Synthesis and characterization of amorphous carbon nanowires

Article abstract of DOI:10.1063/1.125190

By heating a pressed tablet of graphite powder mixed with nickel in a quartz tube mounted inside a high-temperature tube furnace at 1200°C, amorphous carbon nanowires were formed on the inner wall of the quartz tube near a copper cooling finger. Bright-field images of transmission electron microscopy show that the diameters of the nanowires are around 40 nm. Selected-area electron diffraction and Raman study reveal that the nanowires have an amorphous structure.

Full text of DOI:10.1063/1.125190

Synthesis and characterization of amorphous carbon nanowires
Y. H. Tang, N. Wang, Y. F. Zhang, C. S. Lee, I. Bello, and S. T. Lee
Citation: Applied Physics Letters 75, 2921 (1999); doi: 10.1063/1.125190
View online: http://dx.doi.org/10.1063/1.125190
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/75/19?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 75, NUMBER 19
8 NOVEMBER 1999
Synthesis and characterization of amorphous carbon nanowires
Y. H. Tang, N. Wang, Y. F. Zhang, C. S. Lee, I. Bello, and S. T. Leea)
Center Of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials
Science, City University of Hong Kong, Hong Kong, China
͑Received 15 July 1999; accepted for publication 13 September 1999͒
By heating a pressed tablet of graphite powder mixed with nickel in a quartz tube mounted inside
a high-temperature tube furnace at 1200 °C, amorphous carbon nanowires were formed on the inner
wall of the quartz tube near a copper cooling finger. Bright-field images of transmission electron
microscopy show that the diameters of the nanowires are around 40 nm. Selected-area electron
diffraction and Raman study reveal that the nanowires have an amorphous structure. © 1999
American Institute of Physics. ͓S0003-6951͑99͒05045-7͔
Nanoscale carbons, such as C60 ͑Refs. 1 and 2͒ or
fullerenes,3 nanotubes,4 and onions,5,6 have stimulated great
interest among scientists. In terms of chemical bonding, all
these carbon nanoscale materials possess sp2 hybridized
bonds.7 Besides the sp2 bond, sp and sp3 are the other two
types of C–C bonds in carbon materials.8 The sp2 bond cor-
responds to the graphite material. The sp and sp3 correspond
to carbyne and diamond bonding, respectively. Carbon nano-
tubes can be conceptualized as cylinders rolled from a graph-
ite sheet and capped at both ends by hemispheres of
fullerenes. They are hollow one-dimensional ͑1D͒ nanoscale
materials possessing sp2 bonds. The 1D materials with sp3
bonds are particularly interesting since they correspond to
diamond-like nanowires. Although this material has not been
synthesized, nanoscale material with sp2 and sp3 bonds is
also of interest. This sp2/sp3 material corresponds to amor-
phous carbon nanowire ͑CNW͒. In general, amorphous car-
bon material contains a highly disordered network of carbon
atoms that are predominantly sp2 bonded and more than
10% sp3 bonded.9 CNWs have not been reported so far. In
this letter, we report the synthesis of CNWs by a thermal
evaporation technique.
remove the loosely bound particles and then observed di-
rectly with TEM ͑Philips CM20͒.
Figure 1͑a͒ shows a TEM bright-field image of the
nanowires. The diameters of most nanowires are around 40
nm. Some nanowires with diameters less than 10 nm can also
be seen from this image. Unlike carbon fibers which are
straight,12 these nanowires are smooth and curvy. The inset
in Fig. 1͑a͒ gives a selected-area electron diffraction ͑SAED͒
pattern taken from an individual nanowire. The diffuse dif-
fraction ring clearly shows that the nanowire is amorphous.13
It is noticeable that the pattern differs from that of carbon
The evaporation apparatus used in the present experi-
ment is similar to that used to synthesize Si nanowires
͑SiNW͒.10,11 8 g of graphite powder ͑mean size: 30 ␮m,
purity: 99.85%, impurities: O͒ mixed with 0.1 wt % Ni pow-
der was pressed into a tablet by a hydraulic press at room
temperature. The tablet was then put into a quartz tube
mounted inside a high-temperature tube furnace. After
pumping down to a pressure of 4ϫ10Ϫ2 Torr, high-purity
argon ͑99.99%͒ was then passed through the quartz tube at a
flow rate of 50 sccm near atmospheric pressure. The quartz
tube was then heated to 1200 °C and kept at this temperature
for 24 h. A little black soot was collected from the inner wall
of the quartz tube near the copper cooling finger. A Mo grid
for transmission electron microscopy ͑TEM͒ observation
was put in advance inside the quartz tube at a position where
the soot was expected to deposit. After removal from the
tube, the grid was simply blown by a stream of nitrogen to
FIG. 1. Morphologies of the carbon nanowires and SAED pattern. ͑a͒ TEM
bright-field image. The inset is the SAED pattern of a single nanowire. ͑b͒
TEM dark-field image.
a͒
Corresponding author. Electronic mail: apannale@cityu.edu.hk
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0003-6951/99/75(19)/2921/3/$15.00 2921 © 1999 American Institute of Physics
129.22.67.107 On: Thu, 27 Nov 2014 10:54:41

