Influence of Ethylene Thermal Decomposition on Carbon Nanotube Growth: Insights from a Two-Zone Reactor Study

Gas-phase decomposition of 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. A standard synthesis condition was first established using Bayesian Optimization, with the CNT growth zone temperature (Tg) fixed at 700 °C. CNTs were then synthesized at preheating zone temperature (Tp) of 500, 600, 700, 800, and 900 °C, while maintaining Tg at 700 °C under the standard condition. Carbon yield and IG/ID 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 change in the gas-phase composition was observed at above 700 °C, correlating with changes in carbon yield and crystallinity. Based on these results, we propose a mechanism by which differences in gas chemistry lead to changes in the carbon yield and the IG/ID 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.