**1. Introduction**

Carbon nanotubes (CNTs) were first discovered by Iijima in 1991 [1]. CNTs have sparked great interest in many scientific fields such as physics, chemistry, and electrical engineering [2, 3]. CNTs are composed of graphene sheets rolled into closed concentric cylinders with diameter of the order of nanometers and length of micrometers. CNTs are in two kinds, based on number of walls, the single-walled and multi-walled.

The diameter of single walled carbon nanotubes (SWNTs) ranges from 0.4 nm to 3nm and the length can be more than 10 mm that makes SWNTs good experimental templates to study one-dimensional mesoscopic physics system [3]. These unique properties have been the engines of the rapid development in scientific studies in carbon based mesoscopic phys‐ ics and numerous applications such as high performance field effect transistors [4-9], singleelectron transistors [10, 11], atomic force microscope tips [12], field emitters [13, 14], chemical/biochemical sensors [15-18], hydrogen storage [19].

There are three important methods to produce high quality CNT namely laser [20], arc dis‐ charge [21, 22], and Chemical Vapor Deposition (CVD) [23, 24]. Recently, arc discharge in liquid media has been developed to synthesize several types of nano-carbon structures such as: carbon onions, carbon nanohorns and carbon nanotubes. This is a low cost technique as it does not require expensive apparatus [25,26].

However, several techniques such as oxidation, nitric acid reflux, HCl reflux, organic func‐ tionalization, filtration, mechanical purification and chromatographic purification have been developed that separate the amorphous carbons and catalyst nanoparticles from the CNTs while a significant amount of CNTs are also destroyed during these purification processes [27].

© 2013 Jahanshahi and Kiadehi; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Jahanshahi and Kiadehi; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this review paper, synthesis, purification and structural characterization of CNTs based on arc discharge in liquid media are reviewed and discussed. In addition, several parame‐ ters such as: voltage difference between electrodes, current, type and ratio of catalysts, elec‐ trical conductivity, concentration, type and temperature of plasma solution, as well as thermal conductivity on carbon nanotubes production are investigated.

CNTs [33-35]. However, the laser technique is not economically advantageous, since the

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

The chemical vapor deposition (CVD) is another method for producing CNTs in which a hy‐ drocarbon vapor is thermally decomposed in the presence of a metal catalyst. In this meth‐ od, carbon source is placed in gas phase in reaction chamber as shown in Figure 2. The synthesis is achieved by breaking the gaseous carbon molecules, such as methane, carbon monoxide and acetylene, into reactive atomic carbon in a high temperature furnace and

This carbon will get diffused towards substrate, which is coated with catalyst and nanotubes grow over this metal catalyst. Temperature used for synthesis of nanotube is 650 – 900 0

In fact, CVD has been used for producing [37-39] carbon filaments and fibers since 1959. Figure 2 shows a schematic diagram of the setup used for CNT growth by CVD in its sim‐ plest form. CNTs grow over the catalyst and are collected upon cooling the system to room temperature. The catalyst material may be different, solid, liquid, or gas and can be placed

A schematic diagram of the arc discharge apparatus for producing CNTs is shown in Figure 3. In this method, two graphite electrodes are installed vertically, and the distance between the two rod tips is usually in the range of 1–2 mm. The anode and cathode are made of pure

C

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process involves high purity graphite rods, high power lasers and low yield of CNTs.

sometime helped by plasma to enhance the generation of atomic carbon [36].

**2.2. Chemical vapor deposition (CVD)**

range and the typical yield is 30% [36].

**Figure 2.** Schematic diagram of a CVD setup.

**2.3. Arc Discharge**

inside the furnace or fed in from outside [40, 41].

graphite (those are, with a purity of 99.999%).
