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

Abstract

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)/AlOx (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 AlOx layer. In contrast, Fe is reduced sufficiently in H2 in the absence of CO2, thereby remaining on the AlOx 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.