Preparation of Ag core-Cu shell nanoparticles by microwave-assisted alcohol reduction process

Article abstract of DOI:10.1246/cl.2007.154

We have succeeded in rapidly preparing Ag core-Cu shell nanoparticles (denoted as Cu/Ag nanoparticles) having 10-40 nm in size by a microwave (MW)-assisted alcohol reduction process. The core-shell nanoparticles were prepared by step-by-step successive reduction of a silver precursor and then of a copper precursor added after the formation of silver nanoparticles in 1-heptanol. Copyright

Full text of DOI:10.1246/cl.2007.154

154
Chemistry Letters Vol.36, No.1 (2007)
Preparation of Ag Core–Cu Shell Nanoparticles
by Microwave-assisted Alcohol Reduction Process
Takashi Nakamura,1 Yasunori Tsukahara,Ã1 Tomohisa Yamauchi,1
Takao Sakata,3 Hirotaro Mori,3 and Yuji WadaÃ1;2
1Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
2Department of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushima-Naka, Okayama 700-8530
3Research Center for Ultra-High Voltage Electron Microscopy, Osaka University,
7-1 Mihogaoka, Ibaraki, Osaka 567-0047
(Received October 10, 2006; CL-061176; E-mail: ytsuka@jrl.eng.osaka-u.ac.jp, yuji-w@cc.okayama-u.ac.jp)
We have succeeded in rapidly preparing Ag core–Cu shell
heptanol (30 mL) in a three-necked flask. After treated by super-
sonication for completely dispersing the precursor, the 1-hepta-
nol solution was heated by MW irradiation using an MW appa-
ratus MMG-213VP (Micro Denshi Co., Ltd., 500 W) at 413 K for
5 min under N2 bubbling (the obtained sample here was denoted
as S1). The heating temperature was controlled by on-off irradi-
ation of MW. A 1-heptanol solution (30 mL) including copper
myristate (0.2 mmol) was mixed in the S1 1-heptanol solution,
and then, heated at 433 K for 5 and 10 min (the samples obtained
here were denoted as S2 and S3, respectively).
The sizes and morphologies were characterized by a high-
resolution transmission electron microscopy (HRTEM) at
300 kV with a Hitachi H-9000 (Hitachi High-Technologies
Corp.) The solution diluted with hexane was dropped onto a cop-
per grid coated with carbon film, and then the grid was dried un-
der vacuum at 323 K. UV–vis spectra of the solutions diluted by
tenth with hexane were measured with a V-570 (JASCO Inc.).
Figure 1 shows HRTEM images. An image of the sample S1
is shown in Figure 1a. Nanoparticles having 5 nm in size were
demonstrated therein. Observation of electron diffraction (ED)
pattern showed face-centered cubic (fcc) structure of silver
nanoparticles (denoted as Cu/Ag nanoparticles) having 10–
40 nm in size by a microwave (MW)-assisted alcohol reduction
process. The core–shell nanoparticles were prepared by step-by-
step successive reduction of a silver precursor and then of a cop-
per precursor added after the formation of silver nanoparticles in
1-heptanol.
Metal nanoparticles are applied to catalysts,1 magnetic,2
electronic,3 and nonlinear optical materials.4 Monometallic
nanoparticles have been extensively used for these applications.
Nowadays, bimetallic nanoparticles, having core–shell and
alloy nanostructures, are desired as new materials because new
functions and improvements of functions can be expected in
catalytic1 and magnetic properties,5 that cannot be achieved by
monometallic nanoparticles.
Bimetallic nanoparticles having a core–shell nanostructure
(denoted as core–shell nanoparticles) have started to attract
much attention of researchers. Properties and preparation meth-
ods of the core–shell nanoparticles have been reported, e.g., Ag/
Au,6 Pd/Ag,7 Pt/Au,8 Ag/Ni,9 Ag/Co.9 Ag/Cu nanoparticles
have been prepared only by the physical methods such as a laser
ablation in ethanol solvent10 or a thermal evaporation under ultra
high vacuum.11 Chemical preparations in solvents have never
been reported for Ag/Cu or Cu/Ag nanoparticles, while chemi-
cal preparations of these particles by solution reactions should
have advantages in manipulation and tuning of their fine struc-
tures. Furthermore, based on the previous reports as to improve-
ments of catalytic and magnetic function of core–shell nanopar-
ticles,1,5 it is expected that Ag/Cu or Cu/Ag nanoparticles show
new and different functions from sole silver or copper nanopar-
ticles.
(b)
(a)
Our group succeeded in easily preparing silver12 and cop-
per13 nanoparticles in long-chain alcohol solvents by a MW-
assisted alcohol reduction process. Consequently, we have tried
to prepare Cu/Ag nanoparticles by modifying this process to the
consecutive ones of the formations of Ag and Cu: first, silver
nanoparticles are prepared from a silver precursor in a long-
