A green precursor for carbon nanotube synthesis

NEW CARBON MATERIALS Volume 26, Issue 2, Apr 2011 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2011, 26(2):85–88.

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A green precursor for carbon nanotube synthesis S. Paul, S. K. Samdarshi* Department of Energy, Tezpur University, Tezpur-784028, Assam India

Abstract:

The present work aims to explore a natural renewable precursor for the synthesis of multiwalled carbon nanotubes

(MWCNTs), conforming to the principles of green chemistry. MWCNTs were synthesized by chemical vapor deposition using a natural renewable precursor (coconut oil). Nitrogen gas was used as an inert atmosphere as well as a carrier for the evaporated precursor (flow rate: 100 mL/min). The synthesized MWCNTs are characterized by scanning and transmission electron microscopy, electron dispersive X-ray analysis, and Raman spectroscopy. The diameters of the synthesized nanotubes are in the range of 80 nm to 90 nm under optimum conditions. Key Words: Coconut Oil; Natural renewable precursors; Carbon nanotube; CVD

1

Introduction

The pivotal publication of Iijima[1] in 1991 made carbon nanotubes (CNTs) a key component in nanotechnology. Research in the area of CNTs has grown enormously in the last decade, which is clear from the number of publications in the area. As this novel form of carbon has applications in power electronics, molecular electronics, energy storage, biomedicine, and many others[2-5] and as the numerous new applications are being proposed, so is the rapid rise in demand for its large production. This is the high time to explore new environment-friendly natural renewable precursors. The petroleum-based precursors have been investigated in detail, and the easy availability of high-grade precursors have resulted in production and process optimization of different types, structure, dimension, and orientation of CNTs. One paper published recently[6] describes a simple routine for synthesizing large-scale and low-cost CNTs from hexane. However, the naturally occurring hydrocarbon precursors have generated some interest because of the possibility of production of CNTs from the bank of hydrocarbons that are being renewably produced by the nature in a carbon neutral manner. But the high cost and low availability of the renewable raw materials (precursors in case of CNT synthesis) like camphor and turpentine have confined them to research laboratories. So it becomes important to search for new natural renewable precursors that are easily available and have low cost. Of course, it calls for studies related to yield and quality of the CNTs being produced and the applications of the resultant CNTs. Scanty literature has been published on synthesis of carbon nanotubes using natural renewable precursors. Kumar and Ando[7] used camphor and Afre et al.[8] used turpentine oil

as precursor for the synthesis of CNTs. They prepared good quality vertically aligned multiwalled nanotubes (MWCNTs) both from camphor and turpentine oil precursors. Andrews et al.[9] have reported the impact of the precursor’s flux and type on the quality and size of the single-walled carbon nanotubes (SWCNTs) synthesized from camphor and from camphor analogues. Ghosh et al.[10] have reported the synthesis of well-graphitized SWCNTs of reasonable purity from turpentine oil. Chemical vapor deposition (CVD), arc discharge, and spray pyrolysis are some of the common methods used for high temperature synthesis of CNTs. Hydrothermal method is one of the popular low temperature synthesis routes[11]. Transition metals (Fe, Co, Ni etc) are used as catalysts, which provide growth sites for CNTs. Recently Chai et al have successfully synthesized CNTs using nickel oxide as catalyst[12]. The present work aims to utilize renewable precursor Cocos nucifera Lin. oil for the synthesis of CNTs and analyze the product qualitatively and quantitatively. CVD, being the simplest method, has been used for the synthesis of CNTs. Iron was used as catalyst for the growth of CNTs. The catalyst was synthesized using self-propagating high-temperature synthesis (SHS) method.

2 2.1

Materials and method Materials used

For synthesis of iron catalyst, CO(NH2)2 (urea) and Fe(NO3)3·9H2O (iron nitrate nonahyrdate) were purchased from Merck, India. The chemicals were used as received without further purification. Cocos nicifera oil was directly extracted from Cocos nicifera seed by Soxhlet apparatus.

Received date: 20 May 2010; Revised date: 4 April 2011 *Corresponding author. E-mail: [email protected] Copyright©2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(11)60068-1

S.Paul et al. / New Carbon Materials, 2011, 26(2): 85–88

Fig.1

Schematic diagram of CVD unit (1-carrier gas cylinder, 2-gas

regulator, 3-manometer, 4-flowmeter, 5-bubbler, 6-quartz tube, 7-0.5 kW furnace, 8-5 kW furnace, 9- precursor on quartz boat, 10- catalyst on quartz boat)

