Abstract
The jatropha-derived biodiesel, a green precursor was found to be a new and promising precursor for the synthesis of carbon nanotubes (CNTs) and carbon–nitrogen (C–N) nanotubes. The CNTs and C–N nanotubes have been synthesised by spray pyrolysis of biodiesel with ferrocene and ferrocene–acetonitrile, respectively, at elevated temperature under an argon atmosphere. The typical length and diameter of as-grown CNTs are 20 µm and 20–50 nm, respectively. The C–N nanotubes are found in bundles with effective length of ∼30 µm and diameter ranging between 30 and 60 nm with bamboo-shaped morphology. The as-grown CNTs and C–N nanotubes were characterised through scanning and transmission electron microscopes, X-ray photoelectron, Raman and Fourier transform infrared spectroscopic techniques. These investigations revealed that the nanotubes synthesised by jatropha-derived biodiesel are clean from carbonaceous impurities and the bamboo compartment formations in C–N nanotubes are due to nitrogen incorporation. The nitrogen concentration in C–N nanotubes decreases with the increase in synthesis temperature.
1. Introduction
Carbon nanotubes (CNTs) are an interesting class of nanostructures which have been extensively studied since their discovery in 1991 Citation1. Because of outstanding electrical, thermal and mechanical properties, CNTs have potentials for applications in nanoelectronics, sensors, field emission and as reinforcing agents in composite materials Citation2,Citation3. The CNTs have been synthesised by a variety of techniques. Some more prominent ones are arc discharge, laser vapourisation and chemical vapour deposition (CVD) Citation4–7. Among them, CVD is currently the most widely used method because it is a low-cost technique and is capable of producing relatively large yield of CNTs. Till date, several precursors such as carbon monoxide Citation8, methane Citation9, acetylene Citation10, benzene Citation11, xylene Citation12, etc. have been used as a carbon feedstock to synthesise CNTs. These carbon sources are related to fossil fuels which may not be sufficiently available in near future. However, comparatively less number of studies have been made regarding the synthesis of CNTs from natural precursors. Some natural precursor used in earlier studies corresponds to eucalyptus oil and turpentine oil Citation13–15.
Doped CNTs have also attracted considerable attention owing to their outstanding properties Citation16–18. Recently, it has been reported that nitrogen atoms incorporated in CNT can enhance its electrical and mechanical properties and also increase the energy storage capacity Citation19,Citation20. The additional electrons contributed by the nitrogen atoms provide electron carriers to the conduction band; carbon nitrogen (C–N) nanotubes have been found to be either metallic or narrow energy band gap semiconductors, thus offering the possibility of tailoring conductivity as compared to pure CNTs Citation21–23. Thus, the C–N nanotubes of controlled composition may prove extremely advantageous in the fabrication of materials with desired electronic properties.
In this study, we report the synthesis of CNTs and C–N nanotubes by spray pyrolysis method using new green precursor: jatropha-derived biodiesel. Jatropha is domestically cultivated on barren land. In this study, biodiesel was prepared by transesterification of jatropha oil with methanol. The components of biodiesel are carbon, hydrogen, and oxygen, e.g. C18H34O2; whereas details of morphological characteristics were monitored by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The details of nitrogen doping were explored by employing Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) techniques and Raman spectroscopy.
2. Experimental
2.1. Synthesis of carbon and C–N nanotubes
Synthesis of CNTs was carried out using spray pyrolysis-assisted CVD method. We used ferrocene (C10H10Fe) as a source of iron (Fe) which acts as catalyst for the growth of CNTs. The jatropha-derived biodiesel was used as a carbon source. Biodiesel was prepared from jatropha oil using transesterification process Citation24–26. In a typical process, 10 mL of methanol and ∼0.3 mg of sodium hydroxide (NaOH) are mixed into jatropha oil (∼100 mL). Sodium hydroxide reacts with methanol and changes into sodium methoxide (NaOCH3) which acts as a catalyst for transforming jatropha to biodiesel. The mixture was then heated to 70°C and stirred for 18 h under magnetic stirring. After the treatment, solution was washed with water. Due to washing of this solution, glycerol, which is water soluble, gets dissolved. Biodiesel being insoluble in water remains as a separate phase, and it was separated. The spray pyrolysis setup consisted of a nozzle (inner diameter ∼0.5 mm) attached at one end of a container, which was used for storing and releasing precursor solution. The other end of the nozzle was attached to the quartz tube (length 700 mm and inner diameter 25 mm), which was mounted inside a reaction furnace (300 mm long). The outer part of the quartz tube was attached to water bubbler. In a typical experiment, the quartz tube was first flushed with argon (Ar) gas in order to remove air from the quartz tube and then heated to a desired reaction temperature of the range 800–900°C. The accuracy of temperature measurement was ±2°C. The precursor solution i.e. ferrocene with biodiesel (∼30 mg/mL) was sprayed through nozzle into the quartz tube, using Ar as carrier gas.
