Copper Nanoparticles for Printed Electronics: Routes Towards Achieving Oxidation Stability

Abstract

In the past few years, the synthesis of Cu nanoparticles has attracted much attention because of its huge potential for replacing expensive nano silver inks utilized in conductive printing. A major problem in utilizing these copper nanoparticles is their inherent tendency to oxidize in ambient conditions. Recently, there have been several reports presenting various approaches which demonstrate that copper nanoparticles can resist oxidation under ambient conditions, if they are coated by a proper protective layer. This layer may consist of an organic polymer, alkene chains, amorphous carbon or graphenes, or inorganic materials such as silica, or an inert metal. Such coated copper nanoparticles enable achieving high conductivities by direct printing of conductive patterns. These approaches open new possibilities in printed electronics, for example by using copper based inkjet inks to form various devices such as solar cells, Radio Frequency Identification (RFID) tags, and electroluminescence devices. This paper provides a review on the synthesis of copper nanoparticles, mainly by wet chemistry routes, and their utilization in printed electronics.

Full text

Materials 2010, 3, 4626-4638; doi:10.3390/ma3094626
materials
ISSN 1996-1944
www.mdpi.com/journal/materials
Review
Copper Nanoparticles for Printed Electronics: Routes Towards
Achieving Oxidation Stability
Shlomo Magdassi *, Michael Grouchko and Alexander Kamyshny
Casali Institute of Applied Chemistry, the Institute of Chemistry, the Hebrew University of Jerusalem,
Jerusalem 91904, Israel; E-Mail: magdassi@cc.huji.ac.il
Received: 22 July 2010; in revised form: 18 August 2010 / Accepted: 1 September 2010 /
Published: 8 September 2010
Abstract: In the past few years, the synthesis of Cu nanoparticles has attracted much
attention because of its huge potential for replacing expensive nano silver inks utilized in
conductive printing. A major problem in utilizing these copper nanoparticles is their
inherent tendency to oxidize in ambient conditions. Recently, there have been several
reports presenting various approaches which demonstrate that copper nanoparticles can
resist oxidation under ambient conditions, if they are coated by a proper protective layer.
This layer may consist of an organic polymer, alkene chains, amorphous carbon or
graphenes, or inorganic materials such as silica, or an inert metal. Such coated copper
nanoparticles enable achieving high conductivities by direct printing of conductive
patterns. These approaches open new possibilities in printed electronics, for example by
using copper based inkjet inks to form various devices such as solar cells, Radio Frequency
Identification (RFID) tags, and electroluminescence devices. This paper provides a review
on the synthesis of copper nanoparticles, mainly by wet chemistry routes, and their
utilization in printed electronics.
Keywords: printed electronics; copper nanoparticles; conductive inks
1.Introduction
The use of silver nanoparticles (NPs) in conductive inks and their printing by inkjet technology has
been known for years [1]. However, the very high cost of silver limits wide industrial application.
Since copper is much cheaper, but possesses a very high conductivity (only 6% less than that of Ag),
Cu NPs can be considered as a replacement for silver NPs.
OPEN ACCESS

