5744 J. Am. Chem. Soc., Vol. 123, No. 24, 2001
Park and Cheon
obtained by an EQUINOX 55 FT-IR spectrometer (KBr pallet). UV/
vis absorption data were obtained by a Shimadzu UV-3100S spectro-
photometer in EtOH solvent. Powder 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). DC susceptibility and hysteresis measurements were
recorded for powdered samples of nanoparticles in a gelatin capsule.
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
-5 T.
Scheme 1. Synthetic Routes of Core-Shell and Solid
Solution Type Nanoalloys via Transmetalation Reaction
Results and Discussion
The size distribution and crystallinity of the nanoparticles
were studied first by transmission electron microscopy. TEM
images show the presence of nonagglomerated and moderately
monodispersed spherically shaped CoPt3 nanoparticles with an
average diameter of 1.8 nm (σ ) 0.1 nm) (Figure 1). Examina-
tion of single nanoparticles (Figure 1 inset) gives lattice spacings
of 2.23((0.03) Å for the (111) plane and 1.93((0.03) Å for
the (200) plane, which are consistent with the known bulk fcc
values.13 X-ray powder diffraction and selected area electron
diffraction (SAED) patterns also show peaks due to the (111),
(200), (220), and (311) lattice planes of fcc CoPt3. Elemental
analysis of the nanoparticles confirms that the ratio of Co to Pt
is 1.00:2.96. Similar results were obtained for Co1Pt1 with
homogeneous nanoparticles (1.9 nm, σ ) 0.3 nm) by TEM
analyses and a Co-to-Pt ratio of 0.46:0.54 is obtained from
energy-dispersive X-ray analysis (EDAX), which is close to the
expected 1:1 stoichiometry.
In the case of CocorePtshell type nanoalloys, moderately
monodispersed nanoparticles with an average diameter of 6.27
nm (σ ) 0.58 nm) are observed by TEM (Figure 2). The surface
of the CocorePtshell particle is smooth and homogeneous as
examined by HRTEM. This is in contrast to previous attempts
which resulted in rather inhomogeneous surfaces with uneven
adhesions of smaller outer particles as shell layers of core-shell
nanoparticles.14 The measured lattice distance of 2.27((0.03)
Å on the shell of nanoparticles is consistent with the known Pt
lattice parameter (2.265 Å) for the (111) plane (Figure 2). While
the size of the CocorePtshell nanoparticles remains similar to that
of the starting Co nanoparticles, EDAX studies indicate that
large amounts of Pt are present in the CocorePtshell nanoparticles
with an observed stoichiometry of Co0.45Pt0.55. With this
stoichiometry, the Co core is estimated to be roughly 4.75 nm
with the outer Pt layer to be about 1.82 nm thick (∼4 layers)
by simple close packing model simulations. In principle, the
thickness of the Pt shell can be tuned by varying the amount of
Pt(hfac)2.
prepared according to literature methods.10 Toluene and nonane were
purified by distillation under an argon atmosphere over sodium. Ethanol
was purified by distillation over calcium hydride. Solvents were
carefully degassed by the freeze-pump-thaw technique before use.
All other reagents purchased from commercial sources were used as
obtained without further purification.
Synthesis of Nanoalloys. (a) Co1Pt3 Nanoalloys.11 0.25 mmol (0.085
g) of Co2(CO)8 (0.25 mmol, 0.085 g) in 5 mL of toluene was injected
into 5 mL of hot toluene solution containing 0.5 mmol (0.28 g) of
Pt(hfac)2 with 0.15 mL of oleic acid as a stabilizing surfactant. Upon
mixing, evolution of CO gas and a gradual color change from yellow
to brownish black were observed. After 12 h, the resulting solution
was cooled to room temperature and treated with ethanol to isolate the
black nanocrystals from the orange supernatant. The reaction byproducts
were separated from the remaining orange supernatant solution and
further purified by sublimation under vacuum. The orange crystals
obtained were analyzed by spectroscopic methods and elemental
analysis. IR spectra of the orange crystals revealed peaks at 1645 (Cd
O str), 1610 (CdC str), 1564 (CdO str, CdC bend), 1537 (CdO str,
CdC bend), 1485 (CdO str, CdC bend), 1347 (CF3 str), 1258 (CF3
str), 1227 (CF3 str), 1205 (C-H in plane bend), 1146 (C-H in plane
bend), 1095 (C-H out of bend), 805 (C-H out of bend), 744 (C-CF3
str), and 674 (C-CF3 str) cm-1. UV/vis absorption maxima occurred
at 300 nm from a π-π* transition and at 455(sh), 489, 510, and 546
nm from d-d transitions. Elemental Anal. Calcd for C10H2O4F12Co‚
xH2O (x ) 1.5): C, 24.02; H, 1.01; Co, 11.79. Found: C, 24.16; H,
1.24; Co, 11.58. The obtained Co1Pt3 nanoparticles are easily redispersed
in organic solvent such as hexane and toluene.
