**2. Part B: Vertically-aligned carbon nanotubes**

### **2.1. Introduction**

model suggests bulk diffusion of carbon species into the metal particles [26]. The third step is the precipitation of the carbon in the form of CNTs from the saturated catalyst particle.

248 Syntheses and Applications of Carbon Nanotubes and Their Composites

**Figure 5.** TEM images of as-grown CNTs sample synthesized under constant reaction temperature of 650 °C usingr Methyl ester of *Pongamia pinnata* oil at a flow rate of 20 mL per hour. (a) indicates the reshaping of 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 MWNTs [22]. The precursor vapor decomposed on surface of the catalyst particle produces carbon. As the reactivity between the catalyst and the carbon exceeds the thresh‐ old value, carbon atoms loose their mobility in the solid solution, forming metal carbides [23]. These meta stable Fe and Co carbides decomposed and produce carbon which dissolve 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 barri‐ er for further carbon transfer from the gas phase to the bulk of the catalyst since carbon dif‐ fusion is slower through metal carbides [27]. The saturated metal carbide have lower

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 (Fig. 5a). This is consistent with the growth mechanism proposed by Rodriguez [29]. 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 nanotubes [30, 31]. This gives the impression that the catalyst is in liquid state during reaction. The catalyst particle seen inside and at the

melting point and they are fluid like during the growth process [28].

(b) metal particle at the tip of tube.

Aligned carbon nanotubes were first reported by Thess et al.[2]. In the same year the Chi‐ nese academy of science reported that a 50 µm thick film of highly aligned nanotubes had successfully grown by chemical vapor deposition (CVD) [35]. Vertically aligned CNTs are quasi-dimensional carbon cylinders that align perpendicular to a substrate [36]. Vertically aligned with high aspect ratios [37] and uniform tube length made it easy spinning into macroscopic fibres [38] Aligned CNTs are widely used in nano electronics, composite mate‐ rials as reinforcing agents and self-cleaning applications [39-41]. Aligned CNTs are ideal electrode material for biosensors over entangled CNTs, may be due to its high electrical con‐ ductive property [42]. Large CNT arrays have successfully been grown on different sub‐ strates, such as mesoporous silica [43] planar silicon substrate [44] and quartz glass plate [45]. Substrate provides a solid foundation for growing aligned CNTs. The substrate must able to inhibit the mobility of the catalyst particles in order to prevent agglomeration. The most commonly used active catalyst for growing CNTs are magnetic elements such as Fe, Co or Ni. Gunjishima et al. [46] used Fe-V bimetallic catalyst for synthesize of aligned DWCNTs. Recently, there have been appreciable attempt of using ferrocence as a catalyst for synthesis of aligned carbon nanotubes[47]. Here we report fabrication of aligned CNTs by spray pyrolysis on silicon wafer using mixture of Pine oil, Methyl ester of *Jatropha curcas* oil and Methyl ester of *Pongamia pinnata* oil with ferrocence.

#### **2.2. Experimental Methods**

The syntheses of aligned CNTs were carried out using the spray pyrolysis method. In this spray pyrolysis method, pyrolysis of the carbon precursor with a catalyst take place followed by deposition of aligned CNTs occur on silicon substrate. Pine oil, Methyl ester of *Jatropha cur‐ cas* oil and Methyl ester of *Pongamia pinnata* oil were used as carbon source and ferrocene [Fe (C5H5)2] (Sigma Aldrich, high purity 98 %) was used as a source of Fe which acts as a catalyst for the growth of CNTs. n type silicon wafer (100) of size (1x1cm 2 ) was used as a substrate and kept inside the quartz tube. In a typical experiment, the quartz tube was first flushed with ar‐ gon (Ar) gas in order to eliminate air from the quartz tube and then heated to a reaction temper‐ ature. The precursor mixture was sprayed into the quartz tube, using Ar gas. The concentration of ferrocene in carbon precursor was ~25 mg/ml. The flow rate of Ar was 200 sccm/min. The experiments were conducted at 650 ºC with reaction time of 45 min was main‐ tained for each deposition. After deposition, the furnace was switched off and allowed to cool down to room temperature under Ar gas flow. A uniform black deposition on the silicon sub‐ strate was observed. Finely, the substrate containing aligned CNTs was removed from the quartz tube for characterization. The experiments were repeated several times to ensure the re‐ producibility of the formation of vertically aligned carbon nanotubes.

vestigation is going on in our laboratory for a better understanding of the actual growth mecha‐