Materials 2010, 3 4627
Therefore, during recent years, the synthesis of Cu NPs has become of great interest from a
scientific as well as an industrial point of view, due to its huge potential for replacing the expensive
nano silver ink. Since the 1990s, many attempts have been made to synthesize nano copper by wet
chemistry, as well as by gas or solid phase methods. Among these, are the sonochemical method [2],
microemulsion techniques [3-5], polyol processes [6-8], radiation methods [9-13], thermal
reduction [14], reducing flame synthesis [15,16], metal vapor synthesis [17], vacuum vapor
deposition [18,19] and chemical reduction in solution [20,21]. An example of the gas phase synthesis
route is the pioneering work of Luechinger et al. [16], in which various metal nanoparticles were
prepared by the flame-aerosol route, which is based on spraying a metal precursor yielding metal
nanoparticles coated by a carbon layer.
The present review focuses mainly on wet chemical processes which are based on the reduction of
metal ions by reducing agents in liquid media. This route is very convenient since it may result in
various dispersions with controllable particle characteristics, by numerous variable experimental
parameters. The main parameters are the type and concentration of reagents, their redox potentials and
rate of addition, type and concentration of protective agents, temperature, pH and the addition of
preformed seeds [22]. Additional advantages of these processes are that the metallic NPs can be
synthesized with essentially low-cost equipment and in large volumes.
The driving force of the ion metal reduction (mOx
n+
+ Red = mOx
0
+ Red
mn+
) is the difference in
the redox potentials of the reagents (∆E) which correlates with the Gibb's free energy of the reaction at
standard conditions:
∆G
0
= −RTlnK = −nF∆E
0
(where n = the number of electrons in a reaction equation, K = equilibrium constant,
F = Farady’s constant).
The reduction is thermodynamically possible only if the redox potential of the reducer is more
negative than that of the oxidizer (metal precursor) and, respectively, ΔE is positive. This difference
should be larger than 0.3–0.4 V, otherwise the reduction will proceed too slowly and may not result in
the formation of NPs [23]. Therefore, the synthesis of stable copper NPs presents an additional
challenge in relation to gold and silver, which is caused by a relatively low Cu
0
/Cu
2+
redox
potential (+0.34 V).
As presented in Table 1, not too many papers have been published thus far on the various ways of
synthesizing copper NPs by chemical reduction in solution.
Table 1. A list of published methods to synthesize copper NPs by chemical reduction in solution.
#
Solvent
Cu
precursor
Reducer
Stabilizer
Particle
size
Ref.
1
Water
CuSO
4
Sodium
borohydrate
SDS
2–10 nm
Lisiecki
1996 [24]
2
Water + n-
hexanol/cyclohexane
CuCl
2
Sodium
borohydrate
TX-100
5–15 nm
Qi 1997
[5]
3
Water+ n-Heptane,
n-Octane, n-Hexane
CuCl
2
Sodium
borohydrate
HDEHP
40–80 nm
Song 2004
[25]

Materials 2010, 3 4628
Table 1. Cont.
4
Isooctane
Cu(AOT)
2
Hydrazine
Na AOT
10–30 nm
Salzemann
2004 [26]
5
Water
CuCl
2
Hydrazine
CTAB
5 nm
Wu 2004
[27]
6
Ethylene Glycol
CuSO
4
Ascorbic acid
PVP 40
100 nm
(cubes)
Wang
2006 [28]
7
Water
Cu(NO
3
)
2
Ascorbic acid
PVP 58
3 nm
Wu 2006
[29]
8
Di-Ethylene Glycol
CuSO
4
Sodium
phosphinate
PVP 40
45 nm
Park 2007
[7]
9
Octyl ether
Cu(acac)
2
1,2-
hexadecanediol
Oleic acid,
oleyl amine
5–200 nm
Mott 2007
[30]
10
Toluene + water
CuCl
2
Sodium
borohydrate
Lauric acid +
TOAB
3 nm
Kanninen
2008 [31]
11
Di-Ethylene Glycol
CuCl
2
SFS
PVP
50 nm
Khanna
2009 [32]
12
Water
Cu(NO
3
)
2
Hydrazine
PAA Na
20–100
nm
Grouchko
2009 [21]
SFS = sodium formaldehyde sulfoxylate; HDEHP = Bis(ethylhexyl)hydrogen phosphate; SDS = Sodium
dodecyl sulfate; Na AOT = Sodium bis(2-ethylhexyl) sulfosuccinate; CTAB = Cetyl trimethylammonium
bromide; PVP = Polyvinylpyrrolidone; TOAB = Tetraoctylammonium bromide; PAA Na = Polyacrylic acid
sodium salt.
However, a major problem in utilizing these copper NPs is their inherent tendency to oxidize in
ambient conditions [33]. Yanase et al. [34,35] examined the mechanism and kinetics of oxidation and
reduction of copper NPs in oxygen and hydrogen atmospheres by using UV-vis spectroscopy. They
found a fast oxidation process from Cu to CuO
0.67
and a slow one from CuO
0.67
to CuO, confirming the
formerly suggested mechanism reported by Wieder et al. [36]. From the point of view of conductive
printing applications, the presence of copper oxides on the surface of NPs, has two negative
consequences: it increases the required sintering temperature, and reduces the electrical conductivity.
Only a limited number of reports have attempted to address the oxidation problem, which in general
is based on minimizing the exposure of the copper NPs to oxygen, by a protective layer composed of a
second material at the surface of the particles.
These materials can be divided into four groups: (1) carbon-based materials (carbon and graphene),
(2) surfactants and polymers, (3) silica, (4) metals.
In this paper we describe the approaches that are meant to achieve air stable copper NPs.
2. Amorphous Carbon and Graphene Based Materials
Li et al.[37] synthesized copper NPs by means of solid state reduction of cupric carbonate by
glucose at high temperature. The fundamental reactions can be summarized as follows:

