Characterization of superparamagnetic core-shell nanoparticles and monitoring their anisotropic phase transition to ferromagnetic solid solution nanoalloys

Article abstract of DOI:10.1021/ja049649k

The structure, magnetism, and phase transition of core-shell type CoPt nanoparticles en route to solid solution alloy nanostructures are systematically investigated. The characterization of Co_corePt_shell nanoparticles obtained by a redox transmetalation process by transmission electron microscopy (TEM) and, in particular, X-ray absorption spectroscopy (XAS) provides clear evidence for the existence of a core-shell type bimetallic interfacial structure. Nanoscale phase transitions of the Co_corePt_shell structures toward c-axis compressed face-centered tetragonal (fct) solid solution alloy CoPt nanoparticles are monitored at various stages of a thermally induced annealing process and the obtained fct nanoalloys show a large enhancement of their magnetic properties with ferromagnetism. The relationship between the nanostructures and their magnetic properties is in part elucidated through the use of XAS as a critical analytical tool.

Full text of DOI:10.1021/ja049649k

Published on Web 07/01/2004
Characterization of Superparamagnetic “Core-Shell”
Nanoparticles and Monitoring Their Anisotropic Phase
Transition to Ferromagnetic “Solid Solution” Nanoalloys
†
‡
†
‡
§
Jong-Il Park, Min Gyu Kim, Young-wook Jun, Jae Sung Lee, Woo-ram Lee, and
,
§
Jinwoo Cheon*
Contribution from the Department of Chemistry, Korea AdVanced Institute of Science and
Technology, Daejeon 305-701, Korea, Pohang Accelerator Laboratory (PAL) and Department of
Chemical Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea,
and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
Received January 20, 2004; E-mail: jcheon@alchemy.yonsei.ac.kr
Abstract: The structure, magnetism, and phase transition of core-shell type CoPt nanoparticles en route
to solid solution alloy nanostructures are systematically investigated. The characterization of CocorePtshell
nanoparticles obtained by a “redox transmetalation” process by transmission electron microscopy (TEM)
and, in particular, X-ray absorption spectroscopy (XAS) provides clear evidence for the existence of a
core-shell type bimetallic interfacial structure. Nanoscale phase transitions of the CocorePtshell structures
toward c-axis compressed face-centered tetragonal (fct) solid solution alloy CoPt nanoparticles are monitored
at various stages of a thermally induced annealing process and the obtained fct nanoalloys show a large
enhancement of their magnetic properties with ferromagnetism. The relationship between the nanostructures
and their magnetic properties is in part elucidated through the use of XAS as a critical analytical tool.
Introduction
critical step in further enhancing the desired physical properties,
as it has been demonstrated for bimetallic catalysts, ferromag-
The fabrication of advanced nanostructures with exceptional
magnetic and optoelectronic properties is a prerequisite for their
successful utilization in next generation nanotechnology ap-
plications such as tera-level information storage and optical
probes for biomedical applications.1-4 Multicomponent core-
shell or solid solution alloy nanostructures are emerging as the
netic nanostructures, and semiconductor quantum dots.2
,6,11
Although core-shell type heterometallic nanoparticles are of
importance, the traditional synthetic approaches have not been
successful in producing high quality core-shell nanoparticles.
These difficulties are due to several factors including random
nucleation processes and inhomogeneous growth of the hetero-
metallic component on top of the seeding host nanomaterials.13
Similarly, the precise structural characterization and the under-
standing of the alloying process of core-shell nanoparticles
occurring in the nanoscale regime have been very limited, partly
due to the lack of well-defined model systems and difficulties
in characterization.
Here we choose magnetic nanomaterials of CoPt as a model
system in order to understand the alloying process that is so
critical in systems which exhibit magnetic properties such as
high magnetic anisotropy and magnetooptic Kerr effects.14 For
the nanofabrication of well-defined core-shell type CoPt
nanoparticles, a redox transmetalation process has been per-
2
,3,5,6
most promising solutions for fulfilling these requirements.
In particular, multimetallic nanoparticles can exhibit enhanced
nanoscale properties and are of particular interest due to their
potential as highly efficient catalysts and as independent
magnetic components for ultrahigh density memory devices as
well as in the biomedical field as magnetic sensors.
Elucidating the phase transition processes that occur as the
constituent elements form their novel alloy structures is also a
1
,7-12
†
Korea Advanced Institute of Science and Technology.
