Synthesis and surface plasmon resonance properties of carbon-coated Cu and Co nanoparticles

Article abstract of DOI:10.1016/j.materresbull.2011.01.014

Carbon-coated Cu and Co nanoparticles were synthesized by the carbonization of PVA-metal hydroxide complexes. The possible reaction process and surface plasmon resonance (SPR) properties of the Cu and Co nanoparticles enwrapped in carbon layer were explored. The XRD results showed that no byproducts, such as oxides and carbides, were formed in the products, and the Cu and Co nanoparticles were effectively protected against oxidation by the carbon layer. The size ranges of the metal nanoparticles were 20-50 nm for Cu and 15-65 nm for Co, respectively. UV-vis absorption spectra showed that the SPR bands of the Cu and Co nanoparticles coated with carbon were red-shifted mainly due to the increase of the effective dielectric constant of the surrounding medium induced by the carbon layer.

Full text of DOI:10.1016/j.materresbull.2011.01.014

Materials Research Bulletin 46 (2011) 743–747
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis and surface plasmon resonance properties of carbon-coated
Cu and Co nanoparticles
a,b
a,
a
Jing Li , Chun-yan Liu *, Zheng Xie
a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancun,
Beijing 100190, PR China
b
Graduate School of the Chinese Academy of Sciences, Beijing 100806, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon-coated Cu and Co nanoparticles were synthesized by the carbonization of PVA–metal hydroxide
complexes. The possible reaction process and surface plasmon resonance (SPR) properties of the Cu and
Co nanoparticles enwrapped in carbon layer were explored. The XRD results showed that no byproducts,
such as oxides and carbides, were formed in the products, and the Cu and Co nanoparticles were
effectively protected against oxidation by the carbon layer. The size ranges of the metal nanoparticles
were 20–50 nm for Cu and 15–65 nm for Co, respectively. UV–vis absorption spectra showed that the
SPR bands of the Cu and Co nanoparticles coated with carbon were red-shifted mainly due to the increase
of the effective dielectric constant of the surrounding medium induced by the carbon layer.
ß 2011 Elsevier Ltd. All rights reserved.
Received 18 August 2010
Received in revised form 24 November 2010
Accepted 15 January 2011
Available online 26 January 2011
Keywords:
A. Nanostructures
B. Chemical synthesis
D. Optical properties
1
. Introduction
Metal nanoparticles have attracted great interest due to their
distinguishing characteristics of lower cost and near-infrared
plasmon resonance energy [13]. Co nanoparticles have both the
SPR property [14] and good magnetic properties [15,16].
novel optical properties from surface plasmon resonance (SPR),
which occurs when an electromagnetic field drives the collective
oscillation of free electrons from metal nanoparticle into
resonance. This resonance creates large local electric field
enhancement at the nanoparticle surface, which can be used to
manipulate light–matter interactions and boost non-linear phe-
Carbon-coated nanomaterials, especially metals, are of great
significance due to their stability to oxidation and degradation, and
they show unique optical [17–19], catalytic [20,21] and magnetic
properties [15,16,22]. Various techniques have been developed for
the synthesis of carbon-coated metal nanoparticles (metal/C), such
as arc discharge in different atmospheres with a carbon rod as a
cathode and a metal or alloy block as anode [23–25], chemical
vapor deposition [26], magnetron and iron-beam cosputtering
[27], thermal plasma jet [28], and flame synthesis [29]. However,
these methods are apparently not suitable for the preparation of
metal/C efficiently in large scales. In our previous work [30], we
synthesized carbon-coated copper nanoparticles through a solid-
state reduction process, which involved in the mixing of glucose
and cupric carbonate by hand-grinding in an agate mortar and
subsequent calcination under nitrogen. Yet, the solid phase mixing
process faced the drawback of relatively poor uniformity. Recently,
solution-based carbonization approach has been developed, which
is a much more effective route to synthesize carbon-coated metal
nanomaterials, such as carbon-coated Cu and Ni nanoparticles
synthesized in ionic liquid under microwave heating [31], carbon-
coated Te and Se nanocables [32], and carbon-coated Ag
nanocables under hydrothermal conditions [33]. This route
nomena. These properties suggest
including biological and chemical sensing [1,2], nonlinear optics
3], surface-enhanced Raman spectroscopy (SERS) [4] and elec-
tronics [5].
Noble metal nanoparticles, such as Au and Ag, have been
a range of applications,
[
extensively studied for SPR properties and related applications due
to their flexible synthetic routes. They can resonantly absorb and
scatter incident light upon the excitation of their surface plasmon
oscillation, typically in the visible range and can be tuned in the
range of visible and near-infrared by adjusting the size and shape
of metal nanoparticles, as well the optical and electronic properties
of the medium surrounding the particles [6–12]. Whereas, bare
metal nanoparticles, especially active metals (Cu, Co, etc.) can be
easily oxidized in an oxidizing medium, forming an oxidation layer
on the particle surface. Furthermore, in comparison with noble
metal nanoparticles (Au and Ag), Cu nanoparticles have their own
n+
involves the reduction of M to M (metal) accompanied by the
carbonization of carbon source in one reaction system. However,
this method is not mature and requires further research and
optimization. In this context, we reported a simple and effective
*
Corresponding author. Tel.: +86 010 82543573; fax: +86 010 62554670.
E-mail address: cyliu@mail.ipc.ac.cn (C.-y. Liu).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.01.014

