Exotic Supported CoPt Nanostructures: From Clusters to Wires

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Exotic Supported CoPt Nanostructures: From Clusters to Wires Giovanni Barcaro,† Riccardo Ferrando,*,‡ Alessandro Fortunelli,*,† and Giulia Rossi‡ †

Molecular Modeling Laboratory, IPCF-CNR, Via Giuseppe Moruzzi 1, Pisa, I56124, Italy and ‡Dipartimento di Fisica, Universita di Genova, Via Dodecaneso 33, Genova, I16146, Italy

ABSTRACT A bottom-up computational approach for designing exotic phases of supported metallic nanostructures is proposed and exemplified for the CoPt/ MgO(100) system. Magic polyicosahedral CoPt clusters, Frank-Kasper motifs that have no counterpart in bulk CoPt, are singled out in the gas phase and then deposited on a substrate that enhances their stability via a mechanism rationalized in terms of the many-body character of the metal/surface interaction. Both finitesize (nanoclusters) and one-dimensional (nanowires) structures can be so constructed, where one-layer Co interfacial segregation and the small size of the clusters suggest that their peculiar morphology can be reached and maintained via self-organization, while the wires appear to be robust with respect to necking instabilities. SECTION Nanoparticles and Nanostructures

P

redicting a material's structure from the knowledge of interactions at the atomic level is one of the most important and long-standing open problems in condensed matter science,1,2 both from the point of view of basic knowledge and for its practical implications related to the ab initio design of novel structures and phases tailored for specific technological applications.3,4 In the recent years, important advances have been made in the field2 due to the development of effective methodologies for the exploring energy landscapes5 and to the explosive increase of computing power. However, the problem is still far from being solved, so that the development of fresh approaches trying to tackle the complexity of computer-aided materials design is desirable. In this paper, we propose a bottom-up computational approach for designing exotic structures of supported nanoparticles and nanowires for a material, CoPt, which is of great interest for applications in magnetism6 and catalysis7 and is being widely studied at the nanoscale.8,9 In this computational approach (the computational methodology is described in detail in the Supporting Information), we first look for the most important structural motifs of gas-phase clusters by means of a combination of global optimization searches5 and density functional (DF) calculations. In particular, we are interested in singling out structures possessing a magic character,10 that is, presenting an uncommon energetic stability, often associated with highly symmetric configurations. Then, we choose a substrate such that it can enhance the stability of these structures once they are deposited. It turns out that an appropriate substrate is the MgO(001) surface, an oxide support widely used in both model studies and applications.11 As a final step, we use the supported nanoclusters as building blocks to produce extended nanostructures in the form of nanowires. As we show below, both of these types of

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nanostructures are exotic in the sense that they have no counterpart in the bulk phases of CoPt alloys but can be described as either finite-size (clusters) or one-dimensional (wires) analogues of Frank-Kasper phases12 and are thus related to quasicrystals.13,14 We recall that at 50-50% composition, bulk solid CoPt presents an ordered phase of L10 structure, with tetragonal crystal lattice, alternating homogeneous Pt and Co planes along a (001) axis. The structures of CoPt binary nanoparticles (referred to as nanoalloys in the following) are however much more complex; as has been shown both theoretically8,15 and experimentally,6,9 that they can present both crystalline (bulklike) and noncrystalline structures. Our results below confirm these predictions. To begin with, we focus on free CoPt nanoparticles in the size range of Ntot = 20-50 (with Ntot as the total number of atoms) at 50-50% composition. This size range corresponds to an interval in which polyicosahedral (pIh) structures10,15 are expected. pIh are made up of several interpenetrating elementary icosahedra of 13 atoms (see an example in Figure 1). pIh free clusters with five-fold symmetry have been shown to be of special energetic stability in binary systems with atomic size mismatch,10 such as Ag-Cu and Ag-Ni, but the origin of the stability of other classes of pIh structures is still unclear, and no theoretical predictions of surface-supported pIh structures have been reported so far. A nice example of an unsupported magic pIh structure is a cluster of 38 atoms, Co18Pt20, shown in the top row of Figure 1. This is a Frank-Kasper polyhedron12 with a disclination line

Received Date: October 1, 2009 Accepted Date: November 2, 2009 Published on Web Date: November 09, 2009

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DOI: 10.1021/jz900076m |J. Phys. Chem. Lett. 2010, 1, 111–115

