparticle showing bulgy and rounded corners/edges imaged along ⟨100⟩, (b) ceria cuboid imaged close to the ⟨110⟩ direction, and (c) a ceria nanocuboid edge showing a sawtoothlike surface, also along ⟨110⟩. (d) Molecular modeling: {110} edge with {111} facets (Ce yellow and O red in top display; surface Ce monolayer highlighted by large spheres in bottom display).

breakdown of 110 edges into {111} facets in simulations is not a general feature either, at least for a small 4 nm side of a simulated cube particle (see also Figure S2, Supporting Information). However, close inspection of one of the simulated edges shows occasional features, equivalent to the experiment. For example, Figure 3d shows the edge connecting two flat {111} corners, which is printed here as a snapshot from the simulations detailed in section 3.5.2 about morphology models without Ce reduction. The flat {110} surfaces comprise two and three CeO2 units, while on the {111} surfaces we find between four and five CeO2 units. Another remarkable feature of the experimental images is that some edges/corners appear to bulge outward with two facets connecting to form corners or projected edges, devoid of at least one layer of atoms (Figure 3a). Tersoff et al. have shown that the shape of a small crystal is an oscillatory function of the number of atoms in the crystal;48 the authors used analytical calculations matched to experimental observations in the case of a crystalgrowth process where the total number of atoms changes continuously. During growth, attachment of atoms to defective sites (steps, edges, corners) is energetically more favorable compared to attachment on flat facets. During the competing

Figure 4. Irradiation of a typical ceria nanocube by a converged electron beam: (a, b) before irradiation showing a curvy edge; (c, d) after irradiation showing a flat facet.

the roundness of a ceria cuboid is highlighted. What appears as a rounded {111}-type corner, imaged along the [100] zone axis, translates into a round {110} edge accounting for projection (Figure S3, Supporting Information). We also note an incomplete (fractional) monolayer, shown by an arrow. Figure 4b is the magnified image of the region of interest (ROI). The electron beam was converged onto this edge for about 2 min. After 2 min when the beam was spread back to imaging mode, we see that, apart from ablating the carbon film in the ROI, the 24564



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curved edge has turned into a flat edge of type {110}. Close observation reveals, Figure 4d, that most of the atoms, which occupied the round corner/edge positions, have now moved to the facet (via surface diffusion) filling the earlier incomplete monolayer. 3.3. High Resolution EELS Study of Undoped Ceria Cuboid. Stoichiometric fluorite-structured {100} surfaces are intrinsically unstable because of their dipole moment. Accordingly, reconstruction and oxygen depletion are expected to quench the dipole.53 For small {100} facets on octahedral particles, this is well documented.49 However, for extended cube faces, no reports are known. As an indicator for oxygen depletion, we use the reduction of Ce4+ to Ce3+, which we expect for such surfaces to be structurally systematic, rather than an environmentally triggered redox effect. High resolution EELS line scans were taken on one of the ceria nanocubes, as shown in Figure 5. One scan was oriented perpendicularly across the surface (red rectangular region in Figure 5a), and one control scan was taken further inside the particle (outside of Figure 5a). Figure 5a represents a survey image of the edge of a ceria nanocube. EEL spectra were taken from the surface toward the particle center (red rectangular region). The length of the line scan was about 7.5 nm with 0.3 nm pixel size. The scan was corrected for drift and the initial 6 pixels eliminated because of the absence/poor counts of cerium. Figure 5b shows a 4.4 nm line scan starting from the surface (A) to the bulk (G), which is equivalent to 19 pixels of 0.23 nm each. M5−M4 branching ratios from each of those EEL spectra were measured from second derivative spectra, as described in section 2.2. Figure 5c shows the M5/M4 ratio as a function of position. Near the surface, the ratio oscillates (assumed random fluctuations) around 1.2 and then drops toward the particle interior over 2 nm down to around 1.0 and over 4 nm just below 1.0. The M5/M4 ratio for the reference region (taken well inside the particle) always fluctuates below 1.0, with a mean value of 0.93 (±0.04). These values are in fair agreement with the earlier reported standard results for Ce3+ and Ce4+ compounds.27,29 For example, measurements29 by the second derivative method for the Ce-valence standards CePO4 (for Ce3+) and bulk CeO2 (for Ce4+) revealed 1.308 for +3 and 0.901 for +4 compounds. Our values of 1.2 (surface) and 0.9 (bulk) therefore indicate a change in oxidation state of cerium from +3 to +4, associated with moving from the surface to the bulk of the cuboid. Another indication of change in oxidation state of cerium is the shift observed in the M5 and M4 peaks toward higher energy.28,29 Our data shows an increase in the peak position from 881.5 to 883.5 eV from surface to bulk regions, Figure 5d, although absolute energy values are affected by zero-loss drift and are not reliable in isolation. The control line scan within the particle interior has a twofold purpose: to help calibrate the interior peak ratio for Ce4+ and also estimate the significance via separating random fluctuations from systematic trends. There the peak positions remain strictly constant. It is to be noted that the EELS signals are projected measurements through the particle at the position of the electron beam and therefore include two contributions from the top and bottom surface near shells where cerium will likely be equally reduced. With fluctuations of ±0.05 for the M5/M4 ratio, defining the uncertainty, and a total value range of 1.0−1.2, the contributions of these shells vanish into the noise for the particle thickness tP being 8 times the shell thickness tS. In the present case, for tS ∼ 2 nm and particle size of at least 20 nm, the results are therefore reliable for the particle interior (pure Ce4+) and also

