Atomic-Scale Faceting in CoPt Nanoparticles Epitaxially Grown on NaCl

Article pubs.acs.org/crystal

Atomic-Scale Faceting in CoPt Nanoparticles Epitaxially Grown on NaCl Véronique Pierron-Bohnes,*,† Ileana Florea,†,⊥ Ovidiu Ersen,† Corinne Ulhaq-Bouillet,† Christine Goyhenex,† Nadi Braidy,‡,§,∥ Christian Ricolleau,‡ Yann Le Bouar,§ and Damien Alloyeau‡ †

Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS UMR 7504 CNRS-UDS), 23 rue du Loess BP 43 F-67034, Strasbourg Cedex 2, France ‡ Laboratoire Matériaux et Phénomènes Quantiques, (MPQ UMR 7162 CNRS-UP7), Bâtiment Condorcet Case courrier 7021, F-75205 Paris Cedex 13, France § Laboratoire d’Etude des Microstructures (LEM UMR 104 CNRS/ONERA), BP 72 92322 Châtillon Cedex, France S Supporting Information *

ABSTRACT: Sub-10 nm CoPt nanoparticles were slowly grown at 400 °C in epitaxy on a NaCl substrate. Their faceted shape was analyzed using state-of-the-art TEM techniques: aberration-corrected imaging, electron tomography, and probe-aberrationcorrected scanning transmission electron microscopy. These nanoparticles consist in truncated octahedrons with a chemically disordered face-centered cubic (FCC) structure. We evidenced slight variations of the truncation of these nano-octahedrons depending on their size: the largest particles are less truncated than the smallest particles. We also highlighted the up−down symmetry of the NPs, suggesting that the adhesion energy of FCC-CoPt on NaCl is negligible. Energy descriptions of these NPs were made by using quenched molecular dynamics in the framework of the second moment approximation of the tight-binding formalism, while taking into account the random distribution of Co and Pt atoms. In a general manner, this original energy approach for studying faceting in chemically disordered nanoalloys is consistent with experimental results, particularly for small-size clusters. However, as the experimentally observed size-effect on the NPs truncation was not theoretically predicted, this phenomenon could originate from kinetic effects inherent to nanocrystal growth.

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INTRODUCTION It is now widely acknowledged that the shape of inorganic nanoparticles (NPs) significantly affects most of their technologically relevant properties. For a decade, these shape effects have been of major interest in material science, as they potentiate the efficiency of synthetic nanostructures for many applications (optical, magnetic, catalytic, electronic, and biomedical).1−8 This growing interest in controlling the three-dimensional morphology of nano-objects has motivated the development of many chemical and physical synthesis methods.9−12 This interdisciplinary challenge has also required an important collaborative effort between experimentalists and theoreticians to characterize and predict the 3D organization of atoms in finite-size objects. On the one hand, modern developments in X-rays scattering methods,13 near-field microscopy,14 and electron microscopy15,16 have extended the resolution of nano-object 3D-analysis down to the atomic scale.17,18 On the other hand, through the study of the electronic and thermodynamic properties of nanostructures, theoretical considerations have been developed to take into account confinement effects on the shape of NPs.19−25 Particularly, if surface energies alone are sufficient to describe the shape of macroscopic particles, many additional effects (edges, vertices, surface state, and atomic relaxation) arise from the significant increase of the percentage of surface atoms in small clusters.26 In addition, kinetic aspects of NP synthesis and © 2014 American Chemical Society

NP interaction with the environment (coating agents, substrate, and matrix) can give rise to unexpected morphologies that significantly differ from the energetically stable nanostructures.27,28 With respect to monometallic NPs, the mixing of several metals in nanosize objects allows tuning their physical and chemical properties by varying their composition, but this additional parameter in the study of metallic cluster induces a higher variability of their atomic structure and 3D shape. Here, we investigate the shape of chemically disordered CoPt NPs epitaxially grown on the NaCl substrate, both experimentally and theoretically. These atomic scale studies are motivated by the shape effects on the structural (and consequently magnetic) properties recently reported in such magnetic alloy NPs.29 This is achieved by consistently comparing a suite of quantitative transmission electron microscopy (TEM) techniques, including aberration-corrected imaging and tomography with quenched molecular dynamics calculations of similar atomic arrangements.