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interfacial oxidation) might play a decisive role. For fcc-like particles, we concurrently find that Pt has a strong tendency to form a segregated layer at the topmost (100) facet, in agreement with experimental observations.6,16 For these particles, then, one-layer Co surface segregation at the interface together with one-layer Pt surface segregation at the topmost facet translates into L10 chemical ordering and entails that even numbers of layers possess a magic character; the presence of a pure Co first layer in fact favors the formation of a pure Pt second layer, and so on, thus strongly reinforcing the tendency already present in suspended particles.8 However, this also implies that bulk-like gas-phase clusters must be cut to be accommodated on the substrate, as shown in Figure 2. On the contrary, pIh clusters can be satisfactorily accommodated on the substrate while preserving their gasphase shape, simply segregating cobalt at the interface. As a counterintuitive result, the square-symmetry MgO(001) substrate does not favor the L10 phase but, on the contrary, enhances the stability of Frank-Kasper cluster structures. The most representative cluster structures obtained for Ntot = 20, 40, 48, 50, 62, 64, and 80 are depicted in Figure 2, and the corresponding energy results are reported in Table 1 of the Supporting Information. Scrutinizing the results, for small sizes (Ntot = 20), we find that the putative global minimum is a double icosahedron (N = 19) plus a lateral atom (Figure 2a), while an L10-type motif (Figure 2b) and an incomplete pIh (Figure 2c) lie at more than 2 eV. By increasing the size to Ntot = 40, a sharp crossover occurs, pure icosahedral motifs such as that in Figure 2f are still competitive in terms of adhesion energies but are drastically prevailed at the metal bonding level by a pIh motif, which is the gas-phase pIh6 cluster plus two lateral Co atoms at the interface (Figure 2d). In this size range, L10-type motifs such as Figure 2e present an excellent adhesion energy but are higher in energy due to a too low metal binding. The latter can be improved by introducing overhangs and thus better approximating the ideal shape predicted by the Wulff-Kaischev construction,11,17 but at the expense of the adhesion energy, so that the resulting structures are even less stable. One could think that sizes around 38 are somewhat special as they correspond to the perfect, closed-shell pIh6, but it turns out that pIh structures manage to be competitive at larger sizes by combining the pIh6 building block with novel epitaxial forms. For example, at Ntot = 48, the optimal L10type arrangements are a truncated pyramid (Figure 2i) or a structure exhibiting a stacking fault and an overhang at the interface (not shown). However, the putative global minimum is a peculiar pIh-like structure (Figure 2g,h) still based on the perfect 38 atom pIh6 motif but now adsorbed on the surface by tilting its six-fold symmetry axis and by inserting further atoms to fill up the void so created; the added atoms produce a (100) pseudomorphic epitaxy with the substrate to realize the best match with the oxide surface (see a detailed view in Figure 2h). By increasing Ntot to 62-64, a novel type of pIh structure comes into play that can be denoted as double pIh6. It is in fact possible and energetically favorable to match a tilted pIh6 with an adjacent one tilted in the opposite way, creating a structure (shown in Figure 2l,m) which achieves

Figure 1. Schematic pictures of free polyicosahedral (pIh6, top row) and L10 truncated octahedron (TO, bottom row) structures at a composition of Co18Pt20. Top and side views are shown on the left and right side, respectively. The disclination line in the Frank-Kasper pIh6 cluster is indicated. Cobalt atoms are displayed in blue, and platinum atoms are in dark gray.

running along its symmetry axis and can be identified with the “six-fold pancake” (a pIh structure exhibiting a six-fold symmetry axis, hereafter pIh6).10 This size and composition is simultaneously magic for L10 bulk-like structures because it is possible to build up a perfect cluster with (tetragonally distorted) truncated octahedral shape and L10 ordering (see the bottom row of Figure 1). However, even in this case, DF calculations predict that the pIh6 structure prevails by more than 1 eV. Let us now consider supported CoPt nanoparticles. We anticipate the finding that the lowest-energy structures are dominated by one-layer Co segregation at the interface with the substrate. We expect this to be a common occurrence in supported nanoalloys with a sufficiently strong adhesion to the substrate, as for CoPt particles on MgO(100), and its driving force can be explained in terms of novel epitaxial concepts and the physics of the metal bonding. The metalsurface interaction in fact, as any other metal bonding mechanism, possesses a definite many-body character, that is, the adhesion strength of a metal atom to the surface depends appreciably on its coordination number (the number of first-neighbor metal atoms surrounding the interacting one). Such a many-body character is much more pronounced for Pt than for Co; the strong Pt/Pt bonds compete with and quickly weaken the Pt/MgO interaction, quenching it from values appreciably higher than the Co/MgO interaction in the case of a single atom (∼2.5 eV for the Pt atom versus ∼1 eV for the Co atom on the O site) to definitely smaller values already for coordination number = 4 (∼0.45 eV for Pt versus ∼0.75 eV for Co on the O site). This entails that, once a cluster reaches a critical size such that the adhesion energy favors the formation of an interface involving a substantial number of atoms thus exhibiting non-negligible coordination numbers, there will be a strong tendency for Co to form a segregated layer at the interface. Experimental evidence for small CoPt particles is not conclusive in this respect, as it concerns, for example, particles embedded in amorphous carbon or MgO matrixes6 where matrix-cluster interaction (and possible


