The Growth Mechanism of Carbon Nanotubes

The growth mechanisms of carbon nanotubes vary depending on the preparative method applied. Some of them have not been fully elucidated to the present day. Yet a detailed knowledge of this mechanism is crucial for the directed synthesis of certain types of nanotubes to establish optimal conditions for a controlled growth. Some of the popular growth mechanisms and hypotheses concerning them are presented below. 

---

Tessonnier J-P, Su DS. Recent progress on the growth mechanism of carbon nanotubes A review. ChemSusChem. 2011 4(7) 824-847. 

Y.H. Mo, A.K.M.F. Kibria, and K.S. Nahm, The growth mechanism of carbon nanotubes from thermal cracking of acetylene over nickel catalyst supported on alumina, Synth. Met., 122, 443 47 (2001). 

The growth mechanism of carbon nanotubes from a metal catalyst is based on the solvation of carbon vapour into metal clusters. This is due to the ability of metals, such as Ni and Co, to dissolve carbon when liquid [111]. Quantum molecular dynamics (QMD) simulations... 

NiA"-SWNTs contain a mixture of catalyst Ni/Y (38 wt %) with an atomic ratio of 85/15. With this ratio, nanotubes in long bundles are obtained [110]. With an XPS analysis carried out on nanotubes deposited on a substrate covered by a thin gold film, it was possible to check the hypothesis of a segregation of yttrium on the periphery of the catalytic particle (Figure 3.55). Indeed, yttrium was observed only in the oxide form (Y 3d3 at 159 eV) while nickel was only in metallic form (Ni 2p3/2 at 853 eV). This confirms the growth mechanism of carbon nanotubes previously described. 

The direct linking of carbon nanotubes to graphite and the continuity in synthesis, structure and properties between carbon nanotubes and vapor grown carbon fibers is reviewed by the present leaders of this area, Professor M. Endo, H. Kroto, and co-workers. Further insight into the growth mechanism is presented in the article by Colbert and Smalley. New synthesis methods leading to enhanced production... 

As authors supposed from the generally accepted concepts of mechanisms of carbon nanotube growth, the dispersed nickel sputtered must catalyze the growth of these nanotubes. The source for carbon should be carbon from the hydrocarbon that transforms into the vaporous state in the arc zone. It has been supposed to prepare single-wall nanotubes on the nickel particles 1-10 nm in size and the layer of nanotubes up to 1 pm thick on the larger nickel particles. 

One of the first syntheses of carbon nanotubes succeeded by using an arc discharge. The mechanism of this growth type, however, turned out a complex system of single processes that can only be co-ordinated by controlling various parameters... 

The growth mechanism of carbonaceous nanostrucmral materials is generally thought to be via VLS. Cite some recent examples of carbon nanotube/nanofiber growth at temperatures far below the melting point of the nanoparticulate catalyst species. 

Whereas multi-wall carbon nanotubes require no catalyst for their growth, either by the laser vaporization or carbon arc methods, catalyst species are necessary for the growth of the single-wall nanotubes [156], while two different catalyst species seem to be needed to efficiently synthesize arrays of single wall carbon nanotubes by either the laser vaporization or arc methods. The detailed mechanisms responsible for the growth of carbon nanotubes are not yet well understood. Variations in the most probable diameter and the width of the diameter distribution is sensitively controlled by the composition of the catalyst, the growth temperature and other growth conditions. 

Other studies include the determination of the mechanism of growth of carbon nanotubes (Charlier et al., 1997 Bernholc et al., 1998) and the... 

Figure 3.43 Metalotalyzed growth on the open end of a nanotube according to the scooter mechanism (a) insertion of C2-units into carbon-metal bonds, (b) complexing of the metal atom by a cyclic polyyne.

In situ studies of the growth mechanism performed during the deposition of nanotubes from the gas phase revealed that carbon preferably attaches to steps on the crystal surface of the catalyst particles, thus initiating the formation of graphene layers. The continually changing shape of the metal particle also contributes to this process as it is generating new active centers all the time. 

Figure 3.44 Different growth mechanisms for the deposition of carbon nanotubes from the gas phase tip growth (the catalyst is moving upward) or bottom growth (the catalyst remains on the substrate).

Figure 1.6 Schematic representation of (a) the diffusion-precipitation mechanism of carbon filament growth from the gas phase [32], and (b) the carbon-fiber growth mechanism proposed by OberUn et al. [33]. Important details regarding the effects of metal particle size and shape on the chemical reactions occurring at the metal-carbon interface, and thus on the nature and size of the filaments or nanotubes produced, have yet to be sorted out.

---