Materials 2010, 3 4629
COCuCCuO
OHCOHOHC
OHxCOCuOOxHOHCuCuCO
C
C
C
o
o
o
 
 
 
350
2
350
26126
22
350
223
76
)1(2)(
It was found that as the glucose load increases, the carbon layer on the surface of the NPs becomes
thicker, and stable copper NPs, coated by amorphous carbon, are obtained.
Luechinger et al. [16] has demonstrated how deposition of bi- or tri-layers graphene on copper can
be realized on a large scale, and how it enables full protection of the copper metal core under humid
air. The protected copper was synthesized by the reducing flame technique [15,38] which leads to the
formation of ~50 nm copper NPs coated with a ~3 nm layer of graphene. The main advantage of this
method is that it leads to coated nanoparticles in a one-step process. Figure 1a presents a high
resolution transmission electron microscope (HR-TEM) image of copper NPs coated by graphene, and
a corresponding schematic illustration. It was found that the graphene-coated copper NPs are stable to
oxidation up to 165 °C (thermogravimetric analysis, Figure 1b). The formulation of these NPs in an
aqueous ink-jet ink and printing with that ink enabled conductive patterns. As a highly conductive
material, graphene is not expected to decrease the pattern conductivity, even though, the obtained
conductivity was found to be five orders of magnitude lower than that of bulk copper.
Figure 1. (a) Transmission electron microscope (TEM) image of a single copper NP with a
thick graphene layer of 3 nm (left) and a corresponding schematic illustration (right);
(b) Thermogravimetric analysis (TGA) confirms the thermal stability of these NPs up to
165 °C. Reprinted from reference [16], with permission from IOP Publishing LTD.

Materials 2010, 3 4630
3. Surfactants and Polymers
Another way to address this oxidation problem is to coat the obtained copper NPs with a dense
layer of capping agents. Such molecules are usually present at the surface of the NPs to prevent
aggregation and agglomeration in dispersions. However, in order to minimize the penetration of
oxygen to the NPs surface, a dense structure of these molecules is required.
Ang et al. [39] formed 5 to 10 nm Cu NPs coated by alkanethiols, by reducing the copper nitrate in
presence of C
8
to C
12
alkanethiols in ethanol. In a typical procedure copper nitrate was dissolved in
absolute alcohol and the respective alkanethiol was added. Then, a solution of sodium borohydride was
added dropwise into the reaction mixture. The reaction was maintained at room temperature with
vigorous stirring carried out in an inert N
2
atmosphere. A brownish precipitate was isolated by
centrifugation, washed repeatedly with deionized water, toluene, ethanol, and acetone, and vacuum
dried [39]. It was found that such a layer is a good barrier that protects the copper NPs from oxidation
for at least six months. Kannienen et al. [31] used the same approach with a wider range of chain
lengths (C
6
to C
18
), and compared their performance to that of lauric (C
12
) and oleic (C
18
) acids. Thiols
were found to improve oxidation resistance already at a 1:1 ratio, while oxidation resistance improved
with increased chain length, but not with increased concentration. Oleic acid was found to increase
oxidation resistance slightly in comparison to lauric acid. In summary, it was found that oleic
acid-capped particles were clearly superior to thiol-stabilized particles.
More stable dispersions were obtained by the use of polymers as capping agents. For example,
Jeong et al. [40] followed by Engels et al. [41] evaluated the effect of the well established stabilizer,
poly(N-vinylpyrrolidone) (PVP), at various molecular weights (10,000, 29,000 and 40,000 g/mol).
Jeong et al. found that besides the effect of the PVP molecular weight on the NPs size (the particle size
increases as the PVP molecular weight increases), the minimum thickness of the amorphous CuO and
chemisorbed PVP layers was 1.6 nm, obtained using PVP with a molecular weight of 40000 g/mol.
The molecular weight of the PVP capping agent determines their conformation when adsorbed on the
Cu surface, which significantly influences the formation of the surface oxide layer. In addition, their
results confirm that the thickness of the surface oxide layer is the predominant factor which determines
the electrical conductivity of the Cu film obtained after sintering. As presented in Figure 2, when the
molecular weight of PVP increases, the obtained resistivity decreases for all temperature ranges down
to six times the resistivity of bulk copper (under vacuum). Lee et al. [8] used the same polymer,
PVP 40, to form a conductive copper ink, and to ink-jet print with it to form conductive patterns.
According to TGA analysis, the obtained copper patterns were stable to oxidation up to 180 °C .
However, highly conductive patterns (45% of bulk copper) were demonstrated only after heating to
200 °C for 60 min under hydrogen. Kobayashi et al. [42] used polypyrrole (PPY), a conductive polymer,
to coat the copper NPs through the polymerization of pyrrole in aqueous solution. This method produced
PPY-coated copper NPs which were chemically stable, even under air, for a prolonged period of time.
However, no data was shown regarding the stability of these NPs at elevated temperatures.
Pulkkinen et al. [43] evaluated two different polymers, polyethylene imine (PEI) and
tetraethylenepentamine (TEPA), as protecting agents for copper NPs. However, even after pressing and
heating to 250 °C , the obtained resistivities were 3 to 5 orders of magnitude higher than bulk copper.

