Synthesis and characterization of metallic copper nanoparticles via thermal decomposition

Article abstract of DOI:10.1016/j.poly.2008.08.020

Copper oxalate was used as a precursor to prepare metallic copper nanoparticles by thermal decomposition. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and UV-Vis spectroscopy. XRD analysis revealed broad pattern for fcc crystal structure of copper metal. The particle size by use of Debye-Scherrer's equation was calculated to be about 40 nm.

Full text of DOI:10.1016/j.poly.2008.08.020

Polyhedron 27 (2008) 3514–3518
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of metallic copper nanoparticles
via thermal decomposition
a,b,*
, Fatemeh Davar , Noshin Mir b
b
Masoud Salavati-Niasari
a
Institute of Nano Science and Nano Technology, University of Kashan, P.O. Box 87317-51167, Kashan, Islamic Republic of Iran
Department of Chemistry, Faculty of Science, University of Kashan, P.O. Box 87317-51167, Kashan, Islamic Republic of Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper oxalate was used as a precursor to prepare metallic copper nanoparticles by thermal decomposi-
tion. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy, trans-
mission electron microscopy, Fourier transform infrared spectroscopy and UV–Vis spectroscopy. XRD
analysis revealed broad pattern for fcc crystal structure of copper metal. The particle size by use of
Debye–Scherrer’s equation was calculated to be about 40 nm.
Received 18 May 2008
Accepted 14 August 2008
Available online 6 October 2008
Keywords:
Nanomaterials
Ó 2008 Elsevier Ltd. All rights reserved.
Copper nanoparticles
Inorganic materials
Optical materials and properties
1
. Introduction
In recent years, synthesis of transition metal nanoparticles is a
techniques [24], laser ablation [25], polyol method [26], solvother-
mal method [27], DC arc discharge method [28], thermal and sono-
chemical reduction [29].
growing research field in chemical science [1–3]. Nanoparticles
which are ultra fine particles have the diameter from 1 to
Among various techniques for synthesis of inorganic nanoparti-
cles, thermal decomposition is one of the most common to produce
stable monodisperse suspensions with the ability of self-assembly.
Nucleation occurs when the metal precursor is added into a heated
solution in the presence of surfactant, while the growth state take
place at a higher reaction temperature [30]. Recently Zheng and
coworkers reported synthesis of nanosized CuO via thermal
decomposition of copper oxalate [31], but up to now there has
not been any report about synthesis of metallic copper nanoparti-
cles with this precursor.
Very recently, our group reported the synthesis of uniform-
sized nanoparticles of various transition metal oxides from thermal
decomposition of metal-oleate complexes which were prepared
from the reaction of some organometallic complexes [32,33]. In
the continuation of the development of simple synthesis of nano-
sized nanoparticles, we herein report on the synthesis of metallic
copper nanoparticles from a simple, green, low-cost, and reproduc-
ible process from copper oxalate as precursor. In this process,
oleylamine was used as both the medium and the stabilizing
reagent.
1
00 nm and their properties are different form bulk materials. As
the metal particles are reduced in size, the bulk properties of the
particles disappear to be substituted to that of a ‘‘quantum dot’’,
following quantum mechanical rules. It can thus be easily under-
stood that metal nanoparticles’ chemistry differs from that of the
bulk materials [4]. Since with size reduction, the high surface area
to volume ratio lead to enhanced catalytic activity [5].
Among various metal particles, copper nanoparticles have at-
tracted considerable attention because copper is one of the most
important metals in modern technologies [6]. Considerable inter-
est has been focused on copper nanoparticles due to their optical,
catalytic, mechanical and electrical properties [7–12]. Since late
1
980s, copper nano particles have attracted much attention of
researchers because of their applications in catalysis [13]. The
advantages of Cu nanoparticles are being cheap, high yields in mild
reaction conditions and having short reaction times as compared to
traditional catalysts [14,15]. Cu nanoparticles have been synthe-
sized through different methods such as thermal decomposition
[
16–19], metal salt reduction [20], microwave heating [21], radia-
tion methods [22], micro emulsion techniques [23], super critical
2
. Experimental
2.1. Materials
*
Corresponding author. Address: Institute of Nano Science and Nano Technology,
University of Kashan, P.O. Box 87317-51167, Kashan, Islamic Republic of Iran. Tel.:
All the chemicals reagents used in our experiments were of
+98 361 5555333; fax: +98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
analytical grade and were used as received without further
0
277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.08.020

