Evolution of the Magnetic and Optical Properties in CocoreAushell and

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Evolution of the Magnetic and Optical Properties in CocoreAushell and (CoRh)coreAushell Core−Shell Nanoparticles Junais Habeeb Mokkath*,† Department of Physics, Chemical Physics Division, Chalmers University of Technology, 412 96 Gothenburg, Sweden ABSTRACT: The magnetic and optical properties of CocoreAushell and (CoRh)coreAushell core−shell nanoparticles (14 shells, face centered cubic structure, 321 atoms) are investigated using spin-polarized density functional theory in the generalized gradient approximation. For CocoreAushell the properties show a clear dependence on the size of the Co core. For (CoRh)coreAushell we vary the Co:Rh composition. We show that the distribution of the magnetic moments within the nanoparticle depends on strongly/weakly on the chemical environment for the Co/Rh atoms. The optical absorption shows sensitivity to both composition and chemical arrangement. In particular, (CoRh)coreAushell nanoparticles show a blue shift for increasing Co concentration, whereas (RhCo)coreAushell nanoparticles show a red shift for increasing Co concentration. The investigated nanoparticles uniquely couple the magnetism of the core to the optical properties of the Au shell and thus pave the way to optical tracking. improve the biocompatibility.19−23 It is worth mentioning that intriguing magneto-plasmonic effects recently been reported in a variety of nanohybrid structures. For example, ref 24 reported a unique magneto-plasmonic response in multilayered Ag/ CoFeB/Ag films using nanosphere-assisted physical deposition. While ref 25 showed that magnetic and plasmonic effects could be combined in a variety of core−shell nanohybrid structures, such as CoM, FeM, AuM, and AgM (M = Zn or Al) alloys as cores and ZnO or Al2O3 as shells, and ref 26 reported magnetoplasmonic multilayered nanoporous thin films composed of Ag, CoFeB, and ITO layers fabricated on anodic aluminum oxide porous films. From a theoretical point of view, a nanoparticle composed of a 3d and 4d transition metal core and a noble metal shell pose huge complexities, since the local chemical environments of the atoms as well as magnetic proximity effects play important roles.27 It is known that finite size effects enhance the ferromagnetism of the 3d metals28,29 and induce significant spin-polarization in otherwise paramagnetic 4d and 5d metals. Another way to induce magnetism is the formation of intermetallics cores of 3d/4d and 3d/5d elements. Recent experiments30 and theoretical studies31,32 on CoRh nanoparticles have found not only large Co and sizable Rh moments (despite the fact that Rh is paramagnetic in the bulk) but also strong spin orbit coupling and magnetic anisotropy. CocoreAushell nanoparticles with cores of 6.5 nm diameter have been synthesized in ref 33 by heating Co nanoparticles in 1,2-dichlorobenzene, under reflux, with [(C8H17)4N]+[AuCl4]− containing trioctylphosphine as stabilizer. The reaction byproduct CoCl2 indicates that the core−shell structure is due to

I. INTRODUCTION Metallic core−shell nanoparticles combining multiple functionalities are of immense technological interest.1−8 Often the physicochemical properties can be flexibly modified by changing the composition and chemical arrangement.9 Recently, bimetallic and trimetallic core−shell nanoparticles are receiving attention for designing functionalities in a bottomup approach.10,11 In contrast to bimetallic nanoparticles,12,13 trimetallic nanoparticles have been rarely explored, although they can provide a new understanding of the interdependence between the composition, structure, and resulting physical and chemical properties. For example, Gonzalez and co-workers have demonstrated the synthesis of of PdAuAg multishell nanoparticles from Ag nanocubes by sequential galvanic exchange and Kirkendall growth,14 whereas Fang and coworkers have obtained AuPdPt nanoparticles using seedmediated growth, finding an unusually high activity for the electro-oxidation of formic acid.15 AuPdPt nanoparticles show a better catalytic performance than bimetallic core−shell nanoparticles.16 A recent study reported that carbon-supported FePt and FePtSn alloy nanocrystals exhibit increased catalytic activity for methanol oxidation compared with carbon supported Pt catalyst.17 Kim and co-workers have synthesized Cocore(CdSe)shell nanoparticles that complement magnetism with luminescence in the visible spectral range.18 Nanoparticles combining an optical fingerprint with other desired properties, such as magnetism, are particularly attractive, as they enable optical tracking. Indeed, the optical spectrum can be designed by integrating noble metals on the nanoparticle surface, due to localized surface plasmons, which has the additional advantage that the noble metal surface is chemically inert and can be functionalized to enhance the solubility in different media or © XXXX American Chemical Society