Materials 2010, 3 4631
Figure 2. Resistivity of the Cu conductive film as a function of heat treatment temperature
for various PVP MWs. The ink-jet printed Cu nanoparticulate films were heat-treated at
various temperatures between 200 °C and 325 °C under vacuum. Reprinted from [40],
Reproduced with permission of copyright Wiley-VCH Verlag GmbH and Co. KGaA.
4. Silica Coating
Only one attempt has been made to form an inorganic protection layer on copper NPs.
Kobayashi et al. [44] reported his success in coating copper NPs by a thin layer of silica. The x-ray
diffraction (XRD) analysis in Figure 3 shows that while the uncoated copper NPs are accompanied by
clear Cu
2
O peaks (3a), the coated particles have only low Cu
2
O peaks (3b), even after one month (3c).
However, in view point of conductive printing, the presence of silica, an insulating material with high
melting point, is a major obstacle in obtaining a continuous conductive pattern.
Figure 3. XRD patterns of (a) uncoated Cu NPs; (b) Cu/SiO
2
NPs; (c) Cu/SiO
2
NPs one
month after preparation. (●) Metallic Cu, ( ) Cu
2
O. Reprinted from reference [44] with
permission from Elsevier.

Materials 2010, 3 4632
5. Metallic Coating
Core-shell structured, bimetallic NPs can be prepared by the successive reduction of one metal over
the nuclei of another [45]. The preparation and characterization of bimetallic NPs from various
combinations of noble metals have been the subject of numerous papers, examples of which include
the Au-Pd [46], Au-Pt [47], Ag-Pd [48], Ag-Pt [49], Ag-Cu [50], and Ag-Au [51] systems. Until now,
many methods such as electroless plating [52], surface seeding [53], and self-assembly [54], have been
explored to fabricate metallic shells on metallic particles. Generally, such NPs are prepared by the
successive reduction of one metal ion over the core of another [55]. This process often leads to the
formation of a new nuclei of the second metal in solution (in addition to a shell around the first metal
core) and is clearly undesirable from the application point of view. Satry et al. [56] overcame this
problem in a gold-silver core-shell system by immobilizing a reducing agent on the surface of the core
metal which, when exposed to the second metal ions, reduces them, thereby leading to the formation of
a thin metallic shell.
Schmid et al. [46] used hydroxylamine as a reducing agent that is capable of reducing the metal ion,
but the reduction was dramatically accelerated by the Au surfaces. As a result, in the presence of
preformed gold NPs, no new particle nucleation occurred in the solution, and all the added metal ions
served in the formation of the shell.
Furthermore, copper-silver core-shell NPs have been described in several studies, using
electrodeposition [57], thermal evaporation techniques under ultrahigh vacuum [50], and ion exchange
in soda lime glass matrix [58]. Lee et al. [59] demonstrated the formation of various core-shell NPs
through the transmetalation (redox) process. It should be noted that one disadvantage of this method is
that it requires several steps. In the first step, the core particles are synthesized. Then, the core material
serves as a reducer for the reduction of shell atoms on the core particle. The core surface atoms are
oxidized (and are released to the solution as ions), and the shell material ions are reduced. In our
previous report [60], we used this approach to synthesize a copper-silver core-shell structure in a two
step process. As schematically presented in Figure 4; in the first step an aqueous dispersion of Cu NPs
is prepared by reducing Cu(NO
3
)
2
with hydrazine hydrate excess in the presence of polyacrylic acid
sodium salt as a polymeric stabilizer [21]. The large excess of hydrazine prevents oxidation of the Cu
NPs in the aqueous dispersion, but only if it is kept in closed vials. In the second step, the excess
hydrazine is consumed or washed out, and silver salt is added. During the transmetalation reaction, the
reduction of silver ions by the copper metal takes place directly on the surface of CuNPs (Ag to Cu
atomic ratio of 0.12), thus creating a silver shell on the copper core.
Figure 4. Schematic illustration of a single Cu NP synthesis and the formation of a silver
shell by the transmetalation reaction. The surface copper atoms serve as reducing agents
for the silver ions.

