Effects of Buffer Gases on Graphene Flakes Synthesis in Thermal Plasma Process at Atmospheric Pressure.

A thermal plasma process at atmospheric pressure is an attractive method for continuous synthesis of graphene flakes. In this paper, a magnetically rotating arc plasma system is employed to investigate the effects of buffer gases on graphene flakes synthesis in a thermal plasma process. Carbon nanomaterials are prepared in Ar, He, Ar-H-2, and Ar-N-2 via propane decomposition, and the product characterization is performed by transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and the Brunauer-Emmett-Teller (BET) method. Results show that spherical particles, semi-graphitic particles, and graphene flakes coexist in products under an Ar atmosphere. Under an He atmosphere, all products are graphene flakes. Graphene flakes with fewer layers, higher crystallinity, and a larger BET surface area are prepared in Ar-H-2 and Ar-N-2. Preliminary analysis reveals that a high-energy environment and abundant H atoms can suppress the formation of curved or closed structures, which leads to the production of graphene flakes with high crystallinity. Furthermore, nitrogen-doped graphene flakes with 1-4 layers are successfully synthesized with the addition of N-2, which indicates the thermal plasma process also has great potential for the synthesis of nitrogen-doped graphene flakes due to its continuous manner, cheap raw materials, and adjustable nitrogen-doped content.