Synthesis of nickel nanoparticles and carbon encapsulated nickel nanoparticles supported on carbon nanotubes

Article abstract of DOI:10.1016/j.jssc.2005.10.001

Nickel nanoparticles were prepared and uniformly supported on multi-walled carbon nanotubes (MWCNTs) by reduction route with CNTs as a reducing agent at 600 °C. As-prepared nickel nanoparticles were single crystalline with a face-center-cubic phase and a size distribution ranging from 10 to 50 nm, and they were characterized by transmission electron microscopy (TEM), high-resolution TEM and X-ray diffraction (XRD). These nickel nanoparticles would be coated with graphene layers, when they were exposed to acetylene at 600 °C. The coercivity values of nickel nanoparticles were superior to that of bulk nickel at room temperature.

Full text of DOI:10.1016/j.jssc.2005.10.001

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Journal of Solid State Chemistry 179 (2006) 91–95
www.elsevier.com/locate/jssc
Synthesis of nickel nanoparticles and carbon encapsulated nickel
nanoparticles supported on carbon nanotubes
Ã
Jipeng Chenga, , Xiaobin Zhanga, Ying Yeb
aDepartment of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
bDepartment of Earth Science, Zhejiang University, Hangzhou 310027, PR China
Received 3 August 2005; received in revised form 30 September 2005; accepted 3 October 2005
Available online 2 November 2005
Abstract
Nickel nanoparticles were prepared and uniformly supported on multi-walled carbon nanotubes (MWCNTs) by reduction route with
CNTs as a reducing agent at 600 1C. As-prepared nickel nanoparticles were single crystalline with a face-center-cubic phase and a size
distribution ranging from 10 to 50 nm, and they were characterized by transmission electron microscopy (TEM), high-resolution TEM
and X-ray diffraction (XRD). These nickel nanoparticles would be coated with graphene layers, when they were exposed to acetylene at
600 1C. The coercivity values of nickel nanoparticles were superior to that of bulk nickel at room temperature.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Carbon nanotube; Nickel nanoparticles; Microstructure; Carbon encapsulation; Magnetic property
1. Introduction
nanoparticles loaded on CNTs were reported recently,
such as Pt [6], Cu2O [7], Au [8], TiO2 [9], CeO2 [10].
Metallic nickel nanoparticles have attracted much
attention because of their applications as catalyst and
conducting or magnetic materials [11–12]. It is anticipated
that nanoscaled nickel particles should be of unusually
catalytic ability and differ considerably from that of bulk
materials. While the nanoscaled aspects usually cause an
intractable agglomeration, it will constrain the potential
merit to certain degree. So, CNT is an ideal support for
nickel nanoclusters because of its nanoscaled and steady
structure characteristics and morphology, as well as inert
and resistant properties.
Metal nanoparticles are usually activated and tend to be
oxidized in the open air. Carbon encapsulation of magnetic
nanoparticles could retain their intrinsic nanocrystalline
properties [13] and keep them from oxidization [14]. The
carbon coatings can endow these magnetic particles with
biocompatibility and stability in many organic and
inorganic media. Those magnetic systems are suitable for
potential applications like ferrofluids, magnetic recording
media, and medical imaging. Some novel magnetic systems
of carbon encapsulated ferromagnetic materials, e.g., iron,
cobalt and nickel, have successfully been obtained by a
Carbon materials are often used as support for preparing
metal-based catalyst due to resistance to acid/basic media
and easy recovery of precious metals by support burning
[1]. Carbon can present many allotropic forms, which
includes fullerenes [2] and carbon nanotubes (CNTs) [3]
that were discovered decades ago. Recently, CNTs have
generated an intense effervescency in scientific community.
The preparation of nanocomposite materials from CNTs
and metal or metal oxide nanoparticles has important
implications to the development of advanced catalyst and
sensory materials. This category of composite currently
attracted much attention [4]. The modification and
anchoring of nanoparticles on the external surface of
CNTs can dramatically influence their physical properties,
such as electrical conductance, which may lead to novel
changes that are relevant to practical applications. Kong et
al. [5] reported that modification of CNTs with Pd particles
would impact sensitivity of nanotube electrical properties
to fabricate sensors. Various metal and metal oxide
Ã
Corresponding author. Fax: +86 571 87951411.
E-mail address: mseem@zju.edu.cn (J. Cheng).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.10.001

