Carbon nanotubes are an exciting allotrope of carbon and are generally considered to belong to the fullerene family. Carbon nanotubes (CNTs) are nanometer-size cylinders made out of carbon atoms. They can be thought of as a layer of graphite rolled-up into a cylinder. The small scale and the one dimensional structure of carbon nanotubes are directly related to their unique properties which find more and more applications as the understanding and progress in synthesis advances. Carbon nanotubes represent an exemplary system where the bottoms-up approach to synthesis results in perfect structures with sizes less than 10nm, a range which remains inaccessible for advanced techniques.
Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide (HiPCO), and chemical vapor deposition (CVD). Most of these processes take place in vacuum or
with process gases. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable. With the increasing demand in CNTs, a low-cost synthesis is very essential. Another aspect of interest is the development of CNTs that are hydrophilic because they have a pioneering application in the field of biotechnology.
While researchers worldwide are trying to develop newer and better methods for producing CNTs, the age-old practice of making 'kaajal' is the simplest and the cheapest way of making CNTs. It is prepared by the burning of vegetable oil to prepare fresh carbon soot known as `kaajal'. Burning of other edible oils also produces soot, but mustard oil was selected because of its very high un-saturated fatty acids content and for its low cost. Water-soluble carbon nanotube (wsCNT) is prepared by the oxidative treatment of this kaajal. Scanning electron microscopy
Figure 1. A. Armchair SWCNT B. Zigzag SWCNT C. Chiral SWCNT
(SEM) and atomic force microscopy (AFM) were used to structurally characterize the wsCNT. The XRD, Raman, FTIR, UV-Vis-NIR, Fluorescence, and magnetic properties of these wsCNT were presented. To separate the mixture of wsCNT a
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method named gel electrophoresis was used. The discovery of magnetic carbon (present in water soluble CNT) in aqueous solution may open the door for future application as carbon-based spintronics. "Spintronics" is a new field that exploits the 'spin' of the electron (that gives rise to magnetization) rather than its 'charge' to create a remarkable new generation of 'spintronic' devices which will be smaller and more versatile than those currently making up silicon chips and circuit elements.
Nanotubes are categorized as single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). Single-walled nanotubes have a diameter of close to 1 nanometer, with a tube length that can be many thousands of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector (Figure:1). The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m=0, the nanotubes are called "zigzag". If n=m, the nanotubes are called "armchair". Otherwise, they are called "chiral".
Single-walled nanotubes are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multiwalled carbon nanotube (MWNT) variants. Single-walled nanotubes are the most likely candidate for miniaturizing electronics beyond the micro electromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors.
Fig 2. Structure of a multiwall carbon nanotube (MWCNT)
A multiwall carbon nanotube (MWCNT) can similarly be considered to be a coaxial assembly of cylinders of SWCNTs, like a Russian doll, one within another; the separation between tubes is about equal to that between the layers in natural
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graphite (Figure:2). Their diameters range from a few nanometers to around 40nm, depending on the number of concentric tubes.
Multi walled nanotubes can come in an even more complex array of forms, because each concentric single-walled nanotube can have different structures, and hence there are variety of sequential arrangements. The simplest sequence is when concentric layers are identical but different in diameter. However, mixed variants are possible, consisting of two or more types of concentric CNTs arranged in different orders. They can have either regular layering or random layering.
Carbon nanotubes are unique nanostructures with remarkable electronic and mechanical properties. The strength of the sp² carbon-carbon bonds gives carbon
nanotubes amazing mechanical properties. They are the strongest and stiffest materials on earth, in terms of tensile strength and elastic modulus respectively. The Young's modulus of the best nanotubes can be as high as 1000 GPa. These properties, coupled with the lightness of carbon nanotubes, give them great potential in applications such as aerospace. The electronic properties of carbon nanotubes are also extraordinary. Especially notable is the fact that nanotubes can be metallic or semiconducting depending on their structure. Thus, some nanotubes have conductivities higher than that of copper, while others behave more like silicon. There is great interest in the possibility of constructing nanoscale electronic devices from nanotubes. There are several areas of technology where carbon nanotubes are already being used. These include flat-panel displays, scanning probe microscopes and sensing devices. The unique properties of carbon nanotubes will undoubtedly lead to many more applications. CNT is unique in keeping high electrical conductivity, tensile strength, flexibility, elasticity (~18%), good electron field emitting, high aspect ratio (length = ~1000 x diameter) and low thermal expansion coefficient.
Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction," but good insulators laterally to the tube axis. Due to their nanoscale dimensions, electron transport in carbon nanotubes will take place through quantum effects and will only propagate along the axis of the tube (Table:1).
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Because of this special transport property, carbon nanotubes are frequently referred to as "one-dimensional".
Material Young's modulus (GPa) Tensile Strength 3(GPa) Density (g/cm) Single wall nanotube 1054 150 Multi wall nanotube 1200 150 2.6
Steel 208 0.4 7.8 Epoxy 3.5 0.005 1.25 Wood 16 0.008 0.6
Table 1. Properties of carbon nanotubes compared with some other materials
The conventional synthesis of CNT includes electric arc method, laser method, chemical vapor deposition (CVD) method, ball milling technique, diffusion flame synthesis, electrolysis, use of solar energy, heat treatment of a polymer, and lowtemperature solid pyrolysis.
PRESENT WORK
Vegetable oils contain glycerides of a mixture of several types of fatty acids. The raw soot was collected by burning vegetable oil with the aid of a cotton piece in insufficient air (like mustard oil). This soot is classically known as `kaajal' and has been in use as an ointment against common eye ailments and also as eye-liners. The soot was collected and was purified by soxhlet extraction technique using the solvents petroleum ether, toluene, alcohol and water. The purified soot was treated with an aqueous solution of 2.6 molar HNO3. The undissolved residue was separated by centrifugation and the centrifugate was evaporated to dryness on a water bath to yield a black solid. Residual nitrate present in the solid was removed by repeatedly dissolving it in water and evaporating to dryness to yield a nitrate-free black solid. Further purification was done by recrystallization of the sample solution from water with alcohol followed by washing with water. The sample was dried and collected. It has been identified as water soluble (about 20mg/ml) CNT.
Characterization of water soluble CNT (wsCNT) was done to study the morphology and microstructure using different methods namely microscopy (SEM), atomic force microscopy (AFM), XRD study, Raman study, FTIR study, Electrical spectral studies, Fluorescence study, ESR study, Magnetic study and Gel electrophoresis.
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A low magnification (Figure 3a) and high magnification (Figure 3b) SEM micrograph of wsCNT from the soot clearly showed the presence of extensive networked tubular structure with different types of junctions and branching. EDAX analysis confirmed the presence of oxygen along with carbon in the derivatized soot (Figure 3c).
At higher magnification, as shown in Figure 4 , the presence of different types of junctions and branching were clearly visible. These micrographs showed the presence of several turns and bents of complex shapes, sometimes with longer slender tubes. At the end, the slender tube changed its outer diameter to an L-shaped junction (figure 4d), which is very similar to joining two pipes of different diameters. In figures 4e, f, relatively uncomplicated but bent tubes are visible. One can readily recognize several types of junctions present in these micrographs. Thus L-shaped (dipodal) and Y- or T-shaped (tripodal) junctions were recognized. These complex structures have a lot of structural defects. These defects were introduced by the chaotic synthetic conditions of these CNTs under insufficient oxygen, which were augmented by subsequent acid treatment.
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Figure 4. High magnification SEM image of wsCNT representing various bends and junctions .
A typical tapping mode AFM image of wsCNT is shown in Figures 5 and 6. The diameter of the wsCNT was found to vary between 9-30 nm, which supported that they were multi-walled nanotubes. The longest of the observed nanotubes was almost 1 µm in length (figure 6b). The side-walls of the tubules showed the presence of defective sites (figure 5), thus corroborating the findings of SEM studies presented above. These defects were expected because of the introduction of several carboxylic acid group functionalities on the surface of the graphene sheet of the CNT. The introduction of sp3 hybridization arising out of carboxylic acid function introduced defects. These might have been caused by the oxidation process, which introduced extensive functionalization on the side-walls of the nanotube.
Figure 5a, b. AFM topograph of wsCNT from mustard soot (amplitude data: scan range = 80 x 80 nm; Z-range = 0-2 nm).
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Figure 6. AFM topograph of wsCNT from mustard soot: (a) amplitude data: scan range = 100 x 100 nm; Z-range = 0-2 nm), (b) showing a long and branched carbon fiber.
