Influence of ethylene thermal decomposition on carbon nanotube growth: insights from a two-zone reactor study

Gas-phase decomposition of the carbon precursor is a critical yet not fully understood step in the catalytic chemical vapor deposition growth of carbon nanotubes (CNTs). Here, we present a systematic investigation of how the thermal decomposition of C₂H₄ influences CNT growth. Using a custom-designed two-zone reactor with independently controlled preheating (decomposition) and CNT growth zones, we decoupled the effects of gas-phase decomposition from the growth temperature. Standard synthesis conditions were first established using Bayesian optimization, with the CNT growth zone temperature (T_g) fixed at 700 °C. CNTs were then synthesized at preheating zone temperatures (T_p) of 500, 600, 700, 800, and 900 °C while maintaining T_g at 700 °C under standard conditions. Carbon yield and I_G/I_D remained stable at 500, 600, and 700 °C but showed significant changes at 800 and 900 °C. To elucidate these variations, we analyzed the gas-phase composition at various temperatures using micro-GC. Significant changes in the gas-phase composition were observed above 700 °C, correlating with changes in carbon yield and crystallinity. Based on these results, we propose a mechanism by which differences in gas-phase chemistry lead to changes in the carbon yield and the I_G/I_D ratio. This study provides valuable insight into the role of gas-phase decomposition in CNT growth and highlights the potential of tuning gas-phase chemistry for controlled growth of CNTs.