Letter pubs.acs.org/NanoLett
Structural and Magnetic Evolution of Bimetallic MnAu Clusters Driven by Asymmetric Atomic Migration Xiaohui Wei,†,⊥ Rulong Zhou,‡,⊥,○ Williams Lefebvre,§ Kai He,# Damien Le Roy,†,⊥,¶ Ralph Skomski,†,⊥ Xingzhong Li,⊥ Jeffrey E. Shield,∥,⊥ Matthew J. Kramer,∇ Shuang Chen,‡,⊥ Xiao Cheng Zeng,‡,⊥,†,∥ and David J. Sellmyer*,†,⊥ †
Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, United States Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588, United States § UR, Groupe de Physique des Matériaux−GPM, UMR CNRS 6634, 76801 Saint Etienne du Rouvray, France ∥ Department of Mechanical and Materials Engineering, University of Nebraska, Lincoln, Nebraska 68588, United States ⊥ Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, United States # Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ∇ Division of Materials Science and Engineering, Ames Lab, Ames, Iowa 50011-3020, United States ○ School of Science and Engineering of Materials, Hefei University of Technology, Hefei, Anhui 230009, China ‡
S Supporting Information *
ABSTRACT: The nanoscale structural, compositional, and magnetic properties are examined for annealed MnAu nanoclusters. The MnAu clusters order into the L10 structure, and monotonic size-dependences develop for the composition and lattice parameters, which are well reproduced by our density functional theory calculations. Simultaneously, Mn diffusion forms 5 Å nanoshells on larger clusters inducing significant magnetization in an otherwise antiferromagnetic system. The differing atomic mobilities yield new cluster nanostructures that can be employed generally to create novel physical properties. KEYWORDS: Bimetallic nanoparticles, Ostwald ripening, phase transformation, structure−property correlation, structure/composition/morphology evolution, annealing
M
surface segregation of Pt into monolayers-thick nanoshells similar to the effects of chemical/electrochemical treatment and acid leaching,9,10 which significantly enhances the catalysis. When the core in the core−shell structure becomes ordered, the catalysis can be further enhanced11 as a result of the interface strain.12 For RuPt nanocatalysts, after annealing, a population of large Pt clusters appears besides the bimetallic cluster population.3 Possible high temperature exposure induced phase segregation and formation of nanoshells may also modify the optical properties because of the interface strain, lattice distortion, and electronic structure modification. An important mechanism governing the size evolution of nanoparticles is Ostwald ripening (OR),13,14 which describes the transfer of atoms from small to large particles in a liquid or solid solution; this leads to the growth of larger particles as a result of the shrinking and dissolution of smaller particles.15−17 This thermodynamically driven spontaneous process occurs because larger particles are more energetically favorable than
ultielement nanoparticles are important candidates for potential applications in magnetics,1,2 catalysis,3−5 and 6 optics. As these nanoparticles are frequently exposed to high temperature during postdeposition processing and application, the nanoscale structural, compositional, and morphological transformations and associated property evolution become a critical issue. One example is Pt-containing transition metal bimetallic nanoparticles, which have potential applications in magnetic recording.7 To achieve the structural transformation from the disordered fcc to the ordered L10 structure, asproduced nanoparticles are routinely annealed at high temperature. For CoPt nanoparticles, it has been found that annealing induces size-dependent Co concentrations, which vary from 45 to 75 at % as particle size increases from 2 to 10 nm, in contrast to the uniform Co concentration for as-produced clusters.8 Since the L10 structure stabilizes around the equiatomic composition, the largest particles with the highest Co concentrations may contain compositional and structural inhomogeneity, which likely significantly impacts magnetism. However, this has not been investigated extensively. Another group of bimetallic nanoparticles that exhibit high-temperature structural transformations impacting properties are nanocatalysts. For CoPt nanocatalysts, annealing is found to induce © 2014 American Chemical Society
Received: November 27, 2013 Revised: February 5, 2014 Published: February 11, 2014 1362
