Microdischarge-Induced Decomposition of Ammonia and Reduction of Silver Ions for Formation of Two-Dimensional Network Structure

Significance Statement

  A nanoparticle assembly in a network structure has various potentials for functional electronic and optical outputs. In this study, based on interdisciplinary science and engineering which include plasma chemistry, fractal science and metamaterial optics, our experimental results demonstrate a formation process of functional metallic nano-particle networks. A microdischarge or microplasma, which can be installed in various chemical reactors and biomedical instruments working at atmospheric pressure, works to convert ammonia to hydrazine (N₂H₄) at atmospheric pressure [Plasma Sources Sci. Technol. 22 (2013) 032003]. We monitored density of generated hydrazine and used it to reduce AgNO₃ in aqueous solutions in an in-situ system. We observed various 2-dimentional patterns of Ag nanoparticles (~ several hundreds of nano-meters) in a fractal-like structure, and they became both conductors and insulators due to contact conditions in percolation theory. In addition, such spatial patterns show abnormal absorption spectra in the infrared-ray region, which indicates that this self-assembly process has a potential for formation of optical metamaterials; dynamic and nonlinear microwave metamaterials [Plasma Sources Sci. Technol. 21 (2012) 013001, Physical Review E 92 (2015) 033105] may become optical ones in this photomask-free fabrication. Formation of the observed fractal-like patterns is numerically reproduced in the diffusion-limited aggregation (DLA) model, and complicated chemistries in atmospheric-pressure microdischarges are in a complex reaction network comprehended in network theory [AIP Advances 5 (2015) 107140].

This study was first presented in 22nd International Symposium on Plasma Chemistry (Antwerp, Belgium, 2015), and summarized in a journal paper of its special issue in Plasma Chemistry and Plasma Processing.  

 

About the author

Prof. Osamu Sakai ([email protected]) is a full professor at Department of Electronic Systems Engineering, School of Engineering, the University of Shiga Prefecture, Japan. His current research interests are metamaterial science, plasma science and engineering, and complex networks.

Journal Reference

Plasma Chemistry and Plasma Processing, January 2016, Volume 36, Issue 1, pp 281-294.

Osamu Sakai^1,2,Yu Hiraoka²,Naoya Kihara²,Ella Blanquet^1,2,Keiichiro Urabe^2,3,Masanobu Tanaka⁴

1. Department of Electronic Systems Engineering, The University of Shiga Prefecture, 2500 Hassakacho, Hikone, Shiga, 522-8533, Japan

2. Department of Electronic Science and Engineering, Kyoto University, Kyoto-daigaku Katsura Nishikyo-ku, Kyoto, 615-8510, Japan

3. Department of Advanced Materials Science, School of Frontier Sciences, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8561, Japan

4. Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-Ku, Yokohama, Kanagawa, 226-8502, Japan  [/expand]

Abstract

Microdischarge-induced reaction processes working at atmospheric pressure create fractal-like network structure of metal nano-particles which shows variable electric and optical properties. Due to their smallness, microdischarges or microplasmas can be installed in a gas-tubing system, and they enable us to create a compact chemical reduction reactor which includes decomposers of molecules, gas flows, and aqueous solutions with metallic ions at atmospheric pressure. Ammonia (NH₃) gas is successfully decomposed in this reactor, and its products which include mainly hydrazine (N₂H₄) and flow in the downstream induce reduction reactions for AgNO₃ solution. Various parameters in the reactor trigger formation of functional patterns of silver nano-particles like partially transparent layers whose conductivity is variable. Optical properties of this equivalent films show some absorption spectra coming from structure resonances, which can be an optical metamaterials in this self-assembly process.