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number of researchers [13,15–16]. However, the prepara-
tion of carbon encapsulated metallic nanoparticles in a
high yield with a simple method still represents a significant
challenge [14,17–18].
In this paper, a simple method is described for the
preparation of nickel nanoparticles well dispersed on multi-
walled CNTs (MWCNTs). The resultant hybrid is char-
acterized in detail and applied to react with acetylene at
600 1C. These metallic nanoparticles are thus encapsulated
with graphene sheets. The encapsulation procedure is based
on the large body of knowledge developed for the growth
of CNTs. The magnetic properties of as-prepared nickel
nanoparticles are preliminary evaluated.
2. Experimental section
The CNTs used as the support of nickel particles were
fabricated by pyrolysis of acetylene using metals supported
on nanocrystalline CaCO3 [19]. Fe–Co bimetallic catalyst
dispersed on CaCO3 reacted with acetylene at 750 1C to
obtain CNTs. As-prepared CNTs were purified by reflux-
ing in diluted nitric acid for hours to remove residual
impurities at 140 1C. After cooling to ambient temperature,
the black suspension was filtered with a ceramic filter and
washed with adequate distilled water until neutral, and
then dried. Some CNTs (ꢀ1 g) were thoroughly dispersed
into a saturated Ni(NO3)2 solution (ꢀ30 ml) for two days
at room temperature. After filtration with filter paper, the
nickel-impregnated CNT powder was dried at 130 1C.
The nickel-impregnated CNTs were kept at 600 1C for
20 min under nitrogen atmosphere. Then, nickel ions on
CNTs were reduced by its support and transformed into
nanoclusters, which was confirmed by micrometric ana-
lyses. A hybrid of nickel-deposited CNTs was thus
obtained. To encapsulate these nickel particles, an amount
of nickel-deposited CNTs was applied as catalyst to
decompose acetylene at 600 1C, which is based on the
CNT synthesis. These nickel nanoparticles were thus fully
encapsulated by graphene layers.
Transmission electron microscopy (TEM) was used to
observe the nanostructures of CNTs and nickel nanopar-
ticles. X-ray diffraction (XRD) spectra were recorded with
D/max-rA using CuKa radiation. Field emission scanning
electron microscopy (FSEM, FEI sirion) equipped with an
energy dispersive X-ray spectrometer (EDS) was employed
for morphology examination and elemental analysis.
Finally, the magnetic properties of products were obtained
from hysteresis loops recorded in a vibrating sample
magnetometer (VSM) at room temperature.
Fig. 1. SEM image of purified MWCNTs (a), SEM (b) and TEM
(c) images of nickel particle deposited CNTs.
3. Results and discussion
number of nanoparticles homogeneously attached on the
external surface of CNTs were observed with a wide size
distribution, as shown in Fig. 1b. Fig. 1c shows a bright-
field TEM image of nickel particles attached on CNTs with
a high magnification. The morphologies of particles and
CNTs are clearly observed. The size of nanoparticles varies
from 10 to 50 nm. Most particles are quasi-spherical in
Figs. 1a and b show two typical SEM micrographs of
MWCNTs purified by nitric acid and nickel-deposited
carbon tubes, respectively. From Fig. 1a, it can be seen that
the purified CNTs are bare multi-wall with a large diameter
distribution. However, after the deposition of nickel, a

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Ni(111)
C
Ni(200)
CNT
Ni(220)
(a)
Ni
Ni
O
(b)
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
Energy (KeV)
10
20
30
40
50
60
70
80
2 Theta/ degree
Fig. 2. EDS spectrum of nickel-deposited carbon nanotubes.
Fig. 3. XRD patterns of nickel-deposited CNTs (a) and carbon
encapsulated nickel particles (b), showing the diffraction peaks of graphite
and nickel.
shape and tightly dispersed on the surface of CNTs. The
elemental analysis by EDS proves that the resulting
product contains the elements C, nickel, and O in Fig. 2.
The results of EDS indicate that the content of nickel in the
compound is about 12% in weight. Combining these with
SEM and TEM analyses, it is reasonable to conclude that
nickel nanoparticles have been successfully prepared on
CNTs surface. The presence of oxygen comes from some
functional groups that were introduced by nitric acid
oxidation [20]. However, few nickel nanoparticles could be
attached to the as-prepared CNTs by the same procedure.
The inert, pristine surface of as-prepared CNTs makes it
difficult for attachment of metal precursors [21]. It has been
pointed out that most metals would not adhere to them
[22]. Then, two main approaches are developed, i.e. surface
modification and sensitization activation. The former is
associated with the oxidation of the nanotube surface to
create functional groups and increase metal nucleation
[8,21]. The latter consists of the generation of small nuclei
to further promote metal deposits on CNTs [23]. The
reasonable formation process of nickel nanoparticles in our
case is as following. In reflux with nitric acid, the surface
graphene layers of CNTs would react with the oxidants
and produce a high density of various functional groups,
such as hydroxyl, carboxyl and carbonyl [20]. With the
presence of high-density surface functional groups, CNTs
showed much better dispersibility in water compared with
the raw nanotube material [21]. These functional groups
could provide active sites for interaction with nickel cations
during the immersion process. At elevated temperature,
nickel ions congregated on the surface of CNTs and were
reduced by carbon atoms in the inert atmosphere at 600 1C.
Hence, nickel nanoparticles were formed on CNTs. The
method to deposit nickel nanoparticles on CNTs is easily
carried out in our case, avoiding sensitization and
activation [23], as well as hydrogen reduction [7].
The powder XRD profile of nickel-deposited CNTs
(Fig. 3a) clearly shows the presence of a majority of face-
centered-cubic (fcc) nickel (PCPD file: No. 04-0850), with
an individual diffraction peak of graphite (002) at ꢀ261
from CNTs. In addition, there is no nickel carbide of the
cementite phase and nickel oxides detected. In accordance
with above results, one can believe that nickel ions
originally absorbed on CNTs are reduced by carbon atoms
and they agglomerate to form nanoparticles. Koltypin et
al. [24] have also found that nickel ions could be reduced to
nanosized nickel particles with amorphous carbon as
reducing agent in an inert gas atmosphere at 500 1C. Even
micron-sized nickel oxide particles could be reduced by
natural graphite at 950 1C [25]. The broadening of nickel
peaks in the XRD pattern is due to the small particle size.
The size of the nickel nanoparticles is also estimated using
the Scherer equation in a form: t ¼ 0:9l=B cos yB [26],
where t is the particle size, l the characteristic wavelength
of the X-ray used, B the angular width in radians at an
intensity equals to half of the maximum peak intensity, and
yB the Bragg angle in degree at which the diffraction
occurs. The average particle size of the deposited nickel
nanoparticles is estimated as 19 nm, calculated from the
Scherer equation using nickel (111) peak at 2y of 44.51.
This size generally approximates to that of TEM observa-
tions.
A magnified view of a nickel particle is presented in Fig.
4, a typical high-resolution TEM micrography. It displays
fringe patterns, indicating the crystalline nature of these
nickel particles. Nickel nanoparticles dispersed on the
CNTs are all single crystalline. Fig. 4 represents such a
nickel particle. Viewed along the [011] direction, two twin
planes are detected with lattice spacing around 0.2 nm,
which corresponds to the 1 1 1 plane, i.e. (11ꢁ1) and
(1ꢁ11) planes (marked). These planes enclose an angle of

