Analysis of Fe and Co binary catalysts in chemical vapor deposition growth of single-walled carbon nanotubes

Abstract: Metal catalysts play a pivotal role in the growth of single-walled carbon nanotubes (SWCNTs), with binary metallic catalysts emerging as an efficient SWCNT synthesis strategy. Among these, iron (Fe), cobalt (Co), and their alloys are particularly effective. However, prior studies have predominantly employed Fe--Co alloy catalysts with fixed atomic ratios as well as unchanged chemical vapor deposition (CVD) conditions, leaving the influence of variable Fe--Co compositions and CVD growth parameters on SWCNT synthesis poorly understood. This study focuses on the role of Fe--Co catalyst ratios, with the aim of elucidating the distinct contributions of Fe and Co atoms in the growth of SWCNTs. By systematically exploring a wide range of Fe--Co ratios and growth conditions, we identified Fe${0.75}$Co${0.25}$ as a highly efficient binary catalyst at 850~$^\circ$C, primarily forming catalyst clusters with diameters of 2.5--6~nm and yielding SWCNTs with diameters ranging from 0.9--1.1~nm. On the other hand, Fe${0}$Co${1}$ exhibited higher catalytic activity at 600~$^\circ$C, generating smaller catalyst clusters of 1.5--5~nm and producing SWCNTs with reduced diameters of about 0.6--0.9~nm. Transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS) analyses reveal that high SWCNT yields correlate with the formation of uniformly sized Fe--Co catalyst particles with surface-segregated Co that optimizes carbon solubility. Molecular dynamics (MD) simulations further corroborate these findings, demonstrating that the structure and melting behavior of Fe$x$Co${1-x}$ clusters depend on cluster size and composition.