Redox-transmetalation process as a generalized synthetic strategy for core-shell magnetic nanoparticles

Article abstract of DOI:10.1021/ja053659j

Although multicomponent core-shell type nanomaterials are one of the highly desired structural motifs due to their simultaneous multifunctionalities, the fabrication strategy for such nanostructures is still in a primitive stage. Here, we present a redox-transmetalation process that is effective as a general protocol for the fabrication of high quality and well-defined core-shell type bimetallic nanoparticles on the sub-10 nm scale. Various core-shell type nanomaterials including Co@Au, Co@Pd, Co@Pt, and Co@Cu nanoparticles are fabricated via transmetalation reactions. Compared to conventional sequential reduction strategies, this transmetalation process has several advantages for the fabrication of core-shell type nanoparticles: (i) no additional reducing agent is needed and (ii) spontaneous shell layer deposition occurs on top of the core nanoparticle surface and thus prevents self-nucleation of secondarily added metals. We also demonstrate the versatility of these core-shell structures by transferring Co@Au nanoparticles from an organic phase to an aqueous phase via a surface modification process. The nanostructures, magnetic properties, and reaction byproducts of these core-shell nanoparticles are spectroscopically characterized and identified, in part, to confirm the chemical process that promotes the core-shell structure formation.

Full text of DOI:10.1021/ja053659j

Published on Web 10/28/2005
Redox-Transmetalation Process as a Generalized Synthetic
Strategy for Core-Shell Magnetic Nanoparticles
Woo-ram Lee,† Min Gyu Kim,‡ Joon-rak Choi,† Jong-Il Park,§ Seung Jin Ko,†
Sang Jun Oh,| and Jinwoo Cheon*,†
Contribution from the Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea,
Pohang Accelerator Laboratory (PAL), Pohang 790-784, Korea, Research Institute of
Industrial Science and Technology (RIST), Pohang 790-600, Korea, and Korea Basic Science
Institute, Daejeon 305-333, Korea
Received June 3, 2005; E-mail: jcheon@yonsei.ac.kr
Abstract: Although multicomponent core-shell type nanomaterials are one of the highly desired structural
motifs due to their simultaneous multifunctionalities, the fabrication strategy for such nanostructures is still
in a primitive stage. Here, we present a redox-transmetalation process that is effective as a general protocol
for the fabrication of high quality and well-defined core-shell type bimetallic nanoparticles on the sub-10
nm scale. Various core-shell type nanomaterials including Co@Au, Co@Pd, Co@Pt, and Co@Cu
nanoparticles are fabricated via transmetalation reactions. Compared to conventional sequential reduction
strategies, this transmetalation process has several advantages for the fabrication of core-shell type
nanoparticles: (i) no additional reducing agent is needed and (ii) spontaneous shell layer deposition occurs
on top of the core nanoparticle surface and thus prevents self-nucleation of secondarily added metals. We
also demonstrate the versatility of these core-shell structures by transferring Co@Au nanoparticles from
an organic phase to an aqueous phase via a surface modification process. The nanostructures, magnetic
properties, and reaction byproducts of these core-shell nanoparticles are spectroscopically characterized
and identified, in part, to confirm the chemical process that promotes the core-shell structure formation.
Introduction
Among the multicomponent nanostructures, in particular,
bimetallic core-shell nanoparticles are one of the increasingly
important systems due to their enhanced and tunable magnetic,
optical, and catalytic properties.4a-g Although extensive studies
have been conducted to prepare bimetallic core-shell nano-
particles through sequential reduction methods,7 there tends to
be difficulty in producing well-defined bimetallic core-shell
nanoparticles, especially with respect to the small magnetic core
and thin metallic shell layer required for the total particle size
of the sub-10 nm scale regime. Similarly, the precise structural
characterization and understanding of the key processes in the
The fabrication of core-shell nanoparticles is of significance
in materials chemistry due to the multifunctionalities and
enhanced properties that these structures possess as compared
to their mono-elemental counterparts. Multiple functionalities
derived from both core and shell components enable them to
accomplish versatile roles as optical probes,1 magnetic separa-
tors,2 and chemical catalysts.3 Various types of core-shell
nanostructures whose core or shell parts consist of metals,
semiconductors, and dielectric materials have been prepared.4
The dual magnetic core-shell nanoparticle system is one that
has been successfully studied, and FePt@Fe3O4 nanoparticles
demonstrate cooperative magnetic switching behaviors through
exchange coupling between core and shell materials.5 Very
recently, bifunctional nanoparticles comprised of a magnetic
metal core and a semiconductor shell have been reported. In
one such example, Co@CdSe nanoparticles combine the proper-
ties of magnetic nanoparticles and luminescent semiconductor
quantum dots in a core/shell arrangement.6
(4) (a) Sobal, N. S.; Ebels, U.; Mo¨hwald, H.; Giersig, M. J. Phys. Chem. B
2003, 107, 7351. (b) Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Sastry,
M. J. Am. Chem. Soc. 2003, 125, 8440. (c) Mizukoshi, Y.; Fujimoto, T.;
Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (d)
Mallik, K.; Mandal, M.; Pradhan, N.; Pal, T. Nano Lett. 2001, 1, 319. (e)
Srnova-Sloufova, I.; Lednicky, F.; Gemperle, A.; Gemperlova, J. Langmuir
2000, 16, 9928. (f) Sao-Joao, S.; Giorgio, S.; Penisson, J. M.; Chapon, C.;
Bourgeois, S.; Henry, C. J. Phys. Chem. B 2005, 109, 342. (g) Sobal, N.