Pohang University of Science and Technology.
Yonsei University.
‡
§
(
1) (a) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Inter-
science: New York, 2001. (b) Pole, C. P.; Owens, F. J. Introduction to
Nanotechnology; Wiley-Interscience: New Jersey, 2003. (c) Pileni, M. P.
Metal Nanoparticles: Synthesis, Characterization, and Applications; Marcel
Dekker: New York, 2001. (d) Schmid, G. Clusters and Colloid: From
Theory to Application; Wiley-VCH: Weinheim, 1994.
(7) (a) Himpsel, F. J.; Ortega, J. E.; Mankey, G. J.; Willis, R. F. AdV. Phys.
1998, 47, 511. (b) Jeon, I. J.; Kang, D. W.; Kim, E. D.; Kim, D. H.; Choe,
S. B.; Shin, S. C. Appl. Surf. Sci. 2002, 197-198, 639. (c) Pirausx, L.;
George, J. M.; Despres, J. F.; Leroy, C.; Ferain, E.; Legras, R.; Ounadjela,
K.; Fert, A. Appl. Phys. Lett. 1994, 65, 2484.
(8) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. Science 2000, 290,
1131.
(2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000,
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(3) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J.
A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290,
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14.
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(
(
(
4) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science
998, 281, 2013.
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(11) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179.
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Chem. Soc. 1997, 119, 7019.
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9072
9
J. AM. CHEM. SOC. 2004, 126, 9072-9078
10.1021/ja049649k CCC: $27.50 © 2004 American Chemical Society

Superparamagnetic “Core−Shell” Nanoparticles
A R T I C L E S
formed. During this process, the Co nanoparticle, which
constitutes the core structure, is oxidized while the shell-forming
Pt substituent is reduced due to the favorable redox potential
between the two metals. This nanoscale fabrication process
developed previously by us is advantageous by well-defined
colloidal CocorePtshell nanoparticles (6.4 nm, σ ) 0.6 nm) are separated
from the dark red-black solution in waxy powder form after adding
ethanol and centrifugation.
Phase Transitions of CocorePtshell Nanoparticles into fct CoPt
Nanoparticles. An aliquot amount of toluene solution (∼0.01 mM)
containing synthesized CocorePtshell nanoparticles was dropped on the
Si(100) wafer (5 mm × 5 mm) and slowly dried for 30 min. Then,
CocorePtshell nanoparticles coated on Si wafer were thermally annealed
at the constant temperature (600, 700 °C) of an electrical furnace for
12 h inside a quartz tube under vacuum. The sample was a shiny black
colored thin film after annealing and used for further SQUID and XRD
analyses.
For the XAS measurement, the powdered CocorePtshell nanoparticles,
annealed under same condition as described above, were mounted in
aluminum cells and sealed with polyimide tape (KAPTON-500H, 125
µm thickness). Samples were formed with a thickness (d) of 100 µm
complying the condition that ∆µx e 0.1, where x is the effective sample
thickness (d/cos 45°) and ∆µ is the absorbance at both the Co K-edge
and Pt LIII-edge. Sample preparations were carried out in an inert
glovebox to prevent any oxidation or contaminations.
1
5
shell layer growth and a straightforward preparation method.
Rigorous structural characterizations of 0-D nanomagnets, in
particular, bimetallic core-shell and their phase transformation
processes to different structures are important. Although trans-
mission electron microscopy (TEM) has been a powerful tool
for characterization of nanomaterials, it is limited in the amount
of information with respect to the chemical bonding and
structural characterization of complex multicomponent materials
such as nanoalloys. In contrast, X-ray absorption spectroscopy
(
(
XAS) can provide valuable structural and chemical information
e.g., interatomic distance, coordination number, oxidation state
of chemical species) about the nanostructures and supplement
16
the TEM data. While XAS studies on noble metals and their
alloys have been well explored,17 those on magnetic nanopar-
ticles are relatively rare. Here, we demonstrate that XAS is
critical to prove the formation of 0-D core-shell CoPt nano-
structures and to monitor their ferromagnetic phase transitions
to solid solution alloy nanostructures in combination with
additional methods such as TEM, XRD, and SQUID.