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METHODS

Sample Preparation. The CoPt nanostructured thin films were produced by pulsed laser deposition (PLD) on [001] NaCl substrate heated at 400 °C in a high vacuum chamber.30 The pressure in the Received: November 20, 2013 Revised: March 17, 2014 Published: April 9, 2014 2201

dx.doi.org/10.1021/cg4017432 | Cryst. Growth Des. 2014, 14, 2201−2208

Crystal Growth & Design

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Figure 1. CoPt NPs grown at 400 °C on a [001] NaCl substrate and transferred on an amorphous carbon film (a) STEM HAADF image (b) electron diffraction of the NP assembly (the diffraction spots are indexed with the FCC structure oriented along the [001] zone axis). (c) Intensity profile measured across a NP (along the [22̅0] direction) as indicated by the arrow (red online) in (a). The trapezoidal form (blue online) shows the expected profile of a truncated octahedron without probe convolution. The gray zone indicates the width of the {001}-type facets. chamber was better than 10−7 Torr. The NaCl substrate was freshly cleaved and annealed at 400 °C for 1 h in the deposition chamber before the growth of the NPs. A typical target−substrate configuration was used to deposit separately the two metals by PLD using a KrF excimer laser at 248 nm with pulse duration of 25 ns at repetition rates of 10 Hz. The samples were obtained by alternated irradiation of the pure Co and Pt metal targets to produce the plasma of each metal. The deposition rate of each element was controlled by an in situ quartz crystal monitor that indicates the nominal thickness of deposited materials on the quartz surface, in a continuous thin film approximation. As the metallic species do not wet NaCl substrates, NPs are formed instead of a continuous thin film, and the nominal thickness of a sample obviously does not correspond to the particle thickness. To ensure a good homogeneity of the film, the deposited thickness corresponding to two successive steps (one Pt and one Co) was set to 0.1 nm, for a total of 1 nm. We started this alternative deposition with Pt. The average deposition rate was 0.02 and 0.15 nm/ min for Pt and Co, respectively. It is worth noting that after the synthesis, the samples were cooled down, and consequently, the NP shape could no longer change and was quenched till observation. At room temperature, the sample was covered by a ∼5 nm thick layer of amorphous carbon using the same PLD setup, acting as a carbon replica. These carbon-supported CoPt NPs were floated off on a water surface by dissolving the NaCl and transferred to a 3 mm commercial 200 mesh TEM grid. The TEM grid with the carbon-supported CoPt NPs was then immediately transferred to the PLD chamber to add a 5 nm amorphous carbon capping layer to protect the structures from oxidation. TEM Analysis. Aberration-corrected high-resolution TEM (HRTEM) analyses were performed with the JEOL ARM200F microscope, equipped together with an image CEOS aberration corrector and a cold-field emission gun.31,32 Aberration-corrected scanning transmission electron microscopy (STEM) imaging and tomographic experiments were performed using a JEOL 2100F operating at 200 kV and equipped with a CEOS probe corrector. The tomography data set was acquired by tilting the specimen between −70° and 70°, with an increment of 2°, giving a total of 71 images (2048 × 2048 pixels). The data treatment was performed using the IMOD software and the TomoJ33 plug-in implemented in the ImageJ software. The 3D reconstruction of the chosen area was