chain alcohol solvent, and then a copper precursor is reduced
in the presence of the silver nanoparticles in the same solution.
This is the first report on chemical preparation of Ag core–Cu
shell nanoparticles in solutions.
(c)
Figure 1. HRTEM images of the samples prepared only silver
(a) and at atomic fraction Cu:Ag = 2:1 heated at 433 K for
5 min (b). Figure 1c is magnified image of one particle in
Figure 1b. Indicated particles by arrows are nanoparticles having
core–shell nanostructure.
Silver myristate and copper myristate used as the precursors
for preparing nanoparticles were synthesized according to previ-
ous reports.12,13 Silver myristate (0.1 mmol) was dispersed in 1-
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.1 (2007)
155
constant surrounding the silver nanoparticles owing to deposi-
tion of copper nanoparticles onto the surface of the silver nano-
particles. The absorbance at 407 nm was damped by 45% in spite
of the presence of Cu/Ag nanoparticles found by 17% in the
HRTEM image of Figure 1b. The change of the plasmon absorp-
tion seemed complicated and could not be simply explained.
Dash dot line in Figure 2 shows the UV–vis spectrum of the
sample S3. The absorption at 400 nm was damped. Furthermore,
the back ground rose up in the all region caused by scattering
of probe light for measuring the UV–vis spectrum, suggesting
the formation of large particles due to the aggregation.
3.0
2.0
1.0
0.0
200
400
600
800
1000
Wavelength / nm
Figure 2. UV–vis spectra of the samples. Silver myristate
was heated for 5 min at 413 K in 1-heptanol (solid line). Copper
myristate was added into silver nanoparticles solution and
then heated for 5 min (dash line) and 10 min (dash dot line).
The sample of silver was diluted by twentieth with hexane.
In conclusion, we have succeeded in preparing Ag core–Cu
shell nanoparticles by an MW-assisted alcohol reduction process
in a short period of time. MW-heating is advantageous for this
process due to its rapid heating mode.
(Figure 1a, inset). This result agreed with a result of Wada
et al.12 showing preparation of silver nanoparticles having
5 nm in size.
An HRTEM image of the sample S2 is shown in Figure 1b.
Nanoparticles having 10–40 nm in size were observed. Figure 1c
shows a magnified image of the particles in Figure 1b. A particle
having 5 nm was observed with a strong contrast inside the par-
ticle having 15 nm in size. Thickness of the shell was 5 nm. This
inside nanoparticle should be a silver nanoparticle having 5 nm
in size because this diameter of the silver nanoparticles was
close to those observed in Figure 1a. Furthermore, the shell
surrounding the silver nanoparticle should be copper because
of lighter contrast than the contrast of the inside nanoparticle.
The different contrasts can be understood by difference of atom-
ic numbers: a copper atom (Cu, 29) is lighter than a silver atom
(Ag, 47). Therefore, it has been concluded that the nanoparticle
in Figure 1c is an Ag core–Cu shell nanoparticle having 15 nm in
size successfully prepared by step-by-step reduction of the two
precursors. Additionally, nanoparticles of the sample S3 were
aggregated forming the large particles over 100 nm in size (see
Supporting Information Figure S1).14
A lot of Cu/Ag nanoparticles were observed in an image
of the wide sight (Figure 1b) as indicated by the arrows. The
number of the nanoparticles having the core–shell structure
was counted to be 18 in 107. That is, the Cu/Ag nanoparticles
were prepared by 17%. From the contrast of each particle, other
nanoparticles having homogeneous contrast differing from clear
contrast of the Cu/Ag nanoparticles were found. It was difficult
to distinguish between silver and copper nanoparticles from the
fringes of the particles because of small difference of lattice
One of the authors (T. Nakamura) thanks the financial
support of the 21st COE of Osaka University. This work was
supported by a Grant for Practical Application of University
Research and Development Results under the Matching Fund
Method of NEDO, Creation and Support Program for Start-ups
from Universities of JST, and a Grant-in-Aid for Scientific
Research on Priority Areas (417) (No. 17029038) from MEXT
of the Japanese Government.
References and Notes
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UV–vis spectrum of the sample S1 is shown in Figure 2 (sol-
id line). The UV–vis spectrum had a peak at 407 nm attributed to
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Published on the web (Advance View) December 16, 2006; doi:10.1246/cl.2007.154