One-step synthesis of carbon nanotubes with Ni nanoparticles as a catalyst by the microwave-assisted polyol method

Article abstract of DOI:10.1016/j.jallcom.2010.11.131

Carbon nanotubes (CNTs) were firstly synthesized by the microwave-assisted polyol method and magnetic Ni nanoparticles were employed as a catalyst in the process. The structures, morphologies and magnetic properties of the as-synthesized samples were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM), respectively. Our results indicated that CNTs can be synthesized after the observation of a small electric spark with Ni particles used as a catalyst. TEM showed that the length of the hollow carbon nanotubes was of the order of micron. VSM demonstrated that Ni/CNTs composite was ferromagnetic characteristic with hysteretic behavior at room temperature.

Full text of DOI:10.1016/j.jallcom.2010.11.131

Journal of Alloys and Compounds 509 (2011) 2829–2832
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
One-step synthesis of carbon nanotubes with Ni nanoparticles as a catalyst by
the microwave-assisted polyol method
Xian-song Liu∗, Feng Hu, De-ru Zhu, Dao-ning Jia, Peng-peng Wang, Zheng Ruan, Chun-hao Cheng
Engineering Technology Research Center of Magnetic Materials, Anhui Province, School of Physics & Materials Science, Anhui University, Hefei 230039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon nanotubes (CNTs) were firstly synthesized by the microwave-assisted polyol method and mag-
netic Ni nanoparticles were employed as a catalyst in the process. The structures, morphologies and
magnetic properties of the as-synthesized samples were investigated by using X-ray diffraction (XRD),
transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM), respectively. Our
results indicated that CNTs can be synthesized after the observation of a small electric spark with Ni
particles used as a catalyst. TEM showed that the length of the hollow carbon nanotubes was of the order
of micron. VSM demonstrated that Ni/CNTs composite was ferromagnetic characteristic with hysteretic
behavior at room temperature.
Received 11 June 2010
Received in revised form
16 November 2010
Accepted 22 November 2010
Available online 1 December 2010
Keywords:
Carbon nanotubes
Microwave-assisted
Ni catalyst
© 2010 Elsevier B.V. All rights reserved.
Ferromagnetic
1. Introduction
utilized to prepare CNTs, and nano-sized nickel particles were used
as a catalyst in the process. Compared with other approaches,
Since the original report by Iijima in 1991 [1], a consider-
able amount of work has been done on carbon nanotubes (CNTs).
The applications of CNTs and related materials have attracted
great interest over the last decade [2–4]. Owing to their outstand-
ing and unique physical properties derived from a combination
of unique dimensional, structural and topological features [5–7],
such as high flexibility, large aspect ratio, electronic conductive
or semi-conductive behavior, excellent mechanical strength and
high thermal conductivity, CNTs have a wide range of applications
such as hydrogen reservoirs [8], nanoelectrodes [9], mechanical
actuators [10], catalysis [11], sensors [12], electromagnetic char-
acteristics [13], field emission displays [14] and human therapy
[15].
CNTs can be produced by diverse techniques such as chemical
vapor deposition [16–18], arc discharge [19] and laser evaporation
[20]. Up to now, CNTs can be also synthesized by a wide variety of
other routes such as heat treating the polymer [21], flame [22,23],
ion-beam irradiation [24], electrolytic formation [25], in situ syn-
thesis [26], solar synthesis [27] and microwave irradiation heating
method [28]. It is well known that the methods of CNTs synthe-
sis mostly require metallic particles as a catalyst, and magnetic Fe,
Co, Ni nanoparticles are the best catalyst for CNTs growth. In the
present work, the microwave-assisted polyol method was firstly
this method is simple and economical of time. Furthermore, it
does not require any complex or expensive equipments. In this
paper, the effects of surfactant concentration and temperature
were discussed. The magnetism of the nickel nanoparticles was also
investigated. In the case of encapsulated ferromagnetic Ni nanopar-
ticles within carbon nanotubes or any other special components
and structures, it may provide possibilities to develop magnetic
composite materials and find many new applications.
2. Experimental
The materials used in this study were nickel (II) acetate tetrahydrate (99.99%)
and ethylene glycol (>99%). Following our previous work [29–31], before the reac-
tion, 50 ml of a solution of 0.125 M (or 0.250 M) Ni(Ac)2 in ethylene glycol was
prepared in a 100 ml glass flask. Considering that nickel nanoparticles agglomerate
easily while heating [32] and greatly decrease their catalytic efficiency, so-called sur-
factants or dispersants with polyvinylpyrrolidone (PVP, average molecular weight
40,000) had to be added to solve the dispersibility of nickel nanoparticles. During
our experiments, 2–4 g PVP was added. After the solution was ready, the flask was
placed in the center of a microwave oven (Spectra, 900 W), connected to a condenser,
purged by argon for about 20 min, and then the microwave oven was turned on at
the power level of 50% (or 60%, or 70%, or 80%) with a continued flow of the gas. The
argon flowing rate was 80 ml/min. After reacting for about 20 min, the black sus-
pension appeared. If the inner wall of the glass flask was plated with metal nickel,
a small electric spark was observed. One or two minutes later, the microwave oven
was turned off and the gas valve was closed. Then the flask was taken out and corked
up, cooled in ice water immediately. The resulting solid product was washed thor-
oughly with ethanol and centrifuged. All these processes were repeated five times.
The products were dried under vacuum and kept in a glove box.
∗
Corresponding author. Tel.: +86 555 5107674; fax: +86 555 5107674.
E-mail address: xiansongliu@yahoo.com.cn (X.-s. Liu).
The powder X-ray diffraction (XRD) data were collected on a Bruker AXSD*
Advanced Powder X-ray diffractometer by using Cu-K␣ radiation (ꢀ = 0.15418 nm).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.11.131

