(Invited) Controlled Synthesis of Functional Carbon Nanomaterials By Gas-Liquid Interface Plasma

Abstract

Carbon nanomaterials, such as fullerene, carbon nanotubes (CNT), graphene sheets, and so forth, play indispensable roles in nanotechnology research. Due to their unique self-organized nanostructure and unique physical and electrical 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. Y 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. 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. After the synthesis, Pt nanoparticles were supported on their surfaces reducing the 8wt%-H2PtCl6 in H2O, and catalytic performances of Pt-supported nanographene were evaluated. In the cyclic voltammogram, nanographene materials synthesized using ethanol showed only about 10% degradation even after 10,000 cycles of the potential cycling test, although those using 1-butanol showed a drastic degradation. These results indicate that the higher-density Pt nanoparticles can be supported on the higher-crystallinity nanographene material and they show higher durability. Very recently, micrometer sized carbon nanosheets were synthesized using iron phthalocyanine or hemin with ethanol. 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. 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.