XAS Measurement. Co K-edge and Pt L -edge X-ray absorption
III
spectra (XAS) were recorded on the BL3C1 beam line of Pohang light
source (PLS) with a ring current of 120-170 mA at 2.5 GeV. An Si-
(111) double crystal monochromator was used with detuning to 85%
in intensity to eliminate the high-order harmonics. The data were
collected in transmission mode with the nitrogen (85%) and argon (15%)
gas-filled ionization chambers as detectors. Energy calibrations were
carried out with the Co and Pt metal foils, assigning the first inflection
point to 7709 and 11564 eV, respectively. To remove an energy shift
problem, X-ray absorption spectra for Co and Pt metal foils were
measured simultaneously in every measurement as the metal foils were
positioned before the window of the third ion chamber.
Experimental Section
Synthesis of Co Nanoparticles and CocorePtshell Nanoparticles. The
synthesis was carried out by slight modification of a reported method.15
Co nanoparticles were synthesized by the thermal decomposition of
dicobalt octacarbonyl (Co
2-ethylhexyl)sulfosuccinate (NaAOT). CocorePtshell nanoparticles were
synthesized by transmetalation between Pt(hfac) (hfac ) hexafluoro-
acetylacetonate) (0.375 mmol) and 6.3 nm (σ ) 0.5 nm) Co nanopar-
ticles (0.75 mmol) in a nonane solution containing 0.09 mL of dodecyl
2
(CO)
8
) in a toluene solution of sodium bis-
Co K-Edge and Pt L -edge EXAFS Data Analysis. The EXAFS
III
(
data analyses were carried out by the standard procedure. The measured
absorption spectra below the preedge region were fitted to a straight
line, and then the background contributions above the postedge region,
µo(E), were fitted to a fourth order polynomial (cubic spline). The fitted
polynomials were extrapolated through the total energy region and
subtracted from the total absorption spectra. The background-subtracted
absorption spectra were normalized for the above energy region, ø(E)
2
isocyanide (C12H25NC) as a stabilizer. After refluxing for 6 h, the
(
14) (a) Ely, T. O.; Pan, C.; Amiens, C.; Chaudret, B.; Dassenoy, F.; Lecante,
P.; Casanove, M.-J.; Mosset, A.; Respaud, M.; Broto, J.-M. J. Phys. Chem.
B 2000, 104, 695. (b) Shevchenko, E. V.; Talapin, D. V.; Rogach, A. L.;
Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 11480.
3
2
)
{µ(E)µ
o
o
(E)}/µ (E). The normalized k - and k -weighted EXAFS
3
2
spectra, k ø(k) and k ø(k), were Fourier transformed in the k range from
.0 to 14.5 Å to show the contribution of each bond pair on the
(c) Sanchez, J. M.; Moran-Lopez, J. L.; Leroux, C.; Cadeville, M. C. J.
-1
2
Phys.: Condens. Matter 1989, 1, 491. (d) Harp, G. R.; Weller, D.;
Rabedeau, T. A.; Farrow, R. F. C.; Toney, M. F. Phys. ReV. Lett. 1993,
Fourier transform (FT) peak. The experimental Fourier-filtered spectra
could be obtained from the inverse Fourier transformation with the
hanning window function in the r space range between 1.0 and 6.0 Å.
To determine the structural parameters for each bond pair, the curve
fitting process was carried out by using the EXAFS formula, which is
expressed as follows:
7
1, 2493.
(
15) Preliminary synthetic results of this process have been reported. The reaction
byproducts, for example Co(hfac) , have been isolated and confirmed by
various analytical methods. See: Park J.-I.; Cheon, J. J. Am. Chem. Soc.
2
2
001, 123, 5743.
(
(
16) Rehr, J. J.; Albers, R. C. ReV. Mod. Phys. 2000, 72, 621.
17) Previously, XAS has been used to investigate the structural characterization
of noble metals and their alloys. (a) Benfield, R. E.; Grandjean, D.; Kr o¨ ll,
M.; Pugin, R.; Sawitowsli, T.; Schmid, G. J. Phys. Chem. B 2001, 105,
2
i
2 2
j
1
961. (b) Bradley, J. S.; Via, G. H.; Bonneviot, L.; Hill, E. W. Chem.