Received: August 15, 2017 Revised: October 13, 2017

A

DOI: 10.1021/acs.jpcc.7b08146 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C redox transmetalation between Co0 and Au3+. It is the purpose of the present work to investigate such CocoreAushell nanoparticles as well as in (CoRh)coreAushell nanoparticles from a first-principles perspective. We will provide a systematic understanding of the evolution of the magnetic and optical properties.

and shell regions of 2.94, 3.00, and 2.83 Å, respectively, whereas for the Co13Au308 nanoparticle the average core Co−Co, core Au−Co, inner shell Au−Au, and shell Au−Au interatomic distances amount to 2.83, 2.88, 2.86, and 2.88 Å, respectively. Replacing the Au atoms in the core of the pristine Au321 nanoparticle by Co atoms thus helps to reduce the differences in the interatomic distances. To analyze how the composition affects the stability, we compute the binding energy per atom, 1 E bind = [E − mE(Co) − nE(Au)] (1) m+n

II. COMPUTATIONAL ASPECTS We use the Vienna Ab Initio Simulation Package,34 which solves the spin-polarized Kohn−Sham equations at the scalar relativistic level using an augmented plane-wave basis set.35 The Co 3d and 4s, Rh 4d and 5s, and Au 5d and 6s orbitals are treated as valence states and the kinetic energy cutoff is set to Emax = 450 eV. Geometry optimizations are performed using free spin calculations. The discrete energy levels are broadened by a Gaussian smearing of σ = 0.02 eV and the exchange and correlation contributions to the energy are treated within the Perdew-Burke-Enzerhof36 flavor of the generalized gradient approximation. The convergence criteria are set to 10−5 eV for the energy and 5 × 10−4 Å for the atomic displacements. The nanoparticles are placed in simple cubic cells, which have to be large enough that any pair of images, due to the periodic boundary conditions, is well separated and that the interaction thus is negligible. This condition is safely satisfied for the chosen separation of 15 Å. We consider symmetric nanoparticles with face centered cubic structure, composed of a central atom and successive shells with increasing distance from this center. These shells comprise 1, 12, 6, 24, 12, 24, 8, 48, 6, 36, 24, 24, 24, and 72 atoms, yielding a large nanoparticle with a total of 321 atoms, for which it is sufficient to restrict the calculations to the Γ point of the Brillouin zone. Although no constraints are imposed during the structural relaxations, the optimized structures are not guaranteed to be the most stable ones since no systematic sampling of initial structures has been aimed. The optical properties of the structurally optimized nanoparticles are computed using the OPTIC module,37 which evaluates the frequency dependent dielectric ε(ω) with local field effects in the random phase approximation.38 The imaginary part of ε(ω) (ε2(ω)) characterizes the absorption.

where E is the total energy of the nanoparticle. E(Co)/E(Au) is the energy of an isolated Co/Au atom in vacuum, and m/n is the number of Co/Au atoms. We obtain a monotonic increase of Ebind for increasing Co content (2.86, 3.11, 3.38, and 3.70 eV for Co13Au308, Co43Au278, Co79Au242, and Co135Au186, respectively). Optimized structures of CocoreAushell nanoparticles are shown in Figure 1 for representative Co core sizes. Interestingly, the

Figure 1. Top: Optimized structures of CocoreAushell nanoparticles with 321 atoms. Bottom: Shell averaged Co (×) and Au (+) magnetic moments.

