Why Carbon Nanotubes Grow

Abstract

Despite three decades of intense research efforts, the most fundamental question “why do carbon nanotubes grow?” remains unanswered. In fact, carbon nanotubes (CNTs) should not grow since the encapsulation of a catalyst with graphitic carbon is energetically more favorable than CNT growth in every aspect. Here, we answer this question using a theoretical model based on extensive first-principles and molecular dynamics calculations. We reveal a historically overlooked yet fundamental aspect of the CNT-catalyst interface, viz., that the interfacial energy of the CNT–catalyst edge is contact angle-dependent. The contact angle increases via graphitic cap lift-off, drastically decreasing the interfacial formation energy by up to 6–9 eV/nm, overcoming van der Waals cap-catalyst adhesion, and driving CNT growth. Mapping this remarkable and simple interplay allows us to understand, for the first time, why CNTs grow.

The promoter role of sulfur in carbon nanotube growth

“…Although the binding energy of graphene on several different transition metals has been computed, 66 the fcc iron (111) surface is only considered recently. 56 Here we investigated three different configurations of graphene on the fcc (111) surface (Fig. 1); in two of them every second carbon atom resides on top of the Fe surface (top-fcc hollow site and tophcp hollow site) while in the third all carbon atoms are located above the hollows of the surface (fcc-hcp hollow site).…”

“…Therefore, the reduction in the adhesion of the carbon cap can favour tube formation in CNT synthesis. 56…”

“…55 Recent computations show that the key factor in the two competitive processes (cap lift-off and NP deactivation) is the interfacial energy of the CNT-catalyst interface and its contact angle dependency. 56 Sulfur has been found to be an indispensable co-catalyst to avoid the catalyst deactivation in FCCVD, which also affects the morphology, 57,58 wall number 16 and even the conductivity of the synthesized CNTs. 26 Interestingly, sulfur is also known to be beneficial for the supported catalyst CVD of CNTs.…”

“…55 Recent computations show that the key factor in the two competitive processes (cap lift-off and NP deactivation) is the interfacial energy of the CNT–catalyst interface and its contact angle dependency. 56…”

Direct photoconversion of CO2 to carbon nanotubes for oxygen recycling in extraterrestrial exploration

“…Herein, we propose a photochemical Bosch process to recover oxygen from CO2, which occurs under mild conditions and accomplishes C formation in one step (Scheme 1). As a low-cost transition-metal oxide, spinel Co3O4 has been widely applied in thermo-and electro-catalysis 15,16,17 , for example catalyzing the growth of CNTs 18,19 .…”