S.; Hilgendorff, M.; Mo¨hwald, H.; Giersig, M.; Spasova, M.; Radetic, T.;
Farle, M. Nano Lett. 2002, 2, 621. (h) Gerion, D.; Pinaud, F.; Williams, S.
C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem.
B 2001, 105, 8861. (i) Zhang, J.; Liu, J.; Wang, S.; Zhan, P.; Wang, Z.;
Ming, N. AdV. Funct. Mater. 2004, 14, 1089.
(5) (a) Zeng, H.; Sun, S.; Li, J.; Wang, Z. L.; Liu, J. P. Appl. Phys. Lett. 2004,
85, 792. (b) Li, J.; Zeng, H.; Sun, S.; Liu, J. P.; Wang, Z. L. J. Phys.
Chem. B 2004, 108, 14005. (c) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.;
Sun, S. Nano Lett. 2004, 4, 187.
(6) Kim, H.; Achermann, M.; Balet, L.; Hollingsworth, J. A.; Klimov, V. J.
Am. Chem. Soc. 2005, 127, 544.
† Yonsei University.
‡ Pohang Accelerator Laboratory.
§ Research Institute of Industrial Science and Technology.
| Korea Basic Science Institute.
(1) Doering, W. E.; Nie, S. Anal. Chem. 2003, 75, 6171.
(2) Stevens, P. D.; Fan, J.; Gardimalla, H. M. R.; Yen, M.; Gao, Y. Org. Lett.
2005, 7, 2085.
(7) (a) Schmid, G.; West, H.; Mehles, H.; Lehnert, A. Inorg. Chem. 1997, 36,
891. (b) Henglein, A. J. Phys. Chem. B 2000, 104, 2201. (c) Watzky, M.
A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382. (d) Watzky, M. A.;
Finke, R. G. Chem. Mater. 1997, 9, 3083. (e) Besson, C.; Finney, E. E.;
Finke, R. G. J. Am. Chem. Soc. 2005, 127, 8179. (f) Yu, H.; Gibbons, P.
C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem. Soc 2001, 123, 9198.
(3) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.; Lee, J.;
Hyeon, T. J. Am. Chem. Soc. 2004, 126, 5026. Zhou, S.; Varughese, B.;
Eichhorn, B.; Jackson, G.; McIlwrath, K. Angew. Chem., Int. Ed. 2005,
44, 4539.
9
16090
J. AM. CHEM. SOC. 2005, 127, 16090-16097
10.1021/ja053659j CCC: $30.25 © 2005 American Chemical Society

Redox−Transmetalation Process as Synthetic Strategy
A R T I C L E S
h. The 6.5 nm (σ ) 0.6 nm) Co nanoparticles were separated by the
addition of ethanol and centrifuging, then redispersed in an organic
solvent such as toluene.
synthesis of bimetallic core-shell nanoparticles have been very
limited, partly due to the lack of well-defined model systems
and difficulties in analyses of such multilayered nanostructures.
In this article, we examine the general applicability of the
redox transmetalation process and the characterization of a var-
iety of bimetallic core-shell systems with superparamagnetism
and noble metallic properties. We present a highly effective and
generalized synthesis of bimetallic core-shell type magnetic
nanoparticles via a redox transmetalation process. Although this
method has the potential to be an effective protocol for the
fabrication of core-shell nanostructures, there has been only
one reported case of Co@Pt.8 Therefore, in this article, we will
focus on the extension of this process to other metals and the
general applicability toward various core-shell nanoparticles.
We selected Co nanoparticles as the core material and a
combination of materials including gold, palladium, platinum,
and copper to comprise the shell layers. In these core-shell
structures, the core magnetic component possesses various
potential functionalities in areas such as magnetic storage, cell
separation vector, and magnetic resonance imaging (MRI). The
surface properties of such shell layers can be utilized in roles
such as catalysis,9 surface enhanced Raman scattering (SERS)
signal enhancers,10 conjugation layers with biological mol-
ecules,11 and protective layers.12
(b) Co@Au Core-Shell Nanoparticles. Co nanoparticles (14 mg,
0.2 mmol), [(C8H17)4N]+[AuCl4]- (32.2 mg, 0.04 mmol), and TOP
(0.089 mL, 0.2 mmol) were dissolved in 4 mL of ODCB and heated to
180 °C. After 30 min, the solution was cooled to room temperature,
and an excess amount of ethanol was added. The black powder was
isolated by centrifugation and redispersed in an organic solvent such
as toluene. The structure and morphology of the nanoparticles were
analyzed by transmission electron microscopy (TEM), high resolution
TEM (HRTEM), energy-dispersive X-ray spectroscopic analysis (EDS),
and X-ray absorption spectroscopic (XAS) techniques. A metal contain-
ing green inorganic byproduct was dried and purified in a silica column
and analyzed by elemental and mass analyses. Elemental Anal. Calcd
for C48H102Cl2CoP2: C, 66.18; H, 11.80; Co, 6.77. Found: C, 66.40;
H, 12.01; Co, 6.84.
(c) Co@Pd Core-Shell Nanoparticles. A mixture of Pd(hfac)2 (52
mg, 0.1 mmol) and Co nanoparticles (14 mg, 0.2 mmol) was heated at
150 °C for 6 h in a nonane solution containing 0.1 mmol (0.024 mL)
of dodecane isocyanide (C12H25NC) as a stabilizer. The nanoparticle
product was separated by centrifugation after the addition of ethanol.