ø(k) )
N S (k)F (k)exp(-2σ k ) ×
∑
j
j
Mater. 1996, 8, 1895 (Pd-Cu). (c) Harada, M.; Asakura, K.; Ueki, Y.;
Thosima, N. J. Phys. Chem. 1993, 97, 10742 (Pd-Rh). (d) Molenbroek,
A. M.; Haukka, S.; Clausen, B. S. J. Phys. Chem. B 1998, 102, 10680
sin(2kr + Φ (k))
j
ij
exp(-2r /λ (k))
j
j
2
(
)
krj
(
Cu-Pd). (e) Nashner, M. S.; Frenkel, A. I.; Adler, D. L.; Shapley, J. R.;
Nuzzo, R. G. J. Am. Chem. Soc. 1997, 119, 7760 (Pt-Ru). (f) O’Grady,
W. E.; Hagans, P. L.; Pandya, K. I.; Maricle, D. L. Langmuir 2001, 17,
3
2
047 (Pt-Ru). (g) Hills, C. W.; Nashner, M. S.; Frenkel, A. I.; Shapley,
which includes the photoelectron wave vector, k () [8π m(E - E
o
)/
J. R.; Nuzzo, R. G. Langmuir 1999, 15, 690 (Pt-Ru). (h) Nashner, M. S.;
Frenkel, A. I.; Somerville, D.; Hills, C. W.; Shapley, J. R.; Nuzzo, R. G.
J. Am. Chem. Soc. 1998, 120, 8093 (core-shell Pt-Ru). (i) Kolb, U.;
Quaiser, S. A.; Winter, M.; Reetz, M. T. Chem. Mater. 1996, 8, 1889 (Pd-
Pt). (j) Harada, M.; Asakura, K.; Ueki, Y.; Toshima, N. J. Phys. Chem.
2
1/2
2
j
h ] ), the coordination number, N , the amplitude reduction factor, S
i
,
the effective curved wave backscattering amplitude, F (k), the Debye-
j
2, the mean free path of the photoelectron, λ, the
Waller factor, σ
j
interatomic distance, r , and total phase shift, Φij, respectively.
j
1
992, 96, 9730 (Pd-Pt). (k) Toshima, N.; Harada, M.; Yonezawa, T.;
Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448 (Pd-Pt). (l)
Nashner, M. S.; Somerville, D. M.; Lane, P. D.; Adler, D. L.; Shapley, J.
R.; Nuzzo, R. G. J. Am. Chem. Soc. 1996, 118, 12964 (Re-Ir). (m)
Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992,
Theoretical scattering paths could be obtained from the crystal-
lographic description of the known model. The theoretical EXAFS
parameters such as phase shift, backscattering amplitude, and total
central atom loss factor were calculated for a function of wavenumber
9
6, 9927 (Au-Pd). (n) Molenbroek, A. M.; Norskov, J. K.; Clausen, B. S.
J. Phys. Chem. B 2001, 105, 5450 (Ni-Au). (o) Shibata, T.; Bunker, B.
A.; Zhang, Z.; Meisel, D.; Varderman, C. F.; Gezelter, J. D. J. Am. Chem.
Soc. 2002, 124, 11989 (Au-Ag).
for all possible scattering paths by FEFF6.01 code. For simplicity in
2
curve fitting process, the S
i
value was fixed to 0.85 for the Co and Pt
J. AM. CHEM. SOC.
9
VOL. 126, NO. 29, 2004 9073

A R T I C L E S
Park et al.
Figure 1. Redox transmetalation process and TEM analysis of CocorePtshell nanoparticles. (a) Redox transmetalation processes for core-shell nanoparticles.
2
+
During the reaction, Pt of Pt(hfac)2 is reduced to Pt while the surface Co atom of the Co nanoparticles is oxidized via hfac ligand migration to form
Co(hfac)2 as a reaction byproduct. Repeating cycles of this reaction results in core-shell bimetallic nanoparticles. TEM images of (b) 6.3 nm Co nanoparticles,
(c) CocorePtshell nanoparticles, and (d) HRTEM images of 6.4 nm CocorePtshell nanoparticles.
atoms. The coordination number (N) was evaluated by independent
iteration from the Debye-Waller factor although the N value was
correlated with the Debye-Waller factor in the amplitude of EXAFS
Results and Discussion
Synthesis and TEM Analysis of CocorePtshell Nanoparticles.