III. RESULTS AND DISCUSSION We first study CocoreAushell nanoparticles with Co cores of 13, 43, 79, and 135 atoms. Thereafter, we address (CoRh)coreAushell nanoparticles with a CoRh core of 135 atoms and different Co:Rh compositions and atomic arrangements. We note that an Au shell of 186 atoms is sufficient to protect the magnetic core from any interaction with the environment of the nanoparticle. A. CocoreAushell Nanoparticles. While, in general the atomic distribution in bimetallic nanoparticles depends on many factors,9 CocoreAushell nanoparticles realize a core−shell structure ref 33. In order to analyze why this is favorable we address the bulk cohesive energies, surface energies, and atomic radii of Co and Au. Since the bulk cohesive energy of Co (4.39 eV) is larger than that of Au (3.81 eV),39 the Co atoms nucleate in the core to increase the number of Co−Co bonds. In addition, Au atoms favor surface segregation, because their surface energy (1.50 J m2−) is lower that of Co (2.52 J m2−)41 and their atomic radius (1.44 Å) is larger than that of Co (1.25 Å).39 Since different chemical bonds favor different interatomic distances, we obtain for the pristine Au321 nanoparticle, for example, average interatomic distances in the core, inner shell

Co core shows a tendency to structural transformation from face centered cubic to more compact hexagonal close packing, similar to Co nanoparticles when the size increases.40 Figure 1 also demonstrates the evolution of the Co and Au magnetic moments as a function of the distance from the nanoparticle center. As a result of the high symmetry, the magnetic moments of all atoms in the same shell l (l = 1 indicating the central atom and l = 14 the outermost shell) are essentially identical. We obtain a weak dependence of the shell average μl for the Co and very small magnitudes and oscillations for the Au atoms. The Co spins are aligned ferromagnetically in all nanoparticles, where more than 95% of the magnetic moments is carried by the Co 3d states. To discuss the effect of the composition on the electronic structure we plot the density of states (DOS) of the Co 3d and Au 5d orbitals in Figure 2. The shell averaged Co magnetic moments (integrated spin density in the Wigner-Seitz spheres; μCo) are also given. The spin minority Co 3d DOS dominates at the Fermi energy (εF). Pronounced and modest exchange splittings, respectively, are found for Co and Au, as to be expected from the fact that bulk Co is ferromagnetic and bulk B

DOI: 10.1021/acs.jpcc.7b08146 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

for the Co135Au186 nanoparticle, where some of them are located just beneath the outermost Au shell and therefore significantly contribute to the absorption. The shoulder seen in Figure 3 around 2.5 eV can be attributed to the Au shell. The enhanced broadening of the 1.8 eV peak is probably a combination of several factors, including the electron scattering at the core−shell interface.45 Similar effects have been reported in other nanoparticles with Au shells, for example, with a Fe3O4 core.46 Interestingly, in (Fe3O4)core Aushell nanoparticles, the absorption spectrum depends mainly on the surrounding dielectric constant than the electron scattering at the core− shell interface. We note that the nanoparticles with small Co core nanoparticles are useful for magneto-plasmonics applications due to the dominant absorption in the visible spectral range combined with the sizable magnetism of the core. In contrast to AucoreAgshell,47 we find no splitting of the plasmonic peak due to the core−shell arrangement, in agreement with ref 44. B. (CoRh)coreAushell Nanoparticles. The behavior of the CocoreAushell nanoparticles can be modified by partially replacing Co atoms in the core with Rh. While the experiments in ref 48 predict in this case a core−shell structure with a Co core, we consider in the following, for clarity, both (CoRh)coreAushell and (RhCo)coreAushell nanoparticles. We aim at elucidating the interface, magnetic proximity, and alloying effects on the optical properties. Table 1 shows that μCoRh monotonously increases with the Co core size. For the Co13Rh122Au186 nanoparticle we obtained

Figure 2. Co 3d DOS (red) and Au 5d DOS (green) of CocoreAushell nanoparticles with 321 atoms. Positive (negative) values correspond to majority (minority) spin. A Gaussian broadening of σ = 0.1 eV is applied. The Fermi level is shifted to zero and is shown by vertical dashed lines.