Obtained nanoparticles were analyzed by the same tools mentioned in
the previous section. The metal containing reaction byproduct was
isolated from an orange supernatant solution and further purified by
sublimation under vacuum before being analyzed by UV-vis absorption
and infrared spectroscopy. A strong UV-vis absorption band of the
byproduct occurs at 300 nm from a π-π* transition, and IR analysis
shows peaks at 1643 (CdO str), 1612 (CdC str), 1562 (CdO str, C-H
bend), 1535 (CdO str, C-H bend), 1481 (CdO str, C-H bend), 1260
Experimental Procedures
General Methods. All reactions were carried out under an argon
atmosphere using standard airless techniques. Pd(hfac)2 (hfac )
1,1,1,5,5,5-hexafluoroacetylacetonate), Pt(hfac)2, Cu(hfac)2, dodecane
isocyanide, and N,N,N-trimethyl(11-mercaptoundecyl)ammonium chlo-
ride were prepared according to literature methods.13 Toluene, nonane,
and o-dichlorobenzene (ODCB) were distilled over sodium, and ethanol
was distilled over calcium hydride. Solvents were carefully degassed
by a freeze-pump-thaw technique before use. Co2(CO)8, NaAOT
(AOT ) bis(2-ethylhexyl)sulfosuccinate), and trioctylphosphine (TOP)
were purchased from Strem and Sigma-Aldrich, and [(C8H17)4N]+-
[AuCl4]- was obtained from the toluene phase after mixing solutions
of (C8H17)4N+Br- in toluene and aqueous HAuCl4 with vigorous stirring.
Synthesis of Nanoparticles. (a) Co Nanoparticles. Co nanoparticles
were synthesized by thermal decomposition of dicobalt octacarbonyl
(Co2(CO)8) in the presence of NaAOT following a previously developed
method (Figure 1a).14 A toluene solution containing Co2(CO)8 (0.5 M,
4 mL) was injected into refluxing 36 mL of a toluene solution containing
NaAOT (0.089 g, 0.2 mmol) and cooled to room temperature after 6
(CF3 str), and 1207 (C-H in plane bend) cm-1
.
(d) Co@Pt Core-Shell Nanoparticles. Co@Pt core-shell nano-
particles were synthesized by refluxing Co nanoparticles (14 mg, 0.2
mmol) and Pt(hfac)2 (61 mg, 0.1 mmol) in a nonane solution containing
0.1 mmol (0.024 mL) of dodecane isocyanide (C12H25NC) as a stabilizer.
Obtained nanoparticles are stable in air and can be redispersed in typical
organic solvents. The orange colored byproduct was confirmed as
Co(hfac)2 after being separated and analyzed by the same methods
described above.
(e) Co@Cu Core-Shell Nanoparticles. NaAOT was dried at 120
°C in a vacuum for 1 h before use. Co@Cu core-shell nanoparticles
were obtained from Cu(hfac)2 (48 mg, 0.1 mmol) and Co nanoparticles
(14 mg, 0.2 mmol). The reaction proceeded at 140 °C for 2 h in ODCB
solution containing 0.1 mmol (44.5 mg) of NaAOT as a capping mole-
cule. The orange colored byproduct was confirmed as Co(hfac)2 after
being separated and analyzed by the same methods described above.
(f) Water-Soluble Co@Au via Phase-Transfer Reagent. As syn-
thesized Co@Au nanoparticles (∼3 mg, 0.04 mmol) were dissolved in
ODCB (10 mL), and N,N,N-trimethyl(11-mercaptoundecyl)ammonium
chloride (3.3 mg, 0.04 mmol) was added into this solution. After vor-
texing for 30 min, reddish black precipitates were formed and isolated
by centrifugation. The resulting powders were redissolved in the aque-
ous phase. These nanoparticles are stable in water for several months.
TEM and HRTEM Analysis. The size distribution, morphology,
lattice distances, and crystallographic structures of nanoparticles were
studied by TEM performed on an EM 912 Omega operated at 120 kV
and HRTEM performed on a Hitachi H9000-NAR operated at 300 kV.
For the TEM analysis, nanoparticles dissolved in toluene were placed
on a TEM grid and allowed to dry in air. A carbon coated nickel grid
was used for the sampling of Co@Cu nanoparticles, and a carbon coated
copper grid was used for Co@Au, Co@Pt, and Co@Pd nanoparticles.
XAS Measurements. The powdered nanoparticles were mounted
in aluminum cells and sealed with polyimide tape (KAPTON-500H,
125 µm thickness). Sample preparations were carried out in an inert
glovebox to prevent any oxidation or contamination. Co, Cu, and Pd
K-edges and Pt and Au LIII-edges X-ray absorption spectra were
(8) (a) Park, J.-I.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5743. (b) Park,
J.-I.; Kim, M. G.; Jun, Y.-W.; Lee, J. S.; Lee, W.-R.; Cheon, J. J. Am.
Chem. Soc. 2004, 126, 9072.
(9) (a) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165.
(b) Jansat, S.; Gomez, M.; Philippot, K.; Muller, G.; Guiu, E.; Claver, C.;
Castillon, S.; Chaudret, B. J. Am. Chem. Soc. 2004, 126, 1592.