Materials 2010, 3 4633
As follows from the TEM and scanning electron microscopy (SEM) analyses (Figure 5 (a) and (b),
respectively), the size distribution of the obtained Cu-Ag core-shell NPs is in the range of 10 to 50 nm.
The thermal stability of the obtained NPs was measured by TGA (Figure 5 (c)) confirming the
inertness of the copper core up to 187 °C where copper oxides start to appear—leading to a mass
increase of about 5 wt %.
Figure 5. (a) TEM and (b) SEM images of the obtained copper-silver core-shell NPs;
(c) TGA of the copper-silver core-shell NPs under air atmosphere.
By using these oxidation stable Cu NPs, it was possible to further evaluate the use of these NPs in
inkjet printing of conductive patterns. An inkjet water-based formulation, which contained 25 wt %
Cu-Ag core-shell NPs, was printed by a Lexmark office printer. An example of a Radio Frequency
Identification (RFID) antenna, printed on an inkjet photo paper, is presented in Figure 6. However, due
to the presence of organic stabilizer at the NPs surface, the resistivity of the obtained patterns was very
high. To obtain highly conductive patterns, a sintering process should be carried out. In order to
overcome the temperature sensitivity of the substrate (paper, plastics), a new room temperature
sintering process was developed [61,62], based on the neutralization of stabilizing charges at the NPs
surface. The sheet resistance measurements after the sintering process revealed, as expected, that the
sintering is accompanied by a drastic decrease of resistance, down to values of 3 (±0.2) Ω/square. It
should be emphasized that such low sheet resistances were reported until now only for metallic

Materials 2010, 3 4634
patterns which were heated at temperatures of ≥150 °C for prolonged time, while in the present study it
was achieved spontaneously at room temperature.
Figure 6. A flexible RFID antenna printed using copper-silver core-shell ink on inkjet
photo paper.
6. Summary
In summary, there are only a few reports which present various approaches for obtaining copper
nanoparticles that can resist oxidation under ambient conditions. These approaches are based on
coating copper NPs by a proper protective layer, thus enabling copper to be used as a low-cost
non-noble metal. This layer may consist of an organic polymer, alkane chains, amorphous carbon or
graphenes, or inorganic materials such as silica, or an inert metal. Several reports have shown that the
formulation of these NPs as conductive inkjet inks, and their printing, can yield conductive patterns
which are stable to oxidation for at least several months. Naturally, the formation of a low or
non-conductive shell (polymers, surfactants, carbon or silica) on the copper NPs has a negative effect
on the obtained conductivity of the printed pattern which is composed of the copper nanoparticles
(except for cases in which the shell is graphene or a second metal). This inherent contradiction
between conductivity and coating layers is well addressed in the literature. To the best of our
knowledge, so far oxidation resistance accompanied by high conductivity was achieved by the
formation of a PVP shell [40] or a silver shell [60] on the copper NPs. This opens new possibilities for
applications in printed electronics, such as solar cells, RFID tags and electroluminescence
(EL) devices.
Acknowledgements
This research was supported by the European Community’s Seventh Framework Program, through
Collaborative project LOTUS, Project Number 248816.
References
1. Magdassi, S.; Bassa, A.; Vinetsky, Y.; Kamyshny, A. Silver nanoparticles as pigments for
water-based ink-jet inks. Chem. Mater. 2003, 15, 2208-2217.