For nitrogen doping of CNTs, acetonitrile (CH3CN) was also added to biodiesel and ferrocene solution Citation27–29. Acetonitrile was used to have source of nitrogen in the precursor. The optimum solution of biodiesel to acetonitrile was 10:1 (in volume). For 1 mL of this solution, the optimum ferrocene concentration was found to be 30 mg/mL. The solution was sonicated for 15 min to prepare the homogeneous mixture. The optimum flow rate of Ar was found to be ∼100 sccm (standard cubic centimeter per minute). The precursor solution was sprayed at a constant flow rate of ∼0.5 mL/min. A total of 10 mL precursor solution was sprayed for 20 min for each run. Once the solution was sprayed, the furnace was put off and allowed to cool down at ∼10°C/min to room temperature under Ar gas flow. A black deposit was extracted from the reaction zone (centre of the furnace) of quartz tube.
2.2. Characterisation techniques
The as-grown carboneous material was characterised by using SEM (Quanta 200) attached with energy dispersive X-ray (EDX) spectrometer and TEM (Tecnai 20 G2). The morphologies of carbon and C–N nanotubes have been explored by employing SEM studies involving secondary electrons were carried out. For SEM observation, the as-grown soot-like material was directly mounted on the sample holder with silver glue. Samples for TEM studies were prepared by dispersing a small amount of black soot-like material in ethanol and sonication for 10 min. Drops of this solution were placed on holey carbon grid and dried. Raman spectra were recorded using a Renishaw micro-Raman spectrometer (model H45517) with a 514 nm Ar ion laser. FT-IR spectra of the samples were recorded using Perkin Elmer (Spectrum 100, USA) spectrometer. The X-ray photoelectron spectrum was recorded in a VSW ESCA instrument (using Al-Kα radiation with a total resolution ∼0. 9 eV at 2 × 10−9 torr base vacuum).
3. Results and discussion
3.1. Microstructural characterisation
3.1.1. Carbon nanotubes
Spray pyrolysis of biodiesel-ferrocene solution at ∼850°C under Ar gas flow leads to a uniform thick black deposition on the whole of the inner wall of the quartz tube at the hot reaction zone (∼850°C). In the spray pyrolysis reaction, the biodiesel-ferrocene solution was atomised via spray nozzle and sprayed through carrier gas (Ar). At high temperature (800–900°C), the dissociation of ferrocene takes place and leads to the formation of Fe nanoparticles on the walls of the quartz tube. These Fe nanoparticles work as catalyst for the dissociation of biodiesel. At such a high temperature, catalyst-assisted thermal cracking of the biodiesel also takes place. A large number 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 molecule alkanes and alkenes will act as precursor for CNTs in presence of Fe catalyst. The feasible schemes are outlined as follows.
1. Decarboxylation of biodiesel
2. Demethylation of biodiesel
3. Cracking of hydrocarbons
4. Cracking of biodiesel (embodying the above three steps)
It may be pointed out that since alkanes contain carbon-to-carbon single bond, their dissociation will require lower energy (347 KJ/mol); whereas alkenes will require at least one carbon-to-carbon double bond. Therefore, their dissociation will require higher energy (611 KJ/mol). In the light 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 that alkanes like CH4, C2H6, etc., break to give rise to carbon. This carbon gets dissolved in ferrocene-derived Fe nanoparticle catalyst. The carbon atoms then diffuse out of Fe nanoparticles to give rise to CNTs. For successful growth of CNTs, the rate of carbon diffusion should be equal to rate of growth of CNTs.
