A new one-step synthesis method for coating multi-walled carbon
nanotubes with cuprous oxide nanoparticles
Kakarla Raghava Reddy,aByung Cheol Sin,aChi Ho Yoo,aWonjung Park,a
Kwang Sun Ryu,aJae-Shin Lee,bDaewon Sohncand Youngil Leea,*
aDepartment of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea
bSchool of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea
cDepartment of Chemistry, Hanyang University, Seoul 133-791, Republic of Korea
Received 10 December 2007; revised 22 January 2008; accepted 22 January 2008
Available online 7 February 2008
A new method was developed for the synthesis of Cu2O-coated multi-walled carbon nanotubes (MWCNTs) on the basis of Feh-
ling’s reaction. The method involved dispersion of carbon nanotubes in Fehling’s reagent, followed by addition of formaldehyde as a
reducing agent. The Cu2O-coated MWCNTs were characterized by transmission electron microscopy, X-ray diffraction, X-ray
photoelectron, Raman and UV–Vis spectroscopy, and superconducting quantum interference device measurements. These novel
materials could be used in catalysis and optoelectronic applications.
? 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes; Metal nanoparticles; Chemical synthesis; Nanomaterials; TEM
Carbon nanotubes (CNTs) have received much
attention due to their unique atomic structure, high
surface area-to-volume ratio, and excellent electronic,
chemical, thermal, mechanical and optical properties
[1–3]. To optimize the potential applications of CNTs
in nanotechnology fields, it is essential to modify the
sidewalls of CNTs by chemical functionalization and/
or attaching suitable nanostructures [4,5]. Recently, sig-
nificant interest has been directed toward the synthesis
of composites that consist of metal nanoparticles and
CNTs to extend the range of applications from nano-
electronics to sensors and catalysis [6–8]. For example,
the attachment of NiO or RuO2nanoparticles to CNTs
has shown great promise for highly efficient photoelect-
rochemical cells, electrochemical supercapacitors and
even sensor devices [9,10]. Composites of CNTs and Pt
have been shown to be excellent amperometric sensors
for glucose over a wide range of concentrations .
Pd-filled CNTs have been utilized as hydrogen storage
materials . SnO-functionalized MWCNTs have been
used as anode materials for lithium ion batteries .
Composites of metal nanoparticles and multi- or
single-walled CNTs (MWNCTs and SWCNTs) can be
prepared by various strategies. Thus far, coating or
deposition of CNTs with various metal nanoparticles,
such as TiO2, SiO2, Fe3O4, CdS, Au, Pd and Pt, have
been developed, and various methods have been used:
solid-state reactions, capillary action, radiolysis, physi-
cal evaporation, electroless deposition, physisorption,
self-assembly and colloidal chemistry combined with
electrostatic interactions or with sonication in aqueous
solution [14–18]. Unfortunately, only a few reports pro-
vide simple, efficient routes for strongly attaching nano-
particles or nanospheres to CNTs [19,20]. Therefore, the
exploration of novel CNT–metal hybrid nanocompos-
ites is important, and is a challenging subject because
of the combined properties of conductive CNTs and
Cuprous oxide (Cu2O) is a p-type semiconductor with
a direct band gap of 2.12 eV. Among various metal oxide
particles, increasing attention has been focused on Cu2O
nanoparticles due to the redox chemistry, extraordinary
electrical, thermal, catalytic and sensing properties, and
the potential applications in diverse areas from optoelec-
tronic devices to sensors of Cu2O nanoparticles [21–23].
In recent years, Cu2O particle-coated MWCNTs have
attracted interest for their potential applications in solar
cells, electrodes, catalysis, hydrogen production, mag-
netic storage and gas sensors . The composites
prepared via physical or electrochemical methods show
1359-6462/$ - see front matter ? 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
*Corresponding author. Tel.: +82 52 259 2341; fax: +82 52 259
2348; e-mail: email@example.com
Available online at www.sciencedirect.com
Scripta Materialia 58 (2008) 1010–1013
some disadvantages: the deposition rate of the Cu2O par-
ticles on CNTs is poor; the sizes of the Cu2O particles on
the nanotubes are larger, and not of uniform diameter;
some of the particles detach from the nanotubes, result-
ing in composites that are highly aggregated and have
poor characteristics [25,26]. Therefore, the synthesis of
uniform Cu2O nanoparticle-coated MWCNTs compos-
ites is of particular interest.
In this paper, we report a simple, effective and novel
method for the synthesis of composites of MWCNTs
with well-dispersed Cu2O nanoparticles by using copper
sulfate as a precursor. These Cu2O-coated MWCNTs
composites have been structurally characterized by
transmission electron microscopy (TEM), X-ray diffrac-
tion (XRD), X-ray photoelectron spectroscopy (XPS),
Raman spectroscopy, UV–Vis spectroscopy, and super-
measurements. The Cu2O nanoparticles coated on the
MWCNTs are uniform in diameter and are well-dis-
persed. A possible mechanism for the formation of these
Cu2O-coated MWCNTs composites is discussed.
