Reaction Dynamics for Gold Nanoparticles Synthesis in Solution Plasma
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Reaction Dynamics for Gold Nanoparticles Synthesis in Solution Plasma Junko Hieda1, Nagahiro Saito2,3, and Osamu Takai3,4 1 Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan 2 Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan 3 CREST, JST, Furo-cho, Chikusa, Nagoya, 464-8603, Japan 4 EcoTopia Science Research Institute, Nagoya University, Furo-cho, Chikusa, Nagoya, 4648603, Japan ABSTRACT We describe the dynamics of the synthesis of gold nanoparticles by glow discharge in aqueous solutions. Initial [AuCl4]− concentration and the voltage applied between the electrodes were varied. Reduction rates were calculated from changes in concentration of [AuCl4]− vs. discharge time. A pulsed power supply was used to generate discharges in the aqueous solutions. The morphology of the nanoparticles obtained was observed by transmission electron microscopy (TEM). [AuCl4]− was reduced by H radicals or electrons generated by the discharge. Dendrite-shaped nanoparticles ~150 nm in size were formed after discharge for 1 min. The pH of the solution decreased gradually with increasing discharge time. The decrease in pH led to dissolution of gold nanoparticles. The reduction and dissolution rates increased proportionately with the applied voltage. The size of gold nanoparticles decreased during discharge and was 20 nm after discharge for 45 min. When the reduction rate lowered as a result of dissolution, anisotropic nanoparticles were formed and continued to grow in the solution. INTRODUCTION Plasmas in the liquid phase have attracted a great deal of attention because of their applicability to industrial materials processing [1]. In particular, glow discharge in the liquid phase (“solution plasma”) is a useful tool for the synthesis of metal nanoparticles. This method provides extremely rapid reactions using activated chemical species and radicals under high pressure. Many methods for the synthesis of metal nanoparticles (e.g., mechanical synthesis, chemical or physical synthesis in the gas phase, and chemical reduction in the liquid phase) have been reported [1–3]. Using the chemical reduction method, metal nanoparticles with excellent monodispersity of particle size are obtained. However, this method requires a reducing agent to fabricate metal nanoparticles from a solution containing metal ions. Use of a solution plasma would allow rapid fabrication of metal nanoparticles without the need to add reducing agents, because plasma in the liquid phase provides a reaction field with a highly excited energy state. In our previous study, we successfully synthesized gold nanoparticles using glow discharge in aqueous solutions. Gold nanoparticles have been used in nanotechnology and biotechnology because of their optical properties in the visible spectrum and their high bioaffinity [4–6]. Nanoparticles with an average size of ~10 nm were ob
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