Figure 5. (a) HAADF survey image of a typical pure ceria nanocube showing the ROI where the line scan was taken. (b) Higher magnification of the survey image with scan region (0.23 nm/pixel) for a length of 4.0 nm. (c) Ce M5/M4 ratio, scanned from the surface toward the center, with the solid line showing the average M5/M4 ratio of the control scan in the particle interior. (d) Second derivatives of the Ce M4,5 spectra for the pixels from A to G near the surface.

for the surface itself (pure Ce3+), while, for the thin part of the entrance wedge, the measurements are to be seen as projected data. 3.4. High Resolution EELS Study of Doped Ceria Cuboid. Doped cerium oxide is increasingly being seen as a potential alternative to YSZ as an electrolyte in SOFCs in order to reduce high operating temperatures.54 Pure ceria is reported to have a higher electrical conductivity compared to other fluorite oxides like ZrO2, ThO2, and HfO2.54,55 Moreover, various rare earth oxides added to ceria as dopants are found to have a significant effect on the electrical conductivity. For example, Yahiro et al. found that ceria doped with 10 mol % Sm2O3 maximizes electrical conductivity compared to Gd2O3 and Y2O3 dopants.56 Dopants with fixed oxidation state (mostly +3) will sustain neighboring oxygen vacancies, and one could expect therefore that the residual Ce occupies as +4, with RE3+ (rare earth) taking over the role of Ce3+. Here we investigate the effect of 10% Sm doping on the fine structure of the Ce M4,5 EELS edge in comparison to pure ceria above. Figure 6a represents the 24565



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computer simulation was used to generate structural models of nanoparticles of ceria with two complementary aims: the first was to study the preferred distribution of reduced cerium atoms (section 3.5.1), followed by the study of stability, surface morphology, and corners/edges of unreduced simulated cuboids in section 3.5.2. 3.5.1. Distribution of Ce3+. To reveal the Ce3+ distribution, Figure 7 shows the low temperature structure of a polyhedral

Figure 7. Structural models of a nanoparticle of ceria: (a) surface rendered model showing more clearly the CeO2{100} and CeO2{111} surfaces; (b) sphere model representation of the atom positions. Cerium is colored white, oxygen is red, and Ce3+ is green.