Au is nonmagnetic. When the Co core grows μCo decreases, because the width of the Co 3d states increases. Figure 3 addresses the absorption CocoreAushell nanoparticles in terms of imaginary part of the dielectric function. The

Table 1. Average Magnetic Moments in (CoRh)coreAushell and (RhCo)coreAushell Nanoparticles with N = 321 Atoms nanoparticle

μWS CoRh

μWS Co

μWS Rh

Co13Rh122Au186 Co43Rh92Au186 Co79Rh56Au186 Rh13Co122Au186 Rh43Co92Au186 Rh79Co56Au186

0.29 1.12 1.52 1.88 1.47 1.25

0.89 2.09 2.04 1.99 1.89 2.07

0.02 0.78 0.79 0.84 0.57 0.66

a small value of μCoRh = 0.29 μB, which is due to the fact that 12 out of 13 Co atoms have Rh nearest neighbors, which increases the width of the Co 3d bands. In Figure 4 the evolution of Co, Rh, and Au magnetic moments is shown as a function of the shell number l. The Co magnetic moment is close to 2 μB in the center and slightly increases to the CoRh interface. An exception is the Co13Rh122Au186 nanoparticle due to the very small core. For sufficiently large Co cores, significant magnetic moments are induced in the Rh atoms almost independent of the distances from the core. We find a transfer of 0.1 to 0.2 electrons per atom from Co to Rh, which increases (reduces) the number of d holes and thus the magnetic moments of the (Co) Rh atoms. This charge transfer is qualitatively in agreement with higher Pauling electronegativity of Rh (χCo = 1.90 and χRh = 2.28).49 When we move toward the Rh−Au interface, the value of μRh decreases due to the reduced Co proximity effect but, on the other hand, enhances the Rh magnetic moments due to the modest charge transfer from Rh to Au (χAu = 2.54). This combined effect is clearly visible in the shell l = 8 in the case of the Co43Rh92Au186 nanoparticle, see Figure 4. In Co79Rh56Au186 nanoparticle, one observes the interface effects, that is, the induced Rh moments benefit from the proximity of spin-polarized Co atoms gaining sizable local

Figure 3. Imaginary part of the dielectric function for CocoreAushell nanoparticles with 321 atoms.

dielectric properties of the Co core influence the surface plasmon resonance (in the visible spectral range) endowed by the Au coating. The fact that the Co the 3d and 4s states energetically overlap appreciably, whereas there is an energy gap of about 1.5 eV between the Au 5d and 6s states,42 is reflected by ε 2 (ω). According to Figure 3, the Au 321 nanoparticle has a dominant absorption peak at 1.80 eV. The absorption spectra of pristine Co nanoparticles are not shown in Figure 3, however, they exhibit spectral features in the deep ultraviolet region of the electromagnetic spectrum, see ref 43. When Co atoms are substituted into the core, this peak decreases/broadens and slowly a new peak appears at 0.4 eV, which is in good agreement with a recent experimental study.44 The new peak is due to absorption of Co atoms. According to Figure 1, all Co atoms lie essentially in the core region, except C

DOI: 10.1021/acs.jpcc.7b08146 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 5. Frequency-dependent imaginary part of the dielectric function obtained for (CoRh)coreAushell and (RhCo)coreAushell nanoparticles.