By utilizing a redox transmetalation process, the synthesis of
CocorePtshell nanoparticles was performed by refluxing 6.3 nm
(σ ) 0.5 nm) Co nanoparticles with Pt(hfac) , (hfac )
hexafluoroacetylacetonate), in a nonane solution containing
n
spectra. The experimental k ø(k) spectra were fitted with possible
scattering paths showing the substantial amplitude for the corresponding
FT peak. In this case, structural parameters such as the coordination
number, the interatomic distance and the Debye-Waller factor were
used as adjustable parameters in the fitting process for the EXAFS
spectra.
2
dodecyl isocyanide as a capping molecule (Figure 1). During
2+
the reaction, the Pt of Pt(hfac)2 is reduced to Pt while the
TEM, XRD, and Magnetic Measurements. TEM and HRTEM
analyses were carried out on an EM 912 Omega, Hitachi H9000-NAR,
and JEOL ARM-1300S instruments operated at 120, 300, or 1250 kV,
respectively. X-ray diffraction (XRD) spectra were obtained using
graphite-monochromatized Cu KR radiation in a Rigaku D/MAX-RC
diffractometer operated at 40 kV 80 mA. Magnetic measurements were
performed on a SQUID magnetometer (Quantum Design MPMS-7).
The temperature was varied between 5 and 300 K according to a zero-
field cooling/field cooling (ZFC/FC) procedure at 75 Oe, and the
hysteretic loops were obtained in a magnetic field varying from +5 to
surface Co atom of the Co nanoparticles is oxidized via hfac
ligand migration to form Co(hfac)2 as a reaction byproduct
(Figure 1a). Transmission electron microscopy (TEM) images
of the obtained CoPt nanoparticles show nanoparticles with an
average diameter of 6.4 nm (σ ) 0.6 nm) (Figure 1c). HRTEM
images show relatively well-developed lattice fringe patterns
especially in the edge area of the dots (Figure 1d). A measured
mean lattice distance of 2.27 Å is consistent with the known Pt
lattice parameter (2.265 Å) for the (111) plane. The slightly
darker image contrast of the edge area also suggests the
-
5 T.
9074 J. AM. CHEM. SOC.
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VOL. 126, NO. 29, 2004

Superparamagnetic “Core−Shell” Nanoparticles
A R T I C L E S
Table 1. Co K-Edge EXAFS Structure Parameters of Co,
CocorePtshell, and Annealed CocorePtshell Nanoparticles (NPs)
∆
eV
E,
compound
nm Co NPs
path
N
R (Å)a σ2 (×10-3 Å2)a
7
4
Co -Co -4.3 7.1
Co -Co -4.3 5.2
Co -Co -5.1 4.5
2.49(3)
2.49(4)
2.48(2)
2.64(1)
8.92
8.67
12.5
16.1
8.26
2.23
2.69
11.3
13.5
5.89
12.1
3.83
nm Co NPs
CocorePtshell NPs
Co -Pt 2.7
2.6
annealed CocorePtshell NPs A1
-3.1 6.1 [8]b 2.65(4)
A2
B1
B2
C1
C2
D1
D2
5.2
-0.4 1.2 [2] 3.77(4)
9.1 2.4 [4] 4.01(7)
1.5 [4] 2.70(2)
-8.7 4.6 [8] 4.48(8)
-9.4 9.6 [16] 4.64(4)
-5.5 3.5 [8] 5.26(7)
5.7
2.6 [8] 5.27(4)
a
3
3
2
In curve fitting process, the goodness of fit by {Σ(k ødata - k ømodel) }/
3
2
Σ(k ødata) was estimated within allowed error range. The estimated errors
are within (0.02 Å for the interatomic distance and about 15% for the
b
Debye-Waller factor. The expected coordination number of bulk fct CoPt
Figure 2. XAS analysis of CocorePtshell and annealed CocorePtshell nanopar-
model.
3
ticles. Fourier transforms of Co K-edge k ø(k) (filled circle) and Pt LIII-
2
edge k ø(k) EXAFS spectra (open circle) of the CocorePtshell nanoparticles.
XAS Analysis upon Phase Transformation of CocorePtshell
into fct CoPt Alloy Nanoparticles. Tremendous variations in
the physical properties of nanomaterials can result from their
structural phase transitions. By thermally annealing our initially
synthesized core-shell structure under vacuum at 600 or 700
formation of the Ptshell layer and EDS analysis of the nanopar-
ticles indicates the stoichiometry to be Co0.45Pt0.55.18 However,
these analyses do not unambiguously prove that the core-shell
structures were formed since interferences such as Moir e´ fringe
patterns are also possible.
