Synthesis of core-shell PtCo nanocrystals

Article abstract of DOI:10.1021/jp034200j

Monodisperse, bimetallic PtCo nanoparticles with a controllable core - shell structure have been synthesized by a new two-stage route. The process involves thermal decomposition of cobalt carbonyl in the presence of a nanosized platinum (Pt) dispersion, where the thickness of the Co shell can be controlled by changing the amount of cobalt carbonyl. The synthesized particles reveal a narrow size distribution with average diameters of 2.6 for Pt and a maximum of 7.6 nm for PtCo. Transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), optical spectroscopy, and superconducting quantum interference (SQUID) magnetometry were used for characterization of the bimetallic system.

Full text of DOI:10.1021/jp034200j

J. Phys. Chem. B 2003, 107, 7351-7354
7351
Synthesis of Core-Shell PtCo Nanocrystals†
Nelli S. Sobal,*,‡,§ Ursula Ebels,| Helmuth Mo1hwald,§ and Michael Giersig*,‡
Bereich Solarenergieforschung, Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, 14109 Berlin, Germany,
Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, Am Mu¨hlenberg 1, 14476 Golm, Germany, and
UMR SPINTEC, CEA-Grenoble, France
ReceiVed: January 24, 2003; In Final Form: April 25, 2003
Monodisperse, bimetallic PtCo nanoparticles with a controllable core-shell structure have been synthesized
by a new two-stage route. The process involves thermal decomposition of cobalt carbonyl in the presence of
a nanosized platinum (Pt) dispersion, where the thickness of the Co shell can be controlled by changing the
amount of cobalt carbonyl. The synthesized particles reveal a narrow size distribution with average diameters
of 2.6 for Pt and a maximum of 7.6 nm for PtCo. Transmission electron microscopy (TEM), energy-dispersive
X-ray spectrometry (EDX), optical spectroscopy, and superconducting quantum interference (SQUID)
magnetometry were used for characterization of the bimetallic system.
Introduction
any systematic investigation of such crystals, it has not been
possible to find any regularity in their properties or to verify
Many of the physical phenomena that make metallic nano-
particles so widely applicable are highly size-dependent.
Therefore, the precise synthesis and characterization of nano-
scale metallic materials is of great interest for such exploits as
magnetic recording,1,2 catalysis,3-8 nanoelectronics,9-12 nano-
scale optics,13 and materials science.14-16 In particular, a
significant number of recently published papers are concerned
with magnetic colloidal particles, whose attractiveness stems
from their possible application to magnetic data storage media.
Methods for preparing highly disperse individual magnetic
crystals of Co,17-25 Fe,20,26,27 and Ni20 have been described. One
can apply the technique of Sun and co-workers,28 originally
proposed to generate FePt, to prepare alloyed nanoparticles such
as CoPt3,29 FePt,28,30-32 FeCoPt,33 CoPt,34 FeMo,35 and FePd.34
The properties of such small particles depend on their composi-
tion, shape, and method of preparation. Therefore the formation
of alloys thoroughly explains new properties of magnetic
systems, often distinct from those of the corresponding mono-
metallic particles. FePt, PtCo, and Pt3Co alloys show high
magnetic anisotropy and chemical stability in comparison to
pure Fe and Co metallic colloids.36,37 Nunomura et al.38 have
observed a large increase of the magnetic moment in PdNi
particles above a threshold concentration of 6.3 atom % Ni.
Self-assembled films of FexCoyPt100-x-y alloy particles have
lower coercivity than FePt samples.33 The authors of ref 39
noticed a promotion of the fcc to tetragonal phase transition
and an increase in coercivity of the films as a result of adding
Ag to FePt nanocrystals.
chemical structure.
In this paper we focus on a “reverse” core-shell particle
system, where a noble metal core of Pt is surrounded by a
magnetic Co shell. In a previous article50 we have already shown
that Ag-core/Co-shell nanocrystals exhibit optical activity and
magnetic behavior distinct from that peculiar to both noble and
3d metals. In addition, the presence of a noble metal seems to
improve the stability of nanosized Co against oxidation. Any
CoO was found even in 3-month-old Co@Ag samples by
electron energy loss spectroscopy (EELS) analysis.51 Bimetallic
systems of this sort are expected to reveal interesting magnetic
properties as found in analogously constructed continuous films,
for instance, an induced polarization in Pt at the Co/Pt interface52
or a giant magnetoresistance effect as in the case of Pd particles
covered with Ni.53
Experimental Section
The so-called modified “polyol” process has been used for
preparation of bi- and trimetallic magnetic nanocrystals.29-31,33,34
In all cases, the reduction of platinum acetylacetonate with a
long-chain polyol in the presence of stabilizers was combined
with the simultaneous thermal decomposition of cobalt or iron
carbonyl. As a result, alloyed particles with a narrow distribution
were obtained. Farrell et al.54 used tiny particles of platinum as
nuclei for further synthesis of monodisperse iron particles.
Afterwards, the metallic platinum particles are not extracted
from the solution and investigated.
In this paper we present a two-stage procedure for the
preparation of bimetallic Pt@Co particles. Pure platinum
particles with definite diameter were formed at first. Later,
decomposition of cobalt carbonyl on the Pt seeds, if carried
out at relatively low temperature, aborts formation of any alloy
yielding only Pt-core/Co-shell particles. A typical synthesis is
described in detail below.
Besides monometallic or alloy structured particles, core-
shell-type systems are of great interest and have revealed a large
range of ferromagnetic properties. Results have been reported
for metallic40-47 and alloyed48,49 core particles covered with a
noble metal (usually Au or Ag). However, in the absence of
† Part of the special issue “Arnim Henglein Festschrift”.
* Corresponding authors: e-mail sobal@caesar.de and giersig@caesar.de.
‡ Hahn-Meitner-Institut Berlin. Present address: Center of Advanced
European Studies and Research, Bonn, Germany.
In a three-necked round-bottom flask, a solution of platinum
acetylacetonate (99.99%, Aldrich, 0.05 g or 0.125 mmol), 1,2-
hexadecanediol (90%, Aldrich, 0.1 g or 0.38 mmol), oleic acid
(99+%, Aldrich, 40 µL or 0.125 mmol), and oleylamine (70%,
Aldrich, 56 µL or 0.175 mmol) in 5 mL of diphenyl ether (dpe)
§ Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung.
| UMR SPINTEC.
10.1021/jp034200j CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/26/2003

