Letter Cite This: Nano Lett. 2017, 17, 7652−7658
pubs.acs.org/NanoLett
Direct Visualization and Control of Atomic Mobility at {100} Surfaces of Ceria in the Environmental Transmission Electron Microscope M. Bugnet,*,† S. H. Overbury,‡ Z. L. Wu,‡ and T. Epicier*,† †
University of Lyon, INSA Lyon, UCBL Lyon 1, MATEIS, UMR 5510 CNRS, 69100 Villeurbanne Cedex, France Chemical Science Division, Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡
S Supporting Information *
ABSTRACT: Ceria is one of the world’s most prominent material for applications in heterogeneous catalysis, as catalyst support or catalyst itself. Despite an exhaustive literature on the structure of reactive facets of CeO2 in line with its catalytic mechanisms, the temporal evolution of the atomic surface structure exposed to realistic redox conditions remains elusive. Here, we provide a direct visualization of the atomic mobility of cerium atoms on {100} surfaces of CeO2 nanocubes at room temperature in high vacuum, O2, and CO2 atmospheres in an environmental transmission electron microscope. Through quantification of the cationic mobility, we demonstrate the control of the surface dynamics under exposure to O2 and CO2 atmospheres, providing opportunities for a better understanding of the intimate catalytic mechanisms. KEYWORDS: CeO2 nanocubes, atomic mobility, facet, surface, environmental transmission electron microscopy, high resolution transmission electron microscopy
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much information at that time, they suffered from relatively limited detection capabilities to reach further insight into the mobility of atoms and the temporal evolution of single atoms at topmost surface planes. Furthermore, imaging and controlling the surface state often requires a control of the atmosphere. While a strong development toward environmental TEM has occurred since the 70s,15,16 detection capabilities did not allow routine atomic resolution under controlled atmosphere. With the advent of aberration-correction and high speed cameras in the past decade, HRTEM imaging has become a key technique to allow atomic-scale analysis in real time in environmental conditions.17−26 One of the main challenges in observing CeO2 in the TEM is its notorious ability to reduce easily under irradiation from the electron beam. While the reduction state of CeO2 surfaces has been probed using electron energy-loss spectroscopy (EELS) in the scanning TEM (STEM) to highlight the preferential reduction of {100} surfaces as compared to {111} facets,27,28 and controlled cationic alloying,29−34 the ease of irradiationinduced reduction of surfaces over bulk is a challenge that can hardly be completely overcome in high vacuum conditions.35 Working under controlled O2 partial pressures in the ETEM has been proposed as a method to effectively maintain the Ce1/ O2 stoichiometry.36,37
eria (CeO2) is a fascinating material with a wide range of applications in electrochemistry and most notably in catalysis.1−3 It is one of the most widely used catalyst and catalyst support in industry, as well as one of the most studied at the laboratory scale, for a wide range of reactions, such as the oxygen storage in CO oxidation and NO reduction in three-way catalyst,4,5 the steam reforming of methanol,6 hot reformate desulfurization,7 water splitting8 and water−gas shift reactions,9 the promotion of electron exchanges at the anode of solid-oxide fuel cells,10 as well as for its potential use in biotechnology and medicine.3 Ceria has indeed the ability to release or store oxygen atoms from its environment by adapting its 3+/4+ valence state to compensate anionic vacancies in between the Ce cationic lattice.11 This property is enhanced by controlled cationic alloying, especially with Zr.4,12 Nevertheless, despite a huge collection of in-depth experimental and theoretical work related to its catalytic activity, the temporal evolution of reactive surfaces of ceria is still not completely understood. Highresolution transmission electron microscopy (HRTEM) is an appropriate technique to probe the surface reactivity and temporal evolution at the atomic scale and shed light on the motion of atoms and molecules. Smith et al. reported dynamic structural rearrangements in few nanometer Au particles with frames taken every few seconds.13 Iijima and Toshinari used a video recording system to track the motion of surface atomic steps and the random walk of surface atoms in gold nanoparticles at high resolution (2.3 Å), and at exposure times of 1/60 of a second.14 Although these studies provided © 2017 American Chemical Society
Received: August 27, 2017 Revised: November 15, 2017 Published: November 22, 2017 7652
DOI: 10.1021/acs.nanolett.7b03680 Nano Lett. 2017, 17, 7652−7658
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Figure 1. CeO2 nanocubes in the ETEM. Top: low-magnification TEM bright-field image recorded in high vacuum conditions, and highlighting the morphology of the nanocubes. Bottom: HRTEM bright-field images in [110] and [001] zone axes highlighting the sharpness and cleanliness of the facets, and the resolution capabilities of the Cs-corrected ETEM instrument.
