Stepwise Displacement of Catalytically Active Gold Nanoparticles on

Jun 20, 2013 - Department of Physics, Graduate School of Science, Osaka ... The sample is Au/CeO2 powder that has exhibited high catalytic activity ...
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Letter pubs.acs.org/NanoLett

Stepwise Displacement of Catalytically Active Gold Nanoparticles on Cerium Oxide Yasufumi Kuwauchi,†,‡ Seiji Takeda,*,† Hideto Yoshida,† Keju Sun,†,§ Masatake Haruta,∥ and Hideo Kohno‡ †

The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan § Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda, Osaka 563-8577, Japan ∥ Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan ‡

S Supporting Information *

ABSTRACT: Aberration-corrected environmental transmission electron microscopy (ETEM) proved that catalytically active gold nanoparticles (AuNPs) move reversibly and stepwise by approximately 0.09 nm on a cerium oxide (CeO2) support surface at room temperature and in a reaction environment. The lateral displacements and rotations occur back and forth between equivalent sites, indicating that AuNPs are loosely bound to oxygen-terminated CeO2 and may migrate on the surface with low activation energy. The AuNPs are likely anchored to oxygen-deficient sites. Observations indicate that the most probable activation sites in gold nanoparticulate catalysts, which are the perimeter interfaces between an AuNP and a support, are not structurally rigid. KEYWORDS: Gold, nanoparticle, CeO2, interface, heterogeneous catalysis, in situ environmental TEM

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surface exposed metal (gold) atoms was estimated to be 0.24 s−1 at 303 K. The mean diameter of AuNPs was measured to be 4.1 nm with a standard deviation of 1.3 nm. For ETEM observations, the Au/CeO2 powder is supported on a carboncoated microgrid backed by a 3 mm diameter Cu mesh. The ETEM (FEI Titan ETEM G2) is equipped with a corrector for the spherical aberration of the objective lens (Cs corrector) and was operated at 300 kV. Images were recorded with a time resolution of 1.0 s per 1024 × 1024 frame. At a magnification of 1100 × 103, the spatial resolution between pixels was 0.0178 nm; thus the accuracy in measuring the position of an atomic column was limited to 0.02 nm. The residual gas pressure in the ETEM was 1.2 × 10−5 Pa, and the partial pressures of constituent gases were H2O: 1.0 × 10−5 Pa, N2: 0.07 × 10−5 Pa, O2: 0.09 × 10−5 Pa, and CO2: 0.04 × 10−5 Pa, as measured by a quadrupole mass spectrometer. To minimize damage during ETEM imaging, the Au/CeO2 powder was irradiated with an electron flux that was less than 4.0 A/cm2 (2.5 × 105 electrons s−1 nm−2). At this electron flux, no irreversible structural damage was induced in the catalyst sample unless observation time exceeded a few minutes. A preliminary examination of the catalyst samples was performed in vacuum by TEAM 0.5 TEM (300 kV, Cs = −16 μm, Cc = 2.07 mm) at the National Center

oble metals are extremely important as catalysts in science, technology, and industry. The catalytic activity of platinum and palladium is based on their surfaces; thus these metals are usually employed in the form of supported nanoparticles to enlarge their surface area. In contrast, normally inactive gold needs to be supported on selected metal oxides to exhibit catalytic activity,1 because catalysis by gold nanoparticles (AuNPs) is most likely affected by the electronic states2−5 which strongly depend on their contact interface structures. Therefore, numerous studies have focused on the role played by the interface between AuNPs and metal oxide supports.2−12 However, the determination of atomistic structures at the interface has been challenging. By means of in situ aberrationcorrected environmental transmission electron microscopy (ETEM),13 we have found that AuNPs occasionally move stepwise by approximately 0.09 nm on CeO2 supports at room temperature. The stepwise lateral displacement and rotation occurs in a reversible manner during catalytic reactions. This unexpected rigid-body-like motion of AuNPs under electron irradiation can be accounted for by the weak interaction of the gold and the support metal atoms. Furthermore, these weakly bound AuNPs stay preferentially at multiple stable positions, such as oxygen-deficient sites, where they can form localized Au-metal (Ce) bonds. The sample is Au/CeO2 powder that has exhibited high catalytic activity for the oxidation of CO even below room temperature, as is described before.11,14,15 The reaction rate per © 2013 American Chemical Society