Fig.2 TEM image of Fe-catalyst synthesized

aging for 80 minutes at 98 oC inside an oven with limited supply of air. Thereafter the sample was directly put into a preheated muffle furnace at 550 oC for calcinations. Initially, the solution boils, undergoes further dehydration, and foams. After the solution reaches the point of ignition, spontaneous combustion takes place and it begins to burn and release large amount of heat and gases. The oxide particles were again heated in hydrogen atmosphere at 600 oC for 2 h to get pure metal particles. The Fe-catalyst particles were then sonicated. These metal particles were then used as catalyst for the synthesis of carbon nanotubes. 2.3 Synthesis of carbon nanotubes: chemical vapour deposition technique CVD technique, being the simplest and of low energy intensity, was used for the synthesis of CNTs. The schematic diagram of the CVD unit is shown in Fig.1. It includes two cylindrical furnaces: first furnace is of 0.5 kW for evaporating the precursor and the second furnace is of 5 kW for depositing the precursor on catalyst. A quartz tube, 1 m long and 0.03 m ID was used as reactor tube. Nitrogen gas (flow rate: 100 mL/min) was used to create an inert atmosphere and also used as the carrier gas for vaporized precursor inside the CVD reactor tube. Inert atmosphere was ensured by employing bubblers both at the inlet and the outlet of the CVD setup. The flow rate of the gas was controlled using a regulator and a flowmeter. 20 g of Cocos nucifera Lin.(coconut) oil was kept in a quartz boat and evaporated at a temperature of 305 oC in the first furnace. Then, it was pyrolyzed over 0.5 g of catalyst particles on semicylindrical quartz boat (ID 2 cm and length 10 cm) at a temperature of 850 oC in the second furnace for 60 min. 2.4

Fig.3

2.2

SEM and EDX analysis of MWCNTs

Synthesis of catalyst

The catalyst was synthesized by SHS method as described by Suresh and Patil[13], with some modifications. The oxidizing valence of the oxidizer and the reducing valence of the fuel were taken into account for determining the composition of the solution. The metal salt iron nitrate is taken and mixed with urea in a ratio of iron nitrate to urea at 1 to 3, and the mixture was dissolved in a minimum quantity of water in a crucible. The crucible containing the solution was kept for

Characterization

The morphology and size of the particles were investigated by scanning electron microscopy (SEM) using model no. JSM-6390LV of JEOL, Japan. Energy dispersive X-ray analysis was done by EDX (Oxford Instrumentation Ltd, UK) attached to the SEM. Transmission electron microscopy (TEM) analysis was carried out at 100 kV using the model JEM-100CX II of JEOL, Japan. Raman spectroscopy was recorded on Raman spectrometer (Jobin Yovn Horiba LABRAM-HR) with a He-Ne laser (frequency 632.81 nm) source.

3

Results and discusion

The sonicated catalyst particles synthesized were of uniform shape and had a size distribution of 7-12 nm, as shown in the TEM micrograph (Fig.2). The particles are found to be almost spherical and well separated from each other. The SEM along with EDX depicting the surface morphology of the deposited and heat-treated CNTs is shown in Fig.3. The SEM micrograph enables us to have a clear view of the overall structure and growth of the CNTs synthesized. The

S.Paul et al. / New Carbon Materials, 2011, 26(2): 85–88

particular direction nor do they have a long range uniformity in the diameter. Undesirable structures like amorphous carbon are negligible. It confirms that the synthesized MWCNTs are reasonably pure (amorphous carbon free) albeit with some structural defects.

Fig.4 Raman spectrum of Cocos nucifera oil-grown MWCNTs

Cocos nucifera Lin oil consists of lauric acid as the major compound followed by oleic acid. Absence of lauric and oleic acids in other plant precursors (turpentine oil and camphor) used for the synthesis of carbon nanotubes makes it a novel precursor for the synthesis of nanotubes. As synthesis parameters such as catalyst type, carrier gas used, temperature, flow rate, etc. are to be optimized, direct comparison of the precursors for quality and quantity of CNTs is not possible. In future, the parameters can be optimized for CNT synthesis using Cocos nucifera Lin oil and may be reported.

4

Conclusions

The structural features of carbon nanotubes prepared by CVD have delicate dependence on carbon precursor, catalyst, carrier gas, flow rate, reaction temperature, and time, etc. Since this is the first report of MWCNT synthesis from Cocos nucifera Lin oil, no data are available for a direct comparison. The use of natural renewable precursors gives reasonable yield and makes the process environment friendly as well. This method can not only be performed easily at any laboratory or research institution but also be scaled up for mass production of MWCNTs from renewable natural precursor. Fig.5 TEM micrograph of MWCNTs synthesized