(10) (a) Doering, W.; Nie, S. J. Phys. Chem. B 2002, 106, 311. (b) Li, X.; Zhang,
J.; Xu, W.; Jia, H.; Wang, X.; Yang, B.; Zhao, B.; Li, B.; Ozaki, Y.
Langmuir 2003, 19, 4285.
(11) (a) Park, S.-J.; Taton, A.; Mirkin, C. Science 2002, 295, 1503. (b) Hong,
R.; Emrick, T.; Rotello, V. M. J. Am. Chem. Soc. 2004, 126, 13572. (c)
Maxwell, D. J.; Taylor, J. R.; Nie, S. J. Am. Chem. Soc. 2002, 124, 9606.
(d) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286,
1550. (e) Tseng, G. Y.; Ellenbogen, J. C. Science 2001, 294, 1293. (f)
Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541.
(12) (a) Chen, M.; Yamamuro, S.; Farrell, D.; Majetich, S. J. Appl. Phys. 2003,
93, 7551. (b) Guo, Z.; Kumar, C. S. S. R.; Henry, L. L.; Doomes, E. E.;
Hormes, J.; Podlaha, E. J. J. Electrochem. Soc. 2005, 152, D1.
(13) (a) Siedle, A. R.; Newmark, R. A.; Kruger, A. A.; Pignolet, L. H. Inorg.
Chem. 1981, 20, 3399. (b) Bertrand, J. A.; Kaplan, R. I. Inorg. Chem. 1966,
5, 489. (c) Funck, L. L.; Ortolano, T. R. Inorg. Chem. 1968, 7, 567. (d)
Weber, W. P.; Gokel, G. W. Tetrahedron lett. 1972, 13, 1637. (e) Tien, J.;
Terfort, A.; Whitesides, G. Langmuir 1997, 13, 5349.
(14) (a) Park, J.-I.; Kang, N.-J.; Jun, Y.-W.; Oh, S. J.; Ri, H. C.; Cheon,
J.ChemPhysChem. 2002, 3, 543. (b) King, S.; Hyunh, K.; Tannenbaum,
R. J. Phys. Chem. B 2003, 107, 12097.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16091

A R T I C L E S
Lee et al.
Figure 1. TEM and HRTEM images of (a) 6.5 nm Co nanoparticles and (b) 6.4 nm Co@Au core-shell nanoparticles using Co nanoparticles as the seed
material. Lattice distances measured by HRTEM are well-matched to known Au lattice parameters for the (111) plane (inset). The average size of core-shell
nanoparticles is ∼6.4 nm, which is similar to the initial size of the Co nanoparticles because the atom exchange process is the only operative reaction.
Scheme 1. Schematic of Core-Shell Nanoparticle Formation via Redox Transmetalation Processesa
a Metal ions (MII) of reactant metal complexes (MII-Li) are reduced on the surface of MI nanoparticles while neutral MI atoms are oxidized to MIy+ by
forming a MI-ligand complex (MI-Lj) as a resultant reaction byproduct. Repeating this process results in the complete coverage of shell layers on core
metals.
recorded on the BL7C beam line of the Pohang light source (PLS)
with a ring current of 120-170 mA at 2.5 GeV. A Si(111) double
crystal monochromator was employed to monochromatize the X-ray
photon energy. The data were collected in transmission mode with
nitrogen (85%) and argon (15%) gas-filled ionization chambers as
detectors. Higher order harmonic contamination was eliminated by
detuning the monochromator to reduce the incident X-ray intensity by
∼30%. Energy calibration was performed using a standard metal foil.
The data reduction of the experimental spectra was performed by a
standard procedure reported previously.15 The measured absorption
spectra below the pre-edge region were fitted to a straight line, and the
background contribution above the post-edge region was chosen using
a third-order polynomial fitting. The polynomials of the extrapolated
background were subtracted from the total absorption spectra, and the
background-subtracted absorption spectra were normalized for a post-
edge energy region.
Results and Discussion
When a metal-ligand complex in a positive metal oxidation
state (MIIx+Lix-) approaches another metal surface (MI), MIILi
molecules can be reduced through the sacrificial oxidation of
the MI surface atoms to produce MII deposition on the MI metal
surface via a redox transmetalation process (eq 1 and Scheme
1). Since this redox reaction can spontaneously proceed under
favorable redox conditions (i.e., the electrochemical potential
of the redox reaction (∆E) is positive) and occurs selectively
only on the metal surface to be deposited, this method has been
regarded as an efficient route for the selective formation of
bimetallic structures.16 Although such a process is known for
bulk scale materials via chemical vapor deposition (CVD) and
electroless plating processes, their utilization for nanoscale
systems is very rare,8 and the general applicability of this process
has not been examined.
Magnetic Measurements. Magnetic measurements were performed
on a SQUID magnetometer (Quantum Design MPMS-7). The hysteresis
loops were obtained at 5 and 300 K in a magnetic field varying from
+5 to -5 T. The powdered samples were prepared in gelatin capsules
in an inert glovebox.
nMI (metal surface atom) + [MII]x+[Li]x-
f
MI(n-1)@MII + [MI]y+[Lj]y- (1)
Spectroscopic and Elemental Analysis. UV-vis absorption spectra
were obtained with a JASCO V-530 UV/VIS spectrophotometer.
