Journal of Modern Physics, 2011, 2, 538-543
doi:10.4236/jmp.2011.26063 Published Online June 2011 (http://www.SciRP.org/journal/jmp)
Spectroscopic Investigation of Modified Single Wall
Carbon Nanotube (SWCNT)
S. Hussain1, P. Jha3, A. Chouksey3, R. Raman3, S. S. Islam1,
T. Islam2, P. K. Choudhary3, Harsh3
1Department of Applied Sciences and Humanities, Nano Sensor Research Laboratory,
Faculty of Engineering & Technology, Jamia Millia Islamia, India
2Department of Electrical Engineering, Faculty of Engineering & Technology,
Jamia Millia Islamia, India
3Solid State Physics Laboratory, Delhi, India
Received January 14, 2011; revised March 16, 2011; accepted April 1, 2011
Abstract
We have investigated the effects of chemical treatment on Single Wall Carbon Nanotube (SWCNT) before
and after being modified with HNO3/H2SO4 by Raman, FTIR and UV-Vis-NIR spectroscopy. The results
show successful carboxylation of the CNT sidewalls as observed from FTIR and UV-Vis-NIR spectroscopy.
This successful functionalization is achieved in 6-8 hrs of refluxing. We also report changes in the first and
second order Raman spectra of SWNTs functionalized with oxygenated groups. During the experiment, we
observe some important Raman features: Radial breathing mode (RBM), Tangential mode (G-band), and
Disordered mode (D-band); which are affected due to the chemical oxidation of carbon nanotubes. We found
that the ratio of D- to the G-band intensity (Id/Ig), increase after functionalization and the RBM mode in acid
treated SWCNTs is almost disappeared.
Keywords: Single Wall Carbon Nanotubes, Functionalization, Raman Spectroscopy, Ftir and UV-Vis-NIR
Spectroscopy
1. Introduction
Carbon nanotubes (CNTs) have attracted much attention
of researchers worldwide because of their pseudo 1D
structure and exceptional mechanical, electrical, and
thermal properties [1]. Carbon nanotubes become very
useful for many applications such as nanoelectonic de-
vices, chemical sensors, gas sensors, solar cells and so
forth.
CNT based sensors, an emerging application area of
the nanotubes and have caught the attention of a large
number of researchers. Due to the enormously high spe-
cific surface area, SWNTs can absorb large amount of
gases, which makes them a potential candidate for
chemical sensors with very high sensitivity. However,
for any practical purpose, apart from sensitivity, selectiv-
ity must also be introduced on to the CNT surface; for
which the CNT sidewalls/ end caps need to be function-
alized with different functional moieties. Functionaliza-
tion process of CNT decreases van der Waals’ forces,
leading to de-bundling between the CNT, and aiding
binding to other materials [2]. Since carbon nanotubes
remain insoluble in common solvents due to the high
cohesive energy of more than 0.5 eV/nm arising from
tube – tube van der Waals’ attraction, therefore function-
alization is also useful in reducing the magnitude of this
energy and rendering them soluble [3]. The first step to
functionalize the relatively inert CNT surface is to oxi-
dize the CNTs. The various oxidation methods have used
for this purpose [4-8]. Liquid phase oxidation is a very
simple method to make nanotubes more easily dispersi-
ble or soluble in liquids; it is necessary to physically or
chemically attach functional groups to their sidewalls
without changing significant properties and also opens
the closed ends of CNTs [9]. The chemical modification
and oxidation of CNTs has been well represented in lit-
erature [10,11]. During acid oxidation, the carbon-carbon
bonded network of the graphitic layers is broken allow-
ing the introduction of oxygen atoms in the form of car-
boxyl, which has been extensively exploited for further

S. HUSSAIN ET AL.
539
chemical functionalization.
It is observed that in the functionalization process,
several functional groups such as carboxylic (–COOH),
carbonyl (–CO), and hydroxylic (–OH) are formed on the
surface of nanotubes, which is typical of carbon materi-
als [12]. It is thought that acid treated nanotubes have
carboxylic groups at the tubes ends and possibly, at de-
fects on the sidewalls. H. T. Gomes and co-workers [12]
had reported that Carboxylic (–COOH) and carboxylate
groups (-COONa) were successfully grafted to the CNT
surfaces when modified using HNO3, HNO3/H2SO4, and
HNO3/Na2CO3.
