Controlling Carbon Nanostructure Synthesis in Thermal Plasma Jet: Correlation of Process Parameters, Plasma Characteristics, and Product Morphology

TL;DR

This study establishes a detailed correlation between process parameters, plasma characteristics, and the resulting carbon nanostructure morphologies in thermal plasma jet synthesis, enabling controlled production of specific nanostructures.

Contribution

It introduces a comprehensive analysis linking plasma temperature, C2 density, and process parameters to nanostructure morphology, filling a gap in understanding plasma-based carbon nanostructure synthesis.

Findings

Low-density nanostructures favored at high temperature and low C2 density.

Denser nanostructures form at higher C2 density and lower temperature.

Flow rate and pressure control nanostructure morphology from GNFs to GNCs.

Abstract

Thermal plasma has emerged as an effective approach for producing carbon nanostructures without the need for specific catalysts nor substrates. While efforts have focused on the effect of process parameters such as reaction pressure, input power or carbon source, the intricate role and relationship with plasma characteristics like density and temperature are often overlooked due to the complexity of the environment. This study addresses this gap by establishing a correlation between process parameters, plasma characteristics, and product morphology, essential for controlling the growth of carbon nanostructures. We explored the impact of carbon precursor type (CH4 and C2H2), hydrogen, pressure, and flow rate on nanostructure formation. Using in situ optical emission spectroscopy (OES), we mapped the distribution of both temperature and dicarbon molecule (C2) density within the plasma jet. We demonstrate that the growth of low-density nanostructures, such as carbon nanohorns (CNHs), is favoured at dilute C2 local densities and high temperatures, while denser nanostructures, such as onion-like polyhedral graphitic nanocapsules (GNCs), are favoured at higher C2 densities and lower temperatures. The carbon density can be controlled by the flow rate and the pressure, which in turn significantly influence the nanostructure morphology, evolving from graphene nanoflakes (GNFs) to GNCs as either parameter increases. Increasing the H/C ratio from 1 to 8 resulted in a morphological transition from CNHs to GNFs. During the synthesis, the plasma jet temperature surpassed 3,000 K, with crystalline growth occurring 50 to 100 mm below the nozzle.