SEM images as elucidated in show the formation of dense CNTs configuration. Based on previous reported work, it can be said that the yield of CNTs in this study is higher than those involving other hydrocarbon sources, e.g. C6H6 Citation11 . This suggests that more than one alkane (e.g. CH4, C2H6) coming from dissociation of biodiesel may be taken part in the formation of CNTs. As can be seen, the as-grown CNTs are clean and free from other carbonaceous materials. The average length of CNTs is ∼10 µm. The TEM investigation of as-grown CNTs confirms that the CNTs are multi-walled in nature. Typical TEM image of the as-grown CNTs is shown in . The diameter of nanotubes has been found to be varies between ∼20 and 50 nm. The size distribution of Fe catalyst particles is responsible for the variation in nanotubes diameter. The nanotubes growing on these catalyst particles having different sizes would therefore have different diameters. A high-resolution TEM (HRTEM) image () clearly shows well-graphitised layers of a typical multi-walled CNT. The side wall of CNT was found to consist of ∼32 graphitic layers (inset of ).
Figure 1. SEM micrographs (a) and (b) of as-grown CNTs obtained by spray pyrolysis of biodiesel-ferrocene solution at ∼850°C. TEM micrographs (c) of as-grown CNTs and HRTEM micrograph (d) of a typical side wall of CNT.
For preparing CNTs in high yield using this technique, several experimental parameters were optimised. To explore the optimum growth temperature for the synthesis of CNTs, several experiments were performed in the temperature range ∼800–900°C using biodiesel-ferrocene solution (concentration of ferrocene in biodiesel ∼30 mg/mL). Our experiments showed that, CNTs were formed in high yield at 850°C (). No CNTs deposition was observed below 800°C for the present precursor. It appears that below 800°C, pyrolysis of biodiesel does not take place. On the other hand, at higher temperature (900°C), the flaky deposit on the quartz tube surface contained amorphous carbon and hardly any CNTs. In this experiment, the optimised growth temperature was found to be 850°C. Hence, further pyrolysis experiments for biodiesel-ferrocene precursor were carried out at this optimised growth temperature. Also, the catalyst (ferrocene) concentration in biodiesel was varied from ∼10 to 35 mg/mL. The optimum concentration of ferrocene in biodiesel was found to be 30 mg mL. Flow rate of the precursor solution also plays an important role for the growth of CNTs. The optimum flow rate of the precursor solution was found to be ∼0.5 mL/min. It has been observed that synthesis carried out at 850°C with higher flow rate (∼1 mL/min) results in large amount of amorphous carbon and lower flow rate (∼0.2 mL/min) produces short CNTs together with amorphous carbon.
3.1.2. C–N nanotubes
Further, we used biodiesel-ferrocene with small proportion of acetonitrile solution for the formation of C–N nanotubes. The decomposition of acetonitrile (CH3CN → CH3 + CN) on Fe nanoparticles occurs with diffusion of carbon and nitrogen into the catalyst nanoparticles. The precipitation of graphite layers takes place when the carbon concentration inside the metallic particle exceeds super saturation. Due to competing endothermic and exothermic reactions, the local temperature of iron catalyst particle periodically changes, carbon deposition pauses and begins again resulting in appearance of bamboo-like sections. The precursor solution was sprayed at various temperatures from ∼750 to 850°C in the quartz tube under an Ar atmosphere. Here, the optimum growth temperature was found to be 800°C. The somewhat lower temperature in this case seems to be due to lower dissociation temperature of biodiesel-ferrocene with acetonitrile mixture. A dark black deposition takes place along the heating zone of the quartz tube by the spray pyrolysis of precursor solution under the above-mentioned conditions. SEM images of as-deposited material show the formation of bundles of aligned nanotubes (). exhibits a typical SEM micrograph of the bundles of nanotubes. , which is the magnified image of a bundle of reveals that the nanotubes are aggregated in the form of bundles. The average length of these nanotube bundles was found to be ∼30 µm. EDX analysis of as-grown C–N nanotubes at 800°C is shown in . This revealed the content of carbon, nitrogen, oxygen and iron are 92.88, 4.04, 2.75 and 0.32 at%, respectively. TEM images of the as-grown nanotubes are shown in . These nanotubes have bamboo-shaped structures (inset of ) with compartment-like configuration with cylindrical shells of varying outer diameters. The diameters of the nanotubes are varying in between ∼30 and 60 nm. It has been suggested that bamboo-shaped morphologies arise from the incorporation of pyridine-like N atoms within the carbon framework Citation30–32. Also, no encapsulated metal particle was found inside the nanotubes. The tip of C–N nanotubes has been observed to be generally covered by catalyst particles (marked by arrow in ). This is suggestive of the fact that tip growth mechanism is responsible for the formation of the C–N nanotubes in the present case. shows the HRTEM image of C–N nanotube.