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Figure 1. Annealing-induced ordering and formation of L10 structure in MnAu clusters with monotonic size-dependent composition and lattice parameters and their corroboration by density functional theory calculations. (a,b) TEM images for as-produced and annealed MnAu clusters, respectively. The insets show the size distributions. In panel b, the large gray clusters with sizes above 10 nm are silica particles formed from pieces of the silica protective layer during annealing. (c−e) Representative HRTEM images for annealed MnAu clusters with various sizes. (f−h) Monotonic 1363
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Figure 1. continued size dependences for the lattice parameters and tetragonal distortion ratio (c/a) for annealed MnAu clusters. Squares and lines are data and their polynomial fits. (i−k) Cuboctahedron clusters computed using DFT. (l) Comparison of experimental observation of size dependence for the tetragonal distortion ratio (c/a, squares) and DFT calculations (stars). Lines are polynomial fits. The compositional size dependence arising in DFT comes from the addition of layers containing one type of element, while maintaining the L10 structure. Extrapolating the size-dependent composition trend predicted by DFT to experimental particle size ranges (2.5 nm < d < 5 nm) shows that the 3.6 nm (dc) cluster has the equiatomic composition. At equiatomic composition, the unit cell volume (UCV) will minimize, consistent with DFT, as Mn−Au bond lengths are shorter than both Mn− Mn and Au−Au bond lengths. Inset shows the experimentally determined UCV as a function of the particle size. At 3.6 nm, the UCV does minimize, consistent with DFT, implying that the composition extrapolation based on DFT calculations is reasonable. (m) XRD of annealed MnAu clusters showing the formation of L10 structure. (n) Size-dependent diffraction-pattern lines calculated from lattice parameters of annealed clusters with sizes of 1.8, 2.4, and 3.2 nm, respectively. The shift directions of the diffraction lines with cluster size increase are designated by arrows.
atures ranging from 500 to 650 °C, at which temperatures annealing leads to the fcc-L10 structural transformation. Thus, the as-produced MnAu clusters were annealed at 500 °C to achieve the fcc-L10 structural transformation. Figure 1a shows a representative TEM image with its size distribution for the asproduced MnAu cluster ensemble. Size analysis from the TEM image shows an average size of 2.7 nm with a polydispersity (standard deviation divided by the average size) of 30%. Electron dispersive X-ray spectroscopy (EDS) indicates a Mn concentration of 57 at % (Supporting Information Figure S1). TEM image analysis (Figure 1b) for annealed clusters indicates an average size of 2.9 nm with a comparable polydispersity. This indicates that annealing does not induce a significant change in particle size. To determine the structure of annealed MnAu clusters, we have examined individual clusters of various sizes with highresolution TEM (HRTEM). Representative images are shown in Figure 1c−e. Fast-Fourier transform was used to obtain the lattice parameters. Figure 1f−h shows the lattice parameter as a function of particle size. An interesting monotonic size dependence is observed for lattice parameters a and c and the tetragonal distortion ratio c/a. However, the lattice parameters of as-produced clusters do not display any clear size dependence (Supporting Information Figures S2−S4), implying that the size dependence forms after the disordered fcc-L10 structural transformation. DFT calculations are utilized to explore the origin of the size-dependent lattice parameters for the L10 MnAu clusters. The cuboctahedron morphology is adopted.18 DFT calculations well reproduce the size dependence for the lattice parameters and indicate that it originates from the combination of size effect and the coupling between the interatomic separation and magnetism for Mn atoms (Supporting Information Figure S5). Interestingly, DFT calculations also identify a size dependence for the composition, which arises from the preservation of the L10 structure and the cluster morphology (Figure 1l). Extrapolating the size-dependent Mn concentration predicted by DFT to the experimental size ranges, the 2−5 nm L10 MnAu clusters have Mn concentrations ranging from 45 to 55 at %, with the 3.6 nm cluster having nearly the equiatomic composition, which we define as dc. DFT also shows that the Mn−Au bond length is shorter than both the Mn−Mn and the Au−Au bond lengths, and thus, at equiatomic composition (or at dc = 3.6 nm) where the Mn−Au bond ratio is maximized, the unit cell volume will minimize. The experimentally determined unit cell volume does minimize at 3.6 nm (inset of Figure 1l), indicating that the composition extrapolation based on DFT calculations is reasonable. However, experimental verification of the composition for a single sub-5 nm cluster is challenging for current nanocharacterization tools such as nano-EDS because of a 5 at