performed in STEM mode.41 The surface mobility of Ce atoms on {100} and {111} facets in high vacuum has been reported recently,42 demonstrating spontaneous relocations of single atoms and entire atomic rows. Spontaneous atomic migration on {100} surfaces has also been shown on epitaxial CeO2 thin films.43 Nevertheless, environmental conditions are needed to understand the relationship between atomic mobility and catalytic activity, as well as facet selectivity of ceria. In this work, we directly visualize and quantify the mobility of Ce atoms at {100} surfaces in high vacuum, O2, and CO2 atmospheres. We demonstrate the control of surface mobility under O2 and CO2 exposure, proving that the effects of electron-induced irradiation commonly observed in high vacuum can be tuned and controlled in such so-called environmental conditions. These results bring new information on the dynamics of the {100} surface structure of CeO2, and open doors to design in situ catalysis experiments in the environmental TEM to better understand the activity of CeO2 as a catalyst or catalyst support. Ceria nanocubes have been synthesized using conventional chemical methods, allowing precise control of the shape of the nanoparticles,44 as shown in Figure 1. More details regarding the synthesis can be found in refs 45 and 46. The surface
The atomic structure of different facets of CeO2 has been studied using advanced high-resolution microscopy methods. For instance, two surface reconstructions of the polar {100} facets have been demonstrated using high-resolution scanning tunneling microscopy and density functional theory.38 These reconstructions are shown to be in agreement with the intrinsic polarity and important reduction of the {100} planes. Atomic structures of {100}, {110}, and {111} surfaces have been studied using HRTEM, leading to the determination of oxygen atomic positions.39 The {100} facets show a complex structure, with evidence of Ce, O, and reduced CeO terminations, whereas {111} facets show an O termination, and {110} facets display a sawtooth-like structure exposing {111} facet domains and CeO2−x termination. The structural evolution of {110} facets into a sawtooth-like structure has been studied in detail recently in CeO2 nanorods.40 The surface structure of ceria is not static, and the inherent mobility of atoms at ceria surfaces must be assessed for a better understanding of the temporal evolution of the atomic structure of facets. Beside the temporal stability of surfaces governing the efficiency of catalytic processes, the stability of crystalline facets is of interest to understand nanocrystal morphology and growth kinetics, as shown in the case of gold nanorods through atomic counting 7653
DOI: 10.1021/acs.nanolett.7b03680 Nano Lett. 2017, 17, 7652−7658
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Figure 2. High vacuum mobility of cations at {100} surface. Single frames (0.04 s exposure about every 0.2 s) extracted from the dynamic sequence in Video V1 in SI. A small area is shown of the edge-on (001) surface of a ceria nanocube in the [110] orientation under high vacuum at 5 × 10−6 mbar. The contrast of some Ce atomic positions is indicated by arrows, highlighting a significant atomic mobility of cations from one frame to the other.