7352 J. Phys. Chem. B, Vol. 107, No. 30, 2003
Sobal et al.
Figure 1. TEM images and size distribution of individual platinum
nanoparticles (a) and of Pt@Co nanoparticles prepared at molar ratios
Pt:Co2(CO)8 ) 1:2 (b) and 1:4 (c).
was heated to 205 °C. After heating for 60 min, the dark mixture
was cooled to 142 °C, and the solution of the designed amount
of dicobalt octacarbonyl Co2(CO)8 (contains 1-5% hexane as
stabilizer, Alfa Aesar) in dpe was gradually added under a
nitrogen atmosphere. Mixing and heating at this temperature
was then continued for 30 min. The resulting dispersion was
left to cool to room temperature, and ethanol was added to
precipitate the particles. After removal and drying of the
supernatant, the magnetic particles were redispersed in toluene.
Use of other nonpolar solvents such as hexane, octane, or
chloroform is possible as well.
Figure 2. HRTEM images of (a) Pt particles with inserted power
spectrum showing the typical 111 reflexes for cubic structure and (b)
a Pt@Co particle showing the typical lattice plane from a cubic cobalt
shell on the Pt particle surface.
Results and Discussion
Reduction of platinum acetylacetonate in the presence of
surfactants (oleic acid and oleylamine) begins at 180 °C. It was
found that 1 h of heating at 205 °C is enough for complete
reduction of the platinum salt and formation of a dark-brown
colloid of pure platinum. Figure 1a shows the TEM images of
individual Pt nanoparticles and the corresponding size histogram.
The spherical particles are well-separated and monodisperse with
a main size of 2.6 nm (standard deviation, 10%). Figure 1b,c
shows the same particles after decomposition of cobalt carbonyl,
leading to deposition of metallic cobalt on the surface of the Pt
seeds.
Results from examination of uncovered and covered particles
by high-resolution TEM are presented in Figure 2a and clearly
show the lattice planes with the periodicity of 0.22 nm typical
for a cubic crystallographic platinum structure. In Figure 2b,
the Pt@Co particles have a measured lattice spacing of
0.20 nm, corresponding to the fcc structure of cobalt nanoc-
rystals.
The structure of the particles was assessed by a transmission
electron microscope Philips 12CM, operating at 120 kV, which
was equipped with an X-ray fluorescent analyzer for the
determination of the material compositions. Samples were
prepared by drying a drop of the solution on a thin (5 nm)
carbon-film-covered copper grid. The size distribution of
the particles was measured from TEM images with at least
200 particles. The UV-visible spectra were recorded on a
Bruins spectrophotometer with correction for toluene back-
ground absorption. The magnetic measurements were made on
a commercial superconducting quantum interference magne-
tometer (SQUID) from Quantum Design for the M-H loops
and from SHE for susceptibility measurements. M-H loops
were taken for samples prepared by drying several drops of
solution on a Si/SiO2 substrate, while the zero-field-cooled
(ZFC) and field-cooled (FC) data were taken for the liquid
solution.
Additionally, the presence of both Pt and Co metal was
qualitatively confirmed by energy-dispersive X-ray (EDX)
spectrometric analysis, as is depicted in Figure 3. It is evident
that the Pt cores are covered with a brighter layer of cobalt.