Received: March 12, 2013 Revised: May 31, 2013 Published: June 20, 2013 3073

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Figure 1. (a) AuNPs supported on CeO2 in a reaction environment (100 Pa of 1 vol % CO/air at room temperature). An AuNP in the rectangular area I in a is shown in b, while another AuNP is seen in the rectangular area II in c. The rectangular area in b is further enlarged in Figure 2, while c is enlarged in Figure 3. The electron current density during ETEM observation was 4.0 A/cm2.

Figure 2. Lateral, rigid-body displacement of an AuNP (Figure 1b). Observation time is shown. (a) Observed images, corresponding simulated images, and modes for the interface are shown in the top, middle, and bottom rows, respectively. (b) Plan view of the interface model that corresponds to the observation in a. The interface observed in a ranges between two straight lines in b. A and B are the primitive vectors of the coincidence lattice. R and Rp are the displacement vectors of the AuNP relative to the CeO2 support and its projection along the viewing direction of a, respectively. Gold, gray, and red circles designate gold, cerium, and oxygen atoms, respectively. Atomic planes of gold and those of cerium are designated by blue and red arrows, respectively. The coincidence lines, or gold atomic columns at which gold atomic planes and Ce atomic planes intersect, are designated by magenta dots. The coincident lattice points of the two crystals, Au and CeO2, are designated by green dots. In a and b, the CeO2 support is fixed in the viewing areas, while the AuNP moves. More details are in the text.

Figures 2 and 3, one can see the rigid-body displacement of both AuNPs. Figure 2 and Movie S1 document the displacement of an AuNP (viewed laterally) and show both image simulations and interface models. To clarify the movement of the AuNP relative to the CeO2 support, atomic planes of gold and cerium are designated by blue and red arrows, respectively, in Figure 2a. Magenta dots in Figure 2a represent “coincident” atomic columns where the Au and Ce planes coincide and run parallel to the viewing direction. One can see that the location of the coincident atomic column changes with observation time, indicating that the AuNP and the support are “mutually” displaced. During the period 0−2 s, the AuNP and the CeO2 support are mutually shifted from side to side by 0.09 nm (along the [1̅1̅2]Au or [112̅]CeO2 directions) and then returned to the initial position at 43 s. Displacement is stepwise as is summarized in Figure 4a. It is well-known that a coincidence lattice19 appears in a coprojection of crystalline Au and CeO2 along the major [111]Au and [111]CeO2 zone axes; it is represented by a 4 × 4 superlattice in Au and 3 × 3 superlattice in CeO2. Interface models simulate the observations, as is shown in Figure 2a and b in lateral and plan views, respectively. In the plan view, coincidence lattice points are marked by green dots. The primitive vectors of the coincidence lattice, A and B, can be expressed as:

for Electron Microscopy, Lawrence Berkeley National Laboratory, USA. Image analysis and simulation was performed with MACTEMPAS software (Total Resolutions, Berkeley, CA, USA), with the following parameters: 300 kV accelerating voltage, a 1 μm spherical aberration coefficient for the objective lens, a 1.4 mm chromatic aberration coefficient for the objective lens, a 10 nm−1 objective aperture radius, a 0.05 nm standard deviation for the mechanical vibration in the lateral plane, and a −4 nm defocus of the objective lens. Homogeneous degradation of ETEM images due to electron scattering by gases16,17 was not taken into account. Figure 1 shows two AuNPs in a reaction environment (100 Pa of 1 vol % CO/air at room temperature). The AuNPs are seen in the rectangular areas denoted by I and II. The diameters of AuNPs are approximately 3.3 nm in Figure 1b and 2.4 nm in Figure 1c. As reported previously,11 AuNPs in the catalyst sample appear to be faceted well during catalytic reaction. Clearly, the AuNPs in Figure 1 are faceted well regardless of their sizes. Thus the AuNPs in Figure 1 are catalytically active during the ETEM observation. In Figure 1, the typical orientation relationships between AuNPs and crystalline CeO2, that is, (111)[1̅10]Au//(111)[11̅0]CeO2 and (11̅1̅) [11̅0]Au//(11̅1̅)[11̅0]CeO2, are observed in areas I and II, respectively,18 and the CeO2 support is observed with the electron beam parallel to the major [11̅0]CeO2 zone axes. In 3074