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Figure 3. Schematic pictures of supported CoPt nanowires: (a) (16,16) L10-type wire; (b) (22,22) wire based on the single pIh6; and (c) (29,30) wire based on the double pIh6. The labeling convention is (NPt,NCo), where NCo and NPt are the number of Co and Pt atoms in the repetitive unit. Color coding is as that in Figure 2.

growth (Figure 2n) is at lower energy than the best pure L10 structure (Figure 2o). The tilted pIh6 motifs (Figure 2g,h and l,m) lend themselves to interesting speculations in terms of 1D systems, as one can ask whether these motifs can be the seeds of extended systems. As the regular interfacial pattern shown in Figure 2h points to a simple translational symmetry, we investigate this possibility by replicating neighboring tilted pIh6 motifs along the [100] direction of the MgO(100) substrate and joining them together to create stable periodic pIh nanowires, which are extended 1D Frank-Kasper phases. To this aim, we conduct EP global optimizations on systems composed of two adjacent (Figure 2g and l) configurations and a variable number of Pt or Co atoms added in between. From such calculations, the structures depicted in Figure 3b,c result. They belong to two families, one based on the tilted single pIh6 (22,22) and one based on the tilted double pIh6 (29,30). These structures can be compared with unfaulted L10 wires. In Figure 3a, the best (16,16) candidate of the L10 family in this size range, as obtained from the Wulff-Kaischev construction, is also shown. The labeling convention is (NPt,NCo), where NCo and NPt are the numbers of Co and Pt atoms in the repetitive unit. As it is well-known, nanowires are kinetically stabilized objects, on whose synthesis experimental conditions (temperature, mass transport, etc.) play a dominating role. Nevertheless, thermodynamic considerations at 0 K such as the present ones can be helpful to orient the search for novel structures and morphologies. In order to compare the thermodynamic stability of 1D systems, the excess energy per unit length (Eexc) can be usefully defined

Figure 2. Schematic pictures of various supported CoPt clusters: (a) Co10Pt10 double icosahedron (N = 19) plus a lateral Co atom lying on the surface; (b) Co10Pt10 L10-type structure; (c) Co10Pt10 incomplete pIh; (d) Co20Pt20 pIh6-based structure; (e) Co20Pt20 L10-type structure; (f) Co20Pt20 incomplete pIh; (g) Co24Pt24 pIh6based structure with its disclination axis indicated; (h) bottom view of (g) to better show its epitaxial relationship to the substrate; (i) Co20Pt20 L10-type structure; (j) side view of a Co32Pt30 L10-type structure; (k) top view of (j); (l) side view of a Co32Pt32 double-pIh6based structure with its two disclination axes indicated; (m) top view of (l); (n) Co40Pt40 structure based on the double pIh6 plus fcc-type surface growth; and (o) Co40Pt40 L10-type truncated pyramid. Color coding is as that in Figure 1, plus oxygen atoms in light gray and magnesium atoms in white.

Eexc ¼

ð1Þ

where nPt and nCo are the number of Pt and Co atoms per unit length of the wire, ntot = nPt þ nCo, EPt and ECo are the bulk binding energies of pure Pt and Co, respectively, and Ebnd is the wire binding energy per unit length. The most stable 1D structures correspond to the lowest values of the wire excess energy. The Eexc values for several trial configurations are reported in Figure 4. The main result that can be drawn from an inspection of this figure is that the best pIh nanowires present an appreciably lower excess energy with respect to their L10 competitors, which in turn suggests that they might be selectively

shell closure at Ntot = 64. At this size, L10-type configurations are appreciably higher in energy (by ∼1.8 eV; see Table 1 of the Supporting Information). Even at Ntot = 62, which is a magic size for L10 motifs (see Figure 2j,k), an incomplete double pIh6 is still the putative global minimum, although by a smaller amount, ∼0.4 eV. For larger sizes, our predictions become more uncertain. However, at Ntot = 80, we still find that a double pIh6 hybridized with a lateral fcc-type surface