7
44
J. Li et al. / Materials Research Bulletin 46 (2011) 743–747
route to synthesize carbon-coated Cu and Co nanoparticles
through the carbonization of PVA–metal hydroxide complexes.
The solution complex process overcame the drawback of poor
uniformity arising from the solid phase mixing. Additionally, our
method is suitable for the preparation of metal/C in large scales.
The possible reaction process and SPR properties of the Cu
and Co nanoparticles coated with carbon were investigated
subsequently.
obtained with a Bruker D8 Focus under Cu K
˚
a
radiation at
1.54056 A with a scanning speed of 48/min in the 2
u range of 20–
758. The Raman spectra were recorded on Via-Reflex Raman
system, using 532 nm excitation wavelength (He–Ne laser).
Ultraviolet–visible (UV–vis) absorption was recorded on
Shimadzu UV-1601 PC spectrophotometer.
a
3. Results and discussion
2
. Experimental
The possible reaction process for the preparation of Cu/C and
2
+
2+
Co/C was shown in Fig. 1. Metal ions (Cu or Co ) firstly
coordinated with the homogenous polar radicals of hydroxy
groups (–OH) in PVA, and then transformed into metal hydroxide
when the system pH value changed to weak alkaline by the
addition of concentrated ammonia solution. During subsequent
carbonization process, metal hydroxide decomposed into metal
oxide, and PVA (with a carbonization temperature of around 300 8C
[34]) carbonized into carbon around metal oxides, and the
surrounding carbon as a reductant reduced metal oxide to metal
nanoparticles synchronously.
2
.1. Materials
Copper(II) nitrate trihydrate (Cu(NO
3
)
2
ꢀ3H
2
O, A.R.), cobalt(II)
nitrate hexahydrate (Co(NO
3
)
2
ꢀ6H
2
O, A.R.), and polyvinyl alcohol
(PVA, C.R.) with an average molecular weight of 77,000 were
purchased from Beijing Chemical Factory (China), and used as
received.
2.2. Synthesis of carbon-coated copper nanoparticles (Cu/C)
The XRD patterns of the desired products were given in Fig. 2.
The crystalline XRD patterns of the products (Fig. 2a) fitted well
with that of metallic Cu (face-centered cubic, JCPDS 04-0836), and
the patterns in Fig. 2b matched that of metallic Co (face-centered
cubic, JCPDS 15-0806). No characteristic peaks for carbon were
observed due to their amorphous nature.
A copper nitrate aqueous solution (20 mL, 0.2 M) was added to a
PVA aqueous solution (100 mL, 1.5 wt%) under continuous stirring
2+
at 75 8C to coordinate Cu ions with hydroxy groups (–OH) in PVA
2+
thoroughly. The molar ratio of Cu ions to hydroxy groups (–OH)
in PVA is 1:8. The pH value of the solution was adjusted up to about
8
.0 using a concentrated ammonia solution. A precursor was
Regular TEM and HRTEM images were shown in Fig. 3.
Obviously, the nanoparticles were embedded in a matrix of
carbon. The size of the nanoparticles was 20–50 nm for Cu (Fig. 3a)
and 15–65 nm for Co (Fig. 3c). The space fringes detected in the
HRTEM images were well matched with the {1 1 1} planes of Cu
(Fig. 3b) and {1 1 1} planes of Co (Fig. 3d), respectively.
collected after centrifugated at 4000 rpm for 15 min, washed with
water and ethanol, and naturally dried in air. This precursor was
heated to 350 8C with a heating rate of 5 8C min and maintained
for 1 h in a tube furnace, and then cooled to room temperature
under nitrogen, and Cu/C was obtained.
ꢁ
1
We took Cu/C as an example to investigate the effect of the
molar ratio of PVA to metal on the construction of products. The
curve a in Fig. 4 was the XRD pattern of the sample prepared with
the molar ratio of 2:1, in which there existed two weak peaks at
2
.3. Synthesis of carbon-coated cobalt nanoparticles (Co/C)
The synthetic route to Co/C is similar to that of Cu/C, except that
the heat-treatment temperature increased from 350 8C to 800 8C.
2
37.18 and 61.38 indexed to cubic Cu O (JCPDS, File No. 05-0667), in
addition to the clear diffraction peaks at 43.38, 50.48, and 74.18
2.4. Characterization
corresponding to face-centered cubic metallic Cu (JCPDS, File No.
0
4-0836). That meant a trace amount of Cu
indicating the amount of PVA was not enough to protect Cu
nanoparticles completely. The intensity of the Cu peaks
2
O coexisted with Cu,
The TEM images were taken on a JEM 2100F transmission
electron microscope, using an accelerating voltage of 200 kV. The
samples were dispersed in ethanol by sonication and dropped onto
a conventional carbon-coated copper grid. The XRD patterns were
2
O
decreased with the increase of the molar ratio from 2:1 to 4:1
(Fig. 4b). When the molar ratio of PVA to Cu increased to 8:1, the
Fig. 1. The reaction process for the preparation of Cu/C and Co/C.

