morphology of entangled multiwalled carbon nanotubes …rasayanjournal.co.in/vol-7/issue_4/5_...
TRANSCRIPT
Vol. 7 | No.4 |333 – 339 | October-December | 2014
ISSN: 0974-1496 | e-ISSN: 0976-0083 | CODEN: RJCABP
http://www.rasayanjournal.com
http://www.rasayanjournal.co.in
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.
Vol. 7 | No.4 |333 – 339 | October-December | 2014
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.
Vol. 7 | No.4 |333 – 339 | October-December | 2014
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
Vol. 7 | No.4 |333 – 339 | October-December | 2014
ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al 336
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
Vol. 7 | No.4 |333 – 339 | October-December | 2014
ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al 337
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
.
Vol. 7 | No.4 |333 – 339 | October-December | 2014
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.
REFERENCES 1. S. Ijima, Nature, 354, 56 (1991). 2. M.S. Dresselhaus, G. Dresselhous and P. Avouris, In carbon nanotubes: synthesis, Structure,
properties and applications, Chap. 13, Springer, Berlin, Germany (2000). 3. M.S. Dresselhaus, G. Dresselhaus and R. Saito, Carbon, 33, 883 (1995). 4. G.D. Li, Z.K. Tang, N. Wang and J.S. Chen, Carbon, 40, 917 (2002). 5. D. Qian, E.C. Dickey, R. Andrews and T. Rantell, Appl. Phys. Lett., 76, 2868 (2000). 6. V. Derycke, R. Martel, J. Appenzeller and P. Avouris, Nano Lett., 1, 453 (2001). 7. S.K. Patra, and G. Mohan Rao, J. Appl. Phys., 100, Article ID: 024319, (2006). 8. H.M. Cheng, Q.H. Yang and C. Liu, Carbon, 39, 1447 (2001). 9. S. Karthikeyan, P. Mahalingam and M. Karthik, E J. Chem., 6, 1 (2008). 10. Z. Sadeghian, New Carbon Mater., 24, 33 (2009). 11. M. Kumar and Y. Ando, Chem. Phys. Lett., 374, 521 (2003). 12. R.A. Afre, T. Soga, T. Jimbo, M. Kumar, Y. Ando and M. Sharon, Chem. Phys. Lett., 414, 6 (2005). 13. P. Ghosh, R.A. Afre, T. Soga and T. Jimbo, Mater. Lett., 61, 3768 (2007). 14. A.B. Suriani, A.A. Azira, S.F. Nik, R. Md Nor and M. Rusop, Mater. Lett.,63, 2704 (2009).
Vol. 7 | No.4 |333 – 339 | October-December | 2014
ENTANGLED MULTIWALLED CARBON NANOTUBES S. Kalaiselvan et. al 339
15. R. Kumar, R.S. Tiwari and O.N. Srivastava, Nanoscale Res. Lett., 6, 92(2011) 16. S. Paul and S.K. Samdarshi, New Carbon Mater., 26, 85 (2011). 17. S. Karthikeyan and P. Mahalingam, Int. J. Nanotechnol. Appl., 4, 189 (2010). 18. S. Karthikeyan and P. Mahalingam, Int. J. Green Nanotechnol. Phys. Chem., 2, 39 (2010). 19. S. Mageswari, A. Jafar Ahamed and S. Karthikeyan, J. Environ. Nanotechnol., 1, 28 (2012). 20. V.S. Angulakshmi, N. Sivakumar and S. Karthikeyan, J. Environ.Nanotechnol., 1, 40 (2012). 21. V.S. Angulakshmi, K. Rajasekar,
C. Sathiskumar and S. Karthikeyan, New Carbon Mater., 28, 284
(2013). 22. V.S. Angulakshmi, C. Sathiskumar, M. Karthik and S. Karthikeyan, J. Environ. Nanotechnol., 2,
101 (2013). 23. S. Karthikeyan, S. Kalaiselvan, D. Manorangitham and S. Maragathamani, J. Environ. Nanotechnol.,
2, 15 (2013). 24. S. Kalaiselvan, M. Karthik, R. Vladimir and S. Karthikeyan, J. Environ. Nanotechnol., 3, 92 (2014). 25. P. Mahalingam, B. Parasuram, T. Maiyalagan and S. Sundaram, J. Environ. Nanotechnol., 1, 53
(2012). 26. Rajesh Kumar, R.M. Yadav, K. Awasthi, T. Shripathi, A.S.K. Sinha, R.S. Tiwari and O.N.
Srivastava, J. Exp. Nanosci., 8, 606 (2013). 27. T. Luo, J.W. Liu, L.Y. Chen, S.Y. Zeng and Y.T. Qian. Carbon, 43,755(2005). 28. Y. Li, X. Zhang, X. Tao, J. Xu, F. Chen, W. Huang and F. Liu, Chem. Phys. Lett., 386, 105 (2004). 29. R. Brukh and S. Mitra, Chem. Phys. Lett., 24, 126 (2006). 30. S. Hofmann, G. Csanji, A. C. Ferrari, M. C. Payne and J. Robertson, Phys. Rev. Lett., 95, 036101
(2005). 31. C. Ducati, I. Alexandrou, M. Chhowalla, J. Robertson and G. A. J. Amaratunga, J. Appl. Phys., 95,
6387 (2004). 32. M.H. Lee and D. G. Park, Korean Chem. Soc., 24, 1437 (2003). 33. R. Sinclair, T. Itoh and R. Chin, Microsc. Microanal., 8, 288 (2002). 34. B. Ozturk, V.L Fearing, J.A. Ruth and G. Simkovich, Metall. Mater. Trans A., 13, 1871 (1982). 35. A.K. Chakraborty, J. Jacobs, C. Anderson, C.J. Roberts and M.R.C. Hunt, J. Appl. Phys., 100, 084321
(2005). 36. N.M. Rodriguez, J. Mater. Res., 8, 3233 (1993). 37. S. Hofmann, K. Sharma, C. Ducati, G. Du, C. Mattevi, C. Cepek, M. Cantoro, S. Pisana, A. Parvez, F.
Cervantes-sodi, A.C. Ferrari, R. Dunin-Borkowski, S. Lizzit, L. Petaccia, A. Goldoni and J.
Robertson, Nano Lett., 7, 602 (2007) 38. S. Helveg, C. Lopez-Cartes, J. Sehested, P.L. Hensen, B.S. Clausen, J.R. Rostrup Nielsen, F.A.
Pedersen and J.K. Norskov, Nature, 427, 426 (2004). 39. E.F. Kukovitsky, S.G. L’vov and N.A. Sainov, Chem. Phys. Lett., 317, 65 (2000). 40. A. Leonhardt, S. Hampel, B. and Büchner et al., Chem. Vap. Deposition, 12, 380 (2006). 41. C. Bower, Z. Otto, Z. Wei, D.J. Werder and S. Jin, Appl. Phys. Lett., 77, 2767 (2000).
[RJC-1176/2014]