Micron size graphene sheets synthesis by methane pyrolysis in an RF-ICP thermal plasma reactor

Bottom-up synthesis of free-standing graphene using thermal plasma technology often results in flakes with smaller lateral dimensions (hundreds of nanometers) compared to top-down and substrate-based approaches (reaching centimeters in size) Dato (2019<i>J. Mater. Res.</i><b>34</b>214-30). This limitation in size restricts the applicability of graphene in various applications. This study investigates a method to overcome this limitation by studying the reactor's quenching effect on the plasma plume exiting an radiofrequency inductively coupled thermal plasma thermal plasma torch. Local gas phase chemistry and graphene morphology were investigated during methane (CH₄) pyrolysis in argon plasma. Natural quenching suppression led to a production of few-layer (2-5 layers), near-micrometer-sized un-supported graphene sheets (∼2.8<i>µ</i>m perimeter) with less crumpling and a projected area of (2-5) × 10⁵nm². Raman, transmission electron microscopy, thermogravimetric analysis, and x-ray photoelectron spectroscopy (XPS) analysis confirmed the high quality of the synthesized graphene. Sp²carbon composition in the sample was calculated using the<i>D</i>parameter obtained from the differentiated C KLL Auger peak in the XPS spectrum. A correlation between the gas phase chemistry and the graphene morphology demonstrated the significant effect of plasma reactor natural quenching and recirculation on the graphene synthesis and offers a potential for controlling the structure of unsupported graphene. The current study provides valuable insights that can pave the way for the development of reactors with a definite control over the morphology of synthesized graphene.