morphology of entangled multiwalled carbon nanotubes …rasayanjournal.co.in/vol-7/issue_4/5_...

Vol. 7 | No.4 |333 – 339 | October-December | 2014

ISSN: 0974-1496 | e-ISSN: 0976-0083 | CODEN: RJCABP

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ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al

MORPHOLOGY OF ENTANGLED MULTIWALLED CARBON

NANOTUBES BY CATALYTIC SPRAY PYROLYSIS USING

MADHUCA LONGIFOLIA OIL AS A PRECURSOR

S. Kalaiselvan1, K. Anitha

2, P. Shanthi

3, P.S. Syed Shabudeen

4

and S. Karthikeyan*5

1Department of Chemistry, Hindusthan College of Engineering, Coimbatore (TN) India.

2Department of Chemistry, A.P.A. Arts College for Women, Palani (TN) India.

3Department of Chemistry, Kongunadu College of Engineering and Technology,

Thottiam (TN) India. 4Department of Chemistry, Kumaraguru College of Technology, Coimbatore (TN) India. 5Department of Chemistry, Chikkanna Government Arts College, Triupur (TN) India.

*E-mail: [email protected]

ABSTRACT Multi-walled carbon nanotubes were synthesized using methyl ester of Madhuca longifolia oil over Fe-Mo

impregnated alumina support at 650 °C under N2 atmosphere. The characterization of the as-grown carbon materials

were analyzed by Field emission scanning electron microscopy (FESEM), High resolution transmission electron

microscopy (HRTEM) and Raman spectroscopic analysis. We confirmed that well graphitized multi-walled carbon

nanotubes were nicely grown on the chosen catalytic supporting material from Raman Spectroscopic analysis.

Keywords: Carbon nanotubes; Spray-pyrolysis; HRTEM; FESEM; Madhuca longifolia oil. ©2014 RASĀYAN. All rights reserved

INTRODUCTION Carbon nanotubes (CNTs) are an interesting class of nanostuctures which have been received enormous

amount of attention since their discovery in 19911. The carbon nanotubes possess desirable mechanical

properties2, interesting electronic behavior

3, and unique dimensions

4. As a result of these properties

nanotubes have potential applications in many fields, including composite reinforcement5, transistors and

logic circuits6, field emission sources

7, and hydrogen storage

8. In general, CNTs are synthesized by arc

discharge, laser ablation, and CVD or spray Pyrolysis. Among these methods CVD or spray pyrolysis

method is regarded as a promising method to synthesize carbon nanotubes (CNTs) because of its benefits

to achieve a high yield of CNTS and can be easily scaled up for the production of CNTs at a relatively low

cost9. Several papers have been published and describe a simple routine for synthesizing low-cost CNT

arrays in large scale from petroleum-based precursors such as benzene, xylene and hexane10

. These

carbon precursors are related to fossil fuels and there may be a crisis for these precursors in the near

future. There are few reports on the synthesis of CNTs from natural precursors such as camphor11

,

turpentine oil12

, eucalyptus oil13

, palm oil14

, neem oil15

, coconut oil16

, pine oil17

, Jactropha curcas oil18

,

Cymbopogen flexuosus oil19

, Helianthus annuus oil20-21

, Glycine Max Oil22

, Madhuca Longifolia Oil23

and

Brassica Juncea Oil24

. The advantages of the plant derived carbon precursor are that it is a very cheap,

renewable biomaterial, green, and abundantly available and owing to these factors, it has a huge potential

to be used as the carbon source for the synthesis of CNTs, and therefore, the resulting product can be

commercialized at a lower cost.

One more major advantage of the spray pyrolysis approach is that CNTS can be made continuously, which

could provide a very good way to synthesize large quantities of CNTS under relatively controlled

conditions.


ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al 334

In search of identifying non-edible oil rich in hydrocarbon, to produce carbon nanotubes after screening

many oils, we have succeeded in growing of carbon nanotubes from plant based carbon source of methyl

ester of Madhuca longifolia oil by spray pyrolysis method. It is pale yellow liquid chemically it contains

oleic (46.3%) and linoleic (17.9%) acids as the major unsaturated fatty acids and the saturated fatty acids

palmitic (17.8%) and stearic (14.0%) acids.