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A = −2a1 + 2c1 = (3/2)a 2 − (3/2)c 2

(1)

B = 2a1 − 2b1 = −(3/2)a 2 + (3/2)b2

(2)

the data, strongly suggesting that the AuNP simply rotated about the axis normal to the interface that is denoted by a blue dot (Figure 3). Thus the AuNP in Figure 3 is anchored to a single site on the support that is the point of rotation. Figure 4b

where a1, b1, c1 and a2, b2, c2 are the crystal axes of Au and CeO2, respectively. To clarify the stepwise displacement during the period 0−2 s, rectangular regions in the upper two models in Figure 2b are enlarged at the lower left of each model. The AuNP is displaced relative to the CeO2 support by the vector R = (1/12)A or R = (1/12)(A + B) (0.10 nm in the [1̅01]Au or [101]̅ CeO2 directions). In the ETEM images in Figure 2a, the projection of R along the viewing direction Rp = (1/12)A + (1/ 24)B (0.09 nm in the [1̅1̅2]Au or [112̅]CeO2 directions) is measured. Accordingly, all of the coincidence lattice points are shifted laterally on the interface by 4R = (1/3)A. During the intervals 4−9 s and 37−42 s, the images of Au atomic columns are blurred. The image at 39 s (Figure 2a) is due to rotation of the AuNP by 4 degrees about the axis normal to the interface plane. Simulated images based on the models agreed well with the observations at 0, 2, 39, and 43 s (Figure 2a). By referring to the two straight lines in Figure 2b that are fixed to the CeO2 lattice, the relative displacement and rotation of the AuNP can be easily seen. It is most likely that the entire AuNP moved as a rigid-body on the support surface, although the positions of constituent atoms are slightly adjusted, especially near the interface. It is significant that the AuNPs move back and forth from a stable coincident position to another equivalent one on the CeO2 support; this is very different from Ostwald ripening processes of metal nanoparticles at elevated temperatures.20 Figure 3 and Movie S2 show the rigid-body like rotation of an

Figure 4. Displacement and rotation of AuNPs. (a) Measured position relative to the CeO2 support of a gold atomic column indicated by left blue arrows in Figure 2, as a function of observation time. A positive or negative value means that the gold atomic column moves toward the [11̅ 2̅ ]Au or [112]̅ Au directions, respectively, from the initial position at 0 s in Figure 2a. (b) Rotation of an AuNP in Figure 3 over time. The angle represents the interplanar angle between {111}Au of the AuNP and {111}CeO2, both of which are parallel to the viewing direction. Arrows in a and b indicate observation times of the ETEM images in Figures 2 and 3, respectively.

