**5. Morphological and structural characterizations**

To investigate the morphological and structural characterizations of the CNTs, a reduced number of techniques can be used. It is very important to characterize and determine the quality and properties of the CNTs, since its applications will require certification of proper‐ ties and functions [74].

However, only few techniques are able to characterize CNTs at the individual level such as scanning tunneling microscopy (STM) and transmission electronic microscopy (TEM). X-ray photoelectron spectroscopy is required to determine the chemical structure of CNTs in spite of the fact that Raman spectroscopy is mostly introduced as global characterization technique.

#### **5.1. Electron microscopy (SEM & TEM)**

The morphology, dimensions and orientation of CNTs can be easily revealed by using scan‐ ning (SEM) and Transmission Electron Microscopes (TEM) which have high resolution. [70-75] (Figs. 15).

Therefore, the TEM technique is applied as a method for measurement of the outer and in‐ ner radius and linear electron absorption coefficient of CNTs [76].This method is used to study CNTs before and after annealing and notice a significant increase of the electron ab‐ sorption coefficient. The inter shell spacing of MWNTs was studied by Kiang et al. [77] us‐ ing high resolution TEM images.

Fabrication, Purification and Characterization of Carbon Nanotubes: Arc-Discharge in Liquid Media (ADLM) http://dx.doi.org/10.5772/51116 69

**Figure 15.** Electron micrographs of CNT (A) SEM of the CNT. (B) TEM of the CNT [57].

#### **5.2. X-ray diffraction (XRD)**

**4.4. Ultrasonication**

**4.5. Micro-filtration**

ing up of a filter cake [67].

ties and functions [74].

[70-75] (Figs. 15).

**5.1. Electron microscopy (SEM & TEM)**

ing high resolution TEM images.

ed period, the CNTs are also chemically cut. [69].

68 Syntheses and Applications of Carbon Nanotubes and Their Composites

bon nanoparticles) are passing through the filter [65, 69, 70, 72].

**5. Morphological and structural characterizations**

used [67-70].

This technique is based on the separation of particles due to ultrasonic vibrations and also agglomerates of different nanoparticles will be more dispersed by this method. The separa‐ tion of the particles is highly dependable on the surfactant, solvent and reagents which are

When an acid is used, the purity of the CNTs depends on the sonication time. During the tubes vibration to the acid for a short time, only the metal is solvated, but in a more extend‐

Micro-filtration is based on particle size. Usually CNTs and a small amount of carbon nano‐ particles are trapped in a filter. The other nanoparticles (catalyst metal, fullerenes and car‐

A special form of filtration is cross flow filtration. Through a bore of fiber, the filtrate is pumped down at head pressure from a reservoir and the major fraction of the fast flowing solution is reverted to the same reservoir in order to be cycled through the fiber again. A fast hydrodynamic flow down the fiber bore sweeps the membrane surface and prevents build‐

To investigate the morphological and structural characterizations of the CNTs, a reduced number of techniques can be used. It is very important to characterize and determine the quality and properties of the CNTs, since its applications will require certification of proper‐

However, only few techniques are able to characterize CNTs at the individual level such as scanning tunneling microscopy (STM) and transmission electronic microscopy (TEM). X-ray photoelectron spectroscopy is required to determine the chemical structure of CNTs in spite of the fact that Raman spectroscopy is mostly introduced as global characterization technique.

The morphology, dimensions and orientation of CNTs can be easily revealed by using scan‐ ning (SEM) and Transmission Electron Microscopes (TEM) which have high resolution.

Therefore, the TEM technique is applied as a method for measurement of the outer and in‐ ner radius and linear electron absorption coefficient of CNTs [76].This method is used to study CNTs before and after annealing and notice a significant increase of the electron ab‐ sorption coefficient. The inter shell spacing of MWNTs was studied by Kiang et al. [77] us‐

This technique is used to obtain some information on the interlayer spacing, the structural strain and the impurities. However, in comparing CNTs with x-ray incident beam, CNTs have multiple orientations. This leads to a statistical characterization of CNTs [78].

#### **5.3. Raman spectroscopy**

Raman spectroscopy is one of the most powerful tools for characterization of CNTs. Without sample preparation, a fast and nondestructive analysis is possible. All allotropic forms of carbon are active in Raman spectroscopy [79]. The position, width, and relative intensity of bands are modified according to the carbon forms [80].

A Raman spectrum of a purified sample (after applying the purification procedure) is shown in figure 16. The peaks at 1380 cm–1 and 1572 cm–1 correspond to disorder (D-band) and graphite (G-band) bands, respectively. The former is an indication of the presence of de‐ fective material and the latter one refers to the well-ordered graphite [62].