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X.-s. Liu et al. / Journal of Alloys and Compounds 509 (2011) 2829–2832
which demonstrates Ni particles with the particle sizes in the range
(111)
of 20–30 nm. It demonstrated that a suitable amount of PVP as sur-
factant was needed during the microwave-assisted reaction and
prevented the agglomeration. It can be thought that on the one
hand the nucleation of Ni was accelerated with microwave irradi-
ation [34], on the other hand, PVP as surfactant could absorb and
decrease the surface tension. Therefore the single particle was pre-
vented from growing, and several particles were prevented from
coming together with adding a suitable amount of PVP [31]. Thus it
indicates that this technique is an effective and convenient method
to obtain dispersed and active Ni nanoparticles.
In order to provide a direct evidence for carbon nanotubes
formation and to investigate the morphology, the as-synthesized
samples were examined by transmission electron microscopy
(TEM). As a typical example, two magnified TEM images of CNTs
synthesized are shown in Fig. 3. Carbon nanotubes synthesized in
our experiment have a tubular structure with smooth and uniform
walls. We can see that the length of hollow carbon nanotubes is
of the order of micron. The diameters of most of the CNTs are
about 20 nm. Especially, in our experiment a very large carbon
nanotube with a diameter of 40 nm can be also clearly observed.
Carbon nanotubes can be distinguished from their solid fibre coun-
terparts by their wall characteristics. The former typically have
smooth and uniform walls, whereas the latter tends to have bumpy
sides and rarely grows straight [35]. By comparison with earlier
report [4], single-walled nanotubes have diameters in the range
0.4–2 nm. Typical multi-walled nanotubes diameters grown by the
arc-discharge method are about 20 nm in diameter, while CVD
grown nanotubes can have much larger diameters of up to 100 nm.
: NiO
*
(200)
(220)
(d)
(c)
(b)
(a)
*
*
20
30
40
50
θ (degree)
60
70
80
2
Fig. 1. X-ray powder diffraction pattern of the samples generated using different
power levels of the microwave oven: (a) 50%; (b) 60%; (c) 70%; (d) 80%.
The patterns were recorded in the 2-theta (2ꢁ) range from 20◦ to 80◦. Transmission
electron microscopy (TEM) investigations were carried out using a JEOL-JEM 100
SX microscope operating at 100 kV. Samples for TEM observation were prepared by
sonicating in absolute ethanol for 1–2 min, followed by placing a drop of the sam-
ple suspension on a copper grid (400 meshes, Electron Microscopy Sciences) coated
with carbon film. The magnetic measurements were carried out using a BHV-55
type vibrating sample magnetometer (VSM) at room temperature with an applied
magnetic field up to 1 T to reach saturation values.
3. Results and discussion
The XRD patterns of the catalyst after the black suspension
appeared but before a small electric spark can be observed at vari-
ous conditions are shown in Fig. 1. The positions of the diffraction
peaks are in good accordance with the reported JCPDS 04-0850
data. As we can see, when the power level employed was 50%
of the microwave oven, besides the phase of Ni, a phase of NiO
(JCPDS 78-0643) was found by XRD simultaneously (Fig. 1a). This
can be explained by the fact that the power level of 50% was not
enough to obtain nickel particles by reducing reaction. It was also
observed that when the microwave oven was turned on at above
the power level of 50%, a single phase of Ni with face-centered cubic
(fcc) structure was formed in the process of CNTs synthesis and no
other impurity was detected in the XRD patterns (Fig. 1b–d). More-
over, with increasing the power level, the relative intensity of the
diffraction peak was stronger and stronger. All the peaks at different
angles correspond to fcc Ni (1 1 1), (2 0 0) and (2 2 0). The grain size
of nickel particle was calculated according to the Scherrer formula
[33]:
0.9ꢀ
D =
ˇ cos ꢁ
where D is the particle size, ꢀ is wavelength of X-ray, ˇ is full width
at half maximum measured in radians and ꢁ is the Bragg angle of
the (1 1 1) plane. The values obtained are 26.8, 30.6 and 38.2 nm,
respectively. This clearly indicates that Ni particles used as a cat-
alyst in this experiment are nano-sized with enough power level
of the microwave oven, these nanoparticles are very advantageous
for CNTs growth.
In order to reveal the morphology and structure of the catalyst
after reduction, the TEM experiment was performed for the catalyst
formed under a solution of 0.250 M Ni(Ac)2 in ethylene glycol. Dur-
ing the current experiments, some of PVP were added (3 g PVP) and
the microwave oven was turned on at a power level of 60–70%. Two
TEM images of the catalyst after the black suspension appeared but
before a small electric spark can be observed are shown in Fig. 2,
Fig. 2. TEM images of Ni catalyst nanoparticles after the black suspension appeared
but before the small electric spark observed: (a) at the power level of 60% of the
microwave oven; (b) at the power level of 70% of the microwave oven.

