Plasma–Ice Interface as Thermodynamically Size-Tunable Reaction Field: Development of Plasma-Assisted Freeze Templating

Abstract

Interfaces or interfacial layers, such as gas–liquid interfaces, are critical for many physical and chemical reactions and are utilized for designing a wide range of materials. In this study, we propose a plasma-assisted freeze templating (PFT) method for materials processing. It uses a new type of interfacial reaction field, i.e., plasma–ice interface. In PFT, a micro- or nanoscale liquid layer formed on the ice body of a frozen aqueous solution is used as a reaction field in which the solutes are highly enriched and the chemical reactions are initiated by reactive species from the plasma. We demonstrated the synthesis of a self-standing gold nanoparticle (AuNP) film of porous structure by PFT in which a helium cryoplasma jet was irradiated onto a frozen solution of auric ions. This PFT method accomplished a surfactant-free and area-selective synthesis of a AuNP film and was unique in comparison with the conventional chemical synthesis of nanostructured gold materials. Furthermore, simple control of the AuNP film was demonstrated by tuning the thickness of the thin liquid layer. This was done by changing the temperature or concentration of the aqueous solution. PFT was demonstrated as a thermodynamically size-tunable scheme for material design; it exploits the plasma–ice interface and is expected to become a novel technique for a wide range of micro- and nanoengineering applications.

Cryogenic-specific Reddish Coloration by Cryoplasma: New Explanation for Color Diversity of Outer Solar System Objects

“…For the coloration, cryoplasma was radiated at 85 K on methanol (CH3OH) and water (H2O) containing ice by feeding helium (He) with 3% nitrogen (N2). Cryoplasma is non-equilibrium plasma whose gas temperature can be controlled at wide range of cryogenic temperatures below room temperature (Stauss et al 2018), enabling coexistence of plasma with ice without melting (Sakakirbara & Terashima 2017;Sakakibara et al 2019). This study roughly simulated energetic processes in the outer solar system via UV radiation and charged and/or excited species formed by cosmic rays and solar winds (Thompson et al 1991;Imasaka 2004).…”