smaller particles. Recent in-situ transmission-electron microscopy (TEM) has captured the OR for Bi nanoparticles,17 where small Bi particles shrink as big Bi particles grow leading roughly to conservation of the total volume.17 For bimetallic nanoparticles, OR can be more complicated because of the involvement of more than one element, where the different mobilities of the two composing elements can lead to asymmetric exchange of atoms generating systematic and global structural and compositional modifications and unique heterostructures. If the mobility of one element far exceeds the other, this element dominates the atomic diffusion, and its directional migration and redeposition onto larger particles can generate nanoshells. We have focused on MnAu clusters as a representative bimetallic nanoparticle system to study high-temperature structural and morphological evolutions and their impact on relevant properties. For sub-5 nm MnAu clusters, our density functional theory (DFT) calculations predict the ordered L10 structure,18 which is a common ordered structure for many Ptcontaining or Mn-containing bimetallic alloys.19−22 An interesting feature of Mn is the sensitive dependence of its magnetism on the interatomic separation. While Mn in various morphologies typically displays antiferromagnetism (AFM), ferro- or ferrimagnetism (FM) are predicted at large lattice parameters. DFT predicts a large magnetic moment of 3.38 μB/Mn for ferromagnetic Mn13Au20 core−shell clusters because of a 9% lattice dilation.2 Experimentally, a scaling between the ferromagnetic moment and the lattice dilation already has been reported for epitaxial α-Mn thin films.23 Facecentered cubic (fcc) and body-centered cubic (bcc) Mn are also predicted to be ferromagnetic when the lattice parameters go above certain limits where the moment starts to scale with the lattice parameters.24 In this letter, we show that annealing induces global asymmetric atomic migration, which transforms the uniform composition of as-produced bimetallic MnAu nanoparticles into size-dependent compositions. This OR phenomenon, as discussed above, causes Mn to migrate and redeposit onto the surface of larger L10 clusters forming uniform 5 Å monometallic shells. The Mn shell structure induces large magnetic moments in an otherwise antiferromagnetic system. As these structural transformations originate from the different mobilities of the two composing metallic elements in bimetallic nanoparticles, they may also occur in other multimetallic nanoparticles generating significant property modifications. We have produced MnAu clusters in a home-built clusterdeposition system25 from a MnAu alloy target. As-produced clusters have the disordered fcc structure. Previous research on Pt-containing bimetallic nanoparticles has shown that 2−3 nm CoPt nanoparticles have order−disorder transition temper1364
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Figure 2. Annealing-induced Mn migration and formation of L10 MnAu−Mn core−shell structure. (a−c) HAADF-STEM images for annealed MnAu clusters with sizes 5.4 nm (a), 6.2 nm (b), and 6.5 nm (c), respectively, which suggest the presence of a 5 Å shell. (d) EELS element mapping for the representative cluster (c) shows that the radius of Mn is larger than that of Au, confirming the presence of an Mn shell. (e−h) EDS radial intensity profiles from both the center to edge (e,f) and edge to center (g,h) show that at cluster surface only Mn is present, confirming the presence of Mn shell. (i,j) Radial intensity profiles for HAADF-STEM images show a 5 Å intensity drop near the cluster surface (indicated by the arrows) corresponding to the location of the Mn nanoshells. (k,l) Radial intensity analysis for representative as-produced MnAu clusters showing smooth transitions near the cluster surface, suggesting lack of nanoshells. (m,n) Model clusters with homogeneous L10 structure (m) and L10 MnAu−5 Å Mn core−shell structure (n). (o,p) Simulations34,35 for the model clusters confirm that a 5 Å Mn shell does produce a 5 Å lighter shell in HAADFSTEM images.
