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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.
ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al 335
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
ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al 338
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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