516
14 Nanostructures Obtained with Plasma Discharge Processes
Although the electric field in the arc discharge is large, theoretical calculations
show that it is not a major governing factor of the nanotube growth [43–45]. This is
why nanotubes are often aligned parallel to the cathode surface. The majority of the
nanotubes obtained with the arc discharge method is closed at the free end, and the
preferred growth site is the anchored end of the nanotubes, which excludes the fielddriven growth mechanism. This is particularly true when the nanotube is anchored
at a catalyst particle.
The spatial distribution of the various carbon nanostructures is a persistent
problem in the arc discharge experiments [1, 27, 33, 46–48]. On the cathode surface
itself, the temperature distribution is a crucial factor. The nanotube formation zone
exhibits a temperature >3000 K, and on parts of moderate temperature (<2500 K),
the formation of non-tubular pyrolytic carbon is preferred [47]. While the major
products are often collected from the deposit zone of the cathode surface, the carbon
soot deposited in the peripheral parts of the discharge chamber exhibits a different
product profile. Therefore, the regulation of the arc discharge process to obtain just
one typical product has not been solved, although the application of various catalysts
can lead to 80–90% purity in the sense of the preferred formation of single-walled
or multiwalled nanotubes.
Regardless of the specific details of the carbon nanotube synthesis, purification
is an indispensable step of the entire process. This is often an oxidative treatment
in either oxygen-containing gas at moderate temperature (like 1000 K) or in hot
oxidizing acid (like HNO 3 ). Under these conditions, the nanotubes are quite intact
but other carbonaceous derivatives with a large number of terminal carbon atoms can
be decomposed. A comparison of the SEM images before and after purification is
shown in Fig. 14.4.
14.3 Arc Discharge Processes with Difference Gases
The first CNT synthesis was performed with argon [1]. Later, several works used
rather helium gas of pressure ranging from 25 mbar to 1 bar [4, 18, 19, 23, 26–29,
39, 50, 51]. Helium proved to lead to faster nanotube growth, but the increase in He
pressure up to 3 bar did not result is a yield increment as compared to atmospheric He
pressure [4]. A theoretical calculation [46] underpins both the growth rate enhancement in He as compared to Ar by a factor of about three and the saturation of nanotube
growth rate around 1 bar.
The application of helium and argon as noble gases sounds very logical to avoid
the reaction with the carbon radicals that build up the nanotubes. Nevertheless, other
gases have also been applied whose impact can be elucidated together with the
other important experimental conditions such as the application of catalysts. One of
these gases is hydrogen [24, 25]. When it is applied together with He for MWCNT
production (no catalyst applied [52]), the larger H 2 pressure led to both higher anode
consumption rate and larger deposit formation rate at the cathode. However, in the
14 Nanostructures Obtained with Plasma Discharge Processes
Although the electric field in the arc discharge is large, theoretical calculations
show that it is not a major governing factor of the nanotube growth [43–45]. This is
why nanotubes are often aligned parallel to the cathode surface. The majority of the
nanotubes obtained with the arc discharge method is closed at the free end, and the
preferred growth site is the anchored end of the nanotubes, which excludes the fielddriven growth mechanism. This is particularly true when the nanotube is anchored
at a catalyst particle.
The spatial distribution of the various carbon nanostructures is a persistent
problem in the arc discharge experiments [1, 27, 33, 46–48]. On the cathode surface
itself, the temperature distribution is a crucial factor. The nanotube formation zone
exhibits a temperature >3000 K, and on parts of moderate temperature (<2500 K),
the formation of non-tubular pyrolytic carbon is preferred [47]. While the major
products are often collected from the deposit zone of the cathode surface, the carbon
soot deposited in the peripheral parts of the discharge chamber exhibits a different
product profile. Therefore, the regulation of the arc discharge process to obtain just
one typical product has not been solved, although the application of various catalysts
can lead to 80–90% purity in the sense of the preferred formation of single-walled
or multiwalled nanotubes.
Regardless of the specific details of the carbon nanotube synthesis, purification
is an indispensable step of the entire process. This is often an oxidative treatment
in either oxygen-containing gas at moderate temperature (like 1000 K) or in hot
oxidizing acid (like HNO 3 ). Under these conditions, the nanotubes are quite intact
but other carbonaceous derivatives with a large number of terminal carbon atoms can
be decomposed. A comparison of the SEM images before and after purification is
shown in Fig. 14.4.
14.3 Arc Discharge Processes with Difference Gases
The first CNT synthesis was performed with argon [1]. Later, several works used
rather helium gas of pressure ranging from 25 mbar to 1 bar [4, 18, 19, 23, 26–29,
39, 50, 51]. Helium proved to lead to faster nanotube growth, but the increase in He
pressure up to 3 bar did not result is a yield increment as compared to atmospheric He
pressure [4]. A theoretical calculation [46] underpins both the growth rate enhancement in He as compared to Ar by a factor of about three and the saturation of nanotube
growth rate around 1 bar.
The application of helium and argon as noble gases sounds very logical to avoid
the reaction with the carbon radicals that build up the nanotubes. Nevertheless, other
gases have also been applied whose impact can be elucidated together with the
other important experimental conditions such as the application of catalysts. One of
these gases is hydrogen [24, 25]. When it is applied together with He for MWCNT
production (no catalyst applied [52]), the larger H 2 pressure led to both higher anode
consumption rate and larger deposit formation rate at the cathode. However, in the