X.-s. Liu et al. / Journal of Alloys and Compounds 509 (2011) 2829–2832
2831
The combined results of the thickness of the walls and the relatively
large diameters of the nanotubes demonstrate that these nanotubes
are possibly multi-walled. The possible carbon source may be the
ethylene glycol, or acetate ions, or PVP. We are inclined to think
that carbon nanotubes are made from the ethylene glycol. This is
in agreement with previous results using a similar carbon source
with alcohol [36]. During our experiments for synthesizing carbon
nanotubes, it was very key factor that an electric spark was excited
under the microwave owing to the inner wall of the glass flask
plated with metal nickel.
For deep insight into the nature of the as-synthesized samples,
the magnetic properties of the samples have been measured by
VSM at room temperature. The static hysteresis loops of the sam-
ples are shown in Fig. 4. During the experiment, the amount of
nickel was normalized. The magnetic hysteresis loop of Ni nanopar-
ticles obtained after the black suspension appeared but before the
small electric spark, was given with the coercivity HC = 89.9 Oe and
saturation magnetization ꢂs = 38.1 A m2/kg (Fig. 4a). For Ni/CNTs
composites obtained by reacting for 1 min after the small electric
spark was measured. The values obtained are with HC = 119.9 Oe
and ꢂs = 32.4 A m2/kg (Fig. 4b). The result corresponding to Ni/CNTs
sample reacting for 2 min after the electric spark is shown in Fig. 4c,
which has a larger hysteresis loop with coercivity (HC = 162.4 Oe)
and saturation magnetization (ꢂs = 23.4 A m2/kg). The hysteresis
loop of all the samples shows ferromagnetic characteristic. The sat-
uration magnetization value for Ni/CNTs composites is lower than
those of Ni nanoparticles without the small electric spark under the
same measurement conditions. The coercivity is larger for Ni/CNTs
composites regarding Ni nanoparticles and bulk Ni (HC = 0.7 Oe)
Fig. 4. The magnetic hysteresis loops of (a) Ni nanoparticles obtained after the black
suspension appeared but before the small electric spark observed; (b) Ni/CNTs com-
posites obtained by reacting for 1 min after the small electric spark observed; (c)
Ni/CNTs composites obtained by reacting for 2 min after the electric spark.
[37], which is similar to those found in the literature [38]. It can
be thought that CNTs may have a great impact on the magnetic
properties of Ni particles. The reason of this phenomenon is not
fully clear now and the further work is underway.
4. Conclusions
In this study, CNTs of diameter about 20 nm were success-
fully synthesized by the microwave-assisted polyol method and
magnetic Ni nanoparticles were employed as a catalyst. As the
microwave eddy current effect excited an electric spark on the inner
wall of a glass flask which was plated metal nickel in the glass flask,
metal Ni nanoparticles can induce CNTs’ formation in the further
reaction. CNTs can be observed after the observation of the small
electric spark, and the length of hollow carbon nanotubes is of the
order of micron. The further experimental results indicated that
the as-synthesized samples were ferromagnetic. Ni/CNTs compos-
Fig. 3. Magnified TEM images of CNTs synthesized at the power level of 80% of the
microwave oven.

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X.-s. Liu et al. / Journal of Alloys and Compounds 509 (2011) 2829–2832
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We would like to acknowledge the financial support from the
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