nanoparticle, comprised of six {100} surfaces and eight {111} surfaces. Depending on area ratios, one would traditionally describe these either as octahedral truncated by {100} or as cuboid truncated by {111}. Inspection of the nanoparticle using graphical techniques reveals that the Ce3+ species reside preferentially at lowcoordinated positions such as stepped and corner sites (Figure 7). We presume this behavior is thermodynamically driven because corner and edge sites offer Ce3+ ions greater relaxational freedom, as they are larger compared to Ce4+. We also observe that the CeO2{100} surfaces include a high concentration of Ce3+ species. In particular, fluorite {100} surfaces are dipolar and are inherently unstable. To quench the dipole, the surface can undergo a structural rearrangement previously atomistic simulation predicted “−Ce−O−Ce−O− Ce−O−” chains on the surface, with CeO rather than CeO2 stoichiometry. Here, we see evidence of the chain configurations, Figure 7a, but we also note that Ce3+ species can also help quench the dipole via a change in local stoichiometry. 3.5.2. Stabilization of Model Cuboid Morphology. Atomistically simulated images of a nanocube of length 4 nm enclosed by {100} surfaces with rounded and flat corners are displayed in Figure 8. The Ce atoms of the layer below the surface (red) are well ordered, while the surface Ce atoms are slightly off their lattice site. Figure 8b shows the presence of groups of atoms redistributed from their lattice positions in order to quench the dipole. At the bottom left corner, an array of O−Ce−O atoms are rearranged on an ordered {100} surface (shown by an elliptical region). Visual inspection shows that large ordered patches of {100} surfaces are removed with increasing temperature and do not reappear after cooling, with the marked ordered isle of Figure 8b being the last remnant of these. Experimentally, these patches might disappear under local irradiation with the beam. We also see a fractional monolayer on the right side edge of the cube similar to our experimental observations (Figure 4d). Figure 8c emphasizes the location of the most prominent structural relaxations, as seen on the {100} type surfaces and the corner/

Figure 6. (a) HAADF survey image of a typical 10% Sm doped ceria nanocube showing the ROI where the line scan was taken. (b) Higher magnification of the survey image showing 0.13 nm pixel scan up to a length of 2.5 nm. (c) Plot showing reduction in M5/M4 ratios while scanning from the surface toward the bulk. The solid line shows the average M5/M4 ratio well inside the bulk (0.91 ± 0.02). (d) Second derivatives of the Ce M4,5 spectra for the pixels from A to G.

HAADF survey image and the region of interest (ROI). EEL spectra were again taken at each pixel starting from the surface (A) into the particle (G) (as shown in Figure 6b). The Ce M4,5 branching ratio starts from 1.3 ± 0.1 on the surface, which is nominally higher compared to the value for pure ceria nanocubes, however, also related to higher initial random fluctuation for the very thin edge. Nevertheless, the backward extrapolation of smooth trends for a zero-depth value for Figures 5c and 6c (∼1.25 vs ∼1.30) would confirm a slightly higher reduction level in Figure 6c. Mainly, in the doped case, the peakratio decreases twice as fast to below 1.0 along the length of a shorter line scan (2.5 nm) compared to 4.4 nm in the case of pure ceria. Similarly, the peak shift is stronger here, ranging from 880.5 eV on the surface to 883.5 eV in the bulk (Figure 6d). All together, the three criteria (initial surface peak ratio, peak shift, and slope of ratio change) point to a more significant redox change in the doped sample. 3.5. Molecular Modeling. To help further understand the structure and chemistry of CeO2 nanocubes, an atomistic 24566



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Figure 8. Top view of a {100} surface. (a) Only Ce atoms are displayed. Green are the surface Ce atoms, red are the Ce atoms just below the surface, and blue is the rest of the crystal. (b) Surface Ce (white) and O (red) atoms are displayed. (c) Ce and O atom positions shown as small spheres to reveal more clearly the considerable ionic relaxation at the {100} surfaces. (d) Surface rendered model revealing more clearly the cuboidal morphology.

time applied to ceria cuboids, that Ce3+ enriched {100} surface layers exist in all samples. We also saw that the reductive 4+ to 3+ ratio change of Ce ions in the Sm doped cuboids is even more pronounced than in the undoped samples. Earlier studies, using XPS, Raman peak shift, and lattice expansion via XRD also indicated similar increased Ce reduction, after doping ceria with various concentrations of Sm, Gd, and Y.57,58 Our EELS results further prove that the reduction effect is more pronounced and very localized at the surface, while the particle interior +4/+3 ratio is similar to pure ceria. In other words, Sm is not preventing Ce from reduction, but rather the overall level of surface oxygen deficiency and therefore overall need to reduce RE ions is larger in the doped material. The correlation between theory and experiment in this work has enabled understanding of various findings, whether mutually confirmative or complementary: (i) Cubes were simulated with extensive flat {100} surface layers, which do not autoconvert to other morphologies, equivalent to the experimental cube resistance to both heating and irradiation. However, cubes have a tendency to form zigzag shaped edges through subfaceting in both theory and experiment (Figure 4), indicating an underlying instability of straight edges of {110} type. (ii) Atomistic simulations complement and enhance the experimental TEM findings: The models reveal the atomistic structure of the top atomic {100} layer. In particular, the Ce and O atoms do not lie on lattice positions; rather, the model reveals significant ionic relaxation, which is likely to influence the (re)activity of these surfaces. Cube corners (and edges) have the greatest proportion of relaxed and “mobile” atom species (Figure