Ebnd - nPt EPt - nCo ECo pffiffiffiffiffiffiffi ntot

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interpreted as originating from size mismatch and the difference in surface energies among the two elements, both favoring core-shell chemical ordering. The ubiquitous occurrence of pIh motifs in isolated binary clusters (see, e.g., ref 21) can however have a different, so far unexplored, origin. Certain classes of pIh structures, in fact, particularly those exhibiting six-fold symmetry axes such as pIh6, have been shown to maximize the number of mixed bonds.21 This implies that pIh configurations will be favorable also for those binary alloys, such as, for example, CoPt, whose phase diagram22 is characterized by ordered phases essentially driven by the strength of the mixed bonds and thus be expected to be of common occurrence in the field of nanoalloys. Our computational approach is thus able to construct exotic phases of CoPt nanomaterials. These phases are possible because of a specific mechanism by which noncubic structures can be selectively stabilized on checkerboard/ square-symmetry cubic surfaces. This mechanism arises from the many-body character of Pt versus Co/surface interaction. Its steeper decrease in Pt with increasing coordination number favors one-layer Co interfacial segregation. pIh structures can accommodate Co segregation without significantly distorting their geometry, whereas this is more difficult for L10 structures, thus leading to the interfacial stabilization of polyicosahedra. From the point of view of technological applications, two points can be underlined. First, considering that atom exchange kinetics in such small clusters is expected to be faster than that in larger particles23 and considering the substantial thermodynamic driving force toward these structures, it is to be expected that both the morphology and the chemical ordering can be reached and maintained via self-organization without invasive postsynthetic treatments (ion bombardments, plasmonic heating, etc.) that are needed for larger systems with the corresponding unwanted side effects (roughening, contamination, etc.). Second, from energetic considerations, it can be gathered that pIh-based nanowires present a uniform stress along their length, in contrast to the lattice mismatch and the formation of stress-relieving dislocations expected for fcc-type nanowires, and might thus be less liable to breaking or necking phenomena that are fatal to their possible technological use.

Figure 4. Wire excess energy per unit length (in eV) as a function of the number of atoms per unit length for supported wires.

synthetized under proper experimental conditions. This occurs despite the fact that pIh6 nanowires are oriented along the (100) direction of the substrate whereas the fcc ones are oriented along (110), so that structures with a comparable cross-sectional size have a higher number of atoms per unit length and are thus expected to possess a lower wire excess energy. It can be noted in passing that for the L10-type wires, one observes a transition with increasing cross section from structures exhibiting a single overhang [(12,12), (16,16), and (24,26)] to those exhibiting a double [(20,22) and [(26,28)] and finally a triple [(32,32)] overhang, in agreement with the Wulff-Kaischev construction11,17,18 (we emphasize that L10 chemical ordering is by far energetically favored in all cases). In view of the mechanical stability of these objects, which is a general key issue when dealing with nanowires,19 it is important to emphsize that for the pIh systems, a proper junction between the pIh6 motifs is of primary importance as structures with a minimum number of atoms between them, such as (17,17) or (27,27), possess a much higher excess energy than those with a more homogeneous cross section, such as (22,22) or (29,30); see Figure 3b,c. This suggests a greater stability of the pIh nanowires with respect to L10-type ones. L10-type nanowires in fact present a larger lattice mismatch with the substrate that, on the regular MgO(100) surface, will eventually be relieved by developing interfacial dislocations. This will decrease their excess energy, but at the same time, it will also increase the instability of the wires with respect to necking.20 In contrast, the much greater stability of, for example, (22,22) with respect to (17,17) or of (29,30) with respect to (27,27) suggests that pIh6-based nanowires present a uniform stress along their length and should be less liable to breaking or necking phenomena and thus more relevant from the technological point of view once synthetic issues are solved. What is the driving force to pIh structures? Five-fold symmetry core-shell pIh structures have been found in several instances,10 and their stability has been


SUPPORTING INFORMATION AVAILABLE Details of the computational methodology. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed.

ACKNOWLEDGMENT A.F. acknowledges financial support from

the EC VII FP within the ERC-AG SEPON project (ERC-2008-AdG227457). R.F. acknowledges support from Italian MIUR for the PRIN Project No. 2007LN873M_003. DF calculations were performed at Cineca Supercomputing Center (Bologna, Italy) within an agreement with Italian CNR.