plots the measured rotation angle with observation time. Like the lateral displacement in Figure 2, the rotation in Figure 3 occurred in a stepwise fashion. We frequently observed a similar stepwise lateral displacement and rotation of a large proportion of the AuNPs (13 AuNPs among 17 AuNPs, 75%) in the same environment and under the same observation conditions, as is also shown in Figure S1. High-activity Au nanoparticulate catalysts are prepared by the deposition precipitation method.14 In the final preparation step, the catalysts are heated in air at 573 K for 4 h. One may thus infer that the AuNPs are supported on a fully oxygenterminated support surface instead of a reduced, metalterminated surface. It has been suggested that, for Au on TiO2, the AuNPs are weakly bonded to the oxygen atoms on an oxygen-terminated surface, while Au atoms and Au clusters on a defective oxygen-terminated surface are more strongly bonded to relatively reduced metal (Ti) atoms at the oxygen vacancies.7,21 High-angle annular dark-field scanning TEM (HAADF-STEM) analyses attempted to determine the occupancy of oxygen atoms at an interface. For example, according to the HAADF-STEM analyses of an Au/CeO2 model catalyst, the Au−Ce interlayer distance ranges from 0.2 to 0.3 nm;19,22 while in a model Au/TiO2 catalyst, a cluster of gold atoms extends in a planar fashion on the TiO2 surface, forming an array of direct metallic bonds with Ti atoms.23 These model catalysts were prepared by depositing gold on the metal oxide in vacuum, while their catalytic activity is difficult to quantify owing to the insufficient surface area of the samples. In principle, HAADF-STEM analyses can reveal only the interface structure that is projected along an observation direction. Hence there are no firm experimental data on the interface structures. In the present work, the Au−Ce interlayer distance is assumed to be 0.30 nm in the simulated images in Figure 2, within the accuracy of the observations. This distance is similar to that estimated in the optimum oxygen-terminated interface model for Au/CeO2 catalysts by ab initio computation, that is, 0.28, 0.31, and 0.32 nm.19 The Au−Ce interlayer distance of 0.21, 0.22, and 0.28 nm in a Ce-terminated interface model19 is definitely smaller than the 0.30 nm distance here.

Figure 3. Rigid-body rotation of an AuNP around an anchoring point. (a) In situ observation of an AuNP (Figure 1c) with observation time. (b) Fourier transforms of the images in a, revealing rotation of the AuNP about the axis normal to the interface plane. Blue and red lines indicate [111]̅ Au and [111̅]CeO2 directions, respectively. (c) Fourierfiltered images without CeO2 diffraction spots in b. In the models in d, [110]Au and [110]CeO2 directions are indicated by blue and red lines, respectively, and the rotational axis is indicated by a blue dot. Gold, gray, and red circles designate gold, cerium, and oxygen atoms, respectively.

AuNP without lateral displacement. Moiré fringes appear because the AuNP and CeO2 crystals overlap. The fringe moved back and forth relative to the CeO2 support over time. Spots in the Fourier transforms of the images clarify the rotation angles, and Fourier-filtered images in Figure 3c reveal the outline of the AuNP. No lateral displacement was detected in the Fourier-filtered images within the 0.02 nm accuracy of 3075

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can be countered by oxygen absorption from the reaction environment.29 The AuNPs were displaced to a lesser extent in vacuum (see Figure S3) than they were in the reaction environment. This could be because the AuNPs are more anchored to multiple sites created by the additional oxygendeficiencies under electron irradiation in vacuum; therefore, their movement is more restricted. We also observed that an AuNP moved stepwise more frequently in O2 gas (14.4 times/ min) (Figure S4) than in the reaction environment (6.4 times/ min) (Figure 1b). Oxygen vacancies at the interface are compensated for by the oxygen gas more than the other environments, and therefore the AuNP moved stepwise more frequently. Accordingly, we conclude that the frequency of stepwise displacements depends on the number density of anchoring points on the interfaces. In rare cases, stepwise displacement was completely suppressed in the AuNPs that were supported on a V-shaped surface owing to the geometrical restriction, as is shown in Figure S5. Finally, it is noteworthy that closely associated with the stepwise displacements the perimeter interfaces between an AuNP and a support are reorganized structurally by depositing a small amount of energy (Movie S1). This implies that the most probable activation sites in gold nanoparticulate catalysts2,8−12 are not structurally rigid. In conclusion, in situ aberration-corrected ETEM has provided firm experimental evidence of dynamics at a catalysis interface.

