**3. Arc-Discharge in Liquid Media (ADLM)**

The traditional arc-plasma growth method for CNTs necessitates complex gas-handling equipment, a sealed reaction chamber, a liquid-cooled system and time consuming purge cycles. The act of extraction of nanotube's product is so complicated [45]. To be compared, the growth of arc plasma in water requires simple operation and equipment which had made the process of CNTs production noticeable [47].

Ishigami et al. [48] developed a simple arc method in liquid nitrogen for the first time that allow for the continuous synthesis of high-quality CNTs. The materials obtained are mainly MWCNTs, amorphous carbon, graphitic particles and carbonaceous material [48, 49]. Subse‐ quently, an aqueous arc-discharge (arc-water) method was developed. Lange et al. [50] gen‐ erated onions, nanotubes and encapsulates by arc discharge in water. Figure 4 is shown produced CNTs in LiCl 0.25 N media [51].

Fabrication, Purification and Characterization of Carbon Nanotubes: Arc-Discharge in Liquid Media (ADLM) http://dx.doi.org/10.5772/51116 59

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

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‐

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

The traditional arc-plasma growth method for CNTs necessitates complex gas-handling equipment, a sealed reaction chamber, a liquid-cooled system and time consuming purge cycles. The act of extraction of nanotube's product is so complicated [45]. To be compared, the growth of arc plasma in water requires simple operation and equipment which had

Ishigami et al. [48] developed a simple arc method in liquid nitrogen for the first time that allow for the continuous synthesis of high-quality CNTs. The materials obtained are mainly MWCNTs, amorphous carbon, graphitic particles and carbonaceous material [48, 49]. Subse‐ quently, an aqueous arc-discharge (arc-water) method was developed. Lange et al. [50] gen‐ erated onions, nanotubes and encapsulates by arc discharge in water. Figure 4 is shown

tion of the chamber by a diffusion pump, rarefied ambient gas is introduced [43].

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

58 Syntheses and Applications of Carbon Nanotubes and Their Composites

**3. Arc-Discharge in Liquid Media (ADLM)**

made the process of CNTs production noticeable [47].

produced CNTs in LiCl 0.25 N media [51].

and Ajayan [44].

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 current (DC) power supply.

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

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]. Recently carbon nanotubes (CNTs) were fabricated successfully with arc discharge in solu‐ tion by a novel full automatic set up [51].

The arc discharge and consequently consumption of anode result to increase the distance between the two electrodes and degrees the voltage between them as well. In order to re‐ main constant voltage and gap between the two electrodes, the program automatically will compare the initial voltage with the voltage of two electrodes with an accuracy of 0.1 V. Based on the calculated difference the program calculates the proportional coefficient for the pro‐ portional controller. Figure 6 is schematic of the apparatus used for automatic arc dis‐ charge in solution [51].

During the arc discharge, the gap between the two electrodes is maintained at approximate‐ ly 1 mm, while the synthesis time is 60 s [51].

**Figure 7.** SEM image of the product which was fabricated without catalyst [51]

**Figure 8.** SEM image of the product which is fabricated with 5% Ni as catalyst [51].

length and defect structure and the yield is moderate.

Ni catalyst, figure 8, motivate a production of elongated CNTs and springy CNTs with a rel‐ atively good yield while Fe catalysts, figure 9, promote a production of CNTs with short

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61

**Figure 6.** Schematic of the apparatus used for automatic arc discharge in solution [51].

#### **3.1. Catalyst Materials and their ratio**

A metal catalyst is necessary for the growth of the CNTs in all the methods used for synthe‐ sis of CNTs. Catalysts use to prepare CNTs usually include transition metals as a single or mixture of two catalysts such as a single, Fe, Co, Ni or Mo [53] or mixture of two catalysts such as FeNi [54], PtRh [55] and NiY [22]. Hsin et al. [57] firstly reported the production of metalcontaining CNTs by arc discharge in solution.

The catalyst activation is determined in relation to the melting temperature and the boiling temperature thus the melting and boiling temperature of a catalyst can be one of the vital factors in the synthesis of SWCNTs [51].

CNTs are synthesized while the anode is filled by divergent single or bimetallic catalysts [51]. Scanning electron micros copies (SEM) show that the fabricated CNTs without any catalyst, figure 7, is in a very short long, disordered and is faulty grown.