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REFERENCES

(22)

(1)

(23)

(2) (3)

(4)

(5) (6)

(7)

(8)

(9)

(10)

(11) (12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20) (21)

Maddox, J. Crystals from First Principles. Nature 1998, 335, 201. Woodley, S. M.; Catlow, R. Crystal Structure Prediction from First Principles. Nat. Mater. 2008, 7, 937–946. Lord, E. A.; Mackay, A. L.; Ranganathan, S. New Geometries for New Materials; Cambridge University Press: Cambridge, U.K., 2006. Xiong, Y.; Wiley, B. J.; Xia, Y. Nanocrystals with Unconventional Shapes;A Class of Promising Catalysts. Angew. Chem., Int. Ed. 2007, 46, 7157–7159. Wales, D. J. Energy Landscapes with Applications to Clusters; Cambridge University Press: Cambridge, U.K., 2003. Favre, L.; Dupuis, V.; Bernstein, E.; Melinon, P.; Perez, A. Structural and Magnetic Properties of CoPt Mixed Clusters. Phys. Rev. B 2006, 74, 014439. Gauthier, Y.; Schmid, M.; Padovani, S.; Lundgren, E.; Bus, V.; Kresse, G.; Redinger, J.; Varga, P. Adsorption Sites and Ligand Effect for CO on an Alloy Surface: A Direct View. Phys. Rev. Lett. 2001, 87, 036103. Gruner, M. E.; Rollmann, G.; Entel, P.; Farle, M. Multiply Twinned Morphologies of FePt and CoPt Nanoparticles. Phys. Rev. Lett. 2008, 100, 087203. Penuelas, J.; Andreazza, P.; Andreazza-Vignolle, C.; Tolentino, H. C. N.; Santis, M. D.; Mottet, C. Controlling Structure and Morphology of CoPt Nanoparticles through Dynamical or Static Coalescence Effects. Phys. Rev. Lett. 2008, 100, 115502. Rossi, G.; Rapallo, A.; Mottet, C.; Fortunelli, A.; Baletto, F.; Ferrando, R. Magic Polyicosahedral Core-Shell Clusters. Phys. Rev. Lett. 2004, 93, 105503. Henry, C. R. Morphology of Supported Nanoparticles. Prog. Surf. Sci. 2005, 80, 92–116. Frank, F. C.; Kasper, J. S. Complex Alloy Structures Regarded As Sphere Packings. I. Definitions and Basic Principles. Acta Crystallogr. 1958, 11, 184–190. Shechtman, D.; Blech, I.; Gratias, D.; Cahn, J. W. Metallic Phase with Long-Range Orientational Order and No Translational Symmetry. Phys. Rev. Lett. 1984, 324, 1951–1953. Dubost, B.; Lang, J. M.; Tanaka, M.; Sainfort, P.; Audier, M. Large AlCuLi Single Quasicrystals with Triacontahedral Solidification Morphology. Nature 1986, 324, 48–50. Rossi, G.; Ferrando, R.; Mottet, C. Structure and Chemical Ordering in CoPt Nanoalloys. Faraday Disc. 2008, 138, 193– 210. Shapiro, A. I.; Rooney, P.; Tran, M.; Hellman, F.; Ring, K.; Kavanagh, K.; Rellinghauss, B.; Weller, D. Growth-Induced Magnetic Anisotropy and Clustering in Vapor-Deposited Co-Pt Alloy Films. Phys. Rev. B 1999, 60, 12826. Wulff, G. Zur Frage Der Geschwindigkeit Des Wachstums Unter Auflosung Der Kristallfiachen. Z. Kristallogr. 1901, 34, 499–530. Molina, L. M.; Hammer, B. Theoretical Study of COOxidation on Au Nanoparticles Supported by MgO(100). Phys. Rev. B 2004, 69, 155424. Tosatti, E.; Prestipino, S.; Kostlmeier, S.; Corso, A. D.; Tolla, F. D. D. String Tension and Stability of Magic Tip-Suspended Nanowires. Science 2001, 291, 288–290. Burki, J.; Stafford, C. A.; Stein, D. L. Theory of Metastability in Simple Metal Nanowires. Phys. Rev. Lett. 2005, 95, 090601. Rossi, G.; Ferrando, R.; Rapallo, A.; Fortunelli, A.; Curley, B.; Lloyd, L.; Johnston, R. Global Optimization of Bimetallic Cluster Structures. Ii. Size-Matched Ag-Pd, Ag-Au, and Pd-Pt Systems. J. Chem. Phys. 2005, 122, 194309.


115

Baker, H., Ed. Alloy Phase Diagrams; ASM International: Materials Park, OH, 1992. M€ uller, M.; Albe, K. Concentration of Thermal Vacancies in Metallic Nanoparticles. Acta Mater. 2007, 55, 3237–3244.


Exotic Supported CoPt Nanostructures: From Clusters to Wires

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