M. Salavati-Niasari et al. / Polyhedron 27 (2008) 3514–3518
3515
purification. Copper oxalate precursor was synthesized according
to this procedure: the Cu(OOCCH O (2 mmol) was dissolved
into 10 mL of distilled water to form a homogeneous solution. A
stoichiometric amount of potassium oxalate dissolved in an equal
volume of distilled water was dropwise added into the above solu-
tion under magnetic stirring. The solution was stirred for about
aged at 240 °C for 45 min, and was then cooled to room tempera-
ture. The reddish nanoparticles were precipitated by adding excess
ethanol to the solution. The final product was washed with ethanol
several times to remove excessive surfactant. This product could
easily be re-dispersed in nonpolar organic solvents, such as hexane
or toluene (Scheme 1).
3
)
2
Á H
2
1
5 min and a blue precipitate was centrifuged and washed with
ethanol several times. The product was dried at 50 °C. The copper
oxalate, Cu(O ), was characterized by elemental analyses, pow-
der X-ray diffraction (XRD) and thermo gravimetric analysis
TGA) [31,34]. Anal. Calc. for [Cu(O )]: C, 15.84; Cu, 41.93. Found:
C, 15.75; Cu, 41.84%.
3
. Results and discussion
4 2
C
The TGA curve of the as-prepared precursor is shown in Fig. 1.
(
4 2
C
There is only one weight loss step in the temperature range 250–
10 °C. The weight loss at 250–315 °C may be ascribed to the
3
decomposition of the oxalate. The weight loss is about 43.23%,
which is close to the theoretical value.
2.2. Characterization
X-ray powder diffraction (XRD) patterns of Cu nanoparticles at
room temperature shows in Fig. 2. The XRD pattern is consistent
with the spectrum of copper, and no peak attributable to possible
impurities is observed. The peak positions are consistent with
metallic copper. All possible peaks of copper are observed, which
indicates the polycrystalline nature of the product. Bragg’s reflec-
tions for Cu nanoparticles are observed in XRD pattern at 2h value
of 43.6°, 50.7° and 74.45° representing [111], [200] and [220]
planes of fcc structure of copper with the space group of Fm3m
XRD patterns were recorded by a Rigaku D-max C III, X-ray dif-
fractometer using Ni-filtered Cu K radiation. Elemental analyses
were obtained from Carlo ERBA Model EA 1108 analyzer. Scanning
a
electron microscopy (SEM) images were obtained on Philips XL-
3
0ESEM equipped with an energy dispersive X-ray spectroscopy.
Transmission electron microscopy (TEM) images were obtained
on a Philips EM208 transmission electron microscope with an
accelerating voltage of 200 kV. Fourier transform infrared (FT-IR)
spectra were recorded on Shimadzu Varian 4300 spectrophotome-
ter in KBr pellets. Thermogravimetric-differential thermal analysis
(
TG-DTA) were carried out using a thermal gravimetric analysis
À1
instrument (Shimadzu TGA-50H) with a flow rate of 20.0 mL min
and a heating rate of 10 °C min . The electronic spectra of the
À1
complexes were taken on a Shimadzu UV–Vis scanning spectrom-
eter (Model 2101 PC).
2.3. Synthesis of Cu nanoparticles
The current synthetic procedure is a modified version of the
method developed by Hyeon and others for the synthesis of metal
nanoparticles which employs the thermal decomposition of transi-
tion metal complexes [32,33,35]. The synthetic pathway is shown
in Scheme 1. In this synthesis Cu nanoparticles were prepared by
the thermal decomposition of copper oxalate, [Cu(O
amine complex as precursor. First, the [Cu(O )]–oleylamine com-
plex was prepared by reaction 0.6 g of Cu(O ) and 5 mL of
4 2
C )]–oleyl-
4 2
C
4 2
C
oleylamine. The mixed solution was placed in a 50 mL three-neck
distillation flask and heated up to 140 °C for 60 min under an argon
atmosphere. The resulting metal-complex solution was injected
into 5 g of triphenylphosphine (TPP) at 240 °C. As the thermal
decomposition proceeded, the azury solution turned to red, indi-
cating the formation of metallic copper. The reddish solution was
Fig. 1. TGA of the copper oxalate precursor, Cu(O C ).
4 2
Scheme 1. Schematic depiction of the selective formation of Cu nanoparticles.

3
516
M. Salavati-Niasari et al. / Polyhedron 27 (2008) 3514–3518
Fig. 2. XRD pattern of the copper nanoparticles.
(
JCPDS No. 4–0836). The size of the crystallites was estimated from
sorbed on the surface of the copper nanoparticles. The FT-IR of the
copper nanoparticles shows the intense adsorption peaks at 2920
and 2850 cm attributed to the alkyl chains of oleylamine. The
Debye–Scherrer equation is about 40 nm.
À1
FT-IR spectroscopy is a useful tool to understand the functional
group of any organic molecule. The infrared spectra of free TPP,
oleylamine and copper nanoparticles coated with oleylamine and
TPP are shown in Fig. 3. By comparing the infrared spectra of the
nanoparticles with free surfactants in Fig. 3 we can see that the or-
ganic molecules have indeed become a part of the nanoparticles. As
shown in Fig. 3a and b, peaks corresponding to P–ph stretching at
1
x
bands attributed to the antisymmetric and symmetric vibration
À1
modes of –N–H are observed at 3437 and 3232 cm , respectively
(Fig. 3c). Due to the coordination of the unpaired electron couple of
the nitrogen atom, the vibration bands are observed in lower val-
ues than those expected for the free amine group, i.e., 3500 and
À1
3300 cm , respectively. This indicates that oleylamine is bound
À1
710 cm , rocking and bending mode of the methylene group
on the surface through the unpaired electron couple of the amine
group [36]. From these results, a chemical bond can be formed be-
tween N and Cu atoms and a coordinate bond between P and Cu
À1
(CH
2
) at 1270 cm , bending mode of the phenyl group
c
(@CH)
À1
À1
at 742 cm and
t
(benzene ring) at 697 cm can be seen clearly.
The only difference among these characteristic peaks is either the
peak intensity or a slight shift in the peak position. For example,
the peak position of the longitudinal modes of oleylamine and
TPP shifts to lower wavenumbers after TPP and oleylamine are ad-
atoms on the surface of the nanoparticles. Thus, the phenyl stretch-
À1
ing mode
m(P–ph) at 1430 cm is still visible in Fig. 3c. So the
oleylamine and TPP serves as the capping ligand that controls
growth. Triphenylphosphine (TPP) was widely used in the synthe-
sis of phosphine-stabilized cobalt or other metal nanoparticles
Fig. 3. FT-IR spectra of (a) free TPP absorption, (b) free oleylamine and (c) Cu
nanoparticles coated by oleylamine and TPP.
Fig. 4. SEM image of Cu nanoparticles.