Fabrication, Purification and Characterization of Carbon Nanotubes: Arc-Discharge in Liquid Media (ADLM) http://dx.doi.org/10.5772/51116 61

**Figure 7.** SEM image of the product which was fabricated without catalyst [51]

Recently carbon nanotubes (CNTs) were fabricated successfully with arc discharge in solu‐

The arc discharge and consequently consumption of anode result to increase the distance between the two electrodes and degrees the voltage between them as well. In order to re‐ main constant voltage and gap between the two electrodes, the program automatically will compare the initial voltage with the voltage of two electrodes with an accuracy of 0.1 V. Based on the calculated difference the program calculates the proportional coefficient for the pro‐ portional controller. Figure 6 is schematic of the apparatus used for automatic arc dis‐

During the arc discharge, the gap between the two electrodes is maintained at approximate‐

A metal catalyst is necessary for the growth of the CNTs in all the methods used for synthe‐ sis of CNTs. Catalysts use to prepare CNTs usually include transition metals as a single or mixture of two catalysts such as a single, Fe, Co, Ni or Mo [53] or mixture of two catalysts such as FeNi [54], PtRh [55] and NiY [22]. Hsin et al. [57] firstly reported the production of metal-

The catalyst activation is determined in relation to the melting temperature and the boiling temperature thus the melting and boiling temperature of a catalyst can be one of the vital

CNTs are synthesized while the anode is filled by divergent single or bimetallic catalysts [51]. Scanning electron micros copies (SEM) show that the fabricated CNTs without any catalyst,

tion by a novel full automatic set up [51].

60 Syntheses and Applications of Carbon Nanotubes and Their Composites

ly 1 mm, while the synthesis time is 60 s [51].

**3.1. Catalyst Materials and their ratio**

containing CNTs by arc discharge in solution.

factors in the synthesis of SWCNTs [51].

**Figure 6.** Schematic of the apparatus used for automatic arc discharge in solution [51].

figure 7, is in a very short long, disordered and is faulty grown.

charge in solution [51].

**Figure 8.** SEM image of the product which is fabricated with 5% Ni as catalyst [51].

Ni catalyst, figure 8, motivate a production of elongated CNTs and springy CNTs with a rel‐ atively good yield while Fe catalysts, figure 9, promote a production of CNTs with short length and defect structure and the yield is moderate.

**Figure 10.** SEM image of the product which was fabricated a) below zero temperature b) at a high temperature

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The voltage effects on the production of the nanostructures by applying a variety of voltage values in different experiments were investigated by Jahanshahi et al [58]. The SEMs of the synthesized materials, figure 11 shows the formation of fullerene at a voltage of 10 V, while

**Figure 11.** a) SEM images of the produced sample by arc discharge at a voltage of 10 V. (b) SEM images of the pro‐ duced sample by arc discharge at a voltage of 20 V. (c) SEM images of the produced sample by arc discharge at a

(800C), c) at the environment temperature (25 0C)[57].

**3.3. Voltage difference between electrodes**

voltage of 30 V [58].

both CNTs and fullerenes are fabricated at a voltage of 20 V.

**Figure 9.** SEM image of the product which was fabricated with 5% Fe as catalyst [51].

Jahanshahi et al. showed that Mo catalysts motivate a production of CNTs with long length and high crystalline structure but with a wide diameter while it has a relatively good yield. In contrast, Mo-Ni bimetallic catalyst cause the production of CNTs with long length, nar‐ row distribution diameters and crystalline structure without any defect and follow with a good yield [51].

#### **3.2. Plasma Solution Temperature**

The effect of solution temperature on the synthesis of CNTs and the structure of fabricated CNTs was investigated by Dehghani et al [57]. Scanning electron microscopes and transient electron microscopes (TEMs), figure 10, shows that the fabricated CNTs below zero as ther‐ mal condition is not suitable for synthesizing CNTs by the arc discharge method in liquid and CNTs cannot grow under low temperatures, especially below zero. High temperature is also not suitable for synthesizing CNTs by the arc discharge method in liquid media.

In contrast, observations show that in the environment with (25 0 C) temperature, long CNTs are formed with narrow distribution diameter, complete clean, flat surface and arranged structure. Constant temperature around 250 C is the best thermal condition for synthesizing CNTs by the arc discharge method in liquid [57].