(b) Co1Pt1 Nanoalloys. The Co1Pt1 alloy was synthesized using the
same procedure used to synthesize Co1Pt3 nanoalloys except with
modified reagent ratios. Co1Pt1 is obtained from 0.5 mmol (0.170 g)
of Co2(CO)8 and 0.5 mmol (0.28 g) of Pt(hfac)2.
(c) CocorePtshell Nanoalloys. CocorePtshell nanoalloys were synthesized
by refluxing 6.33 nm (σ ) 0.61) Co nanoparticle colloids (0.5 mmol)
and Pt(hfac)2 (0.25 mmol) in a nonane solution containing 0.06 mL of
C12H25NC as a stabilizer. The Co nanoparticles were synthesized from
the thermolysis of Co2(CO)8 in toluene solution.12 After 8 h of reflux,
the colloids are isolated from the dark red-black solution in powder
form after adding ethanol and centrifugation. Reaction byproduct was
separated and analyzed as Co(hfac)2. The nanoparticles obtained are
stable in air and can be redispersed in typical organic solvents.
Characterization. Transmission electron microscopy (TEM) ob-
servations were carried out on an EM 912 Omega and Hitachi H9000-
NAR high-resolution electron microscope operated at 120 or 300 kV,
respectively. Elemental analysis was performed on an inductively
coupled plasma atomic emission spectrometer (Shimadzu ICPS-1000III)
and elemental analyzer (EA1110-FISONS). Infrared spectra were
Magnetic measurements of the nanoalloys were performed
on a SQUID magnetometer. A blocking temperature (TB) of 20
K (Figure 3) and a coercivity (Hc) of 6900 Oe at 5 K (Figure
4) were observed for CoPt3 nanoalloys while a TB of 15 K and
Hc of 5300 Oe at 5 K (Figure 4) were observed for Co1Pt1 nano-
alloys. These values are higher relative to those observed for
pure cobalt nanoparticles of similar size12a,15 as a result of the
increased anisotropy due to alloy formation. These solid solution
type nanoparticles show superparamagnetic behavior at 300 K.
(10) (a) Okeya, S.; Kawaguchi, S. Inorg. Synth. 1980, 20, 65. (b) Weber,
W. P.; Gokel, G. W. Tetrahedron Lett. 1972, 13, 1637.
(11) Our synthetic method appears at a glimpse similar to the FePt
synthesis by Sun et al. (ref 5b), but the two methods in fact employ different
processes. Our process utilizes a redox transmetalation process between
Pt(hfac)2 and Co metals for Pt generation, whereas their work is based on
the “polyol process” in the Pt(acac)2 reduction step.
(12) Park, J.-I.; Cheon, J. Unpublished results. (a) TB ) 10 K and Hc )
260 Oe for 2.2 nm Co; (b) TB ) 100 K and Hc ) 470 Oe for 6.4 nm Co;
(c) TB ) 20 K and Hc ) 370 Oe for 4.0 nm Co nanoparticles, respectively.
(13) X-ray Powder Diffraction Patterns (International Centre for Dif-
fraction Data, Newtown Square, PA, 1996).
(14) (a) Schmid, G.; Lehnert, A.; Malm, J.-O.; Bovin, J.-O. Angew.
Chem., Int. Ed. Engl. 1991, 30, 874. (b) Schmid, G.; West, H.; Mehles, H.;
Lehnert, A. Inorg. Chem. 1997, 36, 891. (c) Wang, Y.; Toshima, N. J.
Phys. Chem. B 1997, 101, 5301.
(15) When compared to other known values of CoPt3, our Hc value is
comparable within the same order of magnitude.