from the video sequence Video V1 and directly illustrates the high mobility of O and Ce atoms in HV conditions (Figure 3 top), and the significantly more stable configurations under an oxidizing atmosphere (O2 or CO2, Figure 3 middle, bottom). The surface mobility is strongly affected upon introduction of O2 in the TEM specimen chamber, as illustrated in Video V1 and in Figure 3 (middle). Even at a relatively low O2 partial pressure of 5 × 10−2 mbar, the (001) surface is now terminated by O columns, which in turn limits the mobility of the underlying Ce atoms. Some Ce atoms, however, are not covered by O atoms, and thus remain mobile on the surface. Nevertheless, to a great extent it is apparent that ceria presents an O-termination on {100} surface under O2 atmosphere, which reflects a saturated state where the oxygen supply compensates the loss of oxygen from electron beam irradiation. The surface mobility is completely stopped when CO2 is introduced in the chamber: the surface state is homogeneous over the areas exposed to the electron beam even during prolonged observations, as illustrated in Video V1 and Figure 3. Interestingly, dark dots corresponding to the positions of oxygen atoms have a greater contrast than oxygen columns in Figure 3 (middle). Given the nature of the surrounding gas, this contrast has to be attributed to carbonates from adsorbed CO2. The exact nature and structure of the adsorbed species leading to these atomic-scale contrasts at surfaces is not clear, given the large number of theoretically predicted adsorbed configurations of CO2 under the form of CO3 carbonates.47−49 Similar live HRTEM observations of {001} surfaces have been performed in ⟨100⟩ orientations to follow the atomic
mobility of a (001) facet at room temperature is illustrated in Figure 2 under high vacuum at 5 × 10−6 mbar. The intensity variation at Ce atomic columns is highlighted in single images recorded at different times, extracted from Video V1 recorded at 25 fps in 4k × 4k and available in Supporting Information (SI), showing a small area of the edge-on (001) surface of a ceria nanocube in the [110] orientation. The light contrast indicated by arrows, corresponding to Ce atoms, changes rapidly from one frame to the next, highlighting the atomic mobility of the cations. This mobility can be further appreciated in Video V1, and the (001) terminating layer appears to be essentially a Ce plane. While oxygen atoms are not seen in the images displayed in Figure 2, they are furtively detected from time to time on single frames in Video V1. Similar observations have been achieved with nanocubes oriented along ⟨001⟩, as shown in Figure S1 and Video V2 in SI. In that zone axis, surface Ce atoms are consistently observed atop two oxygen columns, hence at a position that is similar to their bulk position. The contrast variation of O and Ce columns at the topmost surface layer in Video V2 illustrates the substantial mobility of atomic species, either as single atoms and partial rows at once. Although the frame rate (exposure time 0.04 s/f) is likely lower than the hopping rate of surface atoms, some atomic arrangements such as those shown in inset of Figure S1 are stable over several frames. The surface mobility and termination of a (001) facet under three different environmental conditions, that is, high vacuum (HV) at 5 × 10−6 mbar, O2 (5 × 10−2 mbar), and CO2 (5 × 10−2 mbar), is illustrated in Figure 3. This montage is extracted 7654
DOI: 10.1021/acs.nanolett.7b03680 Nano Lett. 2017, 17, 7652−7658
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Figure 3. Mobility of cations at {100} surfaces. Two second average of 50 frames from the Video V1 (see in SI) showing the edge-on (001) surface of a nanocube in the [110] orientation under high vacuum at 5 × 10−6 mbar, 5 × 10−2 mbar O2 and 5 × 10−2 mbar CO2 (respectively from top to bottom). The contrast at oxygen atomic positions (indicated by arrows) increases from top to bottom and the intensity variation of the most external Ce atomic layer decreases accordingly. Video V1 illustrating the dynamics of the (001) surface in these conditions is available as Supporting Information.
