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Chemistry Letters Vol.38, No.6 (2009)
Preparation of Cu@Ag Core–Shell Nanoparticles Using
a Two-step Polyol Process under Bubbling of N2 Gas
Masaharu Tsuji,ꢀ1 Sachie Hikino,1 Yoshiyuki Sano,2 and Misao Horigome2
1Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580
2Color Science Laboratory, Corporate R&D Department, DIC Corporation, Sakura 285-8668
(Received January 20, 2009; CL-090064; E-mail: tsuji@cm.kyushu-u.ac.jp)
Cu core–Ag shell nanoparticles, denoted as Cu@Ag, were
solved in 27.5 mL of EG solution by continuous stirring in a 100-
mL three-necked flask, which was heated to 110 ꢁC and main-
tained at that temperature for 1 h by bubbling of N2 to remove
O2 dissolved in the solution. Then the solution was heated to
prepared using a two-step polyol reduction process under bub-
bling N2 gas. Formation of Cu@Ag particles with an average
size of ca. 80 nm was confirmed using energy-dispersed X-ray
spectroscopic (EDS) measurements. The Cu particle oxidation
was suppressed greatly by Ag shell covering.
ꢁ
.
196 C; 2.5 mL of 48 mM Cu(OAc)2 H2O in EG was added
drop-by-drop using a syringe pump at an injection rate of 2 mL/
min. The solution was maintained at 196 ꢁC for 2.5 min to pre-
pare Cu core nanoparticles. Subsequently, 4 mL of 15 mM
AgNO3 was injected at an injection rate of 6 mL/min to prepare
In recent years, metal-coated colloidal core–shell composite
particles have received intensive attention for potential applica-
tion as catalysts, sensors, and substrates for surface-enhanced
Raman scattering.1 In contrast to extensive studies of core–shell
particle preparation involving Au, Ag, Pd, and Pt, little work has
been done for Cu.2 Recently, Ag@Cu particles were prepared
using laser ablation in ethanol solvent,3 thermal evaporation un-
der ultrahigh vacuum,4 and microwave-assisted alcohol reduc-
tion processes.5 Although Cu@Ag core–shell particles were also
prepared using two vapor phase deposition techniques,6 they
have not been obtained using a chemical method in solution.
Preparation of Cu@Ag particles in solution is expected to be
more difficult than that of Ag@Cu because the displacement re-
action occurs by the addition of Agþ ions to the Cu core solution
resulting from the higher standard electrode potential of Agþ/
Ag (+0.80 V) than that of Cu2þ/Cu (+0.34 V). Here, the stand-
ard hydrogen electrode is used as a reference electrode.
In this study, we prepared Cu@Ag nanoparticles using a
two-step polyol reduction method. The Cu core and Ag shell
structure formation was confirmed using transmission electron
microscope (TEM)–EDS measurements. Although Cu nanopar-
ticles are anticipated for use as new electronic materials because
of their lower cost than either Au or Ag, low oxidation resistance
limits actual application. We show here that Ag shell coatings
can greatly suppress the oxidation rate of the Cu component in
solution.
.
the Ag shell. The final concentrations of Cu(OAc)2 H2O,
AgNO3, and PVP in EG were 3.53, 1.76, and 882 mM, respec-
tively. The reagent solution was cooled rapidly using a water
bath. During Ag shell fabrication, N2 was bubbled continuously.
Product solutions were centrifuged at 13000 rpm three times
for 15 min each time. The precipitates were collected then redis-
persed in deionized water. For TEM (JEM-2010 and 2100F;
JEOL) observations, samples were prepared by dropping colloi-
dal solutions of the products onto carbon-coated Cu or Au grids.
Absorption spectra of the product solutions were measured using
a spectrometer (UV-3600; Shimadzu Corp.) in the UV–visible
(vis) region.
Figures 1a and 1b depict typical TEM images of products
obtained after preparation of Cu core seeds and Ag shells, where
spherical particles with average diameters of 59 ꢂ 11 and
81 ꢂ 13 nm were obtained, respectively. The greater average di-
ameter implies that Cu@Ag particles might have been formed.
Then TEM–EDS was used (Figure 2) for confirmation. Although
distinguishing between Cu and Ag from the TEM image contrast
depicted in Figure 2a is difficult, EDS data shown in Figures 2b–
2d show that Cu@Ag particles were prepared in a high yield (ca.
80%). Figure 3 portrays distributions of the Cu and Ag compo-
nents along the cross section line shown in Figure 2b. These data
imply that uniform Ag shells were overgrown on spherical Cu
cores. In separate experiments for preparation of Cu and Ag
nanoparticles in EG solutions in the presence of PVP, results
showed that the reduction of Cu2þ was much slower than that
The following techniques and procedures were used to over-
come the difficulties described above for preparation of Cu@Ag
particles. (a) Copper is easily oxidized by O2 dissolved in the
solution. Therefore, N2 was bubbled during preparation of Cu
cores and Ag shells to suppress oxidation. (b) Because of a re-
placement reaction occurring between Cu and Agþ,
(a)
(b)
Cu(s) þ 2Agþ ! Cu2þ þ 2Ag(s)
ð1Þ
Cu cores were partly dissolved as Cu2þ. To suppress such a dis-
placement reaction, Ag shell preparation was conducted rapidly
at a high temperature because the reduction of Agþ occurs more
rapidly than reaction 1. (c) Drop-by-drop injection was used to
overgrow Ag shells on Cu cores. Using this technique, supersa-
turation of Ag0 leading to nucleation and growth of Ag nanopar-
ticles was suppressed.
50 nm
Figure 1. TEM images of (a) Cu and (b) Cu@Ag nanoparticles prepared
using polyol reduction in EG.
In actual experiments, 3.33 g of PVP (MW: 10000) was dis-
Copyright Ó 2009 The Chemical Society of Japan
Tsuji, Masaharu
