C A R B O N 6 6 ( 2 0 1 4 ) 7 2 4 – 7 2 6

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Letter to the Editor

Origin of radial breathing mode in multiwall carbonnanotubes synthesized by catalytic chemical vapordeposition

0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.08.057

* Corresponding author: Fax: +91 11 45609310.E-mail address: bps@mail.nplindia.ernet.in (B.P. Singh).

1 Both authors contributed equally.

Ravi Gupta a,1, Bhanu P. Singh a,*,1, Vidya N. Singh b, Tejendra K. Gupta a,Rakesh B. Mathur a

a Physics and Engineering of Carbon, Division of Materials Physics and Engineering, CSIR-National Physical Laboratory,

New Delhi 110012, Indiab Electron and Ion Microscopy Section, CSIR-National Physical Laboratory, New Delhi 110012, India

A R T I C L E I N F O

Article history:

Received 5 June 2013

Accepted 23 August 2013

Available online 5 September 2013

A B S T R A C T

The origin of radial breathing mode (RBM) in the Raman spectra of multiwall carbon nano-

tubes (MWNCTs) is discussed. In general, RBM is characteristics of single wall carbon nano-

tube (SWCNT). With the help of transmission electron microscope (TEM) and Raman

spectroscopic studies, it is established that the presence of SWCNT in the cavity of MWCNT

is responsible for the appearance of RBM in MWCNT (synthesized by low temperature cat-

alytic chemical vapor deposition technique). The estimated diameter of 8.2 A (from Raman

study) of SWCNT is almost same as that observed (�8.3 A) in TEM studies.

� 2013 Elsevier Ltd. All rights reserved.

Raman spectroscopy has become an indispensible tool for

the characterization of carbon nanotubes (CNTs) and espe-

cially for single wall carbon nanotubes (SWCNTs). Unfortu-

nately, the interpretation of Raman spectra of multiwall

carbon nanotubes (MWCNTs) is complex compared to

SWCNTs [1]. Raman spectrum of SWCNTs shows three impor-

tant bands; (i) radial breathing mode (RBM) in the low fre-

quency region (100–600 cm�1) [2], (ii) D-band (due to

disorder, peak around 1340 cm�1) and (iii) G-band (corre-

sponding to the tangential vibrations of carbon atoms, peak

around 1580 cm�1). The RBM frequency is inversely propor-

tional to the tube diameter. RBM cannot be detected in CNTs

having tube diameter >2 nm [1]. Apart from the above, some

other bands have also been reported at around 2600 cm�1

(second order D band) and some low intensity peaks in the

1700–1800 cm�1 range.

The presence of RBM in the Raman spectra of SWCNTs is a

unique feature and is widely used for the estimation of tube

diameter. The presence of RBM in Raman spectra of MWCNTs

is occasionally reported. There is some ambiguity in the inter-

pretation of Raman data of MWCNTs due to appearance and

disappearance of certain peaks [1]. For example, in the Raman

spectroscopic studies of MWCNTs by Zhao et al. [2] and

Jantoljak et al. [3], peaks in the low frequency region (RBM)

were observed. Both groups proposed that the vibration of

atoms of the thin inner shell of MWCNT is responsible for

the appearance of RBM. In another study, Donanto et al. [4]

suggested that the presence of iron oxide within the tubes

Fig. 2 – The position of SWCNT in the cavity of MWCNT is

shown in the HRTEM micrograph. Inset (a) shows the auto

correlation of bracketed region and inset (b) shows the

profile of auto-correlation. (A colour version of this figure

can be viewed online).

C A R B O N 6 6 ( 2 0 1 4 ) 7 2 4 – 7 2 6 725

is responsible for the appearance of RBM in the Raman spec-

tra of MWCNTs. Thus, it can be concluded that there is differ-

ence of opinion about the occasional appearance of RBM in

the Raman spectrum of MWCNTs.

In this study, with the help of Raman spectroscopy and

high resolution transmission electron microscopy (HRTEM),

it has been conclusively established that the presence of

SWCNT in the cavity of MWCNT (synthesized by low temper-

ature catalytic chemical vapor deposition, CCVD) is responsi-

ble for the appearance of RBM in the Raman spectrum of

MWCNTs.