calculated by using iterative algebraic reconstruction technique (ART) algorithms, using a number of 15 iterations. The studied area and the magnification were chosen in order to analyze several particles using the bright field (BF) TEM mode. These samples suffer from a high contamination rate in the STEMHAADF mode. The background increased of 20−25% with respect to the signal during the time necessary to generate eight images. Given that 71 images are needed for a tomogram, it has not been possible to perform the tomography experiments in the STEM-HAADF mode, which would be, in theory, more suitable than BF-TEM mode, as the former does not suffer diffraction artifacts like the latter. Nevertheless, it is known that the presence of the diffraction contrast which induces artifacts in the BF-TEM reconstructions of crystalline objects does not affect the determination of the external shape of NPs.34−38 Specific details on the 3D shape of NPs were obtained by individually analyzing several typical NPs, for which the corresponding subvolume was extracted from the total reconstruction volume. A classical data segmentation procedure based on a thresholding operation was used to obtain the 3D models of the chosen NPs. Quenched Molecular Dynamics (QMD) Calculations. In order to have an insight in the energetics of the NP shape, the free energies of NPs in different configurations were calculated using QMD in the framework of the second moment approximation of the tight-binding formalism (TB-SMA), which is widely used for transition metals.39 This semiempirical model has the advantage of providing an analytical expression for calculating the total energy of a bimetallic system as a function of interatomic distances. The energy at a given site i is written for a pure metal as Ei = A

∑

rij

e−p( r0 − 1) −

ξ2

∑

j≠i

j≠i

rij < rC

rij < rC

rij

e−2q( r0 − 1)

where rij is the distance between site i and j and r0 is the nearestneighbor equilibrium distance in the bulk. The interaction is set to 0 beyond a cutoff radius (rC) that is fixed as the second distance for the atom with the largest atomic radius (here Pt). For each pure metal species Co and Pt, the parameters A, ξ, p, and q are determined by fitting the potential to the universal equation of state,40 while reproducing accurately the cohesive energy, the lattice parameter and 2202



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the bulk modulus.39 The cross-interaction parameters are determined by taking average values from pure species for p and q. A further fitting to the experimental heats of dissolution of one impurity of Pt into bulk Co (respectively Co into Pt bulk) is performed for obtaining A and ξ. The whole procedure and the corresponding parameters can be found with more details in refs 41 and 42. Here we used the QMD in order to calculate total energies while taking into account atomic relaxations, which can be important at surfaces and other low-coordinated sites in particular. In all calculations, the disordered alloy was simulated on 31 different samples with N atoms prepared with a random distribution of each metal on the FCC lattice sites, and the system was relaxed to minimize the total energy. The standard deviations correspond to the dispersion due to the atomic distribution (we checked its saturation with the number of samples).

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TRANSMISSION ELECTRON MICROSCOPY RESULTS Low magnification high angle annular dark field (HAADF) images reveal that 90% of the NPs present a rectangular- or square-projected shape with a 4 to 9 nm size range (Figure 1a). These facetted shapes drastically differ from the round particles obtained with an amorphous substrate using the same synthesis parameters.30,43 This indicates that faceting is driven by the epitaxial growth on the NaCl substrate. This epitaxial relationship with the [001] NaCl substrate is confirmed by selected-area electron diffraction (Figure 1b), since NP assemblies exhibit a monocrystalline face-centered cubic (FCC) pattern in the [001] zone axis orientation. By comparing the structural information from the direct and reciprocal spaces, we observe that the sides of all the cubic/ rectangular NPs are perpendicular to the [220] and [2̅20] directions. Various polyhedral shapes of FCC metal NPs obtained by epitaxial growth on an [001] substrate (as MgO and NaCl) have been reported and compared to the equilibrium shapes.44 For large unsupported particles, these equilibrium shapes are obtained using the Wulff’s construction,45,46 according to which, in a crystal there is a constant ratio between the surface energy of a facet and the distance of this facet to the center of the crystal. In a general manner, NPs tend to minimize their free energy by exposing facets with the lowest surface energies: (111), (100), and (110) in a FCC cluster. For particles on a substrate, a modification of the Wulff’s construction, known as the Wulff−Kaischew theorem,44 expresses that the change of distance between a facet and the center due to the substrate presence is proportional to the adhesion energy. However, as the energy landscape of NPs varies with the synthesis conditions (composition, size, pressure in the vacuum chamber, temperature, and nature of the substrate) departures from the Wulff’s shape are expected. Therefore, an accurate 3D analysis of the experimentally obtained polyhedrons together with theoretical approaches is necessary to understand the variability of NP shape. In that regards, electron tomography is a method of choice because it allows reconstructing the 3D morphology of small nano-objects with a subnanometer resolution,34 from a series of projected images of the sample. As illustrated in Figure 2, the tomogram of a single NP provides reliable information on shape and faceting in planes both parallel (Figure 2a) or perpendicular (Figure 2, panels b and c) to the substrate. The comparison with NP models demonstrates that the analyzed CoPt NPs are truncated octahedrons with large {111} facets and small {100} facets, due to the truncation of their 6 vertices. This model is coherent with a measured angle of 70° between

