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RESEARCH PAPER
Synthesis of hydrophilic copper nanoparticles:
effect of reaction temperature
P. K. Khanna Æ Priyesh More Æ Jagdish Jawalkar Æ
Yogesh Patil Æ N. Koteswar Rao
Received: 1 March 2008 / Accepted: 27 May 2008 / Published online: 13 June 2008
Ó Springer Science+Business Media B.V. 2008
Abstract Synthesis of hydrophilic copper nanopar-
ticles with an additional coating of an hydrophilic
polymer has been carried out by use of hydrazine
hydrate (HH) and sodium formaldehyde sulfoxylate
(SFS) in aqueous medium. The effect of temperature
on nanoparticles when synthesized in aqueous med-
ium has been studied. It is observed that an ideal
temperature ranges some where between 70 and
80 °C. Nearly phase-pure nanocopper can be obtained
when both sodium succinate and polyvinyl alcohol
(PVA) are used together to provide double capping in
aqueous medium. It is observed that the surface
plasmon resonance (SPR) phenomena is sensitive to
experimental conditions and handling of the nanopar-
ticles. X-ray diffraction measurements (XRD)
revealed a broad pattern for the fcc crystal structure
of copper metal. The particle diameter by use of
Scherrer’s equation was calculated to be about 43 nm.
Thermal analysis (TGA) revealed *10–60% weight
loss due to the presence of surfactants. Scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM) showed that there is clustering of
spherical particles in dry state.
Keywords Chemical synthesis Nanoparticles
Metals XRD Aqueous medium
Introduction
Carboxylic acids and/or their salts have been shown to
be excellent surfactants for the synthesis of nanopar-
ticles of metals and semiconductors (Kang et al.
2006). The use of sodium salts of most carboxylic
acids can impart hydrophilicity in nanoparticles and
we have shown earlier that citrate capped silver and
copper nanoparticles can be easily obtained in aque-
ous medium without much of a difficulty (Khanna
et al. 2007a, b, 2008). However, the effectiveness in
preventing surface oxidation was found to be poorer
in case of copper than silver. One reason for such
differences we believe could be because of the use of
central -COONa of citric acid in case of silver being
mono-valent and use of two terminal -COONa in
case of copper due to divalent nature. Use of terminal
carboxylate group therefore may leave higher scope
for oxygen to penetrate the surface of the particles;
thus, copper nanoparticles often tend to oxidize and
more so in aqueous medium. Nonetheless, we have
shown that synthesis of nanoparticles of copper by use
of sodium citrate and or by use of oleic acid can be
performed in water (Khanna et al. 2008). Thus, it is
Dedicated to Prof. David Cole-Hamilton, University of
St. Andrews, UK on his 60th birthday.
P. K. Khanna (&) P. More J. Jawalkar
Y. Patil N. Koteswar Rao
Nanoscience Laboratroy, Centre for Materials for
Electronics Technology (C-MET), Off Dr. Homi Bhabha
Road, Panchawati, Pune 411 008, India
e-mail: pkkhanna@cmet.gov.in;
pawankhanna2002@yahoo.co.in
123
J Nanopart Res (2009) 11:793–799
DOI 10.1007/s11051-008-9441-9
learnt that the synthesis of metal nanoparticles other
than silver and gold is often more challenging because
their surface is prone to oxidation. Suitable surfactant
would also control nucleation of particles thereby
acting as particle growth terminator. Thus, the
surfactant may play a dual role, i.e., to prevent surface
oxidation of the particles and to control the growth of
the particles.
Nanoparticles of transition metals exhibit novel
optical, electronic, magnetic, and chemical proper-
ties and therefore, have a variety of applications
ranging from pigments to electronics in catalysis
and biological systems. Copper nanoparticles have
potential applications as fillers in polymer, as
lubricants and they are used as ink and in metallic
coating (Yang et al. 2006a, b; Reetz and Helbig
1994). Use of Ag–Pd conductor paste has been well
documented for thick-film hybrid integrated circuits
(Ics). However, its copper counter part has found
lesser mention, but in recent years, interest in copper
conductor has increased considerably because of its
excellent solderability, low electrochemical migra-
tion behavior, and low material cost (Ogawa et al.
1994). For its application in electronics, stability of
nanoparticles and their reactivity are the two
important factors. In addition, colloidal copper is
also useful as antibacterial agent and it has very
interesting optical properties because of visible light
absorption, which shows surface plasma resonance
phenomena.
There are number of reports on the synthesis of
copper nanoparticles by various chemical methods
including the concept of sonochemistry (Kang et al.
2006; Khanna et al. 2007a, b, 2008; Yang et al.
