Synthesis of Few-Layer Graphene via Microwave Plasma-Assisted Methane Pyrolysis: A Combined Experimental and Computational Kinetics Approach

In the context of addressing global energy demands, microwave plasma-assisted methane pyrolysis emerges as a promising method for controlled and energy-efficient decomposition of methane. This electrode-less, substrate-less, and catalyst-free one-step process of microwave plasma plays a pivotal role in the synthesis of few-layer graphene (FLG) with no greenhouse gases as byproducts. In this work, the experimental setup features a commercial microwave plasma torch operating at 2.45 GHz with a microwave power starting from 1 kW, employing argon as a carrier gas. Optical emission spectroscopy was used to analyze the plasma species. Raman spectroscopy, transmission electron microscopy, atomic force microscopy, and Brunauer–Emmett–Teller surface area analysis were used to characterize the synthesized graphene. Numerical simulations using ZDPlasKin and Cantera software facilitated the understanding of plasma thermochemistry and reaction kinetics due to microwave plasma gas heating. Parametric studies were performed to investigate the effect of parameters such as the gas mixture ratio, gas flow rate, and microwave power on the plasma and the synthesized graphene. With a high temperature range from 2500 to 4500 K, growth of pristine graphene is influenced by the acetylene (C2H2) formed by the dehydrogenation process. Results indicate that a 9/1 gas ratio Ar/CH4 gas mixture ratio optimizes methane conversion and promotes the formation of key species such as C2H2, essential for quality FLG synthesis. The best graphene quality was achieved at 1.5 kW power with Raman spectra showing a high I2D/IG ratio of 1.05 and a low ID/IG ratio of 0.36, indicating FLG with minimal defects. The study reveals a significant effect of plasma gas temperature on the dehydrogenation process of C2H2, which in turn affects the quality of the synthesized graphene.