In this study, CNTs were synthesized by the thermal

decomposition of toluene in the presence of iron catalyst (ob-

tained by decomposition of ferrocene) in a quartz reactor. The

furnace having a constant temperature zone of 18 cm was

heated to 750 �C. The feed (toluene + ferrocene) was injected

into the quartz tube along with argon (carrier gas). The flow

rate of feed (0.077 g ferrocene in 1 ml toluene) was 20 ml/h.

Detailed experimental procedures are given elsewhere [5].

CNTs were characterized for its structural properties using

Renishaw inVia Reflex Raman spectrometer, UK (with an exci-

tation source of 514.5 nm and 2.5 mW power). The resolution

of the instrument was less than 1.0 cm�1. Raman spectrum of

as-produced CNTs shows three different regions, i.e. RBM, D-

band and G-band (Fig. 1). The peaks in the RBM region were

intense compared to the previously reported work on the Ra-

man spectroscopic studies of MWCNTs [2–4]. It is known that

the diameter of SWCNTs is related to the frequency of the

peak in RBM region. Several relations have been suggested

for the estimation of diameter of SWCNTs and the details

are given in the references [6–7]. It has been mentioned by

Araujo et al. that for SWCNTs having diameter 61.2 nm, the

following relation can be used for the estimation of diameter

of SWCNTs [6]

xrbm ¼ 227=dt ð1Þ

where xrbm is Raman shift (cm�1), and dt is the tube diameter

of CNT (in nm). It should be mentioned that other relations do

not consider nanotube curvature effects which are important

Fig. 1 – Raman spectrum of as-produced carbon nanotube

using laser excitation of 514.5 nm. (A colour version of this

figure can be viewed online).

in smaller diameter CNTs [6], therefore the above relation has

been used for the estimation of SWNCTs diameter. The esti-

mated diameter of SWCNT was 8.2 A (corresponding to

280 cm�1).

HRTEM studies of CNTs were carried out using Tecnai G20-

stwin, 200 kV instrument and the results are shown in Fig. 2.

HRTEM micrograph shows the presence of SWCNT inside the

cavity of MWCNT. The diameter of SWCNT present in the cav-

ity of MWCNT is 8.3 A. It is to be noted that the diameter of

SWCNT observed in HRTEM micrograph is similar to the

diameter estimated by Raman spectroscopic results. Apart

from the peak at 280 cm�1 (in Raman spectrum), some other

peaks in the RBM region were also observed (Fig. 1). It is pos-

sible that few other SWCNTs are present in the cavity of

MWCNT.

Zaho et al. [8] has reported the presence of SWCNT (diam-

eter 3 A) inside MWCNT grown by high temperature arc dis-

charge method. The experiment was carried out in the

presence of pure hydrogen at a pressure of 8.0 · 103 Pa. It is

well known that, arc discharge method produces CNTs along

with abundant carbonaceous and metallic impurities [9]. In

order to purify such CNTs made by arc discharge method, te-

dious purification process is required [10]. CCVD is the cheap-

est, commercially viable, upscalable and most feasible

method for the production of CNTs [9].

In short, this is the first report on the observation of

SWCNT (having diameter 8.3 A) inside the cavity of MWCNT

synthesized by CCVD. It is to be noted that the reaction tem-

perature was low (750 �C) in the present study. The agreement

between Raman data (diameter 8.2 A corresponding to peak

at 280 cm�1) and HRTEM data (diameter 8.3 A) indicate the

presence of RBM in the MWCNT sample. Thus, presence of

SWCNT inside the cavity of MWCNT is responsible for the

appearance of RBM in the Raman spectrum of MWCNTs.

There is a further scope of research in order to investigate

the exact mechanism of SWCNT growth inside the cavity of

MWCNT synthesized by low temperature CCVD technique.

726 C A R B O N 6 6 ( 2 0 1 4 ) 7 2 4 – 7 2 6

Acknowledgements

Authors wish to express gratitude to Prof. R.C. Budhani, Direc-

tor, CSIR-National Physical Laboratory for his keen interest in

the work. The study was carried out under the CSIR-network

project (PSC0109).

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