Spectroscopic Investigation of Modified Single Wall Carbon Nanotube (SWCNT)

doi:10.4236/jmp.2011.26063

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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

E-mail: [email protected]

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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