MWCNTs were obtained from Nanostructured and
Amorphous Materials, Inc. (purity 95%). Copper(II)
sulfate (CuSO4, Aldrich, USA), tartaric acid, formalde-
hyde (HCHO, 37%) and sodium hydroxide were used as
received. Milli-Q purified water was used for all of the
Cu2O-coated MWCNTs were synthesized by treat-
ment of the MWCNTs with Fehling’s reagent (an alka-
line solution of copper sulfate containing sodium
potassium tartarate) in the presence of formaldehyde
as a reducing agent. In a typical procedure, 150 mg
of purified MWCNTs were first suspended in 30 ml of
1 wt.% copper sulfate solution containing a mixture of
tartaric acid and sodium hydroxide, and then stirred
with ultrasonication for 30 min. Next, formaldehyde
(0.5 ml) was added dropwise to the suspension, and
the reaction mixture was stirred at 60 ?C for 1.5 h. The
product was washed with ethanol and distilled water
several times, filtered and finally dried under vacuum
at 50 ?C for 12 h. The nanocomposite thus obtained is
denoted as Cu2O-coated MWCNTs.
The morphology and particle sizes of the Cu2O-
coated MWCNTs were characterized by TEM, using a
Phillips CM-30 microscope operating at an accelerating
voltage of 120 kV. The phase identification of the sam-
ples was performed by XRD analysis with a Rigaku dif-
fractometer, operating in the Bragg configuration using
nickel-filtered Cu Ka radiation (k = 1.5306 A?). XPS
was performed using a Mg KaX-ray source at 80 eV
pass energy and at 0.75 eV (Kratos Analytical Plc, Man-
chester, UK). Raman spectra of the samples were
recorded in the range of 1000–2000 cm?1using a HOR-
IBA Jobin Yvon HR800 Raman spectrophotometer.
UV–Vis spectra of the samples were recorded on a Beck-
man UV–Vis (DU 7500) spectrophotometer using
quartz cells with a 10 mm path length. A superconduc-
ting quantum interference device (SQUID) magnetome-
ter (Quantum Design, MPMS-X 1) was used to measure
the samples’ magnetic properties at room temperature.
The morphology and size of the Cu2O-coated
MWCNTs were analyzed using TEM. Direct evidence
for the formation of Cu2O nanoparticles on the surface
of MWCNTs is provided in the TEM images. Figure 1a
and b show TEM images of the Cu2O-coated MWCNT
composites, synthesized using 1 and 3 wt.% solutions of
copper sulfate. It is clear from the images that Cu2O
nanoparticles were well dispersed into the CNTs. As
the concentration of copper sulfate was increased from
1 to 3 wt.%, the size of the Cu2O nanoparticles increased
from 21 to 32 nm. A high-magnification high-voltage
TEM image (Fig. 1c) also clearly shows the walls of
the CNTs and the Cu2O nanoparticles coated on the
surface of the nanotubes in the composite. The interac-
tion between the Cu2O nanoparticles and the nanotubes
was quite strong because thorough washing did not
remove the nanoparticles from the surface.
The formation of Cu2O nanoparticles on the walls of
MWCNTs is illustrated in Scheme 1; the formation
follows the principle of Fehling’s reaction, as expressed
in the simplified equation. When MWCNTs were sus-
pended into Fehling’s reagent solution and subjected
to sonication, the CNTs could effectively absorb the
copper ions that were complexed with tartrate. Upon
the dropwise addition of formaldehyde, copper(II) ions
(Cu2+) were reduced to copper(I) oxide (Cu2O) particles
on the surface of the nanotubes. During the reaction,
these CNTs acted as templates and/or substrates for
the growth of Cu2O nanoparticles on their surface.
The successful dispersion of Cu2O nanoparticles on
the CNTs was due to the effect of ultrasonication, which
prevented aggregation of the particles. Cu2O-coated
MWCNTs synthesized using 1 wt.% solution of CuSO4
were used for further phase and surface analyses.
RCHOðaqÞ þ Cu2þþ OH?! Cu2O þ RCOO?ðaqÞ
The crystalline nature of the Cu2O nanoparticles on
MWCNTs was confirmed by the XRD spectrum. An
XRD pattern of Cu2O-coated MWCNTs is shown in
Figure 2. The diffraction peaks confirm that the compos-
ite consists of both Cu2O and MWCNTs. The diffraction
peaks at 2h = 29.61?, 36.53?, 42.44?, 61.57?, 74.06? and
Figure 1. TEM images of Cu2O-coated MWCNTs prepared using (a)
1% and (b) 3% solutions of copper sulfate; (c) high-magnification high-
voltage TEM image of the composites in (a).
K. R. Reddy et al./Scripta Materialia 58 (2008) 1010–1013
77.62? can be assigned to the (110), (111), (200), (220),
(311) and (222) planes of crystalline Cu2O, respectively.
These peaks match with reported data for Cu2O (JCPDS
78-2076) . The peak at 2h = 26? corresponds to the
(002) phase of the graphite structure of the CNTs .