% error. It is interesting that, for L10 FePt nanocluster ensembles, increasing the average Fe concentration from 47 at % to 52 at % also produces systematic and correlated variations for the average lattice parameters,26 implying a similar correlation between composition and lattice parameters. The L10 structure of the annealed MnAu clusters is further confirmed by X-ray diffraction (XRD) (Figure 1m). An unusual feature of the diffraction pattern is a nonuniform broadening. Besides strain and defects, the size-dependence for lattice parameters also plays a significant role. As shown in Figure 1n, the size-dependent lattice parameters result in significant and systematic shifts for the diffraction lines contributing to the nonuniform broadening. Weak Mn diffraction lines also are observed suggesting incipient phase separation.
To verify the location of the redeposited Mn in annealed MnAu clusters, the high-angle-annular-dark-field (HAADF) mode of scanning transmission electron microscopy (STEM) is employed as it provides element contrast for Mn and Au due to the different atomic numbers. It is found that, while smaller clusters (d ≤ 4 nm) display homogeneous L10 structure, larger clusters show brightness contrast suggesting 5 Å shells outside the L10 cores, where the lighter color suggests that the shell consists of Mn atoms (Figure 2a−c). EELS (Figure 2d) and EDS (Figure 2e−h) mappings show that the radius for Mn is larger than that of Au, confirming the presence of an Mn shell. Radial intensity profiles of HAADF-STEM images for annealed clusters show characteristic intensity drops of 5 Å near the cluster surface (Figure 2i,j), which is absent in the intensity profile of the as-deposited clusters (Figure 2k,l). To verify that 1365
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Figure 3. Magnetism evolution with Mn shell structure. (a,b) Hysteresis loops at 10 K and ZFC/FC for clusters with and without Mn shell, respectively. ZFC/FC was measured with an applied magnetic field of 100 Oe. Formation of Mn nanoshells enhances the magnetization from 0.17 to 1.4 μB/Mn (a) and gives rise to a blocking temperature at 120 K (b).
Figure 4. Schematic diagram for the formation of size-dependent composition and L10 MnAu-fcc−Mn core−shell as a result of global asymmetric atomic diffusion during annealing. (a) Smallest metastable clusters dissolve during annealing, providing free Mn and Au atoms. MnAu clusters with sizes below the dc either receive Au or lose Mn to become Mn-poor. MnAu clusters above dc do the opposite, that is, either receive Mn or lose Au to become Mn-rich. Free Mn atoms preferentially redeposit onto the surface of the largest clusters, forming fcc-Mn nanoshells. (b) Schematic diagram of the size-dependent composition and morphology. Clusters with/without nanoshells are represented by the lower or upper sections of the sizedependence curve, respectively.
behaviors were also reported for ferrimagnetic epitaxial α-Mn thin films.23,28 For bcc- and fcc-Mn, DFT has determined the ferromagnetic moment as a function of the lattice parameter.24 According to these calculations, for a representative L10 MnAufcc−Mn core−shell cluster with a core size of 4.6 nm and lattice parameters of a = 3.95 Å and c = 4.38 Å, ferromagnetic moments of 3.2 μB/Mn can be expected.28 Note that our fcc Mn nanoshells have much larger lattice parameters than previously studied bcc- and fcc-Mn films.29,30 We propose the following formation dynamics for the sizedependent composition and Mn nanoshell formation. The smallest clusters are metastable and dissolve in the matrix of silica during annealing, providing a reservoir of Mn and Au atoms to feed the growth of bigger clusters; clusters with sizes below the critical size dc (which has the equiatomic composition) give away extra Mn and/or accept extra Au to become Au-rich, and clusters above dc receive extra Mn or lose Au to become Mn-rich. The free Mn atoms preferentially redeposit onto the surface of relatively larger clusters, especially the largest, forming epitaxial L10 MnAu−Mn core−shell structures. A schematic diagram of this process is shown in Figure 4. Similar annealing-induced composition transformation from uniform to size dependent has been reported for other bimetallic nanoparticle systems including RhPt3 and CoPt.8