Fabrication, Purification and Characterization of Carbon Nanotubes: Arc-Discharge in Liquid Media (ADLM) http://dx.doi.org/10.5772/51116 63

**Figure 10.** SEM image of the product which was fabricated a) below zero temperature b) at a high temperature (800C), c) at the environment temperature (25 0C)[57].

#### **3.3. Voltage difference between electrodes**

**Figure 9.** SEM image of the product which was fabricated with 5% Fe as catalyst [51].

62 Syntheses and Applications of Carbon Nanotubes and Their Composites

good yield [51].

**3.2. Plasma Solution Temperature**

structure. Constant temperature around 250

CNTs by the arc discharge method in liquid [57].

Jahanshahi et al. showed that Mo catalysts motivate a production of CNTs with long length and high crystalline structure but with a wide diameter while it has a relatively good yield. In contrast, Mo-Ni bimetallic catalyst cause the production of CNTs with long length, nar‐ row distribution diameters and crystalline structure without any defect and follow with a

The effect of solution temperature on the synthesis of CNTs and the structure of fabricated CNTs was investigated by Dehghani et al [57]. Scanning electron microscopes and transient electron microscopes (TEMs), figure 10, shows that the fabricated CNTs below zero as ther‐ mal condition is not suitable for synthesizing CNTs by the arc discharge method in liquid and CNTs cannot grow under low temperatures, especially below zero. High temperature is

are formed with narrow distribution diameter, complete clean, flat surface and arranged

C) temperature, long CNTs

C is the best thermal condition for synthesizing

also not suitable for synthesizing CNTs by the arc discharge method in liquid media.

In contrast, observations show that in the environment with (25 0

The voltage effects on the production of the nanostructures by applying a variety of voltage values in different experiments were investigated by Jahanshahi et al [58]. The SEMs of the synthesized materials, figure 11 shows the formation of fullerene at a voltage of 10 V, while both CNTs and fullerenes are fabricated at a voltage of 20 V.

**Figure 11.** a) SEM images of the produced sample by arc discharge at a voltage of 10 V. (b) SEM images of the pro‐ duced sample by arc discharge at a voltage of 20 V. (c) SEM images of the produced sample by arc discharge at a voltage of 30 V [58].

On the contrary, the elongated CNTs were synthesized with high quality at a voltage of 30 V. The results show that the rate of production efficiency and anode consumption is in‐ creased by increasing the voltage amount [58].

#### **3.4. Plasma Solution Concentration**

Liquid nitrogen provides a good environment for the MWCN synthesis, but the strong evaporation cause by the operation of the arc discharge does not allow a good thermal ex‐ change between the synthesized material and its surroundings.

Arc discharge in deionised water and liquid nitrogen are erratic due to their electrical insu‐ lation [50]. The electrical conductivity of LiCl solution is also better than deionised water and liquid nitrogen [59].

Figure 12 shows TEM image of the as-grown MWCN synthesized in LiCl. Investigators have demonstrated the possibility of producing carbon nanostructures in the liquid phase (water, hydrocarbons, dichloromethane, CCl4, in liquid gases) [61].

**Figure 13.** Production of CNTs in NaCl solution [62].

**3.6. The solution electrical conductivity effect**

**3.5. Discharge current**

creased to 28 V [63].

published data).

vides a very simple and cheap method to synthesize CNTs [60].

The optimized conditions to synthesize large quantities of SWCNT by applying arc dis‐ charge in NaCl solution deserve further investigation. Arc discharge in NaCl solution pro‐

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65

Discharge current is another important parameter influencing the products of arc dis‐ charge. If the catalyst percentage is 1 mol% Fe, and the discharge current is intentionally reduced to 20 A, the arc became very unstable, and disappear when the voltage is in‐

