Selectivity of chemical oxidation attack of single-wall carbon nanotubes in solution
Enzo Menna, Federico Della Negra, and Michela Dalla Fontana
Department of Organic Chemistry, CNR ITM– Padova, University of Padova, 1, Via Marzolo, 35131 Padova (Italy)
Moreno Meneghetti*
Department of Physical Chemistry, University of Padova, 2, Via Loredan, 35131 Padova (Italy)
Received 5 December 2002; revised manuscript received 1 August 2003; published 21 November 2003
Chemical oxidation of single wall carbon nanotubes SWNTs, produced with the HiPco method, is studied
by resonance Raman scattering. The analysis of the radial breathing mode makes it possible to assign the
observed bands to metallic and semiconducting SWNTs and to suggest the chiral indices of SWNTs contrib-
uting to the spectra. On this basis we observe that the most important parameter which determines the attack
of the nanotubes is their diameter: Small diameter tubes, due to the stress induced by the curvature, are first
attacked and destroyed. Some reactivity is also observed for larger diameter metallic nanotubes, which, how-
ever, are not easily destroyed.
DOI: 10.1103/PhysRevB.68.193412 PACS numbers: 61.48.+c, 78.30.Na, 33.20.Fb
Among the most interesting nanostructures useful for
nanotechnology, single-wall carbon nanotubes SWNTs
have characteristic properties, in particular related to their
electronic structure.
1,2
The synthesis of SWNTs produces a
variety of nanotubes with a variety of diameters and semi-
conducting or metallic properties. However synthetic proce-
dures are not able, at the moment, to separate different types
of nanotubes. Manipulation of isolated and well character-
ized SWNTs is important for many applications such as na-
noelectronics and photonics and their solubilization is an im-
portant step.
Soluble derivatives of SWNTs have been obtained with
different approaches,
3
many of which start by oxidizing the
micrometer long tubes produced by the synthesis. The oxi-
dation is obtained by a strong attack with concentrated sul-
phuric and nitric acids under sonication and the result is a
shortening of the nanotubes to some hundreds of nanometers
and their functionalization with carboxilic acid groups. The
shortened nanotubes are then further oxidized with sulphuric
acid and hydrogen peroxide etching reaction without soni-
cation in order to purify them from amorphous carbon.
4,5
We present a resonant Raman study of the strong SWNT
oxidation attack in solution. Raman is a powerful technique
for SWNT characterization since a strong resonance en-
hancement of the spectra, which provides both vibrational
and electronic structural information, is observed. Character-
istic features of the Raman spectrum are found at about
1500–1600 cm
-1
tangential bands and can be related to
carbon-carbon stretching.
6,7
The low frequency region, usu-
ally below 400 cm
-1
, shows the radial breathing mode
RBM, a totally symmetric mode characteristic of a nano-
tube, and whose frequency depends on the inverse of the
tube diameter.
8,9
This spectral feature made it possible to
suggest the chiral indices ( n , m ) of isolated SWNTs.
10
This is
a very important result because the chiral indices determine
the properties of any single nanotube. We will use the RBM
bands, in particular, and show how it is possible to gain
information on the oxidative attack on the basis of an assign-
ment of all the bands observed in the spectrum below
350 cm
-1
.
The SWNTs studied in the present paper are HiPco
SWNTs obtained by a disproportionation of CO molecules at
high pressure in the presence of Fe nanoparticles produced
by the decomposition of Fe(CO)
5
.
11
The HiPco method is
promising for large scale production using a continuous flow
gas-phase process. SWNTs obtained by this method are very
interesting since they correspond to nanotubes with a wide
variety of diameters, in particular nanotubes with small
diameters,
12
thus enabling us to study, in the same sample,
the behavior of many tubes. It will be shown that small di-
ameter tubes are easily destroyed, as already noted by using
molecular oxygen.
13,14
The chemical oxidation in solution,
studied here, also makes it possible, within the same step, to
remove almost completely the Fe nanoparticles encaged
within the tubes during their synthesis. This is a useful result
in view of the need for samples produced on a large scale for
example for nanoelectronic applications like densely packed
field-effect transistors FET.
15
Purified HiPco SWNTs were purchased from Carbon
Nanotechnologies Inc.. For the oxidative reaction we used a
suspension of the pristine sample in a 3:1 H
2
SO
4
(98%)/HNO
3
(65%) solution at 55°C and under sonication
at 45 kHz. We analyzed the reaction mixture after some min-
utes and up to 4 h. The solutions were filtered and the
samples etched for half an hour with a 4:1 H
2
SO
4
(98%)/H
2
O
2
30% solution to remove all carbon particles
produced by the first reaction.
4
Sharper Raman spectra were
actually obtained, in particular for samples with longer oxi-
dation times. Yield of the overall procedure is reported in
Fig. 1. After 4 h there was a 9% yield and Raman spectra
showed that the nanotubes were still present. On the other
hand, high-resolution transmission electron microscope HR-
TEM measurements showed, for sample oxidized for 35
min, that Fe nanoparticles had been almost completely re-
moved. However, we were not able to find that bundles of
nanotubes, present in pristine sample, were disentangled. A
lower solubility observed for samples after the oxidative pro-
cedure is in line with the latter observation since it can be
related to a pronounced aggregation of nanotubes.
PHYSICAL REVIEW B 68, 193412R2003
0163-1829/2003/6819/1934124/$20.00 ©2003 The American Physical Society 68 193412-1