Materials 2010, 3 4635
2. Kumar, R.V.; Mastai, Y.; Diamant, Y.; Gedanken, A. Sonochemical synthesis of amorphous Cu
and nanocrystalline Cu
2
O embedded in a polyaniline matrix. J. Mater. Chem. 2001, 11,
1209-1213.
3. Lisiecki, I.; Pileni, M.P. Synthesis of copper metallic clusters using reverse micelles as
microreactors. JACS 1993, 115, 3887-3896.
4. Pileni, M.P.; Ninham, B.W.; Gulik-Krzywicki, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. Direct
relationship between shape and size of template and synthesis of copper metal particles.
Adv. Mater. 1999, 11, 1358-1362.
5. Qi, L.M.; Ma, J.M.; Shen, J.L. Synthesis of copper nanoparticles in nonionic water-in-oil
microemulsions. J. Colloid Interface Sci. 1997, 186, 498-500.
6. Martinez-Miranda, L.J.; Li, Y.Q.; Chow, G.M.; Kurihara, L.K. A depth profile study of the
structure and strain distribution in chemically grown Cu films on AlN. Nanostruct. Mater. 1999,
12, 653-656.
7. Park, B.K.; Jeong, S.; Kim, D.; Moon, J.; Lim, S.; Kim, J.S. Synthesis and size control of
monodisperse copper nanoparticles by polyol method. J. Colloid Interface Sci. 2007, 311,
417-424.
8. Lee, Y.; Choi, J.R.; Lee, K.J.; Stott, N.E.; Kim, D. Large-scale synthesis of copper nanoparticles
by chemically controlled reduction for applications of inkjet-printed electronics. Nanotechnology
2008, 19, 415604.
9. Dey, G.R. Reduction of the copper ion to its metal and clusters in alcoholic media: A radiation
chemical study. Rad. Phys. Chem. 2005, 74, 172-184.
10. Qin, C.; Coulombe, S. Synthesis of organic layer-coated copper nanoparticles in a dual-plasma
process. Mater. Lett. 2006, 60, 1973-1976.
11. Zhou, R.M.; Wu, X.F.; Hao, X.F.; Zhou, F.; Li, H.B.; Rao, W.H. Influences of surfactants on the
preparation of copper nanoparticles by electron beam irradiation. Nucl. Instrum. Methods Phys.
Res. Sect. B 2008, 266, 599-603.
12. Yeh, M.S.; Yang, Y.S.; Lee, Y.P.; Lee, H.F.; Yeh, Y.H.; Yeh, C.S. Formation and characteristics
of Cu colloids from CuO powder by laser irradiation in 2-propanol. J. Phys. Chem. B 1999, 103,
6851-6857.
13. Joshi, S.S.; Patil, S.F.; Iyer, V.; Mahumuni, S. Radiation induced synthesis and characterization of
copper nanoparticles. Nanostruct. Mater. 1998, 10, 1135-1144.
14. Dhas, N.A.; Raj, C.P.; Gedanken, A. Synthesis, characterization, and properties of metallic copper
nanoparticles. Chem. Mater. 1998, 10, 1446-1452.
15. Athanassiou, E.K.; Grass, R.N.; Stark, W.J. Large-scale production of carbon-coated copper
nanoparticles for sensor applications. Nanotechnology 2006, 17, 1668-1673.
16. Luechinger, N.A.; Athanassiou, E.K.; Stark, W.J. Graphene-stabilized copper nanoparticles as an
air-stable substitute for silver and gold in low-cost ink-jet printable electronics. Nanotechnology
2008, 19, 445201, DOI: 10.1088/0957-4484/19/44/445201.
17. Vitulli, G.; Bernini, M.; Bertozzi, S.; Pitzalis, E.; Salvadori, P.; Coluccia, S.; Martra, G.
Nanoscale copper particles derived from solvated Cu atoms in the activation of molecular oxygen.
Chem. Mater. 2002, 14, 1183-1186.