The effect of electrical conductivity of liquid on CNTs production might be important. A ser‐ ies of experiments carried out and the products were fabricated using arc discharge between two graphite electrodes submerged in different aqueous solutions of NaCl, KCl as well as LiCl. In comparative studies, CNTs were synthesized under different electrical conductivity conditions, and the results were analyzed, compared and discussed. The scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy were employed to study the morphology of these carbon nanostructures and reported. LiCl 0.25 N (with 22.7mS as electrical conductivity) media when applied as solution, high-crystalline and a longed multi walled carbon nanotubes, single walled carbon nanotubes and springy carbon nanotubes (SCNTs) were synthesized. This study is one of the first one have demon‐ strated application of an arc discharge in liquid media with electrical conductivity effects upon CNT preparation and deserves further study (Dehghani and Jahanshahi, (2012); un‐

**Figure 12.** TEM image of the as-grown MWCN synthesized in LiCl [58].

In contrast, liquid water besides providing a suitable environment also provides the thermal conditions necessary to retain good quality CNTs in the raw material, while the reactivity of the water with hot carbon does not appear to have any major effects on the reaction [59].

Figure 13 shows CNTs are produced in NaCl [62]. Nevertheless, arc discharge in NaCl solu‐ tion is extremely stable owing to the excellent electrical conductivity induced by Cl and Na+ ions. Too many Na+ ions would hinder carbon ions flying from anode towards the center of cathode. Researchers found that perhaps this is another reason that the length of SWCNT is short and only a single SWCNT [61].

**Figure 13.** Production of CNTs in NaCl solution [62].

The optimized conditions to synthesize large quantities of SWCNT by applying arc dis‐ charge in NaCl solution deserve further investigation. Arc discharge in NaCl solution pro‐ vides a very simple and cheap method to synthesize CNTs [60].

#### **3.5. Discharge current**

On the contrary, the elongated CNTs were synthesized with high quality at a voltage of 30 V. The results show that the rate of production efficiency and anode consumption is in‐

Liquid nitrogen provides a good environment for the MWCN synthesis, but the strong evaporation cause by the operation of the arc discharge does not allow a good thermal ex‐

Arc discharge in deionised water and liquid nitrogen are erratic due to their electrical insu‐ lation [50]. The electrical conductivity of LiCl solution is also better than deionised water

Figure 12 shows TEM image of the as-grown MWCN synthesized in LiCl. Investigators have demonstrated the possibility of producing carbon nanostructures in the liquid phase (water,

In contrast, liquid water besides providing a suitable environment also provides the thermal conditions necessary to retain good quality CNTs in the raw material, while the reactivity of the water with hot carbon does not appear to have any major effects on the reaction [59].

Figure 13 shows CNTs are produced in NaCl [62]. Nevertheless, arc discharge in NaCl solu‐

cathode. Researchers found that perhaps this is another reason that the length of SWCNT is

ions would hinder carbon ions flying from anode towards the center of

and Na+

tion is extremely stable owing to the excellent electrical conductivity induced by Cl-

creased by increasing the voltage amount [58].

64 Syntheses and Applications of Carbon Nanotubes and Their Composites

change between the synthesized material and its surroundings.

hydrocarbons, dichloromethane, CCl4, in liquid gases) [61].

**Figure 12.** TEM image of the as-grown MWCN synthesized in LiCl [58].

**3.4. Plasma Solution Concentration**

and liquid nitrogen [59].

ions. Too many Na+

short and only a single SWCNT [61].

Discharge current is another important parameter influencing the products of arc dis‐ charge. If the catalyst percentage is 1 mol% Fe, and the discharge current is intentionally reduced to 20 A, the arc became very unstable, and disappear when the voltage is in‐ creased to 28 V [63].

#### **3.6. The solution electrical conductivity effect**

The effect of electrical conductivity of liquid on CNTs production might be important. A ser‐ ies of experiments carried out and the products were fabricated using arc discharge between two graphite electrodes submerged in different aqueous solutions of NaCl, KCl as well as LiCl. In comparative studies, CNTs were synthesized under different electrical conductivity conditions, and the results were analyzed, compared and discussed. The scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy were employed to study the morphology of these carbon nanostructures and reported. LiCl 0.25 N (with 22.7mS as electrical conductivity) media when applied as solution, high-crystalline and a longed multi walled carbon nanotubes, single walled carbon nanotubes and springy carbon nanotubes (SCNTs) were synthesized. This study is one of the first one have demon‐ strated application of an arc discharge in liquid media with electrical conductivity effects upon CNT preparation and deserves further study (Dehghani and Jahanshahi, (2012); un‐ published data).