yield in the form of entangled, noodle-like, densely packed CNTs of length 3-4 μm is evident. The CNTs are found to be well distributed throughout the stub used for SEM analysis. The EDX confirms the presence of carbon and iron in the sample. Raman spectroscopy was used to identify the structural features of CNTs synthesized. A He-Ne laser (frequency 632.81 nm) was used for Raman analysis. Fig.4 shows the Raman spectrum. It has the same pattern as reported by other workers[14-15]. The main features of Raman spectra are the first-order ones. The disorder-induced D-band appears at 1 327.3 cm-1, whereas the G-band occurs at 1 576.8 cm-1. The lower ID/IG corresponds for better quality tubes[15]. The ID/IG ratio for synthesized CNTs from coconut oil comes out to be 1.39. The higher ID/IG value may be due to the presence of amorphous carbon and defects in the sample. The G-band revealed a shoulder on the higher energy side at around 1 604 cm-1, making the G-band asymmetric, which may be attributed to multiwalled structure[16]. The TEM micrograph of CNTs from Cocos nucifera Lin oil (Fig.5) reveals them to be hollow tubes of approximate diameter ranging from 80-100 nm. The thick walls of the CNTs are due to the multiple layers of concentric graphene sheets. This reveals that the synthesized nanotubes are MWCNTs. However, the MWCNTs are neither aligned to a

Acknowledgements The authors acknowledge DST and AICTE for financial support to carry out this work. Authors gratefully acknowledge SAIF-NEHU, Shillong and UGC-DAE CSR, Indore for TEM and Raman analysis respectively.

References [1] Iijima S. Helical microtubules of graphitic carbon [J]. Nature 1991, 354: 56-58. [2] Bianco A, Kostarelos K, Prato M. Applications of carbon nanotubes in drug delivery [J]. Current Opinion in Chemical Biology, 2005, 9: 674-679 [3] Carson L, Kelly-Brown C, Stewart M. et al. Synthesis and characterization of chitosan-carbon nanotube composites[J]. Materials Letters, 2009, 63(6-7): 617-620. [4] Chen Ming-liang, Zhang Feng-jun, Oh Won-chun. Synthesis, characterization, and photocatalytic analysis of CNT/TiO2 composites derived from MWCNTs and titanium sources[J]. New Carbon Materials, 2009, 24(2): 159-166. [5] Lee S F, Chang Y P, Lee L Y. Enhancement of field emission from carbon nanotubes by post-treatment with a chromium trioxide solution[J]. New Carbon Materials, 2008, 23(2): 104-110. [6] Sadeghian Z. Large-scale production of multi-walled carbon nanotubes by low-cost spray pyrolysis of hexane[J]. New Carbon Material, 2009, 24(1): 33-38. [7] Kumar M, Ando Y. A simple method of producing aligned car-

S.Paul et al. / New Carbon Materials, 2011, 26(2): 85–88

bon nanotubes from an unconventional precursor-Camphor [J]. [8] Afre R A, Soga T, Jimbo T, et al. Carbon nanotubes by spray pyrolysis of turpentine oil at different temperatures and their studies[J]. Microporous and Mesoporous Materials, 2006, [9] Andrews R J, Smith C F, Alexander A J. Mechanism of carbon nanotube growth from camphor and camphor analogs by chemical vapor deposition [J].Carbon, 2006, 44(2): 341-347. [10] Ghosh P, Soga T, Afre R A, et al. Simplified synthesis of single-walled carbon nanotubes from a botanical hydrocarbon: Turpentine oil[J]. Journal of alloys and compounds, 2008, 462(1-2): 289-293

3519-3521. [13] Suresh K, Patil K C. Combustion synthesis and properties of and Compounds, 1994, 209(1-2): 203-206. [14] Donato M G, Galvagno S, Messina G et al. Raman analysis of MWCNTs produced by catalytic CVD: derivation of a scaling law for the growth parameters[J]. Journal of Raman Spectroscopy, 2008, 39(2): 141-146. [15] Zhu S, Zhang H, Bai R, Microwave-accelerated dissolution of MWNT in aniline [J]. Materials Letters, 2007, 61(1): 16-18.

[11] Manafi S, Nadali H, Irani H R. Low temperature synthesis of carbon

ported nickel oxide catalyst [J]. Materials Letters, 2007, 61 (16):

Ln3Fe5O12 and yttrium aluminium garnets [J]. Journal of Alloys

96(1-3) : 184-190.

multi-walled

[12] Chai S P, Zein S H S, Mohamed A R. Moderate temperature synthesis of single-walled carbon nanotubes on alumina sup-

Chemical Physics Letters, 2003, 374(5-6): 521-526..

nanotubes

via

a

[16] Lee C, Kim D, Lee T, et al. Synthesis of uniformly distributed

sonochemi-

carbon nanotubes on a large area of Si substrates by thermal

cal/hydrothermal method [J]. Materials Letters, 2008, 62(26):

chemical vapor deposition[J]. Applied Physics Letters, 1999, 75:

4175-4176

1721 -1723.