In this paper, we have used oxidation technique for the
modification of SWCNT surface and studied the effect of
the chemical treatment on CNT structure using Raman,
FTIR and UV-Vis-NIR Spectroscopy. In our studies we
have observed that acid treatment duration of only about
6 - 8 hours is sufficient to functionalize the CNTs by
introducing COOH group at the CNT surface. But to the
best of our knowledge most literature cites a reaction
duration ranging from 12 - 24 hours. The Raman spec-
troscopy is a perfect tool for the characterization of car-
bon nanotube to get important information about their
properties without damaging the CNT surfaces [13].
2. Experimental Detail
Different sets of experiments were carried out using 100
mg of SWNTs (purity 95%; length 1 - 2 microns and
diameter < 10 nm; manufactured by NTP China) mixed
with 3:1 ratio of HNO3:H2SO4. The CNTs were dis-
persed in acid mixture by using a multi frequency ultra-
sonicator (Blackstone NEY). The refluxing technique is
used to reflux the solution for better results. For this, the
solution was then transferred to a refluxing flask and
refluxing was carried out for various time periods using
Heidolph LR4011 rotary evaporator. The temperature of
refluxing was maintained at 80˚C.
After acid treatment the solution was quenched in
ice-cold water, and samples were diluted. Then base
neutralization of the acid solution was carried out by
adding 0.1 M NaOH. The neutralization point was de-
tected by using a pH meter (Metler Toledo multi seven).
Thereafter, the CNTs were extracted on a polycarbon-
ate membrane of 0.2-microns pore size using a vacuum
filtration assembly. The resulting CNTs were dried in an
oven at 80˚C for 12 hrs. Fourier Transform Infrared
spectroscopy (FTIR) data was obtained using Biorad
FTS 40 spectrometer. Raman data was recorded on a Lab
RAM HR 800 Raman spectrometer (Horiba Jobin Yvon,)
using Argon (Ar+) laser at 488 nm wavelengths.
UV-Vis-NIR spectrophotometer data was acquired using
Varian Cary 5E spectrophotometer.
3. Result & Discussion
3.1. Raman Analyses
There exist many distinct bands in the SWCNT Raman
spectrum that originate from different aspects of the na-
notube are Radial breathing mode (RBM), Tangential
mode (G-band), and Disordered mode (D-band) [13].
Radial breathing modes (RBM) between 100 and 300
cm–1 depends on the tube diameter and this region shows
much variation with different samples. The higher fre-
quency tangential displacement G modes near ~ 1590
cm–1 and the second order G’ peaks near 2600 cm–1 are
sensitive to the charge exchanged between nanotubes and
the guest moiety. The shape and the intensity of D-mode
at 1290 - 1320 cm–1 corresponds to the sp3 – hybridized
carbon atoms [4], which is correlated with the extent of
nanotube sidewall defects and chemical sidewall func-
tionalization [14]. The D*-mode also serves to show the
disorder and defects of graphitic walls.
Raman spectra were recorded at room temperature on
pristine as well as on acid treated carbon nanotubes using
a micro Raman spectrometer equipped with a 488 nm
laser as shown in Figure 1. The strong background in the
spectrum observed from the acid treated sample is
probably indicative of the fact that the sample surface
was rough due to presence of defects during the acid
treatment. Chemical oxidation creates defects on the
sidewalls of nanotube and attaches some functional
group at the defective area of nanotubes. Typical
SWCNT Raman features are observed for the RBM- and
G-modes at 160 - 1591 cm–1, respectively. On function-
alization there are distinct changes in the Raman spectra
Figure 1. Raman spectra of SWNTs 3:1 H2SO4: HNO3 (a)
pristine SWNT (b) refluxed for 6 h.

540 S. HUSSAIN ET AL.
from the pristine sample as will be discussed subse-
quently.
Figure 2 shows the Raman spectra of the pristine and
functionalized SWNTs; we have observed changes in
relative intensity of the breathing modes in modified
samples. These changes are indicator of chemical altera-
tion of SWCNTs. It can be observed that the absolute
intensities of the radial breathing mode are drastically
reduced after functionalization. Rongmei et al. [15]
found in 6 h sonication of SWNTs that high wave num-
ber RBM intensities decrease after which leads to as-
sumption that destruction of small diameter SWCNT
takes place [13,16] or reduction in resonance enrichment
of functionalized SWCNTs.
In our case, all low and high wave number RBM in-
tensities has been decreased and disappeared. It may be
due to long time acid treatment with 2 hours ultrasonica-
tion followed by 6 - 8 hours acid refluxing. We believe
that loss in the RBM intensities of SWNT is occurring
due to the acid treatment, which breaks the translational
symmetry of the SWNTs.
