Few studies have been reported on materials processing in the solution plasma although this reaction
field is promising. Recently, the synthesis of carbon nanotubes in the solution plasma has been the subject
of vigorous investigation [3–7]. Nomura et al. demonstrated the synthesis of carbon-related materials
with a microwave-generated plasma in a hydrocarbon liquid under evacuation [6]. The evacuation
prevents the oxidation of products such as graphite, silicon carbide, diamond-like carbon, and carbon
nanotubes. The most attractive feature of this process is that the liquid phase plays an important role as
a plasma reactor. However, this method is limited to nonpolar solvents such as hydrocarbons, because
polar solvents adsorb microwaves and prevent plasma formation.
We synthesized nanocolloidal particles by a spark discharge in aqueous and nonaqueous solutions
under an open environment, and determined the fundamental characteristics of the synthesized nanocolloidal
particles. The experimental set-up of the discharge system is shown in Fig. 4, and the photograph
of one type of solution plasma is shown in Fig. 5. Two metal wire electrodes were placed in an
acrylate vessel. A bipolar direct current (DC) pulsed power supply was used to generate the spark discharge
in the solutions.