Figure 2. SEM micrographs (a) and (b) and EDX spectra (c) of as-grown C–N nanotubes obtained by spray pyrolysis of biodiesel-ferrocene with acetonitrile solution at ∼800°C.
Figure 3. Typical TEM micrographs (a) of as-grown C–N nanotubes. Higher magnification (b) image of C–N nanotubes showing Fe catalyst particles encapsulated at the tip (it has been marked by arrow) and HRTEM micrograph (c) of C–N nanotubes.
3.2. Raman analysis
The qualities of as-grown carbon and C–N nanotubes were evaluated using Raman spectroscopy. and (b) shows typical Raman spectra of as-grown CNTs and C–N nanotubes at different temperatures, respectively. In the range of 500–2000 cm−1, two peaks are observed corresponding to G- and D-bands. The sharp G-band appears at ∼1585 cm−1 for as-grown CNTs and 1590 cm−1 for the as-grown C–N nanotubes. This band is usually regarded to arise due to in-plane oscillation of carbon atoms in the graphene wall of CNTs. The D-band peaks at 1347 cm−1 for CNTs and 1350 cm−1 for C–N nanotubes represent the degree of defects or dangling bonds. The presence of sharp G- and D-band peaks suggests highly graphitic nature of the as-grown CNTs and C–N nanotubes. The ratio of intensities of D- to G-bands (I D/I G) can be used as an indicator of the extent of disorder and nitrogen incorporation within the CNTs Citation33. In order to obtain the structural information of the as-grown CNTs, we calculated the I D/I G values from the Raman spectra of CNTs, as shown in . The I D/I G value was found to be 0.62, 0.19 and 0.69 for 800°C, 850°C and 900°C, respectively, for the as-grown CNTs. The I D/I G value of the CNTs grown at 850°C is smaller than that grown at the other temperatures (e.g. 800°C and 900°C). It indicates that CNTs grown at 850°C are relatively of high quality. The I D/I G values were 0.90, 1.17, and 0.81 for C–N nanotubes prepared at 750°C, 800°C and 850°C, respectively. The higher I D/I G value for C–N nanotubes suggests that defects and disorder are introduced in the curved graphene sheets or in sp2-hybridised carbon. This defect and disorder are most likely due to the presence of nitrogen atom in the CNTs.
Figure 4. Raman spectra of as-grown CNTs (a) and C–N nanotubes (b) at different growth temperatures.
3.3. FT-IR analysis
FT-IR study in the range 4000–1000 cm−1 has been carried out to detect the incorporation and binding of N atoms in the bamboo-shaped C–N nanotubes synthesised in this investigation. The FT-IR spectrum of as-grown CNTs is shown in . The peak at 3430 cm−1 is due to the presence of OH group, which indicates the existence of ambient atmospheric moisture in the samples. Another peak at 1650 cm−1 is associated with the vibration of carbon (C=C) skeleton of the CNTs. The other peaks at 1380 and 1080 cm−1 correspond to the single C–N bonds Citation34–36. The peaks at 2270 and 1590 cm−1 can be attributed to C≡N and C=N bonds, respectively Citation34,Citation35. It is expected that substitution of nitrogen atom in place of a carbon atom in a sp2 bonded carbon network will induce strong FT-IR signal and consequently absorption in the 1084–1600 cm−1 region will occur if the nitrogen atoms are incorporated into the carbon network Citation34,Citation37. Thus, the FT-IR spectra of the C–N nanotubes clearly indicate the incorporation of nitrogen atoms in the carbon network of CNTs.
Figure 5. FT-IR spectra of as-grown CNTs (a) and C–N nanotubes (b).