mobility, as reported in Supporting Information (see Video V2 in SI-b). Qualitatively, they show identical results to the ones in ⟨110⟩ azimuth. In particular, the contrast difference between dots located at the oxygen positions defining the terminating layer is quite significant when changing the atmosphere from O2 to CO2 (see Figure S2), which gives more credence to the conclusion that adsorbed carbonates are effectively being imaged. In order to evaluate quantitatively the mobility of atoms, the intensity of atomic columns has been measured. In purely kinematic conditions that are necessarily reached in the thinnest near surface areas where “black atoms” images have been acquired (see Supporting Information SI-c), the intensity of a single column can be considered as inversely proportional to its atomic occupancy. Although this is clearly not achieved rigorously for all counted columns, their intensity is then a qualitative marker of their atomic occupancy (see Supporting Information SI-d for details). The relative variation of atomic occupancy can be visualized over a time frame of about 17s (420 frames) in Video V3. The position of each column in each frame is determined through a peak search algorithm, assigning the center of each column to the barycenter of intensity. The columns where the intensity varies across a large range of the temperature-color lookup table are the ones where the occupancy varies most widely. It should be noted here that recording videos at 25 fps is fast enough to detect such variations clearly associated with atomic mobility, as attested by the Video V1, although there is certainly an averaging effect due to atomic diffusion at a much faster rate, especially under high vacuum. Even though averaging individual video frames clearly points out differences in the behavior of the surface columns in the different environments, as depicted
in Figure 3, there is a risk that a slower acquisition frequency leads to a worst sensitivity of the columns intensity versus the atomic mobility. As the crystal thickness increases toward the center of the nanocubes, the kinematical approximation is no longer valid and in principle the intensity of the columns will no longer reflect their atomic occupancies. However, any variation remains a qualitative indicator of atomic mobility. The atomic mobility is evaluated as follows: We label negatives intensities of Ce columns at the (001) surface as ISCe(n,t), where n is the index of the nth column, t the time within a video sequence counted as the number of frames since the recording start, and superscript S is the surface. Similarly, IBCe(n,t) designates the intensity of a “bulk” Ce column. These intensities are displayed in Video V3 in SI, which corresponds to the areas shown in Video V1 and illustrates the variation of the atomic occupancy of each Ce columns through the variation of their reverse intensity ISCe(n,t) and IBCe(n,t). Keys to read this video are (i) the lower intensity (blue) indicates columns with fewer atoms, and (ii) the blinking effect due to the variation of intensity with time quantifies the atomic mobility (little color change of any given column means low or undetectable mobility). Naming σ[ISCe(n,t)] and σ[IBCe(n,t)] the standard deviations of the intensity of the nth column, respectively at the (001) surface and from the bulk, allows to define the corresponding mean values σSmean and σBmean. With the aim to compare video sequences taken under different environmental conditions, it is necessary to normalize these measurements and define the surface mobility as 7655
DOI: 10.1021/acs.nanolett.7b03680 Nano Lett. 2017, 17, 7652−7658
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Figure 4. Quantitative analysis of Ce mobility at {100} surfaces. Variation of the standard deviation of atomic column intensities of a ceria nanocube viewed along the [110] direction (FEI Titan ETEM 300 kV at 20 °C) under high vacuum at 5 × 10−6 mbar, 5 × 10−2 mbar O2, and 2.6 × 10−2 mbar CO2 (respectively from top to bottom). Each dot images a single atomic column position, and its color indicates the variation of intensity over 420 images recorded at 25 fps on a normalized scale (Blue, low variation; red, high variation). For the (001) surface, the column intensities vary strongly under high vacuum, indicating a high mobility (red). The mobility, also illustrated by the position of each column (see SI-c), is lower under O2 atmosphere and nearly stops when CO2 is adsorbed (blue). The mobility of Ce atoms on {111} surfaces is relatively low independently of the conditions. Missing columns correspond to positions where columns fully disappear within the time frame spanned by the 420 frames.