M. Salavati-Niasari et al. / Polyhedron 27 (2008) 3514–3518
3517
Fig. 5. TEM images of Cu nanoparticles.
[
37]. The phenyl groups in TPP can provide greater steric hindrance
than straight-chained alkyl groups such as tributyl and trioctyl to
control the size of nonmagnetic metal nanoparticles and to stabi-
lize efficiently the magnetic nanoparticles.
The morphology of the Cu nanoparticles is examined by SEM
(Fig. 4). From the micrograph, it was observed that the nanoparti-
cles were agglomerated. The TEM image (Fig. 5) shows the pres-
ence of dense agglomerates. The particles have an irregular
shape, and their distribution, likewise, is not uniform (30–80 nm).
The growth mechanism could be well understood on the basis
of the following reactions and the crystal habits of Cu. In the pres-
4 2
ence of oleylamine as solvent [Cu(O C )]–oleylamine were synthe-
sized. After addition of TPP to these solutions at high temperature
the complexes were decomposed. TPP is a high-boiling point sur-
factant with a patulous long-chain structure providing great steric
hindrance. The addition of TPP into the mixture of oleylamine and
complex as an additional surfactant reduced the particle size much
further and resulted in nanoparticles with very thin arrays. Oleyl-
amine is known as a ligand that binds tightly to the metal nanopar-
ticles surface. The combined effects of TPP and oleylamine were
much more profound than those of individual contributions. On
the other hand these complexes transform to Cu under an argon
atmosphere. When the triphenylphopsphine (TPP) was used as
the complexing agent, the spherical nanoparticles capped by this
surfactant were obtained. The growth of the Cu nanoparticles can
be explained on the basis of the schematic view presented in
Scheme 1. The Cu nanoparticles agglomerated together to form a
spherical structure. In this synthetic rout oleylamine acts not only
as the reaction solution but also as the stabilizer, which prevents
the nanoparticles from oxidizing. Recently, copper oxalate was
used as precursor for synthesizing CuO. In that method copper oxa-
late was calcined at 400 °C for 4 h [31] or 350 °C for 10 h [34] with-
out any capping agent and argon atmosphere. So, metal oxide
nanoparticles obtained. But here with use of capping agent, metal-
lic nanoparticles obtained. We believe that capping agents used
here help to the formation of metal rather than metal oxide.
Nanosized particles exhibit unique optical properties with an
exponential-decay Mie scattering profile with decreasing photon
energy. Some transition metal nanoparticles show a distinct sur-
face-plasmon resonance (SPR). The SPR is very sensitive to metal
size, dielectric constant of metal and embedding medium (here is
oleylamine) [20,29]. For instance, Cu nanoparticles typically exhi-
bit SPR at around 600 nm [38]. However, for the fresh Cu nanopar-
ticles synthesized here show an absorption peak at around 572 nm
Fig. 6. UV–Vis absorption of the Cu nanoparticles.
observed. This observation alludes to the size effect to the Cu nano-
particles. The synthesized particles were stable for ꢀ6 days. This
kind of stabilization of Cu nanoparticles was due to the capping
of particles by oleylamine in thermal treatment. The arrangement
of capping agent, oleylamine surrounding the nanoparticles is size
dependent. After a few days, slow oxidation of copper particles sets
in with the disappearance of absorption peaks [39] and finally the
as-synthesized product becomes black. This indicated that the
sample was oxidized.
4
. Conclusion
In summary, Cu nanoparticles with diameters from 30 to 80 nm
have been successfully synthesized via thermal decomposition of
Cu(O
)]-oleylamine complex. This method employed an inex-
pensive, reproducible process for the large-scale synthesis of cop-
per nanoparticles.
[
4 2
C
Acknowledgment
(Fig. 6). This peak can be assigned to the absorption of nanoparti-
cles of copper [38]. A modest blue shift (about 28 nm) of the
absorption edge relative to that of the bulk copper powder is
Authors are grateful to council of University of Kashan for pro-
viding financial support to undertake this work.

3
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