Synthesis of carbon-based materials by methane pyrolysis in a low-current gliding arc discharge

In this study, a low-current (25-75 mA) gliding arc discharge (GAD) system was utilized for the synthesis of carbon-based materials. We investigated the effects of discharge current (I) and methane concentration (Phi CH4) on the discharge features and the carbon materials properties. We observed a strong influence of the discharge current on the discharge behavior. For the lowest I value (I = 25 mA), the plasma is confined and emits blue light. In this condition, no carbon formation is observed (whatever Phi CH4 is), revealing a too low associated discharge power (PD) to allow for a significant dissociation of CH4. When increasing I, the plasma features are strongly affected with the appearance of a yellow flame, associated with the production of incipient soot, expanding as a function of I. In these conditions, carbon is always generated. We observed that a high enough discharge current (I = 75 mA) is necessary to allow for a stable plasma in the entire Phi CH4 range. Characterization of carbon products reveals the formation of two distinct types of carbon nanomaterials: graphene nanoflakes (GNFs), including single-layer, bilayer, and multilayer structures, predominantly synthesized at Phi CH4 = 10 %, and carbon nanoparticles (CNPs), primarily generated at higher Phi CH4. For multilayer GNFs, two morphologies-"flat platelet" and "wrinkled layer"-were identified through Annular Dark-Field Scanning Transmission Electron Microscopy (ADF-STEM) images. For CNPs, a comparative thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) study with commercial carbon blacks (CBs) reveals that the thermal stability and purity of CNPs improves with increasing PD, shifting their properties closer to those of CBs.