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Fig. 5. TEM image of carbon encapsulated nickel nanoparticles that
originally deposited on CNTs.
1.0
0.5
0
(b)
(a)
Fig. 4. HRTEM micrograph of a nickel nanoparticle, evidencing its
crystalline structure. Twin (111) planes are indicated with arrows. Inset is
a Fourier transform image of the marked area.
70.31, which are consistent with standard fcc nickel crystal.
Fast Fourier transform (FFT) from the marked square
region in the high resolution TEM image of the nickel
nanoparticle shows spots corresponding to the (111) planes
of nickel, as presented in the inset of Fig. 4.
0.5
1.0
After as-prepared nickel-CNT hybrid reacted with
acetylene at 600 1C for 20 min, most nickel nanoparticles
were wrapped by graphene layer, which was proved by
TEM examinations. A TEM picture of the product is
shown in Fig. 5. The metallic core is encapsulated in
polyhedral concentric graphene shells with a varying
number of layers. The XRD pattern of carbon encapsu-
lated nickel particles is presented in Fig. 3b, where only
carbon and nickel are detected. Although it is known that
nickel is easily oxidized to oxides, some possible oxides
such as NiO, Ni2O3 are not observed in the XRD profile.
This is chiefly due to the encapsulation of graphene layers
which protect it from oxidization. Because the sample had
been exposed to the air for a long time before being
examined, the absence of nickel oxides also confirmed the
protection of a closed carbon shell on the material from
oxidation by the air, as reported elsewhere [14]. The
average metallic particle size of encapsulated nickel is
appreciably larger than that of bare nickel particles from
the half angular width in their XRD profiles. This results
from a sintering propensity at an elevated temperature.
Since the catalytic nickel particles have to expose certain
surface areas to decompose acetylene in order to obtain
carbon atoms, it takes some time for nickel particles to be
supersaturated in carbon at such a relatively low tempera-
-10000
-5000
0
5000
10000
Coercivity (Oe)
Fig. 6. Room temperature magnetization loops of nickel nanoparticles (a)
and carbon encapsulated nickel nanoparticles (b) on CNTs.
ture. As a result, the thicker graphene layers tend to be
accumulated on the surface of the catalytic particles [27].
No carbide peaks in XRD have been exhibited, indicating
that excess carbon atoms could not be stabilized as carbide
within the particles. These particles are energetically
favored for encapsulating themselves into carbon cages
that is highly dependent on the experimental conditions, as
well as properties of metallic particles.
The magnetization curves of nickel and carbon encapsu-
lated nickel nanoparticles on CNTs are shown in Fig. 6.
The spectra show a ferromagnetic response for two samples
and the curves are symmetric around H ¼ 0. Its coercivity
(Hc) is determined to 525 Oe for nickel nanoparticles, and
384 Oe for carbon encapsulated nickel particles. The Hc
value of the bulk nickel is about 100 Oe [16]. The observed
increase of Hc with the decreasing of the particles size

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