The in situ ETEM observations provide insight into the nature of interfacial structures. AuNPs moved stepwise back and forth from one position to an equivalent adjacent position relative to the CeO2 lattice in reaction environments (100 Pa of 1 vol % CO/air at room temperature). An AuNP in Figure 3 is anchored to a single site on the surface, while that in Figure 2 is most likely anchored to multiple sites. The AuNP motion is probably induced by energy transfer from the high-energy (300 keV) electrons during ETEM observation. Suppose an AuNP consists of N Au atoms. There are approximately N2/3 interface Au atoms that are bounded to the support surface. As seen in Figure 1, a typical AuNP in the catalyst sample consists of about a thousand Au atoms, so about a hundred Au atoms exist on the interface. Preliminary ab initio computation was performed for infinite interface models that include an Au(111) 4 × 4 slab on CeO2 surfaces with a 3 × 3 superlattice. The bonding energy per interface Au atom was estimated to be 0.035, 0.059, and 0.279 eV on the fully O-terminated (stoichiometric) surface, an O-terminated surface that has a single oxygen vacancy among the nine surface oxygen sites, and the Ce-terminated (strong reduced) surface, respectively. The valence of two surface Ce ions changes from Ce4+ to Ce3+ when one oxygen vacancy is generated at an adjacent site of the ions. This computational result seems consistent with recent experimental results using spatially resolved electron energyloss spectroscopy.24 The energy increase per oxygen vacancy was roughly estimated to be 0.024 eV. Accordingly, the energy that is needed to displace the AuNP permanently from the fully O-terminated surface and a Ce-terminated surface can be estimated to be 3.5 and 27.9 eV, respectively. The maximum energy transfer from a 300 keV electron to an Au atom in a single collision event is estimated to be 4.3 eV. Therefore, an AuNP cannot be displaced even slightly when it is supported on the Ce-terminated surface. While on both the fully Oterminated surface and an O-terminated surface with a few oxygen vacancies, the AuNP can be instantaneously displaced whenever we assume that the deposited energy is only consumed for displacing the AuNP. Along with the observed results (Figures 2−4), we conclude that AuNPs are loosely bound to the CeO2 support, anchored only by preferential sites; thus, they can migrate and rotate on the support with small activation energies that can be supplied by the electron beam. We examined the electron flux dependence of stepwise displacements, as is shown in Figure S2. An AuNP moved occasionally under a smaller flux of electron, that is, 4 A/cm2 (Figure S2a), while the same AuNP moved more frequently under a larger flux, that is, 10 A/cm2 (Figure S2b). The averaged frequencies of nine AuNPs at the different fluxes, 4 A/ cm2 and 10 A/cm2, were estimated to be 1.2 and 2.5 times/min, respectively. This further confirmed that the stepwise displacement is induced by electron irradiation. Regarding the nucleation of AuNPs, it has been suggested21 that Au atoms are first trapped at oxygen vacancies on a reduced metal surface. Since AuNPs are frequently observed on step edges,19,25 during nucleation the gold atoms most likely find oxygen vacancies at step edges as well as flat terraces. Subsequently, gold atoms aggregate at these sites, forming an AuNP. Hence, the AuNP could be anchored to the nucleation site. As a minor detail, CeO2 is damaged much less by electron irradiation than other supports such as TiO2, because it absorbs and releases oxygen.26−28 Nevertheless, oxygen atoms at the interface and on the surface can be displaced by electron irradiation. Oxygen desorption induced by electron irradiation

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ASSOCIATED CONTENT

S Supporting Information *

ETEM movies and images of AuNPs in a reaction environment, vacuum, and O2. 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: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid for Specially Promoted Research, Grant No. 19001005, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This research was partly supported by “Low-Carbon Research Network (Handai satellite)” and the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). The authors thank to T. Akita, S. Tanaka, K. Tanaka, M. Kohyama, and C. Kisielowski for their interest in this study. The authors are indebted to U. Dahmen, National Center for Electron Microscopy, Lawrence Berkeley National Laboratory for his invaluable support.

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REFERENCES

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