Infrared spectra were recorded on an IMPACT 400 FT-IR spectrometer
(KBr pellet). Elemental analysis was performed on an inductively
coupled plasma atomic emission spectrometer (Shimadzu ICPS-1000III)
and elemental analyzer (Flash EA 1112 series/CE Instruments). Mass
analysis was performed on an Agilent 5973N mass spectrometer.
where M ) metal, L ) ligand, and x and y ) oxidation state.
(16) (a) Crane, E.; You, Y.; Nuzzo, R. G.; Girolami, G. S. J. Am. Chem. Soc.
2000, 122, 3422. (b) Lin, W.; Warren, T. H.; Nuzzo, R. G.; Girolami, G.
S. J. Am. Chem. Soc. 1993, 115, 11644. (c) Gu, S.; Atanasova, P.; Hampden-
Smith, M. J.; Kodas, T. T. Thin Solid Films 1999, 340, 45. (d) Gu, S.;
Yao, X.; Hampden-Smith, M. J.; Kodas, T. T. Chem. Mater. 1998, 10,
2145. (e) Lin, W.; Wiegand, B. C.; Nuzzo, R. G.; Girolami, G. S. J. Am.
Chem. Soc. 1996, 118, 5977. (f) Lin, W.; Nuzzo, R. G.; Girolami, G. S. J.
Am. Chem. Soc. 1996, 118, 5988.
(15) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag:
Berlin, 1986.
9
16092 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Redox−Transmetalation Process as Synthetic Strategy
A R T I C L E S
Figure 2. XAS data of Co@Au core-shell nanoparticles. (a) Co K-edge XANES spectra of pure Co nanoparticles and Co@Au nanoparticles. (b) Au
LIII-edge XANES spectra of bulk Au and Co@Au nanoparticles. (c) Fourier transforms (FTs) for Co K-edge EXAFS spectra (black line) and Au LIII-edge
EXAFS spectra (red line) of Co@Au nanoparticles and Co K-edge EXAFS spectra of 6.5 nm Co nanparticles (green dotted line), respectively. These data
suggest the formation of a cobalt core and a gold shell with an interface layer between them.
the local structural variation around the Co atom. Figure 2a,b
shows normalized X-ray absorption near-edge structure (XANES)
spectra of Co-Au bimetallic nanoparticles (blue solid lines).
For the Co K-edge XANES spectrum, a significant change in
Co@Au Core-Shell Nanoparticles. In addition to a cobalt
magnetic core, Au has been selected for use as a shell layer
since it is particularly important as a platform substrate for self-
assembled monolayers (SAM) for applications in molecular
the intensity of the pre-edge band (peak A) and the white line
electronics and also for colorimetric chemical and biological
peak (peak B) as compared to those of pure Co nanoparticles
sensing systems.11 Co@Au nanoparticles are synthesized by
(red dashed line) was observed (Figure 2a). Both the decrease
redox transmetalation between surface Co atoms of Co nano-
in pre-edge intensity and the increase in white line intensity
particles and Au3+ ions of [(C8H17)4N]+[AuCl4]-. Co nanopar-
resulting from electron transfer from Co to Au and population
ticles (14 mg, 0.2 mmol) were reacted with [(C8H17)4N]+[AuCl4]-
rearrangement between the 3d, 4s, and 4p levels of Co are
(32.2 mg, 0.04 mmol) for 30 min at 180 °C in ODCB solvent.
indicative of the presence of Co-Au chemical interactions. Such
After the reaction, pinkish black products were isolated from
results are in good agreement with previous XANES reports of
the reaction solution and analyzed. Monodispersed Co@Au
bulk M1-xPtx (M ) Co and Ni) bimetallic alloys.8b,17 For the
nanospheres are observed by TEM analysis, and the average
Au LIII-edge XANES spectra, the overall peak features beyond
size was determined to be 6.4 nm (σ ) 0.6 nm) (Figure 1b).
the white line become noticeably weaker than that of the Au
The size of Co@Au nanoparticles remains similar to that of
foil, indicating the formation of its metallic cluster (Figure 2b).18
the starting Co nanoparticles of 6.5 nm. The Co@Au nanopar-
ticle consists of 79 mol % Co and 21 mol % Au according to
EDS, and the nanoparticle is estimated to have a 5.7 nm Co
core and 0.4 nm Au shell from a simple close packing model
considering atomic molar volume, particle diameter, component
ratio, and lattice parameter. The formation of Au on Co
Although these XANES spectra show the Co-Au interaction
and the existence of Au cluster formation, they are not adequate
enough to fully support the formation of a core-shell type
structure.
The core-shell structure of the bimetallic Co-Au nanopar-
ticles is supported by the analysis of the Fourier transforms (FTs)
nanoparticles is also verified by a HRTEM study. According
of the EXAFS spectra (Figure 2c). For Co K-edge EXAFS
to HRTEM (Figure 1b, inset), the measured lattice distance of
spectra, the pure Co nanoparticles show a symmetric FT peak
2.35 Å in the surface of nanoparticle is consistent with the
known Au lattice parameter (2.355 Å) for the fcc (111) plane.
at ∼2.2 Å (green dashed line), while there is a new FT peak at
∼2.6 Å in addition to one at ∼2.2 Å for the Co@Au
To further validate the core-shell structures of the synthe-
sized bimetallic nanoparticles, we performed XAS analyses.
