Controllable growth transition from single-walled to double-walled carbon nanotubes using sulfur in an aerosol CVD reactor

Although double-walled carbon nanotubes (DWCNTs) are valued for their electrical conductivity, mechanical strength, and thermal stability, understanding their growth mechanism is still evolving, with ongoing research efforts providing valuable insights. Sulfur plays a crucial role as a growth promoter. This research explores sulfur's effect using a floating catalyst chemical vapor deposition (FC-CVD) system with methane as carbon source and ferrocene as catalyst. Optimized sulfur concentration significantly improves CNT film quality with a sheet resistance of 61.4 Ω/sq at 90 % transmittance after being doped with AuCl 3 , along with a linear increase in yield as sulfur concentration increases. The proposed mechanism suggests that increased sulfur levels result in a growth transition from single-walled CNTs to double-walled CNTs by activating medium-sized catalyst particles, all while not altering the size distribution. The highest DWCNT proportion observed is 87 %, confirmed by high-resolution transmission electron microscopy. Additionally, a change in the catalyst particle size distribution may shift the growth window, resulting in the formation of multi-walled CNTs. Electron diffraction patterns reveal a random chirality and chiral angle distribution of CNTs, indicating minimal sulfur impact on the nanotube atomic structure. These findings underscore sulfur's importance in CNT synthesis and provide new insights into its mechanisms, potentially guiding future advancements in the controlled production of DWCNTs. • Optimized sulfur concentration enhances CNT quality and improves film conductivity. • High sulfur levels induce a growth transition from SWCNTs to DWCNTs. • Sulfur effects activation of catalyst particles without altering size distribution. • ED pattern analysis indicates sulfur doesn't affect chiral-selective growth of CNT.