2006a, b; Reetz and Helbig 1994; Pileni et al. 1998;
Lu and Tanaka 1997; Chen and Sommers 2001; Arul
Dhas et al. 1998). Synthesis of metal nanoparticles
by chemical method normally employs a reducing
agent and a suitable surface capping agent; thus, a
variety of reducing agents and surfactants have been
reported for production of copper nanoparticles of
different size and shape. We have recently shown
(Khanna et al. 2007a, b, 2008) the variation/alter-
ation in the surface plasmon resonance property of
hydrophilic or hydrophobic copper nanoparticles by
simple solution chemistry by effective utilization of
reaction conditions. In fact, we modified a two-stage
reduction process reported by Yang et al. in 2006 to
a straight forward one-step synthesis for oleic acid
capped copper nanoparticles. In addition, we suc-
cessfully demonstrated the surface plasmon
resonance phenomenon in such isolated copper
particles. It has been reported that large particles
can also lead to excellent SPR band when they are
generated via photochemical method (Condorelli
et al. 2003). Water soluble polymer such as PVP
can lead to very narrow particle size distribution of
copper nanoparticles (Wu et al. 2005; Yang et al.
2006a, b). It is observed that the copper nanoparti-
cles were often contaminated with copper oxide.
High air sensitivity of copper nanoparticles needs
extremely careful and challenging approaches to
avoid formation of its oxide in the end product.
Sodium salt of peptide bolaamphiphile has been used
to reduce copper acetate to generate uni-dimensional
copper nanoparticles due to the mild reducing nature
of the sodium salt of peptide, which eventually helps
to avoid aggregation, and long carbon chain associ-
ated with peptide is effective in preventing surface
oxidation (Kogiso et al. 2002).
We have earlier reported that carboxylic acids and
their salts (e.g., sodium citrate, myristic acid, and
oleic acid) are excellent surfactants for the prepara-
tion of nanoparticles of silver and copper and
successful use of SFS for the preparation of copper,
silver, and gold nanoparticles has also been reported
by us in the recent past (Khanna et al. 2007a
, b, 2008,
2005a, b). It is considered that if sodium citrate is
used, then the citrate ions (from one of the three
coordinating sites) offer at least one unattached -
COO moiety, and if oleic acid is used, then it offers
coordination probability via terminal -COO group.
Copper being a divalent cation may offer better
chances of oxide free nanocopper if a divalent
carboxylic acid or its salt is used as a surfactant.
Oxide free and well-stabilized particles would then
show stable SPR phenomena in the form of colloidal
solution. Such a situation would then allow loading of
these tiny particles in polymer to make the nanopar-
ticles more applicable in many common systems e.g.,
coatings on certain automobile parts as well as anti-
microbial coatings on common surfaces (Esteban-
Cubillo et al. 2006; Cioffi et al. 2004, 2005). In the
present article, we have extended our chemistry
expertise to prepare nanocopper by use of a divalent
coordinating carboxylic acid salt, i.e., sodium succi-
nate. This we believe would leave lesser probability
of surface oxidation.
794 J Nanopart Res (2009) 11:793–799
123
Experimental
All chemicals used were of analytical grade. Copper
chloride (CuCl
2
2H
2
O), sodium formaldehyde sulf-
oxylate (SFS), sodium succinate, and hydrazine
hydrate (HH, 99% pure) were obtained commercially
and were used as received. UV–visible spectra were
recorded in aqueous medium on a JASCO spectro-
photometer model V-570. XRD were obtained on
Rigaku Mini Flex model. Scanning electron micros-
copy (SEM) was carried out on a Philips XL-30
machine. Thermo gravimetric analysis of (TGA) the
nanoparticles was done on a Perkin Elmer Model no.7
under nitrogen at a heating rate of 10 °C/min. TEM
was carried out at IIT Mumbai on a Philips ML 200.
Synthesis of copper nanoparticles
Copper (II) chloride dihydrate (2.0 g/20 mL water)
was taken in a beaker and dissolved completely.
Sodium succinate (7.2 g/100 mL water) was added
drop wise to above copper chloride solution. The
reaction mixture was allowed to stir for about 10 min
at 70–80 °C by which time bluish coloured solution
was obtained. To this, polyvinylalcohol (MW
1,25,000) (0.1 g/10 mL water) was added and stirred
for about 5–10 min. A solution of hydrazine hydrate
(99% pure, 3.0 mL/10 mL water) was then added to
the above reaction mixture with rapid stirring and
was further stirred at the same temperature for about
an hour to afford a brown suspension. The Reaction
mixture was cooled down to room temperature, and
was filtered, and washed with several portions of
distilled water and finally with methanol. The brown
residue was dried overnight in an oven to collect the
free flowing brown powder named as CuSuHHPVA4.