Figure 2. Comparison between the 3D reconstruction of a single CoPt NP obtained by electron tomography and a relaxed model truncated octahedron with a size of 7.5 nm along the -type directions and 8.2 nm along the -type (Rt, the truncation ratio defined in Figure 3 equals 0.92). The Co and Pt atoms are shown as spheres of slightly different radii. 3D volumes are observed along (a) the [001] direction, (b) the [01̅0] direction, and (c) the [110] direction. The z = 0, x = 0, x + y = 0 slices extracted from the tomogram are shown in inset.

the {111}-type facets (Figure 2c), and an angle of 45° between the {001} facets and the -type edges (Figure 2b). We note that the resolution of the tomogram makes it difficult to unambiguously determine if the junction between the {111} facets are edges or very small facets, due to a slight truncation along the -type directions. Nevertheless, these 3D analyses bring the experimental evidence that the truncation along all the directions is the same. The resulting centro-symmetric shape presents then identical {100} facets with lateral dimensions ranging from 1.5 to 3 nm, depending on NP size. These results are consistent with the quantitative analysis of STEM-HAADF images, where the intensity of CoPt NPs is a monotonic increasing function of projected thickness. Therefore, the brighter square zone (or brighter rectangle), observed at the center of the square-shaped (or rectangleshaped) 2D images of the NPs, corresponds to the {001}-type facets parallel to the substrate plane in which the NP thickness is maximum and constant (Figure 1a). As illustrated in Figure 1c, intensity profiles taken across the core of the NPs allow measuring the surface of these facets, considering that the profile is a trapezoid convoluted by a Gaussian beam profile. We found the lateral size of (001) and (001̅) facets to be in a 1.5 to 3 nm range. It is worth noting that these truncated octahedrons have a small surface in contact with the substrate. This result and the centro-symmetric shape of the NPs indicate a weak interaction between CoPt and NaCl crystals. From the 2203



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Wulff-Kaischew theorem, we deduce negligible adhesion energy of FCC-CoPt on NaCl. Similarly, Fortunelli and co-workers found weak interactions between L10 CoPt and MgO22 or NaCl47 and showed that the substrate wetting of CoPt is low on both substrates. To provide a deeper insight into the faceting of CoPt NPs, we used aberration-corrected electron microscopy to analyze the surface of the nanostructures with an atomic-scale precision. As expected from selected-area electron diffraction and electron tomography results, NPs oriented along the [001] zone axis exhibit -type edges in the projected images, but highresolution images clearly reveal their truncated vertices, which originate from small (100) and (010) facets (Figure 3). We

