Enhanced CO₂-assisted growth of single-wall carbon nanotube arrays using Fe/AlO_x catalyst annealed without CO₂

Controlling catalyst-particle formation is essential for the growth of single-wall carbon nanotube (SWCNT) arrays with improved alignment, areal mass, and height. We have previously reported the positive effect of CO₂ on SWCNT growth via chemical vapor deposition, and in this study, we found its negative effect on catalyst-particle formation during annealing. A Fe (1 nm)/AlO_x (15 nm) catalyst that was sputter-deposited on SiO₂/Si substrates demonstrated a prolonged lifetime and enabled the growth of SWCNT arrays with better alignment, twice the height, and three times higher areal mass when the catalyst was annealed under 10 vol% H₂/Ar without CO₂ than with 1 vol% CO₂. Detailed analysis indicated that the Fe particles could remain partially oxidized during annealing in H₂ with mildly oxidative CO₂, resulting in the bulk diffusion of Fe into the AlO_x layer. In contrast, Fe is reduced sufficiently in H₂ in the absence of CO₂, thereby remaining on the AlO_x surface and active for SWCNT growth. The findings of this study emphasize the importance of maintaining a highly reductive atmosphere during annealing to achieve active catalyst particles with a higher number density and longer lifetime.