Preparation of double-walled carbon nanotubes from fullerene ...

CARBON 48 (2010) 1312– 1320 1313
and H 2 (2:1, v/v) at 300 Torr, and the gap between the electrodes
was kept at ca. 2 mm during the arcing discharge. For
each run, when FWS-derived anode with length of 3–4 cm
was consumed, about 100–150 mg of DWCNTs film-like sticky
deposite on the wire cage can be obtained.
The as-obtained DWCNTs were examined using scanning
electron microscope (SEM, JSM-6301F), transmission electron
microscopy (TEM and HRTEM, Philips Tecnai G 2 20) and Raman
spectroscopy (JY LabRam HR800 at 632.8 nm).
The typical X-ray diffraction (XRD, Rigaku D/MAX 2400, Cu
Ka, 40 kV and 100 mA) profile of FWS-derived anode is shown
in Fig. 1. The catalyst in the anode is Fe nanoparticles with
diameters of ca. 40 nm that is estimated by Scherrer equation.
From SEM image shown in Fig. 2a, lots of long filament-like
material can be found in the as-obtained DWCNTs. These filaments
entangle into nets with some catalyst nanoparticles
Intensity (a.u.)
•
• Fe
•
10 20 30 40 50 60 70 80
2 Theta
Fig. 1 – XRD profile of anode prepared from FWS.
on the surface, as shown in Fig. 2b. Further HRTEM examination
reveals that the filaments are DWCNT bundles, as shown
in Fig. 2c and d, and the formation of bundles is believed to be
due to the van der Waals interaction between individual
nanotubes [8]. On average, the interlayer spacing between
the inner and outer walls of the DWCNTs is ca. 0.4 nm, which
is bigger than the interlayer spacing of MWCNTs and graphite.
This may be due to the large repulsive force of the adjacent
tube walls in DWCNTs [7]. It has been found that
amorphous carbons on the DWCNT bundles (see Fig. 2c and
d) and most catalyst particles (see Fig. 2b) can be easily removed
from the as-synthesized DWCNT samples through a
purification process [6], resulting in high-purity DWCNT bundles,
as evidenced in Fig. 2e.
Raman spectroscopy is a powerful tool that is capable of
yielding more detailed information about DWCNT’s structure
[7]. Fig. 3a shows a typical Raman spectrum of the as-prepared
DWCNTs. For the as-prepared DWCNTs, the intensity
ratio of G-band at 1593 cm 1 and D-band around 1323 cm 1 ,
i.e., I G /I D is over 30, indicating that the DWCNT product contain
little amorphous carbon and have well-developed graphitic
structures.
DWCNT is made of two coaxial SWCNTs. Because of this,
the G-band of DWCNTs would be associated with vibration
along the circumferential direction in the lower frequency
range (G outer
and G inner
), and is also related to vibration along
the direction of the nanotube axis in the higher frequency region
(G + ) [8]. The G-band of the as-made DWCNTs shown in
Fig. 3b can be fitted with Lorentzian peaks at 1594 cm 1 (G + ),
1572 cm 1 (G outer
), and the broad peak around 1540 (G inner
)
which are present due to existence of metallic tubes in the
product, or due to the overlap of peaks of tubes with different
Fig. 2 – (a) SEM image of raw product; (b) Low magnification TEM image of raw product; (c) and (d) HRTEM images of raw
DWCNTs; (e) TEM image of the purified DWCNTs.