Carbon Nanotubes from Unconventional Resources: Part A: Entangled Multi-Walled Carbon Nanotubes

http://dx.doi.org/10.5772/51073

251

In view of the perspective of green chemistry, we attempt to explore regenerative materials for CNT synthesis with high efficiency. In this research work a well graphitized MWNTs were syn‐ thesized from Pine oil and Methyl ester of *Jatropha curcas* oil using silica supported Fe, Co and Mo catalyst by spray pyrolysis method. The optimum reaction conditions for synthesis of MWNTs were 650 °C and precursor flow rate of 20 mL per hour. Spray pyrolysis of Methyl ester of *Pongamia pinnata* oil over silica supported Fe, Co and Mo catalyst results in formation of MWNTs filled with magnetic nanoparticles, which find potential applications in magnetic re‐ cording, biomedical and environmental protection. Vertically aligned carbon nanotubes were obtained by spray pyrolysis of Pine oil and Methyl ester of *Jatropha curcas* oil and ferrocene mix‐ ture, at 650 ºC on silicon substrate under Ar atmosphere. The use of natural precursors gives sensible yield and makes the process natural world friendly as well. A thick carbon nanotube with poor structure and alignment was observed with mixture of Methyl ester of *Pongamia pin‐*

The studies in this work demonstrate that the carbon materials are potential precursor for CNTs production under suitable experimental conditions and comply with green chemistry princi‐ ples. It is clear that specific carbon nanostructures can be synthesized by suitably altering the experimental parameters. However, it is a challenge to consistently reproduce CNT of same quality and quantity form the precursor of inconsistent composition. Designing of catalyst ma‐ terial and optimization of reaction parameters which is suitable for synthesis of specific mor‐ phological CNTs from a renewable natural precursor of inconsistent chemical composition is

The authors acknowledge the UGC New Delhi for financial support, the Institute for Environ‐ mental and Nanotechnology for technical support and IITM for access to Electron microscopes.

nism of vertically aligned carbon nanotubes.

one of the future prospects in this area of research.

*nata* oil and ferrocene.

**Acknowledgements**

**Author details**

S. Karthikeyan1\* and P. Mahalingam2

\*Address all correspondence to: skmush@rediffmail.com

1 Chikkanna Government Arts College, TN, India

2 Arignar Anna Government Arts College, TN, India

**3. Conclusions, challenges and future prospects**

#### **2.3. Result and Discussion**

The morphology of carbon sample grown on silicon substrate using a mixture of Pine oil and ferrocene at 650 °C can be observed in Figure 6a. The image revel the formation of high abun‐ dance of carbon nanotubes which are forest like and vertically-aligned to the substrate surface. The growth of carbon nanotubes seems to be uniform and reaches up to a length of 10µm. Fig‐ ure 6b shows the SEM image of carbon sample grown on silicon substrate using a mixture of Methyl ester of *Jatropha curcas* oil and ferrocene at 650 °C. The dense, aligned but non-uniform growth of carbon nanotubes was observed. The length of carbon nanotubes grown was found to be varied from 12.5 to 7.5µm. Figure 6c illustrates the SEM image of the carbon naotubes grown at 650 °C using Methyl ester of *Pongamia pinnata* oil. A thick carbon nanotube with poor structure and alignment was observed.

**Figure 6.** Representative SEM images of as-grown vertically-aligned carbon nanotubes at 650 °C using Pine oil (a), Methyl ester of *Jatropha carcus* oil (b) and Methyl ester of *Pongamia pinnata* oil (c).

From the experimental results we suggest that the synthesis of aligned CNTs is very sensitive to the carbon precursors used. Ferrocene on thermal decomposition at high temperature forms Fe nano particles on the silicon substrate surface. During the chemical vapor deposition process, the carbon precursor is catalytically decomposed and the carbon fragments formed diffuse into the Fe catalyst. The Fe particles may thus easily become saturated or supersaturated with car‐ bon atoms, and the precipitation of the carbon from the surface of the Fe particle leads to the for‐ mation of dense carbon nanotubes [48]. The high surface density of the growing nanotubes serves as an additional advantage for the constituent nanotubes to be "uncoiled". The Vander waals forces between the tube keep them aligned. Thus, the Fe catalysts can effectively catalyze the growth of highly dense vertically aligned carbon nanotubes on silicon substrate. Further in‐ vestigation is going on in our laboratory for a better understanding of the actual growth mecha‐ nism of vertically aligned carbon nanotubes.