Figure 4. Size along the -type directions (S̅110) as a function of the size along the -type directions (S̅100). To take into account the slight anisotropy of the NPs along the [110] or [11̅0] directions, error bars are defined as (S110 − S11̅0)/2 and (S100 − S010)/2, along the abscissa and ordinate axes, respectively (highly anisotropic NPs were not measured). Black ■ and ● (red online) correspond to experimental and calculated data, respectively. The truncation of the NPs is characterized by the Rt ratio given by S̅110/S̅100. The dashed line (blue online) corresponds to the nontruncated octahedrons (Rt = 2−1/2), shown in 3D and in [001]-projection in the bottom inset. The gray (red online) area corresponds to the truncated octahedrons with Rt = 1.08 ± 0.02, according to Wulff’s construction (shown in 3D and in [001]-projection in the top inset). The black solid line indicates the mean value of the experimental Rt ratios (0.92 ± 0.05).

nanostructures between 7 and 9 nm have a Rt ratio below 0.92. This ratio is equal to 0.94 ± 0.04 up to a 7 nm NP size and decreases to 0.87 ± 0.03 for larger NPs. This result is highlighted in Figure 3, in which NPs with a lateral size of 4.1 nm (Figure 3b), 6.6 nm (Figure 3c), and 7.4 nm (Figure 3d) present a Rt ratio of 0.93, 0.87, and 0.86, respectively. As no indication of segregation effect has been observed in these NPs,48 the change of shape due to a segregation of one of the species49 can be ruled out.

Figure 3. Aberration-corrected high-resolution imaging of CoPt NPs in (a), (b), and (d) HRTEM mode and (c) bright-field STEM mode (low pass filtered). The white (or black) square in image (a) indicates the NP size along the [100] and [010] (or [110] and [11̅0]) directions (S100 and S010 or S110 and S11̅0, respectively). These atomic-scale measurements are used to quantify the truncation of the octahedrons along the -type directions via the Rt ratio defined as Rt = [(S̅110)/(S̅100)] = {[(1/2)(S110 + S11̅0)]/[(1/2)(S100 + S010)]}.

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QUENCHED MOLECULAR DYNAMICS CALCULATIONS For large particles, the equilibrium shapes of isolated NPs can be obtained using the Wulff’s construction,45 which is deduced from the different specific surface free energies. The surface energies of the most stable facets in FCC particles, (111), (100), and (110), can be calculated using quenched molecular dynamics (QMD) in TB-SMA formalism and give access to the morphology of large size clusters for disordered CoPt NPs. As the size of the particles decreases, the contributions of edges, corners, and surface stress become similar to surface and cohesion contributions,26 which lead to questioning the validity of the Wulff’s construction. Relaxing the clusters with QMD is then necessary to find the most stable configuration and check the sensitivity of the NP shape to its size. It is worth noting that the shape observed and calculated here are metastable as the atomic distribution of atoms on the lattice is random, whereas for such a size (4−8 nm), the equilibrium state is the ordered L10 phase, consisting of successive pure atomic planes along the [001] direction.29 Because the growth kinetics, which fixed the 3D shape of NPs, is driven by the surface atomic migration, whereas the chemical order is obtained by the volume atomic migration that is far slower, a metastable FCC structure is usually observed in as-grown NPs and a thermal treatment at a

exploited the possibility of analyzing NP surfaces without delocalization of the atomic contrast to quantify the truncation along the [100] and [010] directions as a function of the NP size. This truncation of the projected NPs was characterized by measuring the ratio (Rt) of the NP size along the - and -type directions (S1̅ 10 and S̅100). As indicated in Figure 3a, S̅110 corresponds to the average of the size along the [110] and [11̅0] directions (S110 and S11̅0), whereas S̅100 is given by the average of the size along the [100] and [010] directions (S100 and S010). This allows evaluating the truncation of NPs that are slightly elongated along the [110] or [110̅ ] direction. The 2D projection of a nontruncated octahedron is a square with Rt = 2−1/2 (∼ 0.71), whereas when Rt = 1, the 2D projection of the corresponding truncated octahedron is a regular octagon. The Rt ratios of the NPs were measured to be in-between these two cases (Figure 4): Rt = 0.92 ± 0.05, in the whole NPs size range. Nevertheless, these atomic scale studies reveal a slight size dependence of the NPs shape, since all the 2204



