518
14 Nanostructures Obtained with Plasma Discharge Processes
14.4 Plasma Discharge Setups Submerged into Various
Liquids
The concept of liquid-submerged electrodes in the arc discharge process comes from
the simplicity of the system not requiring a pressure- and atmosphere-regulated
vacuum chamber. The possibility of continuous deposit preparation without the
necessity of the chamber purification was also envisioned during the elaboration
of this idea. This is supported by the sustainability of submerged discharges up to
1–2 h [37].
The first study of the liquid-submerged arc discharge electrode arrangement
applied for CNT production applied liquid nitrogen [20]. Although the temperature
gradient around the liquid-submerged discharge is much larger than in discharges
applying buffer gases, the electrical parameters of the discharge were nearly identical
to those applied in gas chambers [20, 31, 63, 64]. The similarity of the plasma conditions to the gas-based plasma is indicated by the fact that the product distribution,
MWCNT vs. SWCNT, depends on the absence/presence of an appropriate catalyst,
respectively. A beneficial aspect of this synthesis method was an enhanced surface
purity of the nanotubes [20].
Arc discharge with electrodes submerged into water also became popular. Pure
deionized water [22, 32, 65–70], hydrogen peroxide solution [71] as well as various
metal salt solutions (LiCl [72], NaCl [73], iron-group metal sulphates [36]) proved
to be suitable as discharge environment. Unlike in traditional electrochemical setups,
the conductivity of the solutions applied has neither a role in the maintenance of the
arc discharge, nor the ions of the electrolyte seem to influence the properties of the
products.
Organic liquids have also been tested for media of liquid-submerged arc discharge
experiments for producing carbon nanostructures. Examples range from ethanol [74,
75], ethylene glycol [76], various aromatic hydrocarbons [33, 77] to highly fluorinated ether derivatives [30]. By using organic liquids as the discharge medium, it
is possible to achieve that the carbon source of the resulting nanostructure is not
the anode but the organic liquid. A decisive experiment is when the arc discharge
is ignited between non-carbonaceous electrodes [33, 75]. Another evidence for the
incorporation of the fragments of the organic liquid used is that its constituents other
than carbon (H, N) can also be detected in the products [33].
It was reported for various liquids that bubble formation around the discharge
zone took place during the experiments. This means that the gas evolved plays a
similar role in the maintenance of the discharge than the buffer gas in the gaseous
environment. Due to the high density of the liquids as compared to the gases, no gas
stream regulation is needed and the buffer gas is automatically refreshed around the
liquid-submerged discharges, which contributes to the stability of the plasma formed.
If the discharge is maintained between carbon electrodes under water [66], this gas
is composed of hydrogen and carbon monoxide as a result of the reaction between
carbon and water (C + H 2 O CO + H 2 . For hydrocarbon media, the buffer gas
14 Nanostructures Obtained with Plasma Discharge Processes
14.4 Plasma Discharge Setups Submerged into Various
Liquids
The concept of liquid-submerged electrodes in the arc discharge process comes from
the simplicity of the system not requiring a pressure- and atmosphere-regulated
vacuum chamber. The possibility of continuous deposit preparation without the
necessity of the chamber purification was also envisioned during the elaboration
of this idea. This is supported by the sustainability of submerged discharges up to
1–2 h [37].
The first study of the liquid-submerged arc discharge electrode arrangement
applied for CNT production applied liquid nitrogen [20]. Although the temperature
gradient around the liquid-submerged discharge is much larger than in discharges
applying buffer gases, the electrical parameters of the discharge were nearly identical
to those applied in gas chambers [20, 31, 63, 64]. The similarity of the plasma conditions to the gas-based plasma is indicated by the fact that the product distribution,
MWCNT vs. SWCNT, depends on the absence/presence of an appropriate catalyst,
respectively. A beneficial aspect of this synthesis method was an enhanced surface
purity of the nanotubes [20].
Arc discharge with electrodes submerged into water also became popular. Pure
deionized water [22, 32, 65–70], hydrogen peroxide solution [71] as well as various
metal salt solutions (LiCl [72], NaCl [73], iron-group metal sulphates [36]) proved
to be suitable as discharge environment. Unlike in traditional electrochemical setups,
the conductivity of the solutions applied has neither a role in the maintenance of the
arc discharge, nor the ions of the electrolyte seem to influence the properties of the
products.
Organic liquids have also been tested for media of liquid-submerged arc discharge
experiments for producing carbon nanostructures. Examples range from ethanol [74,
75], ethylene glycol [76], various aromatic hydrocarbons [33, 77] to highly fluorinated ether derivatives [30]. By using organic liquids as the discharge medium, it
is possible to achieve that the carbon source of the resulting nanostructure is not
the anode but the organic liquid. A decisive experiment is when the arc discharge
is ignited between non-carbonaceous electrodes [33, 75]. Another evidence for the
incorporation of the fragments of the organic liquid used is that its constituents other
than carbon (H, N) can also be detected in the products [33].
It was reported for various liquids that bubble formation around the discharge
zone took place during the experiments. This means that the gas evolved plays a
similar role in the maintenance of the discharge than the buffer gas in the gaseous
environment. Due to the high density of the liquids as compared to the gases, no gas
stream regulation is needed and the buffer gas is automatically refreshed around the
liquid-submerged discharges, which contributes to the stability of the plasma formed.
If the discharge is maintained between carbon electrodes under water [66], this gas
is composed of hydrogen and carbon monoxide as a result of the reaction between
carbon and water (C + H 2 O CO + H 2 . For hydrocarbon media, the buffer gas