XAS Analysis of CocorePtshell Nanoparticles. To achieve a
better evidence of this core-shell nanostructure formation, X-ray
absorption spectroscopic analysis (XAS) was performed.
A strong peak at ∼2.0 Å and a shoulder at ∼2.3 Å (Figure
°
C for 12 h, its phase transition to solid solution alloy was
induced that is crucial for a significant enhancement of its
magnetic properties.
3
Analysis of the FTs of Co K-edge k ø(k) EXAFS spectra
indicates that the CocorePtshell undergoes a transition to structur-
ally different species during the thermal annealing process. Peaks
corresponding to Co-Co (fcc) at ∼2.0 Å and Co-Pt at ∼2.3
Å of the core-shell structure appear to shift to peak positions
corresponding to greater interatomic distances with an increase
in the overall peak intensity (Figure 3a. from bottom to top
2
, filled circle) appearing in the Fourier transform (FT) of Co
3
K-edge k ø(k) extended X-ray absorption fine structure (EXAFS)
spectrum of the CoPt nanoparticles 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 core-shell interfaces.
19
spectra). A relatively strong and broad first peak from the 600
C sample contains three deconvoluted peaks, at 2.0, 2.3, and
.6 Å, corresponding to Co-Co (fcc) and Co-Pt and Co-Co
fct), respectively (Figure 3b middle spectra). The decrease in
intensity of the peak at 2.0 Å and the appearance of a peak at
.6 Å suggest that the fcc packing of the Co core structure now
°
2
(
2
The FT of the Pt LIII-edge k ø(k) EXAFS spectrum (Figure
2, open circle) suggests three different environments are present
2
for Pt atoms. A strong peak at ∼2.3 Å arising from 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. An
additional peak at ∼1.7 Å corresponds to the scattering with
capping molecules on the surface of the Pt shell layer. Structural
parameters (e.g., interatomic distance, coordination number, and
Debye-Waller factor) for the CocorePtshell structures are obtained
from EXAFS refinement (Table 1). These results suggest that
the interatomic distance between the Cocore and the nearest
neighboring Co atom is 2.48 Å which is consistent with the
value for the bulk face-centered cubic (fcc) phase of cobalt (2.50
Å). The interatomic distance between Co atom and the
neighboring Pt atom is determined to be 2.64 Å which is also
in good agreement with the reported value of Co1Pt1 (2.65 Å).
The EXAFS data clearly confirm that the CoPt nanoparticles
obtained by this transmetalation reaction are indeed of a core-
shell structure of Co and Pt.
changes into a different Co-Co interaction due to a c-axis
compressed face-centered tetragonal (fct) structure (vide infra).
Finally for the 700 °C sample, the strong first peak possesses
two deconvoluted peaks occurring at 2.3 and 2.6 Å, correspond-
ing to Co-Pt and fct Co-Co, respectively (Figure 3a and Figure
3
b top spectra). By standard EXAFS fitting process, the
structural parameters of Co-Pt and Co-Co for the first shell
are obtained as 2.65 and 2.70 Å, which are consistent with 2.65
and 2.69 Å of the fct model. Also, other scattering modes of
EXAFS data are well matched with fct Co1Pt1 model structure.
Peak A (∼2.3 and ∼2.6 Å) in Figure 3a is assigned to two
scatterings of Co-Pt and Co-Co for the out-of-plane and in-
plane direction, respectively (Figure 3c, red arrows A1, A2). The
observation of two weaker peaks B (∼3.4 and ∼3.6 Å) suggests
single scattering with a Co atom at the (0, 0, 1) and (1, 0, 0)
sites from the central Co atom (Figure 3c, blue arrows B1, B2)
and double peaks C (∼4.2 Å) correspond to single scatterings
(
19) The higher FT magnitude observed after annealing can be due to an increase
in the static ordering of Co and Pt site redistribution. Even after annealing
at 700 °C, annealed CoPt nanoparticles do not exhibit the formation of a
bulk CoPt alloy, which is confirmed by TEM analysis and the FT amplitude
of annealed CoPt nanoparticles.
(
18) TEM image contrasts depend on the mass-thickness of the samples with
heavier elements appearing darker than lighter elements and the EDS
analysis indicates a moderate degree of Pt incorporation with an estimated
thickness of Cocore and Ptshell to be roughly 4.6 and 1.8 nm, respectively.