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larger than the {111}-type facets, as shown in the top inset of Figure 4. The truncation is then described by a value of the ratio Rt = S1̅ 10/S1̅ 00 equal to 1.08 ± 0.02 in FCC CoPt for large NPs. Truncated Octahedrons with Constant Number of Atoms. In order to check to what extent the Wulff’s law is valid in NPs,45,46 the energy per atom was calculated in truncated octahedrons of FCC CoPt, relaxed using quenched molecular dynamics. To determine the most stable truncation, it was necessary to compare the free energies of NPs with different truncations but with the same number of atoms. We started from a symmetric octahedron with {111}-type facets at a distance q.d111 from the origin (Rt = 2−1/2), where q is an integer and d111 the interplanar distance of the (111) planes. The number of atoms of this octahedron is called Nq. Some asymmetric octahedrons were constructed with 1 to 6 facets placed at (q+1).d111. We verified that the energy per atom of these asymmetric octahedrons vary in continuity with those of symmetric octahedrons. The number of atoms of these slightly larger octahedrons was then decreased down to Nq by first forming {100}-type facets. The facets were chosen as similar as possible, and finally one facet was eroded to obtain truncated clusters with exactly Nq. Only the most stable configuration is reported. Following the experimental approach, the value of Rt = S̅110/ S̅100 was calculated by averaging the sizes for the different observation orientations and by repeating the calculation for 31 independent atomic configurations. The average energy per atom is compared in Figure 5 for two sizes of the initial

higher temperature is necessary to obtain the equilibrium L10ordered NPs.30 This also means that an equilibrium shape for a chemically disordered particle can be properly defined at low temperature, and the aim of this part is precisely to compute this shape using an atomic model. The lattice parameter of the bulk disordered alloy was first optimized with a box size of 10*10*30 (30 atomic planes along [001] and 10 along [110] and [11̅0]). The box size was varied, and the atoms relaxed inside the box (the equilibrium positions fluctuate due to the random decoration of the lattice sites and the size difference between Co and Pt atoms). The lattice parameter is found to be 0.37746 nm, and the cohesion energy, 5.2985 ± 0.0014 eV/atom, smaller than for the ordered L10 phase: 5.3601 eV/atom in agreement with the stability of the ordered phase at low temperature. Surface Energies and Wulff’s Shape. For the different (hkl) high symmetry surfaces, the total energy has been calculated after relaxation for the same atomic distribution in both a bulk and a slab configuration.50 The sample contains 30 atomic planes perpendicular to the surface, in order to avoid any interaction between the two (up and down) surfaces of the slab, and 100 atoms in each plane (10*10 atom cells, either square, rectangular, or diamond shaped). It is worth noticing that absolute surface energies within such semiempirical descriptions are not well recovered. As a general rule, surface energies are found to be the half of the experimental values. However the second-moment frame allows recovering the anisotropy in energy between the main different faces of a transition metal.51−53 Importantly, the CoPt potential used in this study correctly reproduced the energy difference between the pure Co and Pt (111) surfaces.41 Within these considerations, such potentials, implemented in atomistic simulations, have been successfully applied to the structural study of not only pure surfaces and clusters54,55 but also bimetallic systems like CoPt in many configurations: cobaltplatinum-based overlayers,51,56 bulk L10 alloy phase,57,58 and alloy clusters.20−25 In any case, in the following, we will consider the surface energies not as absolute but as relative while comparing their values between the different facets of the considered CoPt particles. Surface energies of fully chemically disordered CoPt structure, obtained by averaging the surface energy of 31 random atomic configurations, are tabulated in Table 1. The Table 1. Surface Energies for the High Symmetry Surfaces of a CoPt Disordered Alloya. surface