The unchanged MWCNT (002) peak position indicates
that the anchoring of Cu2O did not lead to distortion
of the nanotube shells. The intensity of the diffraction
peak corresponding to the MWCNTs was not as strong
as the peaks due to the Cu2O. No other peaks were ob-
served in the XRD patterns. Therefore, we conclude that
the nanocomposite consists of pure crystalline Cu2O and
equation, L = kk/bcosh, where L is the mean dimension
of the particle, b is the full width of at half maximum
of the diffraction peak, h is the diffraction angle, k is the
wavelength of Cu Karadiation (0.154 nm), and k is equal
to 0.89. The calculated average size of the Cu2O particles
in the composite is about 20 nm, which is in good agree-
ment with the results by the TEM (Fig. 1a).
XPS was used to confirm that the Cu2O nanoparticles
were on the surface of the CNTs,. The full-range XPS
spectrum of the Cu2O-coated MWCNTs is displayed
in Figure 3. The two peaks observed at 934.2 and
932.0 eV correspond to Cu 2p3/2and Cu 2p1/2, respec-
tively, which confirms the existence of Cu2O nanoparti-
cles on the surface of the CNTs. The main peak at
284.6 eV is attributed to sp2-hybridized carbon in the
graphite layers of the CNTs .
Raman spectroscopy was used to characterize the
electronic structure of MWCNTs and Cu2O-coated
MWCNTs, and the results are presented in Figure 4. Both
one at 1350 cm?1and the other at 1592 cm?1. The former
is often called the D band, which originates from the level
of disordered carbons . The latter is the G band, which
graphite sheets . Upon formation of Cu2O nanoparti-
cles on the MWCNTs, the intensity of the G and D bands
intensity of the G band (ID/IG) was smaller for the Cu2O-
coated MWCNTs composites than MWCNTs, because
dispersion of Cu2O nanoparticles into CNTs eliminated
the defect sites. Moreover, coating MWCNTs with Cu2O
particles did not change the electronic energy states of
MWCNTs in the composite.
UV–Vis spectroscopy was used to study the contribu-
tion of the MWCNTs to the excitonic or interband tran-
sitions of Cu2O nanoparticles. The UV–Vis absorption
spectra of MWCNTs and Cu2O-coated MWCNTs are
shown in Figure 5. MWCNTs exhibit a featureless spec-
trum that does not show any absorption bands in the
region between 300 and 1000 nm. An additional broad
absorption band appeared at around 500 nm for the
Cu2O-coated MWCNTs composites, corresponding to
an absorption band of the Cu2O nanoparticles. Also,
the composites showed enhanced absorbance due to
the presence of the Cu2O nanoparticles. The band gap
of Cu2O nanoparticles estimated from the UV–Vis spec-
tra is about 2.35 eV, which is larger than the bulk Cu2O
(Eg? 2.1 eV) .
The magnetic properties of Cu2O-coated MWCNT
composites were measured at room temperature using
1000 800600 400200
Binding energy (eV)
Figure 3. XPS spectrum of Cu2O-MWCNTs.
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000
Figure 4. Raman spectra of (a) MWCNTs and (b) Cu2O-coated
MWCNTsCopper complex on MWCNTs surface MWCNTs-Cu2O
Scheme 1. Schematic illustration of the formation of Cu2O nanopar-
ticles on the surface of MWCNTs.
10 2030 4050 60 7080
(2 2 2)
(3 1 1)
(2 2 0)
(2 0 0)
(1 1 0)
(1 1 1)
(0 0 2)
Figure 2. XRD pattern of Cu2O-coated MWCNTs.
K. R. Reddy et al./Scripta Materialia 58 (2008) 1010–1013
a SQUID magnetometer. A plot of magnetization vs. Download full-text
applied magnetic field is shown in Figure 6. The
Cu2O-coated MWCNT composites had saturation mag-
netization (Ms), remanent magnetization (Mr) and coer-
civity (Hc) values of 0.013 emu g?1, 0.0043 emu g?1and
49.24 Oe, respectively. This result indicates that Cu2O-
coated MWCNTs show weak ferromagnetic behavior.
A simple and efficient one-step method based on Feh-
ling’s reaction for the synthesis of a composite of Cu2O
nanoparticles coated on MWCNTs has been demon-
strated. Cu2O nanoparticles of diameters less than
30 nm were well distributed on the walls of CNTs using
this method. The Cu2O nanoparticles coated on the
MWCNTs were confirmed to be phase-pure crystalline
with a face-centered cubic (fcc) structure based on
XRD analysis. With this method, Cu2O nanoparticles
could be dispersed on the surface of many different
substrates, such as polymers or carbon substrates (e.g.
MWCNT composites are expected to find applications
in nanoelectronic devices, catalysis, sensors and electro-
chemical energy storage, and these practical aspects are
currently under investigation.
This work was supported by the 2007 Research
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K. R. Reddy et al./Scripta Materialia 58 (2008) 1010–1013