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7.2 Engineered Nanostructures: Synthesis Methods
7.2.1 Carbon Nanotubes
Carbon nanotubes are nanomaterials with enormous potential applications in fields
such as medicine, automobile and aviation industries, energy, defense, and diagnosis applications (Prasek et al. 2011; Eatemadi et al. 2014; Li and Zhang 2015; Stout
2015; Zhai et al. 2016; Yadav et al. 2017; Hussain et al. 2018). Carbon materials can
have different constructed structures with entirely different properties (Eatemadi
et al. 2014). In carbon nanotubes, carbon presents a sp
2
hybridization, with weak
out-of-plane bonding and strong in-plane bounds (Yadav et al. 2017). Carbon nanotubes can be obtained as fibers, films, yarns, ropes, single-walled carbon nanotubes
(SWCNTs), and also multiwalled carbon nanotubes (MWCNTs).
Several methods can be employed for obtaining carbon nanotubes, and most of
them rely on gas-phase processes (Prasek et al. 2011; Journet et al. 2012; Eatemadi
et al. 2014; Yadav et al. 2017). Some of the currently employed methods for producing carbon nanotubes are illustrated in Fig. 7.1 (Prasek et al. 2011), and also in other
references available in the literature (Journet et al. 2012).
In general, the carbon nanotube syntheses are classified according to the temperature (high, medium, and low) employed. The high-temperature methods are
subdivided in electric arc discharge, laser ablation, and vaporization induced by a
solar beam. The medium-temperature methods include in situ catalysts on gas phase
and synthesis by supported catalysts. Finally, the low-temperature methods include
plasma-enhanced CCVD (PE-CCVD) and laser-assisted catalytic chemical vapor
deposition (LA-CCVD) (Journet et al. 2012).
Perhaps the most traditional techniques for producing SWCNTs and MWCNTs
are the carbon arc-discharge technique (Ando and Iijima 1993; Journet et al. 1997;
Zhao et al. 1997; Shi et al. 1999), laser ablation technique (Zhang et al. 1998; Zhang
and Iijima 1999; Scott et al. 2001), and chemical vapor deposition (CVD) technique
(Kong et al. 1998; Colomer et al. 2000; Hofmann et al. 2003; Kumar and Ando
2010). Although SWCNTs and MWCNTs can be synthesized without transition
metal catalysts, the use of catalysts (such as Ni or Co) can lead to a better control of
carbon nanotube properties, including diameter and chirality, however with the disadvantage of increasing costs (Yadav et al. 2017). Below, we present in more details
some of these techniques employed for producing carbon nanotubes.
The carbon arc-discharge technique: it consists of vaporizing carbon in the existence of catalysts (cobalt, iron, nickel, etc.) under a reduced atmosphere (inert gas
as helium or argon). After the arc-discharge is formed between two electrodes, a
plasma composed of carbon vapor, inert gas, and the catalysts vapor is formed. The
vaporization arises as a consequence of the energy transferred from the arc to the
anode made of graphite doped with catalysts. A considerable amount of impurities
and/or amorphous carbon can be obtained through it.
Laser ablation technique: this is another technique for producing carbon nanotubes, which employs a high-power laser beam to vaporize the carbon from either a
F. F. Pereira et al.
7.2 Engineered Nanostructures: Synthesis Methods
7.2.1 Carbon Nanotubes
Carbon nanotubes are nanomaterials with enormous potential applications in fields
such as medicine, automobile and aviation industries, energy, defense, and diagnosis applications (Prasek et al. 2011; Eatemadi et al. 2014; Li and Zhang 2015; Stout
2015; Zhai et al. 2016; Yadav et al. 2017; Hussain et al. 2018). Carbon materials can
have different constructed structures with entirely different properties (Eatemadi
et al. 2014). In carbon nanotubes, carbon presents a sp
2
hybridization, with weak
out-of-plane bonding and strong in-plane bounds (Yadav et al. 2017). Carbon nanotubes can be obtained as fibers, films, yarns, ropes, single-walled carbon nanotubes
(SWCNTs), and also multiwalled carbon nanotubes (MWCNTs).
Several methods can be employed for obtaining carbon nanotubes, and most of
them rely on gas-phase processes (Prasek et al. 2011; Journet et al. 2012; Eatemadi
et al. 2014; Yadav et al. 2017). Some of the currently employed methods for producing carbon nanotubes are illustrated in Fig. 7.1 (Prasek et al. 2011), and also in other
references available in the literature (Journet et al. 2012).
In general, the carbon nanotube syntheses are classified according to the temperature (high, medium, and low) employed. The high-temperature methods are
subdivided in electric arc discharge, laser ablation, and vaporization induced by a
solar beam. The medium-temperature methods include in situ catalysts on gas phase
and synthesis by supported catalysts. Finally, the low-temperature methods include
plasma-enhanced CCVD (PE-CCVD) and laser-assisted catalytic chemical vapor
deposition (LA-CCVD) (Journet et al. 2012).
Perhaps the most traditional techniques for producing SWCNTs and MWCNTs
are the carbon arc-discharge technique (Ando and Iijima 1993; Journet et al. 1997;
Zhao et al. 1997; Shi et al. 1999), laser ablation technique (Zhang et al. 1998; Zhang
and Iijima 1999; Scott et al. 2001), and chemical vapor deposition (CVD) technique
(Kong et al. 1998; Colomer et al. 2000; Hofmann et al. 2003; Kumar and Ando
2010). Although SWCNTs and MWCNTs can be synthesized without transition
metal catalysts, the use of catalysts (such as Ni or Co) can lead to a better control of
carbon nanotube properties, including diameter and chirality, however with the disadvantage of increasing costs (Yadav et al. 2017). Below, we present in more details
some of these techniques employed for producing carbon nanotubes.
The carbon arc-discharge technique: it consists of vaporizing carbon in the existence of catalysts (cobalt, iron, nickel, etc.) under a reduced atmosphere (inert gas
as helium or argon). After the arc-discharge is formed between two electrodes, a
plasma composed of carbon vapor, inert gas, and the catalysts vapor is formed. The
vaporization arises as a consequence of the energy transferred from the arc to the
anode made of graphite doped with catalysts. A considerable amount of impurities
and/or amorphous carbon can be obtained through it.
Laser ablation technique: this is another technique for producing carbon nanotubes, which employs a high-power laser beam to vaporize the carbon from either a
F. F. Pereira et al.