Figures 7a and b represented the XRD profiles of the soxhlet purified soot and its derivative, wsCNT respectively. Both the samples showed two predominant peaks. In the purified raw soot a high intense peak at 25.6 and a low intense peak at 44.0 were assigned for (0 0 2) and (1 0 0) reflections respectively. These reflections suggested that the tubules obtained by this method have good extent of graphitization. Similarly, in the case of wsCNT a high intense peak at 24.1 and low intense peak at 42.8 suggested that crystallinity is not lost due to oxidative acid treatment.
Figure 7. XRD pattern of (a) soxhlet purified soot and (b) wsCNT from soot.
-1 Raman spectra of soxhlet purified soot and its wsCNT were shown in Figures 8a and b respectively. The Raman spectrum has two prominent peaks at 1590 and 1347 cmwhich corresponds to the E-12g (G-band) and disorder-induced (D-band) modes of graphite, respectively. The spectrum showed much wider band-width of the D-band compared to CNT obtained from graphite. The similarity in the intensity of these two peaks was attributed to the presence of structural defects and more sp3hybridized carbon. In the Raman spectrum of wsCNT, there were two prominent peaks at 1359 and 1612 cm respectively. Both D-line and G-line have been downshifted after oxidative treatment, but still both were of similar intensity as those of the
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Figure 8. Raman spectra (laser wavelength = 514.5 nm) of (a) soxhlet purifed soot and (b) wsCNT from soot.
2raw soot with slight increase of G-line intensity. It is known that the D-line corresponds to stretching vibrations of sp-hybridized carbon atoms and the G-line is associated with disorder-induced symmetry-lowering effects .Generally, functionalized carbon nanotube showed a decrease in the intensity of the G-line compared to the unfunctionalized carbon nanotubes. The increase in the intensity of the D-line also indicated the structural deformation in the tube wall after the treatment with HNO3. No peaks were observed in the lower wavelength region, and this could be due to the larger diameter of these tubules, as carbon nanotubes with diameters larger than 2 nm normally do not exhibit any peaks in the radial breathing mode (RBM).
-1Infrared spectral data were used to study the carboxylic group functionalization of wsCNT. In figures 9a and b, FTIR spectra of the soot and of the wsCNT were reproduced in the range 400-4000 cm-1. Figure 9a is a typical IR spectrum of the sample, which was a carbon-based material with the absence of any absorption in the range 400-4000 cm-1. For wsCNT (Figure 9b), a band at 1725 cm-1 is attributed to (C=O) stretching vibration associated with the presence of carboxylic acid group. A broadband centered at 3436 cm-1 is assigned due to the (O-H) stretching vibration of the carboxylic acid group. A band centered at 1599 cm-1-1 and a shoulder at 1350 cm may be due to (C=C) stretching vibrations. A band at ~1217 cm is associated with (C-O) stretching vibrations. So the IR study directly prove the presence of carboxylic acid groups in wsCNT which corroborate the SEM, AFM and Raman results about the cause of structural defects in wsCNT. The incorporation of sufficient number of carboxylic acid groups per unit area in CNT may transform its hydrophobic property (insolubility in water) into hydrophilic nature resulting in its
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ready solubility in the form of wsCNT. The carboxylic acid groups may be present on the walls of the tubes and on the cap region of the tubes.
Figure 9a. Infrared spectra of (a) soxhlet purified mustard soot
Figure 9b. Infrared spectra of wsCNT from mustard soot (in KBr).
Solid-state ESR spectra of the soot and of wsCNT at room temperature were included in Figure 10a, which clearly indicates the presence of a stable carbon radical in the sample and also in wsCNT. A dilute solution of wsCNT retained the ESR signal (Figure 10b). Surprisingly this carbon radical was so stable that it could not be reduced or oxidized even by reagents like metallic Li or concentrated nitric acid respectively. The origin of the trapped carbon radical in a rigid graphene matrix is due to the presence of several junctions. The negatively curved sp2-bonded nano regions in the carbon structure associated with multipodal junctions in wsCNT may cause spin frustration resulting in trapping of carbon radicals.
Figure 10. (a) Combined room temperature ESR spectra of the soot (black), and wsCNT (red). (b)
Aqueous solution ESR spectra of wsCNT.
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Figure 11a, b. Field-cooled (FC) and zero-field-cooled (ZFC) of Âc vs. T
It is well-known that CNT is diamagnetic in nature. The magnetic measurements of wsCNT from the sample have been recorded in the 10-300 K temperature range by using powder sample in an inert atmosphere. The unusual temperature dependence of the dc magnetic susceptibility (Âc) is shown in Figure 11 at an applied magnetic field of 5 kOe.