this lighter shell in the HAADF-STEM images corresponds to the Mn shell, we built two model clusters: one 5 nm cluster with homogeneous L10 structure (Figure 2m) and one cluster with 4.5 nm L10 MnAu core and ∼5 Å fcc Mn shell (Figure 2n). Simulations show that a 5 Å fcc Mn shell does produce a lighter 5 Å shell in the HAADF-STEM images (Figure 2p). The Mn shell has a symmetric shape, uniform thickness, and a clear contrast with the core indicating a clear interface between the core and the shell. Note that in a previous study, annealinginduced redeposition of metallic atoms also generated a clearcut epitaxial interface.27 We further investigate the effect of the MnAu core−Mn shell structure on magnetism (Figure 3). The MnAu core curve refers to an ensemble of 2.7 nm clusters that were fabricated to contain mainly the L10 structure after annealing. The MnAu− Mn core−shell curve shows the magnetization of a sample containing a wider size distribution to promote OR and thus Mn shell formation. The average magnetization of L10 MnAu clusters is 0.17 μB/Mn. This agrees with our DFT calculations for a model AFM L10 MnAu cluster with a Mn concentration of 57 at % and a diameter of 2.7 nm. In the MnAu−Mn shell sample the average moment is enhanced to 1.41 μB/Mn (Figure 3a), which is also accompanied by a blocking temperature at 120 K for the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves (Figure 3b). Note that similar blocking 1366
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98CH10886, is highly appreciated. HRTEM and STEM performed at Ames Laboratory were supported by the United States Department of Energy (USDOE), Office of Science (OS), Office of Basic Energy Sciences (BES) under Contract No. DE-AC02-07CH11358. W.L. thanks the ANR (Agence Nationale pour la Recherche) for the financial support through the Programme Jeune Chercheur: Jeune Chercheuse TIPSTEM. HAADF-STEM simulations have been performed at the Centre de Ressources Informatiques de Haute-Normandie (CRIHAN) under project No. 2012013. W.L. thanks Pr. M. D. Robertson for providing a parallel version of the HAADFSTEM images simulation code.
In multimetallic nanoparticle ensembles, OR induces the transfer of atoms from small to large particles due to the higher stability of the latter. If the cluster ensemble contains a population of tiny/metastable clusters, OR can be significantly enhanced. This is due to the dissolution of the tiny/metastable clusters, which provide an abundant reservoir of free atoms for the constituent elements to drastically accelerate their migration. This process will be dominated by the element with higher mobility, whose migration from small to large particles results in the scaling of its concentration with particle size. The largest clusters contain the highest concentration of this element, which can form into nanoshells during redeposition. This work shows the first case of utilizing the different mobilities of atoms in a solid matrix as a driving force for nanoscale nanoparticle engineering. By adding small/ metastable clusters of desirable compositions to the initial cluster ensemble, nanoparticles with complex heterostructures can be built up layer by layer. This new strategy augments previous atomic scale structuring of nanoparticles that utilize the different solubilities,31 reduction potentials,32 or oxidation states33 of atoms in the template nanoparticle and in the liquid solution as the driving forces to add/remove/replace atoms in the preformed template nanoparticles. All these techniques employ atom diffusion to achieve nanoscale engineering of nanoparticles and create potentially powerful strategies to fabricate unique nanoheterostructures by modifying preformed nanoparticles.
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ASSOCIATED CONTENT
* Supporting Information S
Materials and methods, Figures S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(D.J.S.) E-mail:
[email protected]. Present Address ¶
(D.L.R.) CNRS, Inst. NEEL, F-38042 Grenoble, France.
Author Contributions
X.W. and D.J.S. designed the experiment, X.W. and D.R.L. fabricated the nanoparticles, and X.W. obtained the magnetization and X-ray data. R.Z., S.C., and X.C.Z. performed the DFT calculations. W.L. examined the clusters with STEM and performed STEM image simulations and EDS element mapping. K.H. performed STEM-EELS mapping and analyzed lattice parameters for individual clusters from HRTEM. X.Z.L., J.S., and M.K. helped in the structural analysis. X.W., R.S., and D.J.S. prepared the manuscript. All coauthors critically read and edited the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Mak Koten for analysis of the HRTEM images. This research is supported by NSF-MRSEC (DMR 0820521); ARO (W911NF-10-2-0099); NCMN, which is supported in part by the Nebraska Research Initiative; and the UN Holland Computing Center. Research (computation and STEM-EELS) carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC021367
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