Low temperature fabrication of hollow carbon nanospheres over Ni/Al2O3 by the catalytic method

Article abstract of DOI:10.1016/j.jallcom.2007.10.090

A mass of hollow carbon nanospheres was fabricated by chemical vapor deposition of methane over Ni/Al₂O₃ catalyst at 600 °C. The products were characterized with scanning electron microscope, transmission electron microscope, and high-resolution transmission electron microscope images. The results showed that the external diameter of the hollow carbon nanospheres was 5-90 nm and the thickness of the wall was about 15 nm. And a possible formation mechanism of the hollow carbon nanospheres was discussed.

Full text of DOI:10.1016/j.jallcom.2007.10.090

Journal of Alloys and Compounds 465 (2008) 387–390
Low temperature fabrication of hollow carbon nanospheres
over Ni/Al2O3 by the catalytic method
Haipeng Lia,b, Naiqin Zhaoa,∗, Chunnian Hea, Chunsheng Shia,
Xiwen Dua, Jiajun Lia
a School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
b School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
Received 10 August 2007; received in revised form 13 October 2007; accepted 20 October 2007
Available online 26 October 2007
Abstract
A mass of hollow carbon nanospheres was fabricated by chemical vapor deposition of methane over Ni/Al2O3 catalyst at 600 ◦C. The products
were characterized with scanning electron microscope, transmission electron microscope, and high-resolution transmission electron microscope
images. The results showed that the external diameter of the hollow carbon nanospheres was 5–90 nm and the thickness of the wall was about
15 nm. And a possible formation mechanism of the hollow carbon nanospheres was discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Fullerenes; Nanostructures; Vapor deposition; Chemical synthesis; X-ray diffraction
1. Introduction
via high temperature with certain catalyzers [16,17], shock com-
pression from C60 fullerene [18], media-reduction route [19,20],
self-assembly approach [21], etc. Complicated processes or haz-
ardous experimental conditions are necessary for the above
methods. Nowadays, it is still a challenge to find an appropriate
route to obtain hollow carbon nanostructures.
In this paper, we report a new process for synthesizing
HCNSs. The method involves the preparation of the catalyst
precursor Ni(OH)2/Al(OH)3 and the catalytic decomposition of
methane over Ni/Al2O3 catalyst. Generally, the Ni nanoparti-
cles favor the CNT growth. However, in this work the formation
of CNTs was totally inhibited. Therefore, it is very worthy to
investigate the structure and growth mechanism of our HCNSs
and lead to the better control of the formation of high quality
HCNSs.
In recent decades, the fabrication of fullerenes [1], carbon
nanotubes (CNTs) [2], and carbon onions [3] has attracted
much attention in nanostructured carbon field. Various methods
have been developed to synthesize these carbon nanomaterials
with different structures [1–5], due to their unique proper-
ties and potential applications in the fields of conductive and
high-strength composites, semiconductor devices, nanotweez-
ers, field emission displays and gas storage media [6–10].
Besides, the synthesis of carbon nanocapsules has practical
interest because they allow the encapsulation of different mate-
rials within the empty core domain. Therefore, hollow carbon
nanospheres (HCNSs), which represent a special class of mate-
rials, are very promising as photonic crystals, excellent supports
for catalysts, and anode in secondary lithium ion batteries, etc.
[11–13].
2. Experimental
Previously, various methods have been demonstrated for
the synthesis of HCNSs [14–21], such as mixed-valent oxide-
catalytic carbonization process [14], chemical vapor deposition
method by using mesoporous silica [15], pyrolysis hydrocarbon
Previously [22,23], we have fabricated the Ni/Al catalyst using a precipita-
tion route. Here we also used this process but excess NaOH (till the pH value of
the solution arrived at 10) to produce binary colloid Ni(OH)2/Al(OH)3 (weight
ratio Ni/Al = 2/3). After that the colloid was calcined in air to yield clusters made
of Ni and Al oxides and then reduced in an H2 flow (100 ml/min) at 600 ◦C for
120 min to form Ni nanoclusters supported on Al2O3.
∗
Corresponding author. Fax: +86 22 27405874.
To synthesize HCNSs, 50 mg NiO/Al2O3 distributed in a quartz boat uni-
formly was placed into a tube furnace. When the furnace reached 600 ◦C under
E-mail address: nqzhao@tju.edu.cn (N. Zhao).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.10.090

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H. Li et al. / Journal of Alloys and Compounds 465 (2008) 387–390
nitrogen, hydrogen (100 ml/min, 99.99% purity) was introduced to reduce the
catalyst for 2 h. Then the hydrogen flow was stopped, and a mixture of CH4/N2
(60/420 ml/min, v/v) was introduced into the tube and maintained for 1 h. After
the growth, the furnace was cooled to room temperature under N2 and black
powder was obtained.
The as-grown HCNSs were characterized using field emission scanning
electron micrograph (FE-SEM) (JEOL JSM-6700F) and transmission electron
microscope (TEM) (Philips Tecnai G2 F20, 200 kV).
3. Results and discussions
The morphologies of the as-grown HCNSs were character-
ized by SEM and TEM. Fig. 1 shows a SEM image of the
as-prepared hollow carbon nanospheres fabricated by the cat-
alytic method over Ni/Al2O3 catalyst. From SEM image it is
found that the most of the spherical nanoparticles are in the
range of 5–90 nm and average is about 35 nm. In addition, we
can find that some carbon nanowires coexist with the spheri-
cal nanoparticles, which may be from the catalytic growth of
active Ni catalyst. In this work, after the synthesis of HCNSs,
50 mg NiO/Al2O3 (i.e. 45.47 mg Ni/Al2O3) catalyst can obtain
52.27 mg C–Ni/Al2O3 composite powder. That means the car-
bon yield is 6.8 mg/1 g NiO/Al2O3 catalyst.
Fig. 1. SEM image of the as-prepared hollow carbon nanospheres fabricated by
the catalytic method over Ni/Al2O3 catalyst.
observations, the proportion of HCNSs in the samples is esti-
mated to be about 65%. The external diameter of the HCNSs is
5–90 nm and the thickness of the wall ranges from 2 to 50 nm.
The diameter and wall thickness are much smaller than that of
the HCNSs reported in the literatures [20,24]. Thus, the HCNSs
may be important application in future nano-industry, especially
for rising biological field.
A representative low-magnification TEM micrograph of the
products is shown in Fig. 2(a). Based on large numbers of TEM
Fig. 2. Typical TEM images of the as-prepared hollow carbon nanospheres fabricated by the catalytic method over Ni/Al2O3 catalyst.