Other reactions were performed similarly using SFS
or HH with or without variation of temperature and
polymer and were named as CuSuSFS3, CuSuHH2,
CuSuHH7, and CuSuHH10.
Results and discussion
The reaction of copper chloride with sodium succi-
nate followed by reduction with hydrazine hydrate or
SFS in water in an open beaker afforded brown
copper colloids leading to brown powders as per
Scheme 1. An aqueous solution of succinate ions first
create an environment of an umbrella around Cu
2+
ions, which were slowly converted to metallic
nanoparticles via reduction process. The overall
reaction and experimental conditions were adopted
in such a way so that maintains enough succinate ions
needed to prevent formation of copper oxide or
copper(I) chloride. The succinate ions along with a
polymer such as PVA can lead to more effective
surface capping of the nanoparticles, thus preventing
the oxidation of the outer surface.
The reaction mixture of all the preparations show
weak SPR in water. When these powders were re-
dispersed, they retain SPR absorption bands. The SPR
is a phenomenon in the nanoparticles that originates
from oscillation of electrons due to their polarization
in the conduction band of the metal because of the
electric field caused by an incoming light wave (in the
present case, Visible light wave). The SPR can be
maintained and made more prominent by suitably
isolating the right form of colloidal solution during the
experiment. This can be done by effective primary
surface capping by succinate ions followed by a
secondary capping by PVA, thus avoiding agglomer-
ation and leading to the formation of uniform particles.
UV–visible absorption spectra of copper nanoparticles
are shown in Fig. 1a and b. The absorption bands for
copper nanoparticles have been reported to be in the
range of 550–600 nm (Condorelli et al. 2003; Yang
et al. 2006a, b). Thus, the present method appears to
be an attractive route to produce nanoparticles that
show stable surface plasmon resonance (SPR) phe-
nomena from the reaction mixture to the final product.
The absorption maximum for nanocopper prepared
from HH was observed at about 590 nm. The absorp-
tion values can be unaltered; however, they follow a
more shallow pattern if the nanoparticles are prepared
in the presence of PVA. This is believed to be due to
the formation of very small particles. Small particle
CH
2
COONa
CH
2
COONa
+CuCl
2
in H
2
O
[CH
2
COOCuOOCCH
2
]
PVA*
“ intermediate stage “
Reduction, Heat
Cu
(0)
Scheme 1 Schematic presentation of formation of nanocopper
(* optional)
J Nanopart Res (2009) 11:793–799 795
123
size is reported to be the reason for the suppression of
SPR (Lu and Tanaka 1997).
Our experiments resulted in the formation of traces
of copper oxide alongside nanocopper metal, which
could also be the reason for suppressed SPR patterns
in the absorption spectrum. Indeed, XRD analysis of
brown powders confirmed the formation of copper
oxide along with metallic copper in the end product.
Figure 2 shows the XRD pattern of copper nano-
powders. Phase-pure nanocopper can be prepared by
use of PVA (Fig. 2c), where Bragg’s reflections are
observed at 2h value of 43.4, 50.52, and 74.0,
representing \111[, \200[, and \220[ planes of
fcc crystal structures of bulk copper. It is observed
that the formation of oxide free copper nanoparticles
from hydrazine hydrate is maximized if the temper-
ature was raised above 120 °C (Fig. 2e).
Similarly, when the reaction was carried at room
temperature, the formation of metallic copper is
dominated by the presence of copper succinate
intermediate or unreacted reagents. Present studies
show that when PVA is not employed, SFS is more
suitable as a reducing agent for preparation at lower
temperature; however, HH has capability to minimize
the formation of copper oxide at boiling temperature of
water. An estimation of particle size by use of
Scherrer’s equation taking full width at half of maxima
at 2h value of 43.4, 50.5, and 74.0 and by averaging the
values revealed a crystallite size of about 43 nm.
SEM measurement of as-prepared copper nanopar-
ticles revealed formation of spherical agglomerates
(Fig. 3a–d). It is observed that the best spherical
morphology of copper nanoparticle is obtained when a
succinate salt is used along with SFS. This is rightly
considered due to the effective slow reducing nature of
SFS as well as its ability to act as a secondary
surfactant. However, when the HH is used as a
reducing agent, the clustering is more pronounced
possibly due to the strong nature of the reducing agent,
allowing the surfactant to be easily displaced during
500 550 600 650 700
0.34
0.36
0.38
0.4
Wavelength in nm
Intensity
590 nm
0.245
0.255
0.265
500 550 600 650 700
Wavelength in nm
Intensity
590 nm
ab
Fig. 1 UV–visible
absorption spectra of
nanocopper (a) succinate
capped and (b) with
additional capping of PVA
0
7000
14000
20 30 40 50 60 70 80
2 theta(degrees)
Intensity (a.u.)