Materials 2010, 3 4636
18. Liu, Z.W.; Bando, Y. A novel method for preparing copper nanorods and nanowires. Adv. Mater.
2003, 15, 303-305.
19. Casella, I.G.; Cataldi, T.R.I.; Guerrieri, A.; Desimoni, E. Copper dispersed into polyaniline films
as an amperometric sensor in alkaline solutions of amino acids and polyhydric compounds. Anal.
Chim. Acta 1996, 335, 217-225.
20. Huang, H.H.; Yan, F.Q.; Kek, Y.M.; Chew, C.H.; Xu, G.Q.; Ji, W.; Oh, P.S.; Tang, S.H.
Synthesis, characterization, and nonlinear optical properties of copper nanoparticles. Langmuir
1997, 13, 172-175.
21. Grouchko, M.; Kamyshny, A.; Ben-Ami, K.; Magdassi, S. Synthesis of copper nanoparticles
catalyzed by pre-formed silver nanoparticles. J. Nanopart. Res. 2009, 11, 713-716.
22. Kamyshny, A.; Magdassi, S. Colloid Stability: The Role of Surface Forces; Wiley-VCH:
Weinheim, Germany, 2007.
23. Goia, D.V.; Matijevic, E. Preparation of monodispersed metal particles. New J. Chem. 1998, 22,
1203-1215.
24. Lisiecki, I.; Billoudet, F.; Pileni, M.P. Control of the shape and the size of copper metallic
particles. J. Phys. Chem. 1996, 100, 4160-4166.
25. Song, X.Y.; Sun, S.X.; Zhang, W.M.; Yin, Z.L. A method for the synthesis of spherical copper
nanoparticles in the organic phase. J. Colloid Interface Sci. 2004, 273, 463-469.
26. Salzemann, C.; Lisiecki, L.; Urban, J.; Pileni, M.P. Anisotropic copper nanocrystals synthesized
in a supersaturated medium: Nanocrystal growth. Langmuir 2004, 20, 11772-11777.
27. Wu, S.H.; Chen, D.H. Synthesis of high-concentration Cu nanoparticles in aqueous CTAB
solutions. J. Colloid Interface Sci. 2004, 273, 165-169.
28. Wang, Y.H.; Chen, P.L.; Liu, M.H. Synthesis of well-defined copper nanocubes by a one-pot
solution process. Nanotechnology 2006, 17, 6000-6006.
29. Wu, C.W.; Mosher, B.P.; Zeng, T.F. One-step green route to narrowly dispersed copper
nanocrystals. J. Nanopart. Res. 2006, 8, 965-969.
30. Mott, D.; Galkowski, J.; Wang, L.Y.; Luo, J.; Zhong, C.J. Synthesis of size-controlled and shaped
copper nanoparticles. Langmuir 2007, 23, 5740-5745.
31. Kanninen, P.; Johans, C.; Merta, J.; Kontturi, K. Influence of ligand structure on the stability and
oxidation of copper nanoparticles. J. Colloid Interface Sci. 2008, 318, 88-95.
32. Khanna, P.K.; More, P.; Jawalkar, J.; Patil, Y.; Rao, N.K. Synthesis of hydrophilic copper
nanoparticles: effect of reaction temperature. J. Nanopart. Res. 2009, 11, 793-799.
33. Xia, X.P.; Xie, C.S.; Cai, S.Z.; Yang, Z.H.; Yang, X.L. Corrosion characteristics of copper
microparticles and copper nanoparticles in distilled water. Corros. Sci. 2006, 48, 3924-3932.
34. Yanase, A.; Komiyama, H. In situ observation of oxidation and reduction of small supported
copper particles using optical-absorption and X-ray-diffraction. Surf. Sci. 1991, 248, 11-19.
35. Yanase, A.; Komiyama, H. Real-time optical observation of morphological change of small
supported copper particles during redox treatments. Surf. Sci. 1991, 248, 20-26.
36. Wieder, H.; Czanderna, A.W. The Oxidation of Copper Films to CuO
0.67
. J. Phys. Chem. 1962,
66, 816-821.
37. Li, J.; Liu, C.Y. Carbon-coated copper nanoparticles: synthesis, characterization and optical
properties. New J. Chem. 2009, 33, 1474-1477.

