(Invited) Recent Progress in the Synthesis of Functional and Three-Dimensional Carbon Nano-Composites By Gas-Liquid Interface Plasma
Authors: Kondo Hiroki , Zhou Han , Tsutsumi Takayoshi , Ishikawa Kenji , Sekine Makoto , Hori Masaru 
Published: September 2023

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. Furthermore, in recent years, heteroatom-doped carbon nano-composite materials that exhibit catalytic activity are attracting much attention as non-platinum catalysts. Especially, nitrogen (N)-doped nanographene materials are well-known to have high catalytic activity. Pyridinic-N is known as a key component to express catalytic activity [1].
There are two ways to obtain nitrogen-doped carbon nanomaterials. One is the addition of N into carbon nanomaterials during their synthesis, and the other is the post-nitridation after the synthesis, which often used nitrogen plasma. Especially in the latter case, the synthesis method of nanographene itself can be divided into two types of methods. One is the direct synthesis of the powder, and the other is the exfoliating from graphene oxide (GO) or bulk graphite in physical or chemical ways. Hydrothermal treatment using microwaves is one of the effective methods for simultaneous exfoliation, three-dimensional nanopore structuring, and nitrogen addition. While electronic device applications generally need high-quality graphene sheets synthesized by epitaxial growth, or chemical vapor deposition (CVD) methods at high temperatures up to 1,000°C, 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 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-off relationship between the synthesis rate and crystallinity, when different types of alcohols were used as a feedstock 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-off 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.
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