CUSUHH2 CUSUSFS3 CUSUHHPVA4
CUSUHH7 CUSUHH10
a
e
d
c
b
Fig. 2 XRD patterns of
copper nano with variation
in temperature and surface
capping (a) succinate/HH at
70–80 °C(b) succinate/SFS
at 70–80 °C(c) succinate/
HH/PVA 70–80 °C(d)
succinate/HH at room
temperature (e) succinate/
HH at boiling temperature
796 J Nanopart Res (2009) 11:793–799
123
the reduction process and penetration of oxygen to
effect slight oxidation of the surface. Similarly, when
the preparation is done at a boiling temperature, the
particles are much clustered and bigger in size, as seen
in Fig. 3d. Elemental composition of the samples
revealed presence of copper, carbon, and oxygen. The
presence of oxygen is mainly supposed to be from the
succinate group; however, in some samples where
copper oxide is formed (based on the XRD pattern),
oxygen is from both the succinate capped copper
nanoparticles as well as from the copper oxide.
EDAX analysis showed that percentage copper
varies from 25 to 45 wt% depending on the ratio of the
surfactant and the degree of the reduction of copper
metal salt by the respective reducing agent. When
copper zero-valency is attained to its maximum level,
the % copper is found to be 45, which matches well
with the weight loss in thermal analysis. Thermal
analysis (TGA) of the powder revealed low weight
loss of about 10–20% without the use of PVA;
however, the weight loss can go as high as about
60% with PVA. If no polymer is used as a secondary
surfactant, only a two-stage weight loss profile is
observed, indicating that the succinate group decom-
position followed by softening of metal particles.
However, the presence of PVA in the sample showed a
three-phase decomposition profile due to an initial
weight loss because of evaporation of trapped solvent/
moisture in the sample followed by decomposition of
carboxylate group along with PVA (Fig. 4).
Low resolution TEM analysis of PVA mediated
copper nanoparticles revealed the formation of nee-
dle-like spherical agglomerates (Fig. 5a, b). Needle-
like morphology of copper nanoparticle may be due
to the presence of succinate salt along with polymer,
which restrict the growth of the surfactant protected
Fig. 3 SEM of nanocopper prepared from (a) succinate/SFS at 70–80 °C(b) succinate/HH at 70–80 °C(c) succinate/HH at room
temperature (d) succinate/HH at boiling temperature
40
50
60
70
80
90
100
30 230 430 630 830
Temperature ( deg. Celcius)
% Wt Loss
Fig. 4 TGA of nanocopper prepared from succinate/HH/PVA
at 70–80 °C
J Nanopart Res (2009) 11:793–799 797
123
nanoparticles in one direction only leading to flowery
needle. The clustered nanoparticle size was difficult
to measure, but from the scale bar, one can easily
assume the particles to be about 40–50 nm, which
matches well with the calculations made from the
XRD pattern. The absence of well-defined concentric
rings in the electron diffraction pattern indicate that
the nanoparticles are quite amorphous when coated
with polymer in addition to the surfactant.
The measurement of particle size distribution of
the sonicated nanocrystals was done by dynamic light
scattering techniques (via a Laser input energy of
632 nm). It was observed that in the prepared sample,
particles have a wide size distribution, but the
majority of them are dispersed within a narrow
range, as shown in Fig. 6. The volume/weight
analysis of the sample gave a distribution of particle
between 40 and 60 nm with an average distribution of
48 nm. This, however, does not rule out the possi-
bility of smaller particles as the light scattering from
bigger particles would suppress the intensity of the
small particle present in the sample.
Conclusion
The present study illustrates simple, convenient, and
significant methods for the synthesis of copper
nanoparticles through the reduction of copper salts
in aqueous medium. Surface plasmon resonance
(SPR) phenomena from as-isolated nanoparticles
was observed under a visible light impact. The
absorption values can be unaltered and follow a more
shallow pattern if the nanoparticles are heated in the
presence of PVA due to the formation of very small
particles, which is the reason for the suppression of
SPR.
XRD analysis indicated the presence of Cu
2
O
when the reaction was carried out at lower than 70–
80 °C or where the surfactant ratio was lower than
the required. SEM and TEM analysis showed that the
small scattered particles at higher scale do indicate
the formation of spherical particles. TEM, however,
showed small clusters of needle-like rings. Particle
size distribution analysis showed the particles to be in
the range of about 50 nm.
Acknowledgements PKK thanks Director, C-MET for
permission. We thank the Department of Information
Technology (Govt. of India) for financial support for this work.
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