MOBE(001) =
σ S mean σ
deviation of atomic column intensities in the plane further away from the surface seems to be slightly higher than in planes closer to the surface, with the exception of the top surface plane. Nevertheless, such differences of atomic mobility are likely to lie within the error of intensity measurement. One also notes that some of the atomic columns at the top surface plane (left and right in Figure 4, bottom) present a distinguishably higher mobility than other atomic columns at the top surface (center in Figure 4, bottom) and in the atomic planes below the surface. The physical reasons behind these observations will be subject to further investigations that are beyond the scope of the present report. Further illustration of this phenomenon is reported in Figure S8, where the intensity variation of a couple of representative “Surface” and “Bulk” columns is represented as a function of time (i.e., 420 frames recorded at 25 fps) for each environmental condition. These plots provide direct visualization of the decrease of mobility when changing the atmosphere from HV to O2 and to CO2. The corresponding standard deviations lead to mobility factors MOBHV(001) = 2.65, MOBO2(001) = 2.00, and MOBCO2(001) = 1.03, in good agreement with the average values presented in Table 1, and demonstrate unambiguously the greater surface mobility under high vacuum and, conversely, the greater stability under CO2. The dynamics of the catalytically active {100} surface of ceria nanocubes has been investigated in situ under various atmospheres at room temperature in the environmental TEM. The mobility of Ce atoms is substantial in high vacuum, and decreases drastically in an O2 environment, then is undetectable when exposed to CO2. In situ high-resolution TEM observations provide evidence that {100} facets are oxygen-terminated in O2 atmosphere. When exposed to CO2, the contrasts in high-resolution TEM images suggest the adsorption of carbonates forming the termination layer, thereby suppressing the surface mobility. The direct visualization of the
B mean
where the superscript E represents the environment around the sample (high vacuum, O2 or CO2). The intensity variation per column is displayed in Figure 4. In HV conditions (Figure 4, top), the mobility is distinguishably higher at the topmost (001) surface plane, as compared to the bulk and {111} facets, which is quite low. Furthermore, the mobility seems to decrease when approaching the edge/corner of the {100} facet. Goniakowski and Noguera recently proposed that the electrostatic field induced by the polarity of a {100} facet varies from the center to the edges,50 and could be at the origin of the observed inhomogeneity of the mobility in the topmost plane of cations in Figure 4 (top). The variation of intensity at {100} facets decreases in O2 atmosphere (Figure 4, middle), and even further under CO2 atmosphere (Figure 4, bottom). These observations are summarized in Table 1, which reports the value of the mobility parameter MOBE(001). The average mobility of the (001) surface plane decreases accordingly when the atmosphere changes from HV to O2, and then to CO2. More subtle effects are also visible under CO2 in Figure 4 (bottom). On average over a single cerium plane, the standard Table 1. Mobility of (001) Surface Ce Atoms under Various Environmental Conditions at 20°Ca environmental conditions −6
HV 5 × 10 mbar, 20 °C O2 5 × 10−2 mbar, 20 °C CO2 2.6 × 10−2 mbar, 20 °C
MOBE(001) 2.63 ± 0.26 2.16 ± 0.19 1.04 ± 0.12
a
The accuracy is deduced from the numerical accuracy in measuring the intensity of atomic columns (more details are available in SI). 7656
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cationic mobility is supported by a quantitative analysis, and provides valuable information regarding the effect of the surrounding environment at {100} surfaces of CeO2, which is of importance to control and foster the reactivity of this major catalyst material. This work suggests that the atomic-scale mobility could be used as an indicator of the desorption/ adsorption of molecular species during in situ TEM experiments to better understand the role of {100} surfaces in the case of catalytically activated reactions at elevated temperature.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b03680. Materials and methods, in situ TEM characterizations, interpretation of atomic column contrast, evaluation of the atomic mobility, supporting figures (PDF) Evolution of the (001) surface of a ceria nanocube observed along the [110] viewing direction under high vacuum (HV) at 5 × 10−6 mbar, O2 (5 × 10−2 mbar), and CO2 (2.6 × 10−2 mbar) (AVI) Evolution of the (001) surface of a ceria nanocube observed along the [100] viewing direction under HV at 1.5 × 10−6 mbar (AVI) Evolution of the cerium mobility in the area displayed in Video V1, with superimposed colored reverse intensities of atomic columns (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.B.). *E-mail:
[email protected] (T.E.). ORCID
M. Bugnet: 0000-0002-8272-8964 S. H. Overbury: 0000-0002-5137-3961 Z. L. Wu: 0000-0002-4468-3240 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.B. and T.E. acknowledge funding from INSA Lyon through a BQR project THERMOS. S.H.O. and Z.W. are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The electron microscopy work presented here has been performed on a FEI Titan ETEM at the CLYM: Lyon-St-Etienne center for electron microscopy; the assistance of M. Aouine in preparing the microscope is gratefully acknowledged.
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REFERENCES
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