The D-band a useful diagnostic of disorder in the hex-
agonal framework of the tubes, whereas the relative in-
tensity of the D- and G -bands is an important indicator
of the amount of defects introduced upon chemical
treatment [17]. The G-band, which is close to that for
graphite, is related to the tangential (in-plane) mode of
two carbon atoms in one graphene unit cell vibrating
against one another, the so-called E2g symmetry of the
inner layer mode that represents the degree of crystallin-
ity. This is a circumferential mode that vibrates perpen-
dicularly to the tube axis. Figure 3 depicts the compari-
son of the intensity of D and G bands for the pristine and
acid treated SWNTs. In comparison to the pristine tubes,
after acid treatment, the intensity of the sp3 C hybridiza-
tion at ~1348 cm–1 (D-band) has increased relative to the
sp2 C hybridization, in case of the acid treated sample
due to the introduction of some functional groups on the
side wall. This fact is verified by our data obtained from
FTIR and UV-Vis-NIR spectroscopy. D-band peak (1348
cm–1) can be activated by disordering in the sidewall of
the SWCNT and identified with sidewall defects. These
defects disturb the aromatic system of electrons. G-band
is the most intensive mode of SWCNTs which is ob-
served at ~1600 cm–1 and the atomic disarticulation is
occurred along the circumferential direction in this mode
[17]. HNO3 influences the most in the disorder band (D
band), while H2SO4 is responsible for affecting graphite
band (G band) as well as the second order Raman band
(G’ band) [18]. The downshift in G-mode is interpreted
as a softening and elongation of the C–C bonds upon
electron transfer to the SWCNTs, whereas electron with-
drawal from the sp2 lattice leads to a contraction and
Figure 2. Raman Spectra of RBM mode of 3:1 H2SO4:
HNO3 (a) pristine SWNT (b) refluxed for 6 h.
Figure 3. Raman Spectra of D- & G- mode of 3:1 H2SO4:
HNO3 (a) pristine SWNT (b) refluxed for 6 h.
hardening of the lattice. As nitric acid and sulfuric acid
are known to lead to a p-type doping of SWCNTs, the
observed upshifts of the tangential modes in acid-treated
SWCNTs are very likely induced by intercalated acid
molecules [13].Therefore, due to HNO3 and H2SO4 acids,
there will be increase or decrease in the intensity of
D-and G-band and also broadened G mode with a lower
frequency. This broadening is related to the presence of
free electrons in the metallic nanotubes and Breit-Wigner-
Fano (BWF) line is usually used to fit this G-mode
broadening feature [11,17]. According to the Kataura-Plot,
different photon energies determine whether the small
diameter tubes that are resonantly enhanced are metallic

S. HUSSAIN ET AL.
541
or semiconducting [13]. At λexc = 488 nm (Figure 3), the
preferentially functionalized small diameter SWCNTs
are semiconducting, therefore the G-band shows an in-
crease in BWF asymmetry.
This BWF line is observed in n-doped graphite inter-
calation compounds (GIC), n-doped fullerenes, as well as
metallic SWNTs [19]. The ratio of the D- to the G-band
intensity i.e. Id/Ig, is usually used for a measurement of
the disordered sites on carbon nanotubes walls, and it is
the indicator of the level of the covalent functionalization
of the CNTs [14]. On comparing the modified and non
modified tubes we have observed an increase in the ratio
of intensities R = Id/Ig from 0.125 to 0.40, for the modi-
fied tubes as is shown in Table 1.
Full width at half maximum (FWHM) of D- and
G-peak was analyzed for pristine and functionalized
sample of SWCNT (Table 1). For the G peak, the
FWHM of the chemically treated SWCNT (i.e. ΔωG ~
46.0 cm–1) was much larger than that of the untreated
samples (i.e., ΔωG ~28.9 cm–1) and for D peak, the
FWHM of the chemical treated SWCNT (i.e. ΔωD ~ 47.4
cm–1) was larger than that of the untreated samples (i.e.,
ΔωD ~31.7 cm–1). This shows that increase in the FWHM
of the D peak (i.e. ΔωD) is less than the G peak (i.e.
ΔωG).
It is found that the ratio of the D- to G-mode intensity
increase after functionalization [13]. Aryl-nanotube bond
formed during the functionalization, which increases the
intensity of D-peak and due to the electronic resonance,
decreases the intensity of the tangential mode (G-peak).