Materials 2010, 3 4637
38. Grass, R.N.; Stark, W.J. Gas phase synthesis of fcc-cobalt nanoparticles. J. Mater. Chem. 2006,
16, 1825-1830.
39. Ang, T.P.; Wee, T.S.A.; Chin, W.S. Three-dimensional self-assembled monolayer (3D SAM) of
n-alkanethiols on copper nanoclusters. J. Phys. Chem. B 2004, 108, 11001-11010.
40. Jeong, S.; Woo, K.; Kim, D.; Lim, S.; Kim, J.S.; Shin, H.; Xia, Y.; Moon, J. Controlling the
thickness of the surface oxide layer on Cu nanoparticles for the fabrication of conductive
structures by ink-jet printing. Adv. Func. Mater. 2008, 18, 679-686.
41. Engels, V.; Benaskar, F.; Jefferson, D.A.; Johnson, B.F.G.; Wheatley, A.E.H. Nanoparticulate
copper—routes towards oxidative stability. Dalton Trans. 2010, 39, 6496-6502.
42. Kobayashi, Y.; Ishida, S.; Ihara, K.; Yasuda, Y.; Morita, T.; Yamada, S. Synthesis of metallic
copper nanoparticles coated with polypyrrole. Colloid. Polym. Sci. 2009, 287, 877-880.
43. Pulkkinen, P.; Shan, J.; Leppanen, K.; Kansakoski, A.; Laiho, A.; Jarn, M.; Tenhu, H.
Poly(ethylene imine) and tetraethylenepentamine as protecting agents for metallic copper
nanoparticles. ACS Appl. Mater. Interface. 2009, 1, 519-525.
44. Kobayashi, Y.; Sakuraba, T. Silica-coating of metallic copper nanoparticles in aqueous solution.
Colloid. Surface. A 2008, 317, 756-759.
45. Ah, C.S.; Do Hong, S.; Jang, D.J. Preparation of AucoreAgshell nanorods and characterization of
their surface plasmon resonances. J. Phys. Chem. B 2001, 105, 7871-7873.
46. Schmid, G.; West, H.; Malm, J.O.; Bovin, J.O.; Grenthe, C. Catalytic properties of layered
gold-palladium colloids. Chem. Eur. J. 1996, 2, 1099-1103.
47. Lizmarzan, L.M.; Philipse, A.P. Stable Hydrosols of metallic and bimetallic nanoparticles
immobilized on imogolite fibers. J. Phys. Chem. 1995, 99, 15120-15128.
48. Michaelis, M.; Henglein, A.; Mulvaney, P. Composite Pd-Ag particles in aqueous-solution.
J. Phys. Chem. 1994, 98, 6212-6215.
49. Torigoe, K.; Nakajima, Y.; Esumi, K. Preparation and characterization of colloidal silver platinum
alloys. J. Phys. Chem. 1993, 97, 8304-8309.
50. Cazayous, M.; Langlois, C.; Oikawa, T.; Ricolleau, C.; Sacuto, A. Cu-Ag core-shell
nanoparticles: A direct correlation between micro-Raman and electron microscopy. Phys. Rev. B
2006, 73, 113402.
51. Liu, M.Z.; Guyot-Sionnest, P. Synthesis and optical characterization of Au/Ag core/shell
nanorods. J. Phys. Chem. B 2004, 108, 5882-5888.
52. Kobayashi, Y.; Salgueirino-Maceira, V.; Liz-Marzan, L.M. Deposition of sliver nanoparticles on
silica spheres by pretreatment steps in electroless plating. Chem. Mater. 2001, 13, 1630-1633.
53. Oldenburg, S.J.; Jackson, J.B.; Westcott, S.L.; Halas, N.J. Infrared extinction properties of gold
nanoshells. Appl. Phys. Let. 1999, 75, 2897-2899.
54. Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mohwald, H. Electrostatic self-assembly of silica
nanoparticle—polyelectrolyte multilayers on polystyrene latex particles. JACS 1998, 120,
8523-8524.
55. Srnova-Sloufova, I.; Lednicky, F.; Gemperle, A.; Gemperlova, J. Core-shell (Ag)Au bimetallic
nanoparticles: Analysis of transmission electron microscopy images. Langmuir 2000, 16,
9928-9935.

Materials 2010, 3 4638
56. Sastry, M.; Swami, A.; Mandal, S.; Selvakannan, P.R. New approaches to the synthesis of
anisotropic, core-shell and hollow metal nanostructures. J. Mater. Chem. 2005, 15, 3161-3174.
57. Ng, K.H.; Penner, R.M. Electrodeposition of silver-copper bimetallic particles having two
archetypes by facilitated nucleation. J. Electroanal. Chem. 2002, 522, 86-94.
58. Manikandan, D.; Mohan, S.; Nair, K.G.M. Annealing-induced metallic core-shell clusterization in
soda-lime glass: An optical absorption study—experiment and theory. Phys. B 2003, 337, 64-68.
59. Lee, W.R.; Kim, M.G.; Choi, J.R.; Park, J.I.; Ko, S.J.; Oh, S.J.; Cheon, J. Redox-transmetalation
process as a generalized synthetic strategy for core-shell magnetic nanoparticles. JACS 2005, 127,
16090-16097.
60. Grouchko, M.; Kamyshny, A.; Magdassi, S. Formation of air-stable copper-silver core-shell
nanoparticles for inkjet printing. J. Mater. Chem. 2009, 19, 3057-3062.
61. Magdassi, S.; Grouchko, M.; Berezin, O.; Kamyshny, A. Triggering the Sintering of Silver
Nanoparticles at Room Temperature. ACS Nano. 2010, 4, 1943-1948.
62. Magdassi, S.; Grouchko, M.; Kamyshny, A. Conductive ink-jet inks for plastic electronics: Air
stable copper nanoparticles and room temperature sintering. NIP 2009, 25, 611-613.
© 2010 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).