J. Li et al. / Materials Research Bulletin 46 (2011) 743–747
745
Fig. 4. XRD patterns of the products prepared with different PVA/Cu molar ratios:
a) 2:1, (b) 4:1 and (c) 8:1.
Fig. 2. XRD patterns of (a) Cu/C and (b) Co/C.
(
ꢁ1
faint Cu
2
O peaks disappeared, and all peaks of the sample indexed
1375 cm (disordered D band) can be attributed to the A1g model,
to face-centered cubic metallic Cu (Fig. 4c), indicating the Cu
nanoparticles were effectively protected by carbon.
The Raman spectra of the samples confirmed the presence of
carbon on the Cu and Co nanoparticles (Fig. 5a and b). The peak at
and is associated with the structural defects and disorder in the
ꢁ
1
carbon material. The peak at 1580 cm (G band) has E2g symmetry
2
and is related to the vibration of sp -bonded carbon atoms in a
two-dimensional hexagonal lattice, from the stretching modes of
Fig. 3. (a) TEM images of Cu/C, (b) HRTEM images of Cu/C, (c) TEM images of Co/C and (d) HRTEM images of Co/C. The carbon coating was indicated with arrows in the TEM
images.

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J. Li et al. / Materials Research Bulletin 46 (2011) 743–747
Fig. 6a and b, the SPR bands for Cu/C and Co/C were obviously red-
shifted to 608 nm and 424 nm, respectively, which were mainly
due to the surface carbon layer. According to the Drude model, the
SPR band position
l is related to the effective dielectric constant of
the surrounding medium by the relation [38]:
2
2
p
1
l
¼
l
ð
e
þ 2eeff
Þ
where
plasma wavelength,
due to interband and core transitions,
constant of surrounding medium. For Cu/C and Co/C,
l
is the position of the SPR band,
l
p
is the metallic bulk
1
e
is the high frequency dielectric constant
eff is the effective dielectric
e
eeff can be
described as [39]:
eeff
¼
e
waterð1 ꢁ
FÞ þ ecarbon
F
where ewater and ecarbon is the effective dielectric constant of water
and amorphous carbon, and is the volume fraction of the carbon
F
layer. Because the refractive index of carbon (ncarbon = 1.6–2.0) is
Fig. 5. Raman spectra of Cu/C (a) and Co/C (b).
higher than water (nwater = 1.33), the surface carbon layer would
2
lead to the increase of
shift of the SPR band.
e
eff (=n ) and consequently induce the red
C 55 C bonds [35]. The relative intensities of these two bands reflect
the graphitization degree [36], and the fact that these two broad
peaks overlap strongly indicates the amorphous structure of the
carbon layer prepared in the present work.
4. Conclusions
The SPR band of Cu nanoparticles distributes in the region of
In conclusion, we have synthesized Cu/C and Co/C through the
5
60–590 nm [13,37]. And a recent study showed that the bare Co
carbonization of PVA–metal hydroxide complexes. The SPR bands
of the Cu/C and Co/C were obviously red-shifted mainly due to the
increase of the effective dielectric constant of the surface carbon
layer. This method can also be used to prepare other metal
nanoparticles coated with carbon, if the following conditions are
sufficient: (1) the metal ions interact strongly with hydroxy groups
nanoparticles exhibited a SPR band at 270 nm [14]. As shown in
(
–OH) in PVA; (2) the metal oxides can be reduced by the carbon at
a suitable temperature; (3) the metal does not react with carbon to
form metal carbides during a following heating treatment.
Acknowledgement
The author thanks the support of 973 Program and Chinese
Academy of Sciences.
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