The most characteristic features are summarized as following:


Raman spectroscopy is considered an extremely powerful tool for characterizing CNT, which gives qualitative and quantitative information on its diameter, electronic struc‐ ture, purity and crystalline, and distinguishes metallic and semiconducting material as well as chirality.

ized water or aqueous solution. This method is not requiring vacuum equipment, reacted gases, a high temperature furnace and a heat exchange system. Consequently, this method is

Fabrication, Purification and Characterization of Carbon Nanotubes: Arc-Discharge in Liquid Media (ADLM)

http://dx.doi.org/10.5772/51116

71

As it has been deeply investigated above, synthesis, purification and characterization of CNTs based on arc discharge in liquid media were described and discussed in this review paper. The observations of CNT growth under electron microscopy and other analytical techni‐ ques by different groups suggested that the mechanism are extremely sensitive to each fabri‐ cation parameter such as voltage difference between electrodes, current, type and ratio of catalysts, electrical conductivity, concentration, type and temperature of plasma solution and thermal conductivity. All these parameters were reviewed and studied herein. To the best of our knowledge the current review is the first one has discussed all aspects of arc discharge method in liquid media for CNT preparation and this technique deserves further attention.

The authors are grateful to Prof. M. Shariaty Niasar (Tehran Uiversity, Iran), Prof. J. Raoof (The University of Mazandaran, Iran), Dr. H. Molavi and PhD student Mrs R. Jabari Sheresht for their previous collaborations and productive discussion during preparation of this paper.

Nanotechnology Research Institute and Faculty of Chemical Engineering Babol University

[1] Iijima, S. (1991). Helical Microtubules of Graphitic Carbon. *Nature*, 354, 56-58.

[2] Dresselhaus, M. S., Dresselhaus, G., & Eklund, P. C. (1996). Science of fullerenes and

[3] Dekker, C. (1999). Carbon nanotubes as molecular quantum wires. *Physics Today*, 52,

[4] Tans, S. J., Verschueren, A. R. M., & Dekker, C. (1998). Room-temperature transistor

and Asieh Dehghani Kiadehi

carbon nanotubes., Academic Press, New York.

based on a single carbon nanotube. *Nature*, 393, 49-52.

\*Address all correspondence to: mjahan@nit.ac.ir

extremely simple and cheap.

**Acknowledgements**

**Author details**

Mohsen Jahanshahi\*

**References**

22-30.

of Technology, Babol,, Iran

**Figure 16.** Raman spectrum showing the most characteristic features of CNTs produced by arc discharge method in liquid followed by acid treatment [62].

### **6. Conclusion**

Carbon nanotubes (CNTs), a new structure of carbon element, are composed of graphen sheets rolled into closed concentric cylinders with diameter of the order of nanometers and length of micrometers. CNTs are attracted significant attention because of their unique physical and mechanical properties. These properties have been the engines of the rapid de‐ velopment in scientific studies in numerous applications such as in fuel cell and electrocata‐ lyst, nanobiosensors, gas adsorptions and membrane separation [82-88].

Three methods, laser, arc discharge and chemical vapor deposition are used to synthesize CNTs. The laser method is also known as the laser-furnace method. The quantities CNTs in this method are large but this technique is not economically advantageous, since the process demanded considerable power. The chemical vapor deposition is another method for pro‐ ducing CNTs. It could produce CNTs at temperatures above 700 0 C in large quantities, but the walls of the CNTs frequently contain many defects. Traditional arc discharge requires a complicated vacuum and heat exchange system. The yields of the laser and traditional arc discharge methods are very low (mg/h). From the application perspective, researchers are continuously trying to devise improved methods for CNTs fabrication.

Arc discharge in liquid media is a new method of synthesizing CNTs developed recently. All that is required is a dc power supply and an open vessel full of liquid nitrogen, deion‐ ized water or aqueous solution. This method is not requiring vacuum equipment, reacted gases, a high temperature furnace and a heat exchange system. Consequently, this method is extremely simple and cheap.

As it has been deeply investigated above, synthesis, purification and characterization of CNTs based on arc discharge in liquid media were described and discussed in this review paper. The observations of CNT growth under electron microscopy and other analytical techni‐ ques by different groups suggested that the mechanism are extremely sensitive to each fabri‐ cation parameter such as voltage difference between electrodes, current, type and ratio of catalysts, electrical conductivity, concentration, type and temperature of plasma solution and thermal conductivity. All these parameters were reviewed and studied herein. To the best of our knowledge the current review is the first one has discussed all aspects of arc discharge method in liquid media for CNT preparation and this technique deserves further attention.
