522
14 Nanostructures Obtained with Plasma Discharge Processes
SiC nanofibers of 10–20 nm in diameter were highly crystalline and exhibiting an
α- or β-SiC structure, occasionally covered with an SiO 2 shield. Niobium carbide
nanoparticles [95] were obtained by using Nb electrodes and various carbohydrides
as carbon source gas. The shape of the NbC nanoparticles of a few tens of nm in
diameter could be regulated with the gas phase composition.
The preparation of silicon nanoparticles with arc discharge was successful by
plasma etching of either the anode [96] or the cathode [97] under liquid. The discharge
under nitrogen [96] resulted in Si nanoparticles, while the application of aqueous
solution can lead to either Si particles covered with a thin oxide layer (when acidic
solutions are applied) or SiO 2 (when the solution is alkaline) [97]. The crystallinity
of the particles was the larger, the larger their size. The mean diameter of the Si
nanoparticles obtained can be easily diminished with a chemical etching process in
HF solution and particles with as low a 4 nm diameter can be obtained [96].
Etching of metal anodes leads to various nanoparticles. If the anode is a noble metal
(Ag, Au, Pt), nanoparticles can be produced in discharges submerged into deionised
water. The resulting nanoparticles were reported as being fully metallic [98–100] and
they form a stable suspension that is often termed as nanofluid. This property of the
particles gave rise to a speculation that the surface bonds may be influenced by the
CO 2 dissolved in water and give the particle some charge, and ionic reaction products
(like Ag
+ ) are often found in the solution without specifying the corresponding anion.
A zeta potential sufficiently large for the suspension stabilization was also measured
in some cases [98]. The mean particle diameter is between 6 and 25 nm.
Discharges in liquid result in noble metal nanoparticle formation if the precursor
is a dissolved noble metal salt (Ag: AgNO 3 , Au: HAuCl 4 ) and the electrodes used are
non-reactive (Ti or W). Various plasma excitation mode (d.c. arc discharge [101],
RF pulsed arc discharge [102] or MW plasma [103, 104]) are easily applicable
for this goal since the effect in all cases is the fast evaporation of the solution in
the near-plasma region. Although the mean diameter of the noble metal particles
produced with any of the above-mentioned methods was below 20 nm, the lowest
mean particle size (4–5 nm) was obtained by applying a reverse micelle solution of
HAuCl 4 in which the micelles were reported to act as limited-volume nanoreactors
[102]. This was evidenced by the experience that the increase in the water/surfactant
ratio leads to an enhancement of the size of Au particles as the micelle size increases.
When the arc discharge method was used for the preparation of alloyed non-noble
metallic particles, the precursor material was a master alloy comprised in the anode.
Even though the experiments were conducted in a reducing atmosphere containing
H 2 , the resulting nanopowder exhibited a core-shell structure due to the oxidation of
the particles upon removal from the discharge chamber. The post-synthesis oxidation
is selective, and one of the alloy components is the preferred metal forming the outer
oxide shell (Sn in Fe–Sn [105] and Cu in Co–Cu [106] particles).
The last group of materials can be classified as oxide nanorods and nanoparticles. The common feature of the synthesis method that the media was of oxidizing
character, either because of the oxygen present in the gas phase [107] or water when
the plasma was submerged into the liquid phase [97, 100, 103, 108]. The precursor
material can be the elemental form of the metal to be oxidized (Ta [108], W [100],
14 Nanostructures Obtained with Plasma Discharge Processes
SiC nanofibers of 10–20 nm in diameter were highly crystalline and exhibiting an
α- or β-SiC structure, occasionally covered with an SiO 2 shield. Niobium carbide
nanoparticles [95] were obtained by using Nb electrodes and various carbohydrides
as carbon source gas. The shape of the NbC nanoparticles of a few tens of nm in
diameter could be regulated with the gas phase composition.
The preparation of silicon nanoparticles with arc discharge was successful by
plasma etching of either the anode [96] or the cathode [97] under liquid. The discharge
under nitrogen [96] resulted in Si nanoparticles, while the application of aqueous
solution can lead to either Si particles covered with a thin oxide layer (when acidic
solutions are applied) or SiO 2 (when the solution is alkaline) [97]. The crystallinity
of the particles was the larger, the larger their size. The mean diameter of the Si
nanoparticles obtained can be easily diminished with a chemical etching process in
HF solution and particles with as low a 4 nm diameter can be obtained [96].
Etching of metal anodes leads to various nanoparticles. If the anode is a noble metal
(Ag, Au, Pt), nanoparticles can be produced in discharges submerged into deionised
water. The resulting nanoparticles were reported as being fully metallic [98–100] and
they form a stable suspension that is often termed as nanofluid. This property of the
particles gave rise to a speculation that the surface bonds may be influenced by the
CO 2 dissolved in water and give the particle some charge, and ionic reaction products
(like Ag
+ ) are often found in the solution without specifying the corresponding anion.
A zeta potential sufficiently large for the suspension stabilization was also measured
in some cases [98]. The mean particle diameter is between 6 and 25 nm.
Discharges in liquid result in noble metal nanoparticle formation if the precursor
is a dissolved noble metal salt (Ag: AgNO 3 , Au: HAuCl 4 ) and the electrodes used are
non-reactive (Ti or W). Various plasma excitation mode (d.c. arc discharge [101],
RF pulsed arc discharge [102] or MW plasma [103, 104]) are easily applicable
for this goal since the effect in all cases is the fast evaporation of the solution in
the near-plasma region. Although the mean diameter of the noble metal particles
produced with any of the above-mentioned methods was below 20 nm, the lowest
mean particle size (4–5 nm) was obtained by applying a reverse micelle solution of
HAuCl 4 in which the micelles were reported to act as limited-volume nanoreactors
[102]. This was evidenced by the experience that the increase in the water/surfactant
ratio leads to an enhancement of the size of Au particles as the micelle size increases.
When the arc discharge method was used for the preparation of alloyed non-noble
metallic particles, the precursor material was a master alloy comprised in the anode.
Even though the experiments were conducted in a reducing atmosphere containing
H 2 , the resulting nanopowder exhibited a core-shell structure due to the oxidation of
the particles upon removal from the discharge chamber. The post-synthesis oxidation
is selective, and one of the alloy components is the preferred metal forming the outer
oxide shell (Sn in Fe–Sn [105] and Cu in Co–Cu [106] particles).
The last group of materials can be classified as oxide nanorods and nanoparticles. The common feature of the synthesis method that the media was of oxidizing
character, either because of the oxygen present in the gas phase [107] or water when
the plasma was submerged into the liquid phase [97, 100, 103, 108]. The precursor
material can be the elemental form of the metal to be oxidized (Ta [108], W [100],