520
14 Nanostructures Obtained with Plasma Discharge Processes
known to fill up carbon nanostructures (in particular, nanotubes [85]) with other materials, occasionally arc discharge synthesis can lead to nanostructures encapsulated
in carbon. Such carbon-coated composite nanorods and nanoparticles containing a
metal core are of great interest for their magnetic properties, electrochemical performance in electrodes of lithium ion batteries and controlled-release metal carriers.
The relative intactness of the carbon-encapsulated metal originates from the low
reactivity of the nearly perfect graphite layers covering them.
In some cases, the experimental setup for producing carbon-coated metal particles
is nearly identical to those used for nanotube preparation with the exception that the
metal content of the anode is higher than what is required to catalytic nanotube growth
[25, 79, 86–88]. Since the discharge current is in the same range as for the catalytic
preparation of the non-filled nanotubes or nanoonions, it is difficult to categorize the
peculiar criteria that favour metal encapsulation instead of empty nanotube formation.
In other cases, however, neither of the electrodes contained carbon, and the carbon
content of the coating layer comes from the media surrounding the discharge, should
it be either one of the components of the gas phase [89–91] or the liquid itself in
which the discharge is generated [75]. The coating efficiency is generally higher for
metals that do not form carbides. Representative TEM images of the metal-filled
carbon nanostructures are presented in Fig. 14.5.
Both the diameter of the encapsulated nanoparticles and the thickness of the
carbon coating layer vary in a wide range (and hence, so does their ratio). The particle
diameter ranges from about 4 to 200 nm, while the lowest number of covering carbon
layers is about three. The relationship between the experimental conditions and the
characteristic length scales is not totally clear. The particle diameter seems to increase
with the metal content of the anode [91]. The pressure of the carbon source gas has
a dual impact on the particles since its increase leads to the enlargement of both the
metal particles and their coating thickness [86, 89].
Ferromagnetic particles coated with carbon layers exhibit superparamagnetic
behaviour since the carbon shield effectively hinders the exchange interaction
between the electrons of the core metal particles [87, 90]. The carbon shield also
prevents dipole-type particle interaction, which is important for applications. At
the same time, the total magnetization of the nanoparticle assembly is consequently
lower than that calculated merely on the basis of its metal content due the diamagnetic
contribution of the carbon shield.
14.7 Non-carbonaceous Nanostructures Obtained
by Plasma Synthesis
The plasma synthesis of non-carbonaceous nanomaterials became a wide and very
diverse field for which it is difficult to offer a complete summary in the framework of
the present book. A short highlight of the field is given below with examples ranging
from doping of carbon through metal particle synthesis to ceramic particles.
14 Nanostructures Obtained with Plasma Discharge Processes
known to fill up carbon nanostructures (in particular, nanotubes [85]) with other materials, occasionally arc discharge synthesis can lead to nanostructures encapsulated
in carbon. Such carbon-coated composite nanorods and nanoparticles containing a
metal core are of great interest for their magnetic properties, electrochemical performance in electrodes of lithium ion batteries and controlled-release metal carriers.
The relative intactness of the carbon-encapsulated metal originates from the low
reactivity of the nearly perfect graphite layers covering them.
In some cases, the experimental setup for producing carbon-coated metal particles
is nearly identical to those used for nanotube preparation with the exception that the
metal content of the anode is higher than what is required to catalytic nanotube growth
[25, 79, 86–88]. Since the discharge current is in the same range as for the catalytic
preparation of the non-filled nanotubes or nanoonions, it is difficult to categorize the
peculiar criteria that favour metal encapsulation instead of empty nanotube formation.
In other cases, however, neither of the electrodes contained carbon, and the carbon
content of the coating layer comes from the media surrounding the discharge, should
it be either one of the components of the gas phase [89–91] or the liquid itself in
which the discharge is generated [75]. The coating efficiency is generally higher for
metals that do not form carbides. Representative TEM images of the metal-filled
carbon nanostructures are presented in Fig. 14.5.
Both the diameter of the encapsulated nanoparticles and the thickness of the
carbon coating layer vary in a wide range (and hence, so does their ratio). The particle
diameter ranges from about 4 to 200 nm, while the lowest number of covering carbon
layers is about three. The relationship between the experimental conditions and the
characteristic length scales is not totally clear. The particle diameter seems to increase
with the metal content of the anode [91]. The pressure of the carbon source gas has
a dual impact on the particles since its increase leads to the enlargement of both the
metal particles and their coating thickness [86, 89].
Ferromagnetic particles coated with carbon layers exhibit superparamagnetic
behaviour since the carbon shield effectively hinders the exchange interaction
between the electrons of the core metal particles [87, 90]. The carbon shield also
prevents dipole-type particle interaction, which is important for applications. At
the same time, the total magnetization of the nanoparticle assembly is consequently
lower than that calculated merely on the basis of its metal content due the diamagnetic
contribution of the carbon shield.
14.7 Non-carbonaceous Nanostructures Obtained
by Plasma Synthesis
The plasma synthesis of non-carbonaceous nanomaterials became a wide and very
diverse field for which it is difficult to offer a complete summary in the framework of
the present book. A short highlight of the field is given below with examples ranging
from doping of carbon through metal particle synthesis to ceramic particles.