J. AM. CHEM. SOC.
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VOL. 126, NO. 29, 2004 9075

A R T I C L E S
Park et al.
3
Figure 3. (a) Fourier transforms of Co K-edge k ø(k) EXAFS spectra for CocorePtshell nanoparticles annealed at 600 °C and 700 °C for 12 h and (b) their
peak deconvolutions. The intensity of the first shell Co-Co (fcc) (∼2.0 Å) scattering peak of CocorePtshell gradually decreases and finally fades out as the
annealing temperature increases while the intensities of the first shell Co-Pt (∼2.3 Å) and Co-Co (fct) (∼2.6 Å) scattering peaks increase, indicating phase
transformation from CocorePtshell to fct CoPt solid solution alloys. (c) Face-centered tetragonal crystal model of Co1Pt1 and scattering modes (A1,2, B1,2, C1,2,
D1,2) corresponding to the EXAFS spectra of 700 °C sample.
of Pt at the (1, 1/2, 1/2) site and Co at the (1/2, 1/2, 1) site
edge XANES spectra (Figure 4 inset), the successive decrease
in white line peak intensity, caused by increased charge transfer
from Co to Pt, is indicative of the progression of Co-Pt alloying
due to the annealing process. In the Co K-edge XANES spectra
(Figure 4), broad white line peaks of CocorePtshell nanoparticles
are separated into distinct peaks labeled B and C after the
annealing process, which is a characteristic feature of alloyed
(
Figure 3c, green arrows C1, C2). Peak D (∼5.0 Å) results from
the overlap of two multiple scatterings for out-of-plane and in-
plane, namely, Co(1, 0, 0) f Pt(1, 1/2, 1/2) f Co(1, 1, 1) f
Pt(1, 1/2, 1/2) triple scattering and Co(0, 0, 1) f Co(1/2, 1/2,
1
) f Co(1, 1, 1) double scatterings, respectively (Figure 3c,
black arrows D1, D2). Scattering modes of this peak A and of
the smaller peaks labeled B, C, and D at higher atomic distances
20
CoPt.
(Figure 3a top spectrum) are well matched with a model
Thermally induced dynamic phase transitions of CocorePtshell
nanoparticles [Co-Co(fcc) and Co-Pt] to a solid solution alloy
of anisotropically ordered fct CoPt nanoparticles [Co-Pt and
structure of fct Co1Pt1 alloy (JCPDS 43-1358) whose lattice
parameters are a ) 3.803 Å and c ) 3.701 Å under the space
group of P4/mmm of tetragonal symmetry (Figure 3c).
Figure 4 shows normalized X-ray absorption near edge
structure (XANES) spectra of CoPt nanoparticles. In the Pt LIII-
(
20) Hill, E. K.; Baudoing-Sanois, R.; Moraweck, B.; Renouprez, A. J. J. Phys.
Chem. 1996, 100, 3102.
9076 J. AM. CHEM. SOC.
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Superparamagnetic “Core−Shell” Nanoparticles
A R T I C L E S
Figure 4. Changes of normalized Co K-edge XANES and Pt LIII-edge
XANES (inset) spectra upon thermal annealing of CocorePtshell nanoparticles
for 12 h. Appearance of distinct peaks of B, C and the successive decrease
of peak intensity (inset) indicates the progression of the Co-Pt solid solution
alloy formation.
Co-Co(fct)] through a Cocore(CoPt)ordered-alloyPtshell intermediate
state via inter-diffusion processes proceed.21 These XAS results
are also consistent with the fct structure obtained from X-ray
diffraction studies (Figure 5a).
Enhanced Magnetism of Phase Transformed CoPt Alloy
Nanoparticles. The effect of the structural phase transition on
the magnetic properties is critical. Although CocorePtshell nano-
particles show superparamagnetism with zero magnetic coer-
civity (Hc) at room temperature (Figure 5c), the Hc at 5 K was
remarkably enhanced from 330, to 1300, and finally to 7000
Oe after annealing the CocorePtshell nanoparticles at 600 and 700
°
C for 12 h. The sample annealed at 700 °C shows ferromag-
netism with a Hc of 5300 Oe at room temperature (Figure 5d).