Since XAS is sensitive to atomic local sites and does not depend
on the long-range order of the atomic arrangement, it is a
powerful technique for analyzing the local structure of these
mutlicomponent nanoparticles. The local structures around the
Co core and the Au shell atoms of the nanoparticles are
investigated by comparing the Co K-edge and Au LIII-edge
X-ray absorption spectra with that of pure metallic Co nano-
particles and are found to have spectral changes indicative of
nanoparticles. These FT peak features suggest that the first shell
around the central Co atom includes two different kinds of
neighboring atoms of core-shell type nanoparticles. While the
shorter FT peak at ∼2.2 Å can be assigned to single scattering
between inner Co atoms themselves within the core region, the
(17) (a) Hlil, E. K.; Baudoing-Savois, R.; Moraweck, B.; Renouprez, A. J. J.
Phys. Chem. 1996, 100, 3102. (b) Moraweck, B.; Renouprez, A. J.; Hlil,
E. K.; Baudoing-Savois, R. J. Phys. Chem. 1993, 97, 4288.
(18) Lopez-Cartes, C.; Rojas, T. C.; Litran, R.; Martinez-Martinez, D.;
de la Fuente, J. M.; Penades, S.; Fernandez, A. J. Phys. Chem. 2005, 109,
8761.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16093

A R T I C L E S
Lee et al.
Figure 3. Scheme of ligand exchange by using linker molecules with strong Au-S linkage and high water solubility. Photos taken (a) before and (b) after
aqueous phase-transfer reaction. (c) Surface plasmon absorption band at 528 nm in a UV-vis absorption spectrum of water-soluble Co@Au core-shell
nanoparticles.
longer FT peak at 2.6 Å corresponds to single scattering between
a Co atom in the core surface and a Au atom in the outer shell
layer. In the Au LIII-edge EXAFS spectra (red line), the FT
peaks at ∼2.6 and ∼2.8 Å are due to the scattering between
Cocore atoms and Aushell atoms in the interface region of Co@Au
and the scattering between Aushell atoms and Aushell atoms within
the Au shell layer, respectively. The XAS analysis suggests that
the obtained nanoparticles are consistent with a Co core and a
Au shell nanostructure.
The reaction byproduct data are valuable to support the
occurrence of redox transmetalation reactions. The metal
containing byproducts analyzed by elemental and mass analyses
(see Supporting Information) were identified as CoCl2(TOP)2.
On the basis of our reaction byproduct analysis, the transmeta-
lation reaction between Co nanoparticles and Au3+is proposed
to be the following (eq 2):
mg, 0.04 mmol) in ODCB and N,N,N-trimethyl(11-mercapto-
undecyl)ammonium chloride (3.3 mg, 0.04 mmol) for 30 min.
Similar phase-transfer processes of nanoparticles from an
organic phase into an aqueous phase exist.19 After the ligand
exchange, the core-shell nanoparticles become soluble in water
since the hydrophilic cationic part of the surfactant molecule
dissolves the nanoparticle in water, while the thiol group of the
molecule attaches to the gold shell and stabilizes the nanoparticle
(Figure 3). This successful phase transfer of Co@Au nanopar-
ticles is, in part, confirmed by their characteristic magnetic
attraction by a typical laboratory magnet and also by the surface
plasmon absorption band of the Au shell at 528 nm.
Co@Pd, Co@Pt, and Co@Cu Core-Shell Nanoparticles.
Co nanoparticles (14 mg, 0.2 mmol) and Pd(hfac)2 (52 mg, 0.1
mmol) were heated at 150 °C for 6 h in nonane solvent. The
Co@Pd nanoparticles with an average diameter of 6.6 nm
(σ ) 0.8 nm) were observed by TEM (Figure 4a).
The initial reactant molar ratio was 2:1, but the content ratio
of the Co@Pd nanoparticle was 1:1 because half of the Co metal
was oxidized to the Co2+ ion and consumed during the
transmetalation reaction. EDS analysis of the nanoparticles
supports this with a Co/Pd ratio of 47:53, which is close to the
expected 1:1 stoichiometry. The lattice distance measured by
HRTEM is 2.24 Å and is congruous with the lattice parameter
for the fcc (111) plane of Pd (Figure 4a, inset), and Co47@Pd53
is estimated to have a 4.9 nm Co core and a 0.9 nm Pd shell.
Further structural analyses are conducted by XAS studies.
The reaction byproduct is analyzed by UV-vis and IR spec-
troscopy (see Supporting Information). The UV-vis absorption
from the π-π* transition of the hfac ligand, and the observed
10Co + 2[(octyl)4N]+[AuCl4]- + 6TOP f
Co7@Au2 + 3CoCl2(TOP)2 + 2(octyl)4NCl (2)
The expected ratio of Co to Au in the nanoparticle is 7:2 from
the formula of Co7@Au2, and this theoretical ratio is roughly
consistent with the observed ratio of 79:21 by EDS.
Water-Soluble Co@Au Core-Shell Nanoparticles. The
versatility and robustness of as-prepared Co@Au nanoparticles
are tested by aqueous phase transfer experiments. In particular,
the aqueous phase transfer of as-prepared Co@Au nanoparticles
in organic solvents is of interest since magnetic components
containing water soluble Au nanoparticles can be key materials
for various biological sensing and detection systems. The phase-
transfer process is carried out by replacing the capping ligand,
TOP, on the surface of Co@Au with N,N,N-trimethyl(11-
mercaptoundecyl)ammonium chloride. Ligand-exchanged Co@Au
nanoparticles are obtained in the form of reddish black
precipitates by mixing as-synthesized Co@Au nanoparticles (3
(19) (a) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Eychmuller, A.; Weller,
H. Nano Lett. 2002, 2, 803. (b) Negishi, Y.; Tsukuda, T. J. Am. Chem.
Soc. 2003, 125, 4046. (c) Templeton, A. C.; Hostetler, M. J.; Warmoth, E.