edge region, corresponding as well to the locations of most mobile atoms underpinning the irradiation experiments of Figure 4. The simulated 3D cube model to accompany the tomographic experimental result of Figure 2b is printed as Figure 8d. One {110} type edge of this cube has been printed as Figure 3d for direct comparison with experimentally observed nanofaceting of such edges. 3.6. Interpretation. Experimentally, ceria nanocubes exist owing to the synthetic protocol, which favors the energetically least favorable {100} surfaces and promotes the fastest growth in ⟨111⟩ directions. Once the metastable “frozen-in” cubes are formed, the transformation to the polyhedral morphology enclosed by {111} surfaces might occur when the energy associated with mass transport is provided. This process is likely to have high energy barriers. Indeed, the {100} cube faces are experimentally found stable under both irradiation and heat: (i) during electron beam irradiation, the cuboids do not change morphology toward octahedral; (ii) during in situ TEM heating holder testing, we found no morphology changes when heating over 3 h up to 600 °C in a vacuum. What does change experimentally through irradiation (where a beam is converged to a corner or edge region) is the facilitation of the transformation of the roundish edges of a cuboidal nanoparticle into facets (Figure 4). Such faceting would be expected by thermodynamics because CeO2{110} and CeO2{111} are energetically more stable compared to CeO2{100}. As the direct imaging only shows Ce atom fingerprints, it was found useful to study Ce valence with lattice-resolution spectroscopy, to estimate the amount of oxygen depletion at surfaces. EELS spectrum imaging indeed confirms, for the first 24567



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8 vs Figure 4c), equivalent to the experimental tracking of irradiation induced atom mobility. We assume that the irradiation provides the kinetic acceleration of an otherwise energetically driven process, supported by our molecular dynamics calculations which show similar faceting on annealing (Figure S2, Supporting Information). This is important because surface ion mobility, triggered by an electron probe, has been proposed as a powerful gauge of surface activity.49,50 Converged electron beam irradiation enhances Ce mobility following ease of oxygen extraction.50

4. CONCLUSIONS Cuboidal ceria nanoparticles, enclosed by large {100} faces and prepared using a hydrothermal method, are found to be structurally homogeneous and atomically flat for the great majority of faces and edges imaged. Some significant reconstructions, such as subfaceting, have been reproduced by molecular modeling in agreement with experiment. The functional properties of cuboid nanoparticles of ceria for all application fields involving Ce redox buffering and oxygen extraction/storage functions depend on identifying the most active sites.59 In addition to the known “high-activity”{100} faces due to local reconstructions and energetic instability relative to {111}, the role of corners and edges has been emphasized by both experiment and theory as a further complicating factor, possibly exceeding activity of any flat faces. Doping of ceria with rare earth elements of preferred +3 valence is found not to lower the amount of reduced Ce ions near the surfaces. Furthermore, we have shown how the observation of round and faceted corners/edges and dynamical transition between these can provide a modern illustration of Tersoff’s theory48 of shape oscillations of nanocrystals.

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ASSOCIATED CONTENT

* Supporting Information S

Further images underpinning the tomographic tilt series, molecular modeling of ceria nanoparticles, and a 3D sketch of general indexing of faceted cubes are provided complementing the main Figures 2, 3, 4, and 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: g.moebus@sheffield.ac.uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank EPSRC, U.K., for funding the microscopy and modelling parts (EP/H001298, EP/H001220, and EP/ H005838/1) as well as NSF NIRT for Nanoceria research (NSF International supplement: CBET 1028996). The computations were performed on the HECToR facility via the HPC Materials Chemistry Consortium funded by EPSRC (EP/ L000202).

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REFERENCES

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Morphology and Surface Analysis of Pure and Doped Cuboidal Ceria

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