14.4 Plasma Discharge Setups Submerged into Various Liquids
519
formed in situ was reported to be a mixture containing acetylene and hydrogen as
the major components [77].
Liquid-submerged arrangements can be made suitable for the synthesis of both
MWCNTs and SWCNTs, depending on the application of an appropriate catalyst
(see the next subchapter). However, liquid discharge media proved to be particularly
suitable for the production of nanoonions [22, 66] and other nanoparticles [30, 31].
This type of product can be collected from the surface of the liquid reservoir, unlike
nanotubes that accumulate on the cathode.
14.5 Experimental Conditions Affecting the Type
Distribution of Carbon Nanotubes
Carbon nanotubes obtained with arc discharge between carbon electrodes in the
absence of any catalyst are typically MWCNTs. If an appropriate catalyst is applied,
the preferred growth mode can be modified and single-wall or double-wall nanotubes
can be grown. The widespread mode of the introduction of the catalysts is to drill
a cavity in one of the carbon electrodes (mostly in the anode) that is filled up with
the fine-grained catalyst mixed with carbon powder. During the anode erosion, the
catalyst is also evaporated, and the nanotube growth proceeds at the condensed
particles. Catalysts play a very important role in the regulation of the product profile
of the arc discharge method since single-wall or double-wall nanotubes can be made
dominant. Nevertheless, even catalysts can lead to a product of a preferred nanotube
type but cannot significantly narrow the tube diameter distribution.
The first catalysts applied for the SWCNT synthesis were elemental 4d transition
metals (Fe [5], Co [28, 78], Cu [79]). Later the Ni–Y-type mixed catalysts became
nearly a standard [26, 29, 39, 51, 57, 80, 81] by using either a mixture of the two
metals or their alloy. Molybdenum [82], tungsten [54], cerium [83] and holmium [84]
were occasionally applied as a minor component in the catalyst, while calchogenide
elements (S and Se) were identified as promoters increasing the nanotube yield
[81]. While the Fe–Mo catalyst was first observed to promote the growth of double
wall nanotubes [53], the catalysts applied for DWCNT growth are usually contain a
number of elements (Ni, Co, Fe and S, either in elemental form or as metal sulphides)
[19, 55, 56].
14.6 Arc Discharge Synthesis of Carbon-Encapsulated
Nanostructures
Carbon nanostructures are prominent candidates to host various other nanostructures
due to their cavity size of appropriate dimension. Although many ex situ methods are
519
formed in situ was reported to be a mixture containing acetylene and hydrogen as
the major components [77].
Liquid-submerged arrangements can be made suitable for the synthesis of both
MWCNTs and SWCNTs, depending on the application of an appropriate catalyst
(see the next subchapter). However, liquid discharge media proved to be particularly
suitable for the production of nanoonions [22, 66] and other nanoparticles [30, 31].
This type of product can be collected from the surface of the liquid reservoir, unlike
nanotubes that accumulate on the cathode.
14.5 Experimental Conditions Affecting the Type
Distribution of Carbon Nanotubes
Carbon nanotubes obtained with arc discharge between carbon electrodes in the
absence of any catalyst are typically MWCNTs. If an appropriate catalyst is applied,
the preferred growth mode can be modified and single-wall or double-wall nanotubes
can be grown. The widespread mode of the introduction of the catalysts is to drill
a cavity in one of the carbon electrodes (mostly in the anode) that is filled up with
the fine-grained catalyst mixed with carbon powder. During the anode erosion, the
catalyst is also evaporated, and the nanotube growth proceeds at the condensed
particles. Catalysts play a very important role in the regulation of the product profile
of the arc discharge method since single-wall or double-wall nanotubes can be made
dominant. Nevertheless, even catalysts can lead to a product of a preferred nanotube
type but cannot significantly narrow the tube diameter distribution.
The first catalysts applied for the SWCNT synthesis were elemental 4d transition
metals (Fe [5], Co [28, 78], Cu [79]). Later the Ni–Y-type mixed catalysts became
nearly a standard [26, 29, 39, 51, 57, 80, 81] by using either a mixture of the two
metals or their alloy. Molybdenum [82], tungsten [54], cerium [83] and holmium [84]
were occasionally applied as a minor component in the catalyst, while calchogenide
elements (S and Se) were identified as promoters increasing the nanotube yield
[81]. While the Fe–Mo catalyst was first observed to promote the growth of double
wall nanotubes [53], the catalysts applied for DWCNT growth are usually contain a
number of elements (Ni, Co, Fe and S, either in elemental form or as metal sulphides)
[19, 55, 56].
14.6 Arc Discharge Synthesis of Carbon-Encapsulated
Nanostructures
Carbon nanostructures are prominent candidates to host various other nanostructures
due to their cavity size of appropriate dimension. Although many ex situ methods are