K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.;
Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (d) Gittins, D. I.; Caruso,
F. Angew. Chem., Int. Ed. 2001, 40, 3001.
9
16094 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Redox−Transmetalation Process as Synthetic Strategy
A R T I C L E S
Figure 4. TEM and HRTEM images of (a) 6.6 nm Co@Pd, (b) 6.4 nm Co@Pt, and (c) 6.5 nm Co@Cu core-shell nanoparticles. Measured lattice spacing
(inset figures) of 2.24, 2.27, and 2.09 Å by HRTEM are clearly matched to each shell lattice parameters known for the (111) plane of Pd, Pt, and Cu,
respectively.
Figure 5. Co K-edge XANES spectra of Co@Pd, Co@Pt, and Co@Cu
core-shell nanoparticles.
IR modes are clearly matched with those of Co(hfac)2.20 Since
the major byproduct of this reaction is confirmed as Co(hfac)2,
it is reasonable to propose that transmetalation reaction occurs,
and the overall reaction is given in eq 3.
Figure 6. (a) FT peak feature of Co K-edge EXAFS spectra (black line)
2Co + Pd(hfac)2 f Co@Pd + Co(hfac)2
(3)
and Pd K-edge EXAFS spectra (red line) of Co@Pd nanoparticles, (b) FT
of Co K-edge (black line) and Pt K-edge (red line) EXAFS spectra of Co@Pt
nanoparticles, and (c) FT of Co K-edge (black line) and Cu K-edge (red
line) EXAFS spectra of Co@Cu nanoparticles.
As developed previously by us, Co@Pt core-shell nanopar-
ticles are also prepared from the reaction between Pt(hfac)2 (61
mg, 0.1 mmol) and Co nanoparticles (14 mg, 0.2 mmol). The
6.4 nm (σ ) 0.6 nm) Co@Pt nanoparticles are seen by TEM
analysis (Figure 4b). The composition ratio of Co/Pt is 45:55
by EDS, and these nanoparticles are roughly estimated as 4.6
nm Co core and 0.9 nm Pt shell. The inset shows the HRTEM
image of the Pt lattice of 2.27 Å, which is consistent with known
parameters for the Pt (111) planes. Similarly, the Co@Cu core-
shell nanoparticles are obtained from the reaction between Cu-
(hfac)2 (48 mg, 0.1 mmol) and Co nanoparticles (14 mg, 0.2
mmol). The 6.5 nm (σ ) 0.7 nm) Co@Cu nanoparticles are
observed by TEM analysis (Figure 4c). The composition ratio
of Co/Cu is 54:46 based on EDS. HRTEM analysis shows a
lattice distance of 2.09 Å due to the lattice parameter of a fcc
copper (111) plane, and these core-shell nanoparticles are
roughly estimated as 5.2 nm Co core and 0.7 nm Cu shell.
Consistent with the case of Co@Pd, the major byproduct is
identified for both Co@Pt and Co@Cu cases as Co(hfac)2 by
UV-vis absorption and IR analyses20 (see Supporting Informa-
tion).
Therefore, the transmetalation reaction indeed does occur for
all these cases, and the chemical reaction can be summarized
as eq 4. During this redox transmetalation process, M2+ of
M(hfac)2 is reduced on the surface of the Co nanoparticle, while
the surface cobalt atom is oxidized to Co2+ in the form of
Co(hfac)2, and the electron transfer between the two metals
results in core-shell type nanoparticles.
(20) (a) Morris, M. L.; Moshier, R. W.; Sievers, R. E. Inorg. Chem. 1963, 2,
411. (b) Nakamoto, K.; Morimoto, Y.; Martell, A. E. J. Phys. Chem. 1962,
66, 346. (c) Fackler, J. P., Jr.; Cotton, F. A.; Barnum, D. W. Inorg. Chem.
1963, 2, 97.
2Co + M(hfac)2 f Co@M + Co(hfac)2
M ) Pd, Pt, Cu
(4)
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16095

A R T I C L E S
Lee et al.
Figure 7. Hysteresis loops of (a) Co nanoparticles and core-shell nanoparticles (b) Co@Au, (c) Co@Pd, (d) Co@Cu, and (e) Co@Pt at 5 K on a SQUID
magnetometer. (f) Hysteresis loop of Co@Cu nanoparticles at 300 K shows superparamagnetism.
Core-shell structures of Co@Pd, Co@Pt, and Co@Cu are
further confirmed through XAS. When Co K-edge XANES
spectra of Co@Pd, Co@Pt, and Co@Cu nanoparticles are
compared with that of pure Co nanoparticles, respectively, the
intensities of peak A (the pre-edge band) decrease, while those
of peak B (the white line peak) increase (Figure 5). These
changes show the presence of Co-Pd, Co-Pt, and Co-Cu
chemical interactions.
single scattering of Cocore-Ptshell atoms in the interface region
is consistently observed, while the shoulder at ∼2.7 Å is due
to the Ptshell-Ptshell interaction within the Pt shell layer. For the
Co@Cu nanoparticles, although clear FT peak separation is not
feasible due to the nearly indistinguishable bond distances of
Co-Co and Cu-Cu, it is highly likely that the strong single
peaks from Co and Cu K edge, respectively, support the
simultaneous presence of Co and Cu with possible core-shell
structures (Figure 6c). All these EXAFS data consistently
support that the Co@M nanoparticles obtained by this trans-
metalation reaction are core-shell structures of Co, Pd, Pt, and
Cu.