3.4. XPS analysis
The composition and nature of chemical bonding of the as-grown nanotubes were studied by XPS measurements. The results show the presence of carbon and nitrogen in nanotubes. The XPS spectra of C–N nanotubes are shown in . shows the C 1s peak at 284.2 eV and shows the N 1s peaks at 398.2, 400.9 and 404.9 eV for different growth temperatures. We estimated the ratio of carbon to nitrogen by taking the ratio of the integrated peak areas under the C 1s and N 1s signals and dividing them by the atomic sensitivity factor. shows the average concentration of nitrogen in the C–N nanotubes at different growth temperatures. The percentage (atomic) nitrogen contents present in the nanotubes are 9.54%, 4.16% and 2.32% for growth temperatures 750°C, 800°C and 850°C, respectively, and their relative composition comes out to be C10N, C24N and C43N, respectively. Comparison of atomic content of N2 obtained by XPS (4.16 at%) and EDX (4.04 at%) () are nearly the same at 800°C. The C 1s peak at 284.2 eV indicates that carbon is mostly in the form of graphite Citation38. The peaks at 398.2, 400.9 and 404.9 eV correspond to N 1s related to the three different chemical environments of the nitrogen atoms in the as-grown C–N nanotubes. The 398.2 eV feature is characteristic of pyridinic nitrogen, which was probably trapped in the hollow core Citation39. For all growth temperatures, the peak centred at 400.9 eV is due to nitrogen present in the graphene sheet Citation40,Citation41. This corresponds to pyrrolic-type trivalent nitrogen replacing the carbon in the hexagonal structure. In the case of pyridinic-N, the nitrogen atom contributes one p-electron to the π-system. A weak peak at ∼404.9 eV also exists that is consistent with presence of graphitic quaternary nitrogen, corresponding to highly coordinated nitrogen atoms substituting inner carbon atoms within graphitic sheet Citation42. It is found that pyrrolic nitrogen structure is more dominant than pyridinic nitrogen. Thus, for all temperatures, 400.9 eV peak is invariably present. This clearly indicates that N is present in graphene sheet and replaces carbon in the hexagonal network. These nitrogen atoms distort the graphene sheets and are responsible for the capping of CNTs. Above and below 800°C, i.e. at 750°C and 850°C, there are additional peaks at 398.2 and 404.9 eV, respectively, which show that N is also available in the hollow core and in-between graphene sheets. The presence of N only in the graphitic sheet at 800°C may be important since this will correspond to N doping of CNTs.
Figure 6. XPS spectra of C–N nanotubes C 1s (a), N 1s (b) and variation of nitrogen percentage (c) (at%) with growth temperature.
4. Conclusions
The synthesis of CNTs and C–N nanotubes are possible by utilising natural green precursor jatropha-derived biodiesel mixed with ferrocene and acetonitrile. Multi-walled CNTs have been prepared with high yield by spray pyrolysis of biodiesel-ferrocene solution at 850°C under Ar atmosphere. The C–N nanotubes have been synthesised by modifying the biodiesel-ferrocene precursor with acetonitrile at 800°C. The presence and location of nitrogen in C–N nanotubes have been confirmed by Raman, FT-IR and XPS spectroscopy. Raman spectroscopic studies suggest that the as-grown CNTs are highly graphitic. On the other hand, C–N nanotubes show a higher value of I D/I G suggesting higher disorder in C–N nanotubes due to the presence of nitrogen in these CNTs. FT-IR spectra reveals the presence of C–N, C=N and C≡N bonds. The XPS studies clearly suggest that for the synthesis temperature of 750°C and 850°C, the N is present in the graphitic sheet, in the hollow core as well as in between the graphitic sheets. On the other hand, for the synthesis temperature of 800°C, N is present only in the graphitic sheet.
Acknowledgements
The authors are extremely grateful to Prof. C.N.R. Rao, Prof. P.M. Ajayan, Prof. A.K. Raychaudhary and Prof. D.P. Singh (vice chancellor Banaras Hindu University) for their encouragement and support. We are grateful to Prof. A.C. Pandey (Allahabad University, Allahabad, India) for performing Raman spectroscopy of our samples. The financial support from the DST (UNANST: Banaras Hindu University), 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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