Scalable synthesis of aligned carbon nanotubes bundles using green natural precursor: neem oil
© Kumar et al; licensee Springer. 2011
Received: 6 September 2010
Accepted: 18 January 2011
Published: 18 January 2011
Practical application of aligned carbon nanotubes (ACNTs) would have to be determined by a matter of its economical and large-scale preparation. In this study, neem oil (also named Margoaa oil, extracted from the seeds of the neem--Azadirachta indica) was used as carbon source to fabricate the bundles of ACNTs. ACNTs have been synthesized by spray pyrolysis of neem oil and ferrocene mixture at 825°C. The major components of neem oil are hydrocarbon with less amount of oxygen, which provided the precursor species in spray pyrolysis growth of CNTs. The bundles of ACNTs have been grown directly inside the quartz tube. The as-grown ACNTs have been characterized through Raman spectroscopy, scanning and transmission electron microscopic (SEM/TEM) techniques. SEM images reveal that the bundles of ACNTs are densely packed and are of several microns in length. High-resolution TEM analysis reveals these nanotubes to be multi-walled CNTs. These multi-walled CNTs were found to have inner diameter between 15 and 30 nm. It was found that present technique gives high yield with high density of bundles of ACNTs.
Advanced carbonaceous materials have drawn great attention throughout the world because of their particular microstructures, unique properties, and great potential applications in many fields. Several carbon species such as methane, acetylene, benzene, xylene, toluene, etc., have been used as a carbon feedstock to synthesize CNTs [1–7]. These carbon precursors are related to fossil fuels which may not be sufficiently available in near future; so in order to develop a more competitive carbon material, it is necessary to consider developing carbonaceous materials from the natural resource. Recently, there have been reports on the use of natural precursor: camphor (C10H16O), turpentine oil (C10H16), eucalyptus oil (C10H18O) and palm oil (C67H127O8) for synthesis of CNTs [8–16]. More recently, different groups have used chemical vapor deposition method and prepared aligned carbon nanotubes (ACNTs) [17–23]. The ferrocene acts as in situ Fe catalyst precursor and forms nanosized Fe particles for the growth of ACNTs on Si substrates. Despite the mentioned efforts aiming at efficient synthesis of CNTs, further research is necessary to improve yield and purity of CNTs. The main components of neem oil are hydrocarbons containing low amount of oxygen which provides the precursor species in catalytic CVD growth of CNTs.
In this article, we report the synthesis of aligned CNTs bundles using neem oil as the carbon source using the spray pyrolysis technique. To the best of our knowledge, there has been no report on the use of this green bio-hydrocarbon in producing aligned CNTs bundles. Neem oil being a natural source which is renewable and cheap has the potential to be the green alternative for industrial scale production of CNTs.
Synthesis of ACNTs bundles was carried out using spray pyrolysis-assisted CVD method. Neem oil was used as carbon source and ferrocene [Fe (C5H5)2] as a source of Fe which acts as catalyst for the growth of ACNTs. The spray pyrolysis setup consisted of a nozzle (inner diameter 0.5 mm) attached to a ferrocene--neem oil (concentration 20 mg/ml) supply used for releasing the solution into a quartz tube (700 mm long and inner diameter 25 mm), which was mounted inside a reaction furnace (300 mm long) . The outer part of the quartz tube was attached with water bubbler.
The as-grown carbon materials were characterized using scanning electron microscope (SEM) (Philips XL 20) and transmission electron microscopy (TEM) (Tecnai G2 20). Raman spectroscopy was carried out with an excitation wavelength of 448 nm from Ar ion laser with typical acquisition time of 600 s. For SEM observation, the black soot-like material was directly mounted to the sample holder with silver glue which is electrically conductive. Sample for TEM studies were prepared by dispersing a small amount of black soot-like materials in ethanol with sonication for 10 min. Drops of the dispersion were placed onto a holey carbon grid and dried.
Results and discussion
In Figure 7b, Raman-shift range 200-2000 cm-1, two peaks are observed at 1348 and 1585 cm-1 corresponding to D and G bands, respectively. The G band corresponds to the tangential stretching (E 2g) mode of the highly oriented pyrolytic graphite and suggests the CNTs to be composed of crystalline graphitic carbon. Higher intensity of G band indicates the higher degree of crystallinity/graphitization. On the other hand, the D band at 1348 cm-1 originates from disorder in the sp 2-hybridized carbon and indicates lattice distortions in the curved graphene sheets, tube ends, etc. The intensity ratio of D and G peaks (I D/I G) is used to characterize the degree of carbon materials, i.e., smaller ratio of I D/I G corresponds to higher degree of CNTs graphitization [25, 26]. The relative intensity (I D/I G) of the as-grown CNTs is 0.265. This value reveals a higher degree of graphitization when compared to those values reported for the CNTs grown by thermal decomposition of acetylene (e.g., I D/I G ~ 0.84-1.3) , atomic layer deposition of iron sources and oxidants (e.g. I D/I G ~ 0.74-0.90) , and spray pyrolysis of natural precursors (I D/I G ~ 0.3-0.68) [13, 15, 29].
In summary, the growth of high-quality ACNTs bundles could be achieved by adjusting several processing parameters, such as ferrocene concentration in neem oil and temperature, and the same could be evidenced by TEM morphology and Raman spectroscopy. The bundles of ACNTs have been successfully prepared in large scale at 825°C under Ar atmosphere. Dense ACNT bundles with length in the range of of 20-50 μm have been formed directly inside the quartz tube. The as-grown well-crystallized multi-wall CNTs have an outer diameter in the range of 15-30 nm. The present technique gives higher yield and high density of bundles of CNTs. Graphitization of these CNTs is fairly good, and the presence of catalyst particles in the as-grown CNTs is almost negligible.
aligned carbon nanotubes
Energy dispersive X-ray
selected area electron diffraction
scanning electron microscopy
transmission electron microscopy.
The authors are grateful to Prof. C. N. R. Rao, Prof. A. K. Raychaudhary, Prof. A. K. Sood, Prof. P. M. Ajayan, and Prof. D. P. Singh (Vice-Chancellor B.H.U) for their encouragement and support. The authors are also grateful to Prof. A. C. Pandey (Allahabad University) for providing the Raman facility. The financial support from the DST (UNANST: B.H.U), CSIR, UGC, and MNRE-New Delhi, India, is gratefully acknowledged. One of the authors, Rajesh Kumar, is grateful to the Council of Scientific and Industrial Research, Government of India for providing a senior research fellowship.
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