Unveiling the pathways of ethanol decomposition to carbon radicals by nanosecond pulse bubble discharge in liquid: a two-step hybrid model

In recent years, bubble discharge in liquid has become a novel approach for the synthesis of carbon nanomaterials; however, the fundamental discharge process and synthesis mechanism are still not well understood. In this work, we build a two-step simulation model (combining 2D fluid dynamics and zero-dimensional plasma kinetics) to investigate nanosecond pulse discharge in an Ar bubble immersed in liquid ethanol and chemical reaction processes inside. The 2D simulation results show that discharge develops along the gas‒liquid interface where ethanol decomposes, resulting in much higher densities of active species (CH₃, C₂H₅ and OH). The electric field of the selected reference point near the interface obtained by the 2D model is transmitted into the 0D model. The numerical results show that the decomposition of ethanol mainly occurs at the discharge stage, in which electron impact dissociation (e.g. C₂H₅OH + e → CH₂OH + CH₃ + e) and Penning dissociation (e.g. C₂H₅OH + Ar* → CH₂OH + CH₃ + Ar) dominate. The density of all carbonaceous species rapidly increases during discharge, while that of some carbon radicals (CH and C) continues to increase due to neutral species reactions when discharge ceases. By quantitative analysis of the reaction contributions, the dominant pathways of C₂, CH and C are revealed, i.e. C₂H₅OH → C₂H₅ → [C₂H, C₂H₂, C₂H₃] → C₂ and C₂H₅OH → CH₃ → CH₂ → CH → C. In addition, the formation pathways of H and OH radicals, which are indispensable for the transformation of carbonaceous intermediates, are also analysed.