(Invited) Synthesis, Functionalization, and Three-Dimensional Structuring of Carbon Nanomaterials By Gas-Liquid Interface Plasma
Authors: Kondo Hiroki , Tsutsumi Takayoshi , Ishikawa Kenji , Sekine Makoto , Hori Masaru 
Published: November 2022

Carbon nanomaterials, such as fullerene, carbon nanotubes (CNT), graphene sheets, and so forth, play indispensable roles in nanotechnology research and applications. Due to their unique self-organized nanostructures and properties, various types of applications using them are expected and have been developed. One of recent hottest topics among them is graphene sheets and their electric device applications. High quality graphene sheets for such the device applications, epitaxial growth or chemical vapor deposition (CVD) methods at high temperature up to 1,000°C, are used as a synthesis method in general. On the other hand, some kinds of applications, such as sensors, batteries, additives to polymers, and so forth, need a large amount of nanographene power. For that purpose, a reduction of graphene oxide (GO) is well known, but the quality of the synthesized graphene is not high enough. Recently, we have established a high-speed synthesis method of nanographene materials with high crystallinity by a plasma discharge at gas-liquid interfaces with alcohol sources. By this method, a synthesis rate of nanographene over 1 mg/min and higher crystallinity of nanographene than the reduced GO have been realized. On the other hand, there is a trade-of relationship between the synthesis rate and crystallinity, when different types of alcohols were used as a feed stock gas. When ethanol,1-propanol, and 1-butanol were used, it was found that the higher synthesis rates were obtained by the higher-molecular weight alcohols, while its crystallinity was lower. In the comparison between hexane (C6H14) and hexanol (C6H13OH), in the case of hexane, the synthesis rate is about twice as high as that in the case of hexanol, but the crystallinity is lowered. These results indicate that this trade-of relationship is attributed to a ratio of carbon (C) and oxygen (O) atoms. O-related radicals (O, OH, etc.) in plasma could have etching effects of amorphous or low-crystallinity carbon components. Actually, according to the results of plasma diagnostic measurements and residual liquid analyses, it was found that crystallinity of nanographene materials degraded with decrease in OH intensity in plasma. Furthermore, small radicals such as C2 and CHx contribute to the synthesis of nanographene rather than by-products with a six-membered ring structure. Furthermore, we have also found functionalization and structural control of nanographene materials by additive agents to alcohol sources at in-liquid plasma processes. Using an iron phthalocyanine with ethanol, size of carbon nanosheets increased up to micrometer. And they showed excellent catalytic characteristics thorough 4-electron reduction pathway. According to the verification results of dependence on synthesis conditions such as the type of additive, such the catalytic activity is induced by pyridinic C-N bonds. In the case of this way, to increase pyridinic C-N bonds and improve catalytic performance, iron phthalocyanine is much better than Hemin, even which also included Fe and N. These knowledges obtained in this study will open the way to the next-generation green energy solutions, such as high-performance catalytic electrode for the fuel cell.