Unlike the value found for nearly all other forms of carbon, it showed a positive value. At room temperature (300 K), Âc is equal to 2.5Ã-10-7 emu.g-1 Oe-1. It increased slightly with temperature and showed an upturn below 50 K. This behavior is directly related to the presence of unpaired electron in the system as has already been observed by ESR study. The field dependence of the magnetization was also measured at temperatures 10, 25, 50 and 300 K (Figure 12).
Figure 12. (a) Magnetic hysteresis loops for wsCNT(b) Enlarged part of the magnetization curve in the low-magnetic field region.
The magnetic hysteresis in large magnetic field range (at 10 K and 300 K) is also shown (Figure 13). A relatively small diamagnetic contribution due to straw and
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Figure 13. (a) Magnetic hysteresis loops for wsCNT(b) Hysteresis loop for wsCNT at 300 K
Teflon were subtracted from the original data. Our wsCNT from the soot showed characteristics of a soft magnet. With the increase in temperature, the magnetization value in the present case decreased while the magnetic state retained ferromagnetic loop up to room temperature. The value of magnetization depended on the outer diameter of the electron's spin orbit, which is fixed by the nanotube diameter and the number of layers present in the multi-walled wsCNT. This magnetic property of wsCNT can only be due to the presence of its intrinsic junctions and defective structures.
The fluorescence emission spectra of wsCNT from the soot in water upon excitation at 364 nm were shown in Figure 14. Strong luminescence at room temperature has been previously detected in nanotube solutions. Here, in our system,
Figure 14. (a) pH-dependent Fluorescence emission spectra of wsCNT from soot in water at 364 nm excitation wavelength and, (b) ionic strength variation.
the origin of this luminescence has been tentatively attributed to the existence of extensive conjugated electronic structures and excitation-energy trapping associated with defects in the nanotubes. Our luminescence studies in water indicated that the
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emission spectrum is dependent on pH (Figure 14a) and on the ionic strength of the sample solution (Figure 14b).
Due to the nicking of CNT by oxidative stress to form wsCNTs we could observe several types of shapes comprised of different lengths as seen in its SEM and AFM micrographs. Thus the wsCNT should comprise of several fractions with different molecular weights. We use high molecular weight DNA as marker and to introduce gel electrophoresis for the separation of the mixture of wsCNT. The carboxylic acid group present in these wsCNTs are readily available in anionic form like the DNA. Using gel electrophoretic separation technique we could separate three bands out of the mixture of wsCNTs. Comparing their relative movements under identical conditions using super-mixed DNA (having very high base pairs) as molecular marker we could get a rough idea about the three bands of the separated wsCNTs. For the double-stranded DNA the average molecular weight of a unit of DNA with the pair
Figure 15. Left lane: Agarose gel containing wsCNT and super-mix DNA marker, right lane: DNA with base pair (bp) marked.
of four nucleotides has unit molecular weight as 660 Da. Using 660 Da as the unit molecular weight of one base pair of a double-stranded DNA, the total base pairs present in a DNA band in gel electrophoresis would provide the idea about the molecular weight of DNA. When wsCNTs along with super-mix DNA (containing known DNA mixture with several base pairs) as molecular marker were subjected to gel electrophoresis (using TAE buffer, pH 8, and applied 40 V), three black bands from the wsCNT were identified.
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Using such DNA bands as marker, our preliminary experiments provided the approximate molecular weight of these three bands. With EDAX ratio obtained between oxygen and carbon and the relative intensity of Raman bands, a rough idea of the number of carboxylic acids present in a particular fraction of wsCNT with a resolved mass (separated by gel electrophoretic purification) may be achieved.
CONCLUSION
In this research we have proved that the soot made by burning vegetable oils contains CNT. The presence of magnetic carbon present in wsCNT helps in new opening for carbon-based spintronics. REFERENCE: 1. R H Baughman, A A Zakhidov and W A de Heer, Science 297, 787 (2002); Acc.
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(2001) 3. S S Wong, J D Harper, P T Jr. Lansburry and C M Lieber, J. Am. Chem. Soc. 120,
603 (1998) 4. G Che, B B Lakshmi, E R Fisher and C R Martin, Nature 393, 346 (1998) 5. P Poncharal, Z Wang, D Ugarte and W Heer, Science 283, 1513 (1999)
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