Synthesis of Core-Shell PtCo Nanocrystals
J. Phys. Chem. B, Vol. 107, No. 30, 2003 7353
Figure 3. Typical EDX spectrum of Pt@Co particles, which are shown
in the inset. The carbon and copper maxima belong to the TEM grids.
Figure 4. Absorption spectra of colloidal solution of Pt in toluene-
diphenyl ether (200:1) mixture and of bimetallic Pt@Co particles in
toluene.
This result, in agreement with theory, contradicts previously
published results for core-shell systems40-47 wherein the
covering of magnetic cores by metals was proposed but not
unequivocally presented.
The thickness of the relatively uniform Co shell can be
controlled by varying the amount of dicobalt dicarbonyl. Figure
1b,c shows two samples of Pt@Co particles prepared at molar
ratios Pt:Co2(CO)8 ) 1:2 and 1:4, respectively. The average
diameter of the particles in Figure 1b was 5.1 nm and the size
distribution remained relatively narrow. The maximum thickness
of Co (2.3 ( 0.2 nm) deposited in this way was reached by the
ratio platinum:cobalt dicarbonyl 1:4 (Figure 1c). In this case,
the main diameter of the spherical bimetallic particles increased
to 7.5 nm, while the size distribution increase to 18%. With
more generous amounts of cobalt precursors Pt/Co2(CO)8 ) 1:5,
the synthesis yields the expected Pt-core/Co-shell particles and
some individual Co particles. Incomplete coverage was observed
due to cobalt island formation on the platinum surface caused
by an accidental decrease in the initial ratio of Pt:Co dicarbonyl
below 1:2.
Figure 5. (a) Magnetization vs field measurements at various tem-
peratures for Pt@Co particles deposited on a Si substrate; (b) variation
of the coercive field Hc as a function of temperature; (c) ZFC/FC
magnetization curves measured on 7.5 nm Pt-core/Co-shell nanoparticles
dissolved in toluol. The small increase of the magnetic moment in the
FC and ZFC data at low T (below 15 K) might be due to the presence
of paramagnetic Pt phase in the solution.47
From the magnetic measurements, the particles reveal a
superparamagnetic behavior at room temperature, while below
150 K a clear magnetic hysteresis is obtained (see Figure 5a)
with a coercivity that falls off with increasing T (see Figure
5b). The blocking temperature TB, below which ferromagnetism
sets in, is not defined well due to the distribution of the particle
volume and possibly the magnetic properties (such as anisotropy
constants). From the zero-field-cooled (ZFC) and field-cooled
(FC) magnetization curves of the 7.5 nm Pt@Co particles
(Figure 5c) and the magnetization versus applied field measure-
ments (Figure 5a), the blocking temperature is estimated to lie
between 175 and 225 K. This value is much higher than
observed for similar-sized â-Co nanocrystals: 55 K for 7.6 nm
agglomerated particles24 and 63 K for self-assembled and 58 K
The optical spectra in Figure 4 of Pt and Pt@Co solution
show an increased absorption within the shorter wavelength
region with no maximum in the region 290-750 nm. In the
low wavelengths before 300 nm, the actual information of the
particles could not be detected because of the high absorption
of toluene.

7354 J. Phys. Chem. B, Vol. 107, No. 30, 2003
Sobal et al.
for isolated 5.8 nm Co,55 but is similar to values found for
7-8 nm Co core particles surrounded by a CoO shell.56
From the blocking temperature, the anisotropy energy density,
responsible for blocking the spins into a ferromagnetic state
below Tb, can be estimated by 25kBTB ) KV as described by
Bean and Livingston.57 Using for the Co shell volume 157 and
127 cm3, which corresponds to the 7.5 and 7 nm particles
respectively (with a 2.5 nm Pt core) and kB the Boltzmann
constant, the anisotropy constant K is on the order of
4.8 × 106 erg/cm3. This value, is close to both the bulk hcp Co
value (K ) 5 × 106 erg/cm3)58 and the bulk fcc value (K )
2.7 × 106 erg/cm3).55 Since TEM observation indicates an fcc
Co phase, the slightly larger value might be explained by
additional energy terms such as surface anisotropy or exchange
bias for oxidized Co-surface or dipolar interaction between the
particles. These contributions have not been taken into account
and need further investigation.
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