2922
Appl. Phys. Lett., Vol. 75, No. 19, 8 November 1999
Tang et al.
FIG. 4. TEM image of metallic catalysts on the tips of the nanowires.
formed without Ni. The diameters of the nanowires were
dependent on the concentration of Ni powder in the tablet.
This phenomenon can be seen in Fig. 4. When the concen-
tration of Ni was high enough ͑1 wt %͒, irregular shapes of
nanoparticles were observed on the tips of the nanowires.
These nanoparticles are composed of C and Ni as detected by
energy-dispersive x-ray ͑EDX͒ spectroscopy mounted on the
TEM. It should be noted that nanowires with smaller diam-
eters have no catalyst balls on the tips ͑pointed to by the
arrow in Fig. 4͒. When the concentration of Ni in the graph-
ite powder is below 1 wt %, the nanowires are of a smaller
diameter and the metallic particles were less abundant. We
speculate that the small-diameter nanowires might also have
metallic particles on their tips initially. The disappearance of
the metal particles was due to a metal-consuming process
during growth, so that the size of the metallic ball shrinks
gradually as the nanowire grows in length.
FIG. 2. TEM micrograph of the cross section of the nanowires.
nanotubes, in which several diffraction spots of (hk0) ap-
pear in the pattern.4,14 Figure 1͑b͒ shows the corresponding
dark-field image of the same region in Fig. 1͑a͒. All nano-
wires have a bright contrast which is opposite to that in Fig.
1͑b͒. The complementary contrast of Figs. 1͑a͒ and 1͑b͒ fur-
ther supports that the nanowires are not crystalline.
As the nanowires change their growth directions, their
cross section can be observed at the turning points which are
marked by arrows in Fig. 2. It can be seen from the image
that the nanowires have a circular solid cross section. This is
a common characteristic of nanowires, such as SiC
nanowires15 and Si nanowires.16,17
Raman spectroscopy offers an effective method to iden-
tify the structure of carbon materials. We used a Renishaw
2000 micro-Raman system with 1 cmϪ1 resolution and 0.4
cmϪ1 reproducibility to study the nanowires. The excitation
source was the 514.5 nm line of an argon-ion laser with a
spot size of 10 ␮m in diameter and a power of 4 mW. The
measurement was performed at room temperature. The Ra-
man spectrum, shown in Fig. 3, revealed the characteristic
bands centered at 1352 and 1586 cmϪ1, which are the typical
Raman peaks of amorphous carbon materials.18
Baker19 reported the gasification of carbon using Pt cata-
lysts, and discussed the formation of carbon whiskers. Simi-
larly, a CO-assisted growth model of carbon nanowires is
proposed in this work, which is schematically shown in Fig.
5. In this model, graphite reacts first with the oxygen or
water trapped in the graphite tablet at high temperature:
2CϩO2→2CO,
and
Metallic catalysts ͑Ni͒ were found to play an important
role in the growth of the nanowires. CNW could not be
CϩH2O→COϩH2.
Then, the CO molecule was carried by the Ar gas to the
colder finger where it decomposes into graphitic atoms and
carbon dioxide molecules at the surface of the metallic cata-
lyst:
2CO→CϩCO2.
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FIG. 3. Raman spectrum of the carbon nanowires. FIG. 5. CO-assisted growth model of amorphous carbon nanowires.
129.22.67.107 On: Thu, 27 Nov 2014 10:54:41

Appl. Phys. Lett., Vol. 75, No. 19, 8 November 1999
Tang et al.
2923
The gaseous CO2 was then evacuated by the mechanical
pump. The temperature of the colder finger was about
100 °C. The temperature of the substrate where CNWs grew
was between 800 and 900 °C. The Ni nanoparticles could
transport to the substrate possibly due to the following reac-
tion:
This work was supported by the Strategic Research
Grants of the City University of Hong Kong and the Re-
search Grants Council of Hong Kong.
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4
entered the catalyst solution and joined the carbon nanowire
at the other edge of the solution according to the vapor–
liquid–solid ͑VLS͒ growth mechanism.20,21 With the help of
CO as an intermediate product, the whole growth process
can be sustained. This growth mechanism is supported by the
sintering experiment of the tablet. After sintering at 1200 °C
for 72 h, the tablet was put into another quartz tube and the
thermal evaporating experiment was repeated. However, no
carbon nanowires could be obtained. In this case, nearly all
the oxygen trapped in the tablet was removed by sintering at
high temperature for a long time. So the intermediate mate-
rial CO could not be formed and the growth process could
not start.
In conclusion, amorphous carbon nanowires with diam-
eters around 40 nm have been synthesized at 1200 °C by
thermally heating graphite mixed with a Ni catalyst. The
diameters of some nanowires are smaller than 10 nm. SAED
and Raman study show that these carbon nanowires are in an
amorphous state. A CO-assisted growth model is proposed to
explain the growth process. The experimental results provide
a kind of nanoscale carbon material synthesized by simple
thermal evaporation.
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