**2. Synthesis of CNTs**

#### **2.1. Laser vaporization**

The laser vaporization method was developed for CNT production by Smalley's group [28, 29]. The laser is suitable for materials with high boiling temperature elements such as car‐ bon because of its high energy density. The quantities of CNTs, in this method are large. Smalley's group further developed the laser method also known as the laser-furnace method [28]. Fullerenes with a soccer ball structure are produced only at high furnace temperatures, underlining the importance of annealing for nanostructures [28]. These discoveries were ap‐ plied to produce CNTs [29] in 1996, especially SWNTs. A beam of high power laser imping‐ es on a graphite target sitting in a furnace at high temperature as Figure1 shows.

The target is vaporized in high-temperature buffer gas like Ar and formed CNTs. The pro‐ duced CNTs are conveyed by the buffer gas to the trap, where they are collected. Then CNTs can be found in the soot at cold end.

This method has several advantages, such as high-quality CNTs production, diameter con‐ trol, investigation of growth dynamics, and the production of new materials. High-quality CNTs with minimal defects and contaminants, such as amorphous carbon and catalytic met‐ als, have been produced using the laser-furnace method together with purification processes [30-32] but the laser has sufficiently high energy density to vaporize target at the molecular level. The graphite vapor is converted into amorphous carbon as the starting material of CNTs [33-35]. However, the laser technique is not economically advantageous, since the process involves high purity graphite rods, high power lasers and low yield of CNTs.

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

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

The laser vaporization method was developed for CNT production by Smalley's group [28, 29]. The laser is suitable for materials with high boiling temperature elements such as car‐ bon because of its high energy density. The quantities of CNTs, in this method are large. Smalley's group further developed the laser method also known as the laser-furnace method [28]. Fullerenes with a soccer ball structure are produced only at high furnace temperatures, underlining the importance of annealing for nanostructures [28]. These discoveries were ap‐ plied to produce CNTs [29] in 1996, especially SWNTs. A beam of high power laser imping‐

The target is vaporized in high-temperature buffer gas like Ar and formed CNTs. The pro‐ duced CNTs are conveyed by the buffer gas to the trap, where they are collected. Then

This method has several advantages, such as high-quality CNTs production, diameter con‐ trol, investigation of growth dynamics, and the production of new materials. High-quality CNTs with minimal defects and contaminants, such as amorphous carbon and catalytic met‐ als, have been produced using the laser-furnace method together with purification processes [30-32] but the laser has sufficiently high energy density to vaporize target at the molecular level. The graphite vapor is converted into amorphous carbon as the starting material of

es on a graphite target sitting in a furnace at high temperature as Figure1 shows.

thermal conductivity on carbon nanotubes production are investigated.

56 Syntheses and Applications of Carbon Nanotubes and Their Composites

**2. Synthesis of CNTs**

**Figure 1.** Schematic drawing of a laser obtain.

CNTs can be found in the soot at cold end.

**2.1. Laser vaporization**

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

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

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

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

#### **2.3. Arc Discharge**

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

The anode is drilled, and the hole is filled with catalyst metal powder then the chamber is connected to a vacuum line with a diffusion pump and to a gas supply [43]. Like the anode in a DC electric arc discharge reactor, CNT is synthesized of graphite rod. After the evacua‐ tion of the chamber by a diffusion pump, rarefied ambient gas is introduced [43].

**Figure 3.** Schematic diagram of apparatus for preparing CNTs.

When a dc arc discharge is applied between the two graphite rods, the anode is consumed, and fullerene is formed in the chamber soot [43]. The mass production of multi wall carbon nanotubes (MWCNTs) by this dc arc discharge evaporation was first achieved by Ebbesen and Ajayan [44].

**Figure 4.** Carbon nanotubes produced in LiCl 0.25 N [51].

**Figure 5.** Schematic device of arc discharge in liquid.

current (DC) power supply.

The arc discharge in liquid is initiated between two high purity graphite electrodes. Figure 5 is shown schematic device of arc discharge in liquid. Both electrodes are submerged in the liquid in a beaker. At first, the electrodes touch each other and are connected with a direct

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

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

59

The cathode is usually 20 mm in diameter, while the anode is 6 mm in diameter. Then the arc discharge is initiated by slowly detaching the moveable anode from the cathode. The arc gap is kept at the proper value (about 1 mm) that the continuous arc discharge could be obtained [52].