ES (eV/nm2)

ES (eV/atom)

(111) (100) (110)

6.59 ± 0.06 7.48 ± 0.07 8.19 ± 0.09

0.407 ± 0.004 0.533 ± 0.005 0.826 ± 0.008

Figure 5. Average energy per atom for 4579 atom (●, S̅110 = 5 nm) and 16269 atom (□, S̅110 = 8 nm) NPs with various truncations, quantified by the averaged length ratio: Rt = S̅110/S̅100. The vertical narrow arrows show the positions of the energy minimum. Rt value from the Wulff’s construction is shown by a large full arrow with a width corresponding to the error bar.

a

The error bars are the standard deviations of the distribution due to the occupancy fluctuations.

symmetric cluster: S110 = 5 nm (4579 atoms; q = 9; Rt = 0.98 ± 0.04) and S110 = 8 nm (16269 atoms; q = 14; Rt = 1.04 ± 0.02). The energy density fluctuations due to the chemical configurations increase when the particle size decreases, making difficult such an analysis at a size smaller than 5 nm. A small dependence of the shape of the NPs is nevertheless evidenced from the MD results, the bigger the particle, the larger the truncation. The QMD results are compared to the experimental results in Figure 4. The Rt ratio calculated for a 5 nm particle is in good agreement with the truncation ratio observed in small

(111) surface has the smallest energy and the (110) surface the largest energy, as expected from the number of cut bonds in first coordination shells: (111), 3; (100), 4; (110), 5. Note that in the L10 ordered alloy using the same method, we found an energy per atom in the (001) surface of 0.785 eV/atom for a Co surface and 0.500 eV/atom for a Pt surface.57,58 From these values of the surface energies, using the Wulff’s construction, the stable shape for large particles is a truncated octahedron with {100}-type facets at a distance 1.135 times 2205



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1.08 ± 0.02. The equilibrium shape of 5 and 8 nm NPs were calculated by varying the truncation at constant number of atoms. This energetics approach for studying faceting in chemically disordered CoPt NPs is consistent with experimental results, particularly for 5 nm clusters. As the experimentally observed size-effect on the NPs truncation was not theoretically predicted, this phenomenon could originate from kinetic effects inherent to nanocrystal growth. More generally, this work emphasizes the necessity of combining experimental and theoretical investigations to understand the variability of NP shape.

particles, up to 7 nm. However, experimental and theoretical results slightly diverge for larger sizes. As the calculation is well stable at 8 nm and in agreement with the Wulff’s limit, we assume that the experimental size effect on the NP shape is due to kinetic effects occurring in the largest particles. Indeed, large particles are made of atoms which have been deposited on a large area of the substrate. Therefore, some atoms have a very long diffusion path before attaining the particle and will not be able to reach an equilibrium position. Another source of large particles is the coalescence of two small particles, but the time to reach equilibrium shape by surface diffusion is known to increase with the particle size to the power four.59 As a conclusion, at a given temperature, a much longer time is needed to reach the equilibrium shape of large particle when compared to smaller ones. Finally, the particle shapes observed in out-of-equilibrium conditions are expected to strongly depend on the kinetic processes. The atomic mobility is well-known to be higher on the (111) extended surfaces than (100) ones in close-packed metallic systems due to simple coordination effects and possibly to the occurrence of atom-exchange process on (100) surfaces as suggested in the literature for Pt surface self-diffusion.60,61 More recently, it has also been shown within classical molecular dynamics simulations of self-diffusion on a Pt-truncated octahedron, that the crossing of an edge from a {111} facet toward a {100} facet has a very low barrier which is about 0.1 eV lower than the barrier for adatom diffusion from {111} to neighboring {111} facet.62 Therefore kinetics effects can induce a faster growth for {100} facets, leading to shapes near to a plain octahedron.44 This rationale made on a Pt cluster should also be more generally valid for other metallic systems including CoPt. It would be, however, advisable to explore more deeply the effects of alloying on activation energies and diffusion since differences between Co and Pt, for instance their atomic size, may lead to different behaviors toward atomic surface diffusion.42