H. Li et al. / Journal of Alloys and Compounds 465 (2008) 387–390
389
Typical HRTEM images of the HCNSs are presented in
Fig. 2(b) and (c). Broken sphere (marked with arrow) and
the strong contrast between the dark edge and pale center are
revealed in Fig. 2(b), providing further proof of the hollow
nature. Fig. 2(c) shows one typical HCNSs in high magnifica-
tion. It indicates that the hollow spheres are composed of carbon
sheets, and the average wall thickness of HCNSs is about 10 nm.
The HCNSs shell consists of about 32 graphitic layers. Fig. 2(c)
also reveals that the sheets are well graphitized and the interlayer
spacing is about 0.34 nm, which is consistent with the d value
of (0 0 2) plane of hexagonal graphite.
Except for the HCNSs, a few carbon coated Ni nanoparticles
can also be found in Fig. 3(a), and the carbon coated Ni nanopar-
ticles have almost the same inner and outer diameters as the
HCNSs. HRTEM images of the carbon coated Ni nanoparticles
are presented in Fig. 3(b) and (c). These images further reveal
the presence of carbon coated Ni nanoparticles in the sample.
The nanoparticles are composed of a metallic core and a carbon
shell. The carbon shell present in the nanocapsules has a crys-
talline graphite structure. And the interlayer spacing between
graphitic sheets is 0.34 nm, consistent with the ideal graphitic
interlayer space (0.34 nm). The thickness of the coating shell of
the nanocapsules is in the range of 5–50 nm. These thin carbon-
coating shells can give effective protection of corrosion for the
metal nanoparticles and not degrade the magnetic properties of
the metal nanoparticles too much.
To achieve a controllable growth of the HCNSs with high
quality, understanding of their growth mechanism is of impor-
tance, which still remains an open question. Recently, many
studies on the HCNSs have discussed the reason for the forma-
tion of hollow core. Katcho et al. [25] have produced HCNSs by
the chlorination of ferrocene at 900 ◦C, and proposed a specu-
lation to elucidate their experimental results. They pointed out
that iron might play a role in the making of the hollow car-
bon spheres. However, they have not found evidence by TEM
of the presence of iron metallic particles that could catalyze the
growth of the hollow spheres in a way similar to that occurring in
the catalytic synthesis of carbon nanotubes. On the other hand,
Genga et al. [26] have also obtained hollow carbon nanospheres
through a ZnSe nanoparticle template route. They first pro-
duced carbon encapsulated ZnSe nanoparticles by non-catalytic
one-step thermal evaporation method and then obtained carbon
hollow spheres by a thermal conversion process. For the growth
mechanism of HCNSs, they suggested that the thermal decom-
Fig. 3. Typical TEM images of a few carbon coated Ni nanoparticles coexisted with the hollow carbon nanospheres fabricated by the catalytic method over Ni/Al2O3
catalyst.

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H. Li et al. / Journal of Alloys and Compounds 465 (2008) 387–390
position of the toluene took place and produced carbon shells
on the surface of the ZnSe nanoparticles. With increasing tem-
perature, ZnSe pyrolysis took place and led to form the carbon
hollow nanostructures. However, two above hypotheses cannot
beusedtoexplainourresultsduetodifferentsynthesisprocessof
HCNSs in this work. Furthermore, another characteristic of our
results consists in possessing two main different kinds of carbon
nanostructures (HCNSs and carbon coated Ni nanopartilces),
which also suggests that the growth of our HCNSs fabricated
by CVD may follow a different mechanism. Therefore, a possi-
ble formation mechanism of our HCNSs and carbon coated Ni
nanoparticles is proposed. At the beginning of the reaction, the
methane decomposes into hydrogen gas and carbon atoms over
active Ni catalyst. Then, these freshly formed carbon atoms grow
into small graphite sheets because the growth rate of graphite
along the c-axis is much lower than that along any other axis.
When the graphite layers closed upon themselves due to that the
Ni particles acted as a catalyst, inducing curvature in graphite
layers, some Ni particles might be displaced before closure,
while some others were trapped inside and could not escape,
then the two different kinds of carbon nanostructures could be
obtained.
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4. Conclusion
A mass of hollow carbon nanospheres was fabricated by
chemical vapor deposition of methane over Ni/Al2O3 cata-
lyst at 600 ◦C. The products were characterized with scanning
electron microscope, transmission electron microscope, and
high-resolution transmission electron microscope images. The
results showed that the external diameter of the hollow carbon
nanospheres was 5–90 nm and the thickness of the wall was
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Acknowledgement
The authors acknowledge the financial support by Chinese
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