EXPERIMENTAL The scheme of our home-made experimental spray-pyrolysis set-up used for the synthesis of carbon

nanotubes is represented in Figure-1.

Fig.-1: The schematic diagram of spray pyrolysis set-up. (A) Heating source, (B) Spray nozzle, (C) Carbon feed

stock inlet, (D) Nitrogen gas, (E) Quartz tube.

One of the most important components of this experimental set-up is the sprayer (atomizer). The scheme

of this sprayer is given in Figure 2. The sprayer consists of a pyrex nozzle (capillary end of the inner tube)

having an inner diameter of 0.75mm, and an outer pyrex tube which has an exit diameter of 2mm. This

outer tube directs the carrier gas-flow (N2) around the nozzle. The inner tube of the sprayer is attached at

one end to the solution (methyl ester of Madhuca longifolia oil) container. The other end of the nozzle is

fixed to a quartz tube (reactor) by means of a polished glass-to-glass connection. The quartz tube has an

inner diameter of 20 mm and it is placed in a 300 mm long electrical furnace. The quartz tube plays also

the role of the support for the reaction products which appear pyrolytic decomposition of the starting

materials (methyl ester of Madhuca longifolia oil) over Fe-Mo impregnated alumina support. The used

electrical furnace is able to assure a uniform temperature up to 800 °C.

Fig.-2: Schematic diagram of the Sprayer 1. Gas inlet; 2. Solution inlet; 3. Tightening; 4. Polished glass-to-glass

connection; 5, 6-Inner and outer pyrex tube.



The alumina supported Fe/Mo catalyst was prepared by the metal ion impregnation method Fe(SO4)3

(0.3g alrich, technical grade) and (NH3)2MoO4 (0.04g with 99.95% purity) were dissolved in

approximately 50 mL of deionised water, and approximately 2g of alumina was added to the solution

giving a Fe:Mo:Al2O3 ratio 1:0.13:13. The water was removed by rotary evaporation, and the solid dried

overnight in an oven at 100 °C the resulting powder was ground thoroughly using a mortar and pestle.

The fine powders were then calcined for 1 h at 450°C and then re-ground before loading into the reactor.

The prepared catalyst was directly placed in a quartz boat and kept at the centre of a quartz tube which

was placed inside a tubular furnace. The carrier gas nitrogen was introduced at the rate of 100 mL per

minute into the quartz tube to remove the presence of any oxygen inside the quartz tube. The temperature

was raised from room temperature to the desired growing temperature. Subsequently, methyl ester of

Madhuca longifolia oil was introduced into the quartz tube through spray nozzle and the flow was

maintained at the rate of 0.5 mL/min. Spray pyrolysis was carried out for 45 minutes and thereafter

furnace was cooled to room temperature. Nitrogen atmosphere was maintained throughout the

experiment. The morphology and degree of graphitization of the as-grown nanostructures were

characterized by scanning electron microscopy, (Hitachi SU6600), high resolution transmission electron

microscopy (JEOL-3010), Raman spectroscopy (JASCO NRS-1500W, green laser with excitation

wavelength 532 nm). The as-grown products were subjected to purification process as follows. The

sample material was added to 5% HF solution to form acidic slurry. This slurry was heated to 60 °C

and stirred at 600 rpm. The sample was filtered and washed with distilled water. The collected sample

was dried at 120 °C in air for 2 hours25.

RESULTS AND DISCUSSION Influence of transition metal as catalyst on CNT synthesis The growth of carbon nanotubes is catalyzed by transition metals i.e. d- block elements, Fe (Group VIII in

periodic table) and Mo (Group VI in periodic table). Due to the existence of covalent bonding, they have

high heats of sublimation i.e. they require large amount of energy to change from solid to vapor state. The

metal ions, because of their comparatively larger size and low charge density, do not get hydrated easily.

Thus, on account of their high heat of sublimation, high ionization energies, low heats of hydration of

their ions and high boiling and melting point, the transition elements have a little tendency to react. They

have rather a tendency to remain unreactive or noble. Hence these metals can be employed as a catalyst to

the synthesis of carbon nanomaterials26

.