In Figure 6a, FT peaks at ∼2.3 Å for Co K-edge EXAFS
spectra and ∼2.2 Å for Pd K-edge EXAFS spectra of Co@Pd
nanoparticles reveal the interfacial interaction between Co core
atoms and Pd shell atoms, while the peaks at ∼2.0 Å for Co
K-edge EXAFS and ∼2.5 Å for Pd K-edge EXAFS result from
their core atoms’ and shell atoms’ own interactions, respectively.
Similarly, in the case of Co@Pt, a strong peak at ∼2.0 Å and
a shoulder at ∼2.3 Å (Figure 6b, black line) appearing in the
FT of the Co K-edge indicates that the first shell around the
Co atom includes two different types of neighboring atoms. The
peak at ∼2.0 Å is assigned to single scattering by Cocore-Cocore
atoms within the Cocore region, and the shoulder at ∼2.3 Å
corresponds to single scattering by the Cocore-Ptshell at the core-
shell interfaces. The FT of the Pt LIII-edge EXAFS spectrum
(Figure 6b, red line) suggests that three different environments
are present for Pt atoms. A strong peak at ∼2.3 Å arising from
According to our results, the redox transmetalation processes
appear to be effective as a general protocol for the fabrication
of well-defined bimetallic nanoparticles, while the average
diameters of all the core-shell nanoparticles remain similar to
that of starting Co nanoparticles. In addition, the calculated
standard potential values are 1.80 V for the cell Co/Co2+//Au3+
/
Au, 1.23 V for the Co/Co2+// Pd2+/Pd, 1.46 V for the Co/Co2+//
Pt2+/Pt, and 0.62 V for the Co/Co2+//Cu2+/Cu. Since all
theoretical redox potential values are positive, redox transmeta-
lation is thermodynamically favorable and experimentally
confirmed in the cases of Co@Au, Co@Pd, Co@Pt, and
Co@Cu nanoparticles.
9
16096 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Redox−Transmetalation Process as Synthetic Strategy
A R T I C L E S
Magnetic Measurements. Any possible effects of shell layers
on the magnetism of core cobalt are briefly examined by SQUID
measurements. Hysteresis loops of the nanoparticles at 5 K show
coercivity (Hc) of 2090 Oe for 6.5 nm pure Co nanoparticles,
1310 Oe for Co@Au, 840 Oe for Co@Cu, 660 Oe for Co@Pt,
and 740 Oe for Co@Pd core-shell nanoparticles, respectively
(Figure 7). The coercivity values of all core-shell nanoparticles
are smaller than that of pure Co nanoparticles of 2090 Oe.
Considering that the estimated average diameter of cobalt in
each type of nanoparticles is 6.5, 5.7, 5.2, 4.7, and 4.9 nm for
Co, Co@Au, Co@Cu, Co@Pt, and Co@Pd, respectively, the
trend suggests that core-shell nanoparticles with larger Co core
sizes have higher coercivity. According to these data, the core
magnetic material appears to contribute to the overall magnetism
of the core-shell nanoparticles with little, if any, effect on its
magnetism from the noble metal shell layer.6,21 All of these
nanoparticles show superparamagnetic behavior at room tem-
perature.
reactant metal complexes are reduced on the surface of Co
nanoparticles, while neutral Co atoms are oxidized to Co2+ via
ligand migration to form a Co-ligand complex as a reaction
byproduct. Since shell layer formation and core metal consump-
tion occur simultaneously, the average particle size of the
original Co nanoparticles is retained after the reaction process.
Although there are slight core size dependent coercivity changes,
the room temperature superparamagnetism is clearly well-
preserved in all cases. The transfer of Co@Au into the aqueous
phase is effective, and the core part of the Co@Au nanoparticles
retains its magnetism, while the shell layer of the particles
consists of highly versatile gold. We believe that this redox
transmetalation strategy can be generally applicable to a variety
of core-shell type multimetallic nanoparticle fabrications.
Acknowledgment. We thank Dr. Y. J. Kim (KBSI) for high-
voltage TEM (JEM-ARM 1300S), K. T. Son (KBSI) for TEM,
and E. M. Kim (KIST) for mass analyses. This work is supported
by the Korea Research Foundation (R02-2004-000-10096-0).
Conclusion
In this paper, we demonstrate the fabrication of various types
of core-shell nanomaterials such as Co@Au, Co@Pd, Co@Pt,
and Co@Cu through redox transmetalation reactions. Efficient
shell growth proceeds on the core metal through the reaction
without any additional reducing agent. During the transmeta-
lation processes, metal ions (Au3+, Pd2+, Pt2+, or Cu2+) of the
Supporting Information Available: EDS spectra of all core-
shell nanoparticles, mass data of the reaction byproduct of
Co@Au nanoparticles, and spectral analyses of reaction byprod-
ucts of Co@Pd, Co@Pt, and Co@Cu nanoparticles. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Vestal, C. R.; Zhang, Z. J. Nano Lett. 2003, 3, 1739.
JA053659J
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16097