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ASSOCIATED CONTENT

S Supporting Information *

Segregation effect details and figures; effect of missing wedge in the tomography reconstruction details; experimental images and profiles of [001]- and [112]-oriented particles; effect of missing wedge on the tomography recontruction. 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]. Present Addresses ∥

Génie Chimique et Génie Biotechnologique, Université de Sherbrooke, 2500 Boul. de l’Université, Sherbrooke, QC, J1K 2R1, Canada. ⊥ Laboratoire de physique des interfaces et des couches minces (LPICM, UMR 7647 CNRS/Ecole polytechnique), Route de Saclay, Bâtiment 408, 91128 Palaiseau Cedex, France. Author Contributions

N.B., D.A. were in charge of the sample preparation; C.U.B. and V.P.B. performed the STEM-HAADF and STEM-BF, I.F. and O.E. performed the electron tomography, D.A., C.R., and Y.L.B. performed HRTEM. V.P.B. and C.G. achieved QMD. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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CONCLUSION CoPt NPs slowly grown at 400 °C in epitaxy on a NaCl substrate were studied using state-of-the-art TEM techniques. These faceted particles consist in truncated octahedrons with chemically disordered FCC structures. By exploiting aberrationcorrected HRTEM and particularly the absence of contrast delocalization at the NP surfaces, we evidenced slight variations of the truncation of these nano-octahedrons, depending on their size. The largest particles are less truncated (truncation ratio of 0.87 ± 0.03) than the smallest ones (truncation ratio of 0.94 ± 0.04). By using electron tomography, we also highlighted the up−down symmetry of the NPs: the (001)facet at the interface with the substrate is very similar to the (001)-facet in vacuum. This suggests that for a random distribution of species in CoPt, the interface energy CoPt/NaCl is very similar to the surface energy of CoPt (i.e., the adhesion energy of FCC-CoPt on NaCl is negligible). Some energetics descriptions of these NPs were made by using QMD in the framework of the TB-SMA formalism, while taking into account the random distribution of Co and Pt atoms. To the best of our knowledge, it is the first time that a simulation of such a random alloy was performed with a statistical analysis of the energy fluctuations due to the occupation distribution. After calculating surface energies, Wulff’s construction demonstrated that the equilibrium form at large size is a truncated octahedron with a truncation ratio of

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from ANR within ANR-07-NANO-018-04 (ETNAA) is gratefully acknowledged. The COST MP0903 “Nanoalloy” and the CNRS GdR 3182: “Nanoalliages: synthèse, structure et propriétés” are acknowledged as framework for fruitful discussions. This work was also supported by the Région Ile-de-France (convention SESAME E1845 for the JEOL ARM 200F electron microscope installed at the Paris Diderot University).

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REFERENCES

(1) Cao, L.-F.; Xie, D.; Guo, M.-X.; Park, H.-S.; Fujita, T. Size and shape effects on Curie temperature of ferromagnetic nanoparticles. Trans. Nonferrous Met. Soc. China 2007, 17 (6), 1451. (2) Hao, N.; Li, L.; Zhang, Q.; Huang, X.; Meng, X.; Zhang, Y.; Chen, D.; Tang, F.; Li, L. The shape effect of PEGylated mesoporous silica nanoparticles on cellular uptake pathway in Hela cells. Microporous Mesoporous Mater. 2012, 162 (0), 14−23. (3) Kim, C.; Lee, H. Shape effect of Pt nanocrystals on electrocatalytic hydrogenation. Catal. Commun. 2009, 11 (1), 7−10.

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Atomic-Scale Faceting in CoPt Nanoparticles Epitaxially Grown on NaCl

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