Mechanisms of single-walled carbon nanotube nucleation, growth, and healing determined using QM/MD methods

Efficient defect healing in catalytic carbon nanotube growth

The energetics of topological defects (TDs) in carbon nanotubes (CNTs) and their kinetic healing during the catalytic growth are explored theoretically. Our study indicates that, with the assistance of a metal catalyst, TDs formed during the addition of C atoms can be efficiently healed at the CNT-catalyst interface. Theoretically, a TD-free CNT wall with 10(8)-10(11) carbon atoms is achievable, and, as a consequence, the growth of perfect CNTs up to 0.1-100 cm long is possible since the linear density of a CNT is ∼100 carbon atoms per nanometer. In addition, the calculation shows that, among catalysts most often used, Fe has the highest efficiency for defect healing.

Dynamic evolution of supported metal nanocatalyst/carbon structure during single-walled carbon nanotube growth

Single-walled carbon nanotubes (SCWNTs) have outstanding properties that depend on structural features such as their chirality. Thus, developing a strategy to control chirality during SWCNT synthesis is critical for the exploitation of nanotube-based technologies in fields such as electronics and biomedicine. In response to this need, tuning the nanocatalyst structure has been envisioned as a means to control the nanotube structure. We use reactive classical molecular dynamics to simulate nanotube growth on supported Ni(32), Ni(80), and Ni(160) nanoparticles at various metal/support interaction strengths (E(adh)). The initial carbon ring formation is shown to correlate to the nanoparticle surface structure, demonstrating the existence of a "template effect" through a dominant occupation of hollow sites. The E(adh) strength alters the dynamic/structural behavior of the nanoparticle, in turn influencing the interplay between nanotube and nanoparticle structures. For example, the contact region between the nanoparticle surface and the growing nanotube decreases as E(adh) increases because capillary forces that raise the metal into the nanotube are counteracted by the strong metal/support interaction. The nanoparticle mobility decreases as E(adh) increases, eliminating a possible inverse template effect but hindering defect annealing in detriment of the nanotube/nanoparticle structural correlation. On the other hand, the contact between the