Iron Catalysts for the Growth of Carbon Nanofibers: Fe, Fe₃C or Both?

Abstract

Iron is a widely used catalyst for the growth of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) by catalytic chemical vapor deposition. However, both Fe and Fe−C compounds (generally, Fe3C) have been found to catalyze the growth of CNTs/CNFs, and a comparison study of their respective catalytic activities is still missing. Furthermore, the control of the crystal structure of iron-based catalysts, that is α-Fe or Fe3C, is still a challenge, which not only obscures our understanding of the growth mechanisms of CNTs/CNFs, but also complicates subsequent procedures, such as the removal of catalysts for better industrial applications. Here, we show a partial control of the phase of iron catalysts (α-Fe or Fe3C), obtained by varying the growth temperatures during the synthesis of carbon-based nanofibers/nanotubes in a plasma-enhanced chemical vapor deposition reactor. We also show that the structure of CNFs originating from Fe3C is bamboo-type, while that of CNFs originating from Fe is not. Moreover, we directly compare the growth rates of carbon-based nanofibers/nanotubes during the same experiments and find that CNFs/ CNTs grown by α-Fe nanoparticles are longer than CNFs grown from Fe3C nanoparticles. The influence of the type of catalyst on the growth of CNFs is analyzed and the corresponding possible growth mechanisms, based on the different phases of the catalysts, are discussed.

Temperature-dependent chemical state of the nickel catalyst for the growth of carbon nanofibers

“…The direct evidence for carbide-assisted growth was firstly given by in situ TEM observation that Fe 3 C was able to induce the growth of CNFs at the temperatures of 600 -700 o C. [15,16] After that, comparative ex situ TEM analyses also demonstrated that both Fe and Fe 3 C could induce the catalytic growth of CNFs. [17] M A N U S C R I P T…”

Catalyst faceting during graphene layer crystallization in the course of carbon nanofiber growth

“…The present growth experiments were carried out in situ in the transmission electron microscope (TEM), on catalyst particles around which a nanofiber had already developed during a previous direct-current plasma-enhanced chemical vapor deposition (dc-PECVD) growth in a home-made reactor [18]. In order to dispose of a source of carbon atoms inside the electron microscope, we either amorphized the existing nanotube by electron irradiation [19] or used the carbon present -or injected [14] -in iron carbide (Fe 3 C, cementite) catalysts.…”

“…The gas mixture we used for the first growth was made of isopropyl alcohol, water vapor and hydrogen; see details in Ref. [18]. We study iron-based catalysts because iron allows one to play with three types of catalysts: a-Fe, c-Fe and iron carbides [13,18].…”

“…[18]. We study iron-based catalysts because iron allows one to play with three types of catalysts: a-Fe, c-Fe and iron carbides [13,18]. Also, compared to nickel, it appears to favor in certain conditions the growth of real MWCNTs (instead of nanofibers), where the graphene layers are parallel to the tube axis [18,23].…”

“…They may travel to the surface schematized in Fig. 3d either following a transport path along the sidewalls of the particle (at the interface with the encapsulating graphene layers) as described by Begtrup [16], or by direct bulk diffusion [14], the carbon diffusion coefficient in a-Fe being very high at 650°C [18] (See schematic of C-diffusion in Supplemental material, Fig. S2).…”