By these effects, increase in their peaks ratio (Id/Ig), in-
dicates an increased disorder of the graphitic structure of
the modified nanotubes [8], which shows that the nano-
tubes were covalently modified. Raman can detect
changes in C–C bond length. Due to electrochemical
charge injection, the intensities of both the modes vary
with the emptying/depleting or filling of the bonding and
anti bonding states.
3.2. FTIR Spectroscopy
FTIR studies have been performed in the range 400 to
4000/cm−1 for the identification of the functional group
attached on the surface of the CNTs. FTIR spectroscopy
has been used extensively in the structural determination
of molecules. Figure 4 shows a comparative FTIR data
for the pristine and refluxed samples. As observed in the
pristine sample there is almost no signal except for a
small C-C stretch, however, on acid treatment quite a
number of new peaks appear. The bands due to the C=O
stretch are very prominently seen in the range 1740 cm–1
for the carboxylated SWNT. The sample refluxed in 3:1
H2SO4:HNO3 acid for 6 - 7 hrs shows a distinct band at
Table 1. Comparison of the Raman spectral features of
pristine and functionalized SWNT samples.
FWHM (cm–1)
Sample Id/Ig
G band (ΔωG) D band (ΔωD)
Pristine 0.125 28.9 31.7
Functionalized0.40 46.0 47.4
(a)
(b)
Figure 4. FTIR spectra of SWNTs 3:1 H2SO4:HNO3 (a)
refluxed for 6 h (b) pristine SWNTs.
1740 cm–1, which can be assigned to the acid car-
bonyl-stretching mode (Figure 1(a)). Other bands seen
in this sample are a small one at 2950 cm–1 and another
at 3450 cm–1, that are characteristic of C-H and O-H,
stretches respectively. The C-C vibrations occur due to
the internal defects, and the O-H vibration is associated

542 S. HUSSAIN ET AL.
with the amorphous carbon because amorphous carbon
easily forms a bond with atmospheric air. However, the
intensity of this O-H peak is relatively lower and shows
that a lesser amount of amorphous carbon formed during
growth [20].
Along with this an anti symmetric stretch C-O is also
clearly distinguishable at 1659 cm–1. The bands at 1235
cm–1 and 1400 cm–1 are representative of C=C, this shift
in band from 1600 cm–1 indicates the changes in the CNT
structure upon carboxylation [21,22].
3.3. UV-VIS-NIR Spectroscopy
Figure 5 depicts the absorption spectra of pristine
SWNT and functionalized SWNTs. The pristine sample
shows two distinct peaks at 960 nm and 750 nm. These
are the S11 and S22 transitions respectively, and they cor-
respond to the first and second pair of van Hove singu-
larities of the semiconducting nanotubes. The transitions
corresponding to the metallic nanotubes are not ob-
served.
Upon functionalization, the S22 peak vanishes. It is
also seen that the intensity of the S11 peak decreases as
we move from the pristine sample to the refluxed sample.
This implies that there is a change in the electronic
structure of the CNTs on acid treatment. It has been re-
ported [22,23] that on covalent modification the CNTs
results in a disruption of the aromatic system, which in
turn causes a change in the conductivity. This gives rise
to the changes observed in the van Hove singularities in
the spectra. This is indicative of the fact that sidewall
opening has taken place during acid treatment so as to
accommodate the carboxyl group. An important consid-
eration of this methodology is neutralization of the re-
maining acid. The sample degradation starts when ex-
posed to prolonged acid exposure. Our experience with
another sample validated the premise, that when kept
overnight in the quenched state without neutralization the
CNTs completely solubilise and pass through the filtra-
tion membrane without leaving a functionalized CNT
residue upon it.
4. Conclusions
In our studies we have observed that acid treatment dura-
tion of only about 6 - 8 hours is sufficient to functional-
ize the CNTs by introducing COOH group presumably at
the CNT sidewalls. But to the best of our knowledge
most literature cites a reaction duration ranging from 12 -
24 hours. In our studies, we have also observed that, re-
fluxing duration of more than 10 hours completely dam-
ages the CNTs as observed by the intensity of the defect
band in Raman spectra and complete disappearance of
Figure 5. UV comparison of (a) 6 h refluxed sample, (b) and
pristine SWNTs.
the characteristic S11 and S22 peaks of CNTs in the
UV-Vis-NIR spectra.
5. Acknowledgements
The authors gratefully acknowledge the financial support
provided by Ministry of Communication & Information
Technology, Govt. of India, through its Grant No.
20(14)/2007/NANO, dated. 20th April 2010.
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