Co concentration, whereas in the second class, it shows a red shift for increasing Co concentration. While for the Co13Rh122Au186 nanoparticle ε2(ω) has a distinct peak around 0.9 eV, this peak appears for the Rh13Co122Au186 nanoparticle at 0.6 eV. The unique optical characteristics can be related to the different atomic distributions among the (CoRh)coreAushell and (RhCo)coreAushell nanoparticles, see Figure 5. In the former (latter), Co (Rh) atoms show up just below the Au shell and contribute a unique peak in the low energy spectral region and at the same time pushes the Au peak further to high energy range, see the broad peaks at the higher energies shown in Figure 5. These differences suggest that absorption spectroscopy can be employed to determine the composition and structure of a blend of such nanoparticles.

Figure 4. Optimized (CoRh)coreAushell and (RhCo)coreAushell nanoparticles with 321 atoms and shell averaged Co (circles), Rh (squares), and Au (triangles) magnetic moments.

moments at the thin Rh layer, that is, μ7 (Rh) 1.1 μB. It is also worth to notice that in Rh-rich nanoparticles with small Co core the reduction of the Co magnetic moments at the interface is not compensated by the induced Rh magnetic moments. For instance, in Co13Rh122Au186 not only μ = 0.29 μB is small but also μWS CoRh = 0.89 μB. In general, the Rh magnetic moments couple ferromagnetically to the Co magnetic moments providing a significant contribution to the average nanoparticle magnetization μ. For the (RhCo)coreAushell nanoparticles the Co magnetic moments are almost constant and show only weak oscillations with respect to l, see Figure 4, whereas the Rh magnetic moments show significant oscillations with respect to the Co layer thickness. The Rh13Co122Au186 nanoparticle demonstrates that, since all 13 Rh atoms are completely surrounded by Co atoms, the local Rh magnetic moments increases due to the Co proximity effects (thick Co layer). The CoAu interface,The chemical environment may enhance the Co magnetic moments on a small scale due to the combined effects of the charge transfer from Co to Au and the fact that the Co atoms far from the Rh atoms have more Co than Rh nearest neighbors, so that the Rh-induced d-band broadening is weaker, for example. The same nanoparticle also shows how the Co interface enhances the Rh moments. For instance, we find a highly enhanced μ4(Rh) = 0.77 μB (l = 4 is closest to the Co interface) and reduced μ1(Rh) = 0.54, μ2(Rh) = 0.25, and μ3(Rh) = −0.16 μB. Turning to the optical absorption of (CoRh)coreAushell and (RhCo)coreAushell nanoparticles, we address ε2(ω) in Figure 5, finding sensitivity to both composition and chemical arrangement. In the first class, ε2(ω) shows a blue shift for increasing

IV. CONCLUSION The magnetic and optical properties of CocoreAushell and (CoRh)coreAushell nanoparticles have been investigated in the framework of spin polarized density functional theory. Effects of the composition and local chemical environment have been analyzed and quantified. We find, on the one side, significant proximity effects of the spin-polarized Co atoms on the nonmagnetic Rh atoms. For sufficiently large Co cores significant magnetic moments are induced in the Rh atoms almost independent of the distances from the core. We find a transfer of 0.1 to 0.2 electrons per atom from Co to Rh, which increases (reduces) the number of d holes and thus the magnetic moments of the (Co) Rh atoms. On the other side, the dominant absorption peak of the Au shell shows systematic variations with respect to the core size, composition, and chemical arrangement. In particular, (CoRh)coreAushell nanoparticles show a blue shift for increasing Co concentration, while (RhCo)coreAushell nanoparticles show a red shift for increasing Co concentration. Thus, the Au shell enables optical tracking of the nanoparticles. Experimental verification of these computational predictions is highly desirable.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].; [email protected]. ORCID

Junais Habeeb Mokkath: 0000-0001-8889-5889 D

DOI: 10.1021/acs.jpcc.7b08146 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Present Address

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†

Department of Physics, Kuwait College of Science And Technology, Doha Area, 7th Ring Road, P. O. Box 27235, Kuwait. Notes

The author declares no competing financial interest.

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DOI: 10.1021/acs.jpcc.7b08146 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b08146 J. Phys. Chem. C XXXX, XXX, XXX−XXX