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Gold nanoparticles with exotic shapes, such as triangle, pentagon, and hexagon, have been synthesized by glow discharge in aqueous solutions. A pulsed power supply was used to generate discharges in the aqueous solutions. Pulse width and frequency were 2 s and 15 kHz, respectively. Discharges were generated at applied voltages of 1600 and 3200 V. The shapes of the gold nanoparticles and electron diffraction patterns were observed by transmission electron microscopy. The nanoparticles obtained were about 20 nm in diameter. In particular, at the higher voltage of 3200 V, nanoparticles with anisotropic shapes were synthesized. In the initial stages of synthesis, diameter decreased with discharge time as the nanoparticles redissolved in the solution. After discharge for 25 min, nanoparticles with anisotropic shapes appeared. This discharge led to the generation of H 2 O 2 and a decrease in pH as a result of the consumption of OH radicals during the generation of H 2 O 2 and electron donation of H radicals to the solution. After the pH stopped decreasing, H radicals mainly reacted as a reducing agent. The decrease in pH allowed redissolution of the gold nanoparticles. The gold dust particles that were not completely dissolved acted as new seeds for nucleation. Thus, the two reaction steps, nucleation and nuclear growth, occur during the formation of gold nanoparticles with exotic shapes.

Although solution-plasma processing enables room-temperature synthesis of nanocarbons, the underlying mechanisms are not well understood. We investigated the routes of solution-plasma-induced nanocarbon formation from hexane, hexadecane, cyclohexane, and benzene. The synthesis rate from benzene was the highest. However, the nanocarbons from linear molecules were more crystalline than those from ring molecules. Linear molecules decomposed into shorter olefins, whereas ring molecules were reconstructed in the plasma. In the saturated ring molecules, C–H dissociation proceeded, followed by conversion into unsaturated ring molecules. However, unsaturated ring molecules were directly polymerized through cation radicals, such as benzene radical cation, and were converted into two- and three-ring molecules at the plasma–solution interface. The nanocarbons from linear molecules were synthesized in plasma from small molecules such as C2 under heat; the obtained products were the same as those obtained via pyrolysis synthesis. Conversely, the nanocarbons obtained from ring molecules were directly synthesized through an intermediate, such as benzene radical cation, at the interface between plasma and solution, resulting in the same products as those obtained via polymerization. These two different reaction fields provide a reasonable explanation for the fastest synthesis rate observed in the case of benzene.

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