Our annealed CoPt nanoparticles retain their size (∼ 6 nm)
without forming bulk CoPt22 and are observed to be nanoscale
ferromagnets that could potentially be used as magnetic bits
for teralevel information storage applications.2
3
The large enhancement of the magnetic properties is closely
related to the alloy formation and consistent with the XAS
results. Most nanoparticles below the critical size typically show
superparamagnetic behavior at room temperature due to a higher
Figure 5. XRD analysis and magnetic properties of CocorePtshell nanopar-
ticles. (a) XRD pattern of CocorePtshell nanoparticles annealed at 700 °C for
12 h. All peaks are well matched to reference fct Co1Pt1 alloys (dashed
line). (b) Thermal alloying of CocorePtshell nanoparticles to anisotropic fct
structure. Hysteresis loops of CocorePtshell nanoparticles measured at 300 K
7a
thermal fluctuation energy (kT) than anisotropic energy (KuV).
In contrast, the enhanced ferromagnetism of our sample is
originated from the increased magnetocrystalline anisotropy
(c) before and (d) after the annealing process at 700 °C. The magnetic
7
3
(
anisotropy constant (K) of bulk fct CoPt is 4 × 10 erg/cm )
coercivity of the nanoparticles is significantly enhanced from 0 to 5300 Oe
exhibiting room-temperature ferromagnetism.
of the fct structure which possesses a shorter Co-Pt bond
distance in the c-axis and an increased number of neighboring
Co atoms on the Pt atom when compared with fcc packing
modes of core-shell structures (Figure 3c). These anisotropic
structural changes induce strong ferromagnetic coupling on Pt
and Co atoms due to the hybridization of Pt 5d and Co 3d states
(
21) Similar formation of ordered inter-diffused CoPt alloy layers in Co/Pt
layered structures at high temperatures (500-700 °C) have previously
reported, which is consistent with our observations. See: (a) Train, C.;
Beauvillain, P.; Mathet, V.; Penissard, G.; Veillet, P. J. App. Phys. 1999,
8
6, 3165. (b) Moon, D. W.; et al. App. Phys. Lett. 2001, 79, 503.
(22) Our EXAFS results are quite consistent with the TEM analysis of annealed
CoPt where most CoPt retained their sizes except a very small portion
14a,24
and spin polarization of Pt atoms.
(< ∼8%) of sintered particles (Figure S4 in Supporting Information).
(23) Weller, D.; Moser, A. IEEE Trans. Magn. 1999, 35, 4423.
(24) Koote, A.; Haas, C.; Groot, R. A. J. Phys.: Condens. Matter 1991, 3, 1133.
J. AM. CHEM. SOC.
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VOL. 126, NO. 29, 2004 9077

A R T I C L E S
Park et al.
Conclusion
and Engineering Foundation (Grant 1999-1-122-001-5) and the
U.S. Department of the Navy (Grant N62649-03-1-0008). We
thank KBSI for the TEM and magnetic measurements of
samples. We are also grateful to authorities of Pohang Light
Source (PLS) for X-ray absorption spectroscopic measurements.
Nanoscale characterization and correlation of nanostructures
to their magnetic properties during the anisotropic phase
transitions were successfully examined. The core-shell structure
is clearly confirmed and subsequent thermally induced dynamic
phase transitions of fcc CocorePtshell nanoparticles to a solid
solution alloy of anisotropically ordered fct CoPt nanoparticles
are monitored. Our obtained CoPt alloy nanostructures show a
large enhancement of their magnetic properties by exhibiting
ferromagnetism at room temperature and overcoming the
superparamagnetic limitations of common magnetic nanoma-
terials. The redox transmetalation process is effective for the
formation of core-shell nanostructures, and further synthetic
and characterization approaches of this kind may facilitate the
development of a wide range of advanced core-shell or alloy
nanomaterials and help to attain a better understanding of the
intricate relationship between their structure and functionality.
Supporting Information Available: Figures of Co K-edge
k3-weighted EXAFS spectra of CocorePtshell nanoparticles and
the annealed samples at 600 °C and 700 °C for 12 h (S1), (S2)
3
Co K-edge k -weighted experimental and best-fitted EXAFS
spectra of CocorePtshell nanoparticles and their Fourier transforms
3
(
S1), Co K-edge k -weighted experimental and best-fitted
EXAFS spectra of annealed CocorePtshell nanoparticles at 700
C and their Fourier transforms (S3), and a representative TEM
°
image of annealed CocorePtshell nanoparticles at 700 °C (S4). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was supported by the National
R&D Project for Nano Science and Technology, Korea Science
JA049649K
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