Spray pyrolysis of methyl ester of Madhuca longifolia oil at 650 °C under nitrogen gas flow leads to

formation of carbon nanotubes on alumina supported Fe/Mo catalyst nanoparticles. During catalyst-

assisted thermal cracking of the methyl ester of Madhuca longifolia oil, large numbers of reactions are

possible which include decarboxylation, demethylation, leading to the formation of carbon monoxide,

carbon dioxide at the earlier stages forming alkanes and alkenes. Subsequently, these long-chain alkanes

and alkenes will act as precursor for CNTs in the presence of Fe/Mo nanoparticles supported on alumina.

It may be pointed out that since alkanes contain carbon-to-carbon single bond, their dissociation will

require lower energy (347-356 kJ/mol); whereas alkenes will require at least one carbon-to-carbon double

bond. Therefore, their dissociation will require higher energy (611-632 kJ/mol) about two times stronger

than carbon-to-carbon single bond. In the view of this, the alkanes resulting as by product of dissociation

such as CH4 (methane), C2H6 (ethane) and C3H8 (propane) are the most likely candidates to lead to the

formation of CNTs. It should be pointed out those alkanes like methane, ethane etc., and break to give rise

to carbon. This carbon gets dissolved in Fe/Mo nanoparticles supported on alumina. The carbon atoms

then diffuse out of Fe/Mo nanoparticles to give rise to CNTs. For the successful growth of CNTs, the rate

of carbon diffusion should be equal to rate of formation of CNTs.

FESEM and HRTEM observations of CNTs Figure-3a and Figure-3b show the field emission scanning electron microscopy image of the as-grown

nanostructures over Fe-Mo bimetallic catalyst, impregnated in alumina at 650 °C under the flow of



nitrogen by CVD assisted spray pyrolysis method. FESEM image clearly reveals that CNTs grew nicely

on the surface of the alumina particles.

Fig.-3: FESEM micrographs of as grown MWCNTs at 650 °C

Fig.-4: HRTEM micrographs of as grown MWCNTs at 650 °C (a) a tubular structure with

hollow inner (b) magnified view on MWCNTs.

In Figure 4a and 4b we have presented the high resolution HRTEM images of MWCNTS grown over Fe-

Mo bimetallic catalyst impregnated on alumina support at 650 °C with a flow rate of methyl ester of

Madhuca longifolia oil at 0.5 mL per minute. Further, HRTEM image (Figure 4a) of the CNTS clearly

confirms that the wall of the tube was clean and well graphitized. The well-graphitized wall of MWNTs

with thickness about 12 nm and the inner and outer diameter of the tube thickness about 18 and 30 nm

respectively. The diameters of the nanotubes were in the range of 30-35 nm. The side wall of some part of

tubes was so thick and inner diameters of those tubes were so small that the tubular structure observed

clearly. The side walls of CNT were found to consist of ≈ 23 graphitic layers clearly visible in HRTEM

(Figure-4b).

The Raman spectroscopy is a strong tool for characterizing carbon nanotubes. Figure 5 typical Raman

spectra of MWCNTs indicating two characteristic peaks. The G band peak at 1580 cm-1

corresponds to in-

plane oscillation of carbon atoms in the graphene wall of MWCNTs and lower D-band peak at 1356 cm-1

a b

a b



represents the degree of defects or dangling bonds. G-peak is assigned to E2g mode of graphite lattice and

D-peak corresponds to an A1g mode due to the structural defects of the graphite crystal27-28

. The intensity

ratio of D and G peaks (ID/IG) is used to characterize the degree of graphitization of carbon materials, i.e.,

smaller ratio of ID/IG of the as-grown CNTs is ≈ 0.5. This value reveals a high degree of graphitization.

1000 1200 1400 1600 1800 2000

130

132

134

136

138

140

142

144

146

148

150

Inte

ns

ity

(a

.u.)

Ramanshift (cm-1)

Fig.-5: Raman spectroscopic analysis of as grown MWCNTs at 650 °C.

Growth Mechanism of MWNTs The mechanism of CNT nucleation and growth is one of the challenging and complex topics in current

scientific research. Presently, various growth models based on experimental and quantitative studies have

been proposed. It is well established, that during CNT nucleation and growth, the following consecutive

steps were taken place29

.

1. Carbon species formation by decomposition of precursor over the catalyst

2. Diffusion of carbon species through the catalyst particle

3. Precipitation of the carbon in the form of CNTs

The first step involves formation of carbon species by catalytic vapor decomposition of vapors of the

precursor material over the catalyst. In the second step the diffusion of carbon species through the catalyst

particle takes place. The catalyst surface may exert a diffusion barrier. It is still unclear whether carbon

species diffuse on the particle surface30

, on the particle bulk29

or whether surface and diffusion compete.

The most accepted growth model suggests bulk diffusion of carbon species into the metal particles31

. The

third step is the precipitation of the carbon in the form of CNTs from the saturated catalyst particle.

Based on experimental results, a possible growth mechanism of MWNTs was proposed. It is known from

the fact that catalytic centers on catalyst particle act as nucleation site for the growth of MWNTs32. The

precursor vapor decomposed on surface of the catalyst particle produces carbon. As the reactivity

between the catalyst and the carbon exceeds the threshold value, carbon atoms loose their mobility in the

solid solution, forming metal carbides33

. These metastable Fe and Co carbides decomposed and produce

carbon which dissolves in these metal particles. The dissolved carbon diffuses through the metal particle

and gets precipitated in the form of crystalline graphene layer. This carburized surface acts as a barrier for

further carbon transfer from the gas phase to the bulk of the catalyst since carbon diffusion is slower

through metal carbides34

. The saturated carbides have lower melting point and they are fluid like during

the growth process35

.



If the rate of precursor decomposition and the rate of diffusion of carbon are equal, then the metal raise

through a capillary action and tube growth occurs. The fact that long carbon nanotubes observed have

their catalyst particles partially exposed, indicates that the direct contact of catalyst surface with carbon

precursor is essential for continuous CNT growth consistent with the growth mechanism proposed by

Rodriguez36

. In case the decomposition rate exceeds the diffusion rate, more of carbon produced forms a

thick carbide layer over the surface of metal which acts as a barrier for further carbon transfer from the

gas phase to the bulk of the catalyst. However, the thick carbide layer crystallizes out as graphene layer

which encapsulate the metal particle. When a catalyst particle is fully encapsulated by layers of graphene

sheets, the carbon supply route is cut and CNT growth stops resulting in short MWCNTs. The catalyst

particle undergoes several mechanical re shaping during the tip growth of multi-walled nanotubes37-38

.

This gives the impression that the catalyst is in liquid state during reaction. The catalyst particle seen

inside and at the tip of tube could be the solidified form of the liquid phase metal particle. Thus the

growth process is by the vapor–liquid–solid (VLS) mechanism39

.

The CNTs grow with either a tip growth mode or a base growth mode. Base growth mode is suggested

when the catalyst particle remain attached to the support, while tip growth happens when the catalyst

particle lifts off the support material. These growth modes depends on the contact forces or adhesion

forces between the catalyst particle and support40

, while a weak contact favors tip-growth mechanism, a

strong interaction promotes base growth41

. Catalyst particle seen at tip of CNTs indicate tip growth mode.

These catalysts particles have lifted off the support and elongated due to the flow nature and stress

induced by the carbon surrounding the catalyst.

CONCLUSION We have examined the simple and inexpensive method for the formation of MWNTS by the

decomposition of methyl ester of Madhuca longifolia oil using spray pyrolysis with a Fe-Mo / Al2O3

catalyst prepared by wet impregnation method. The present synthesis does not require any pretreatment

of the catalyst precursor to increase its activity. Thus we have simply avoided toxic chemicals like

acetylene, xylene and benzene and gases like CO and Hydrogen. Thus, this technique found to be

effective in synthesizing MWCNTS. A large number of available catalyst sites and efficient catalyst

reactivity were the two crucial factors to control nanotubes growth. The resulting MWCNTS had a

diameter ranging from 30 – 35 nm.

ACKNOWLEDGMENTS The authors acknowledge the UGC New Delhi for financial support, the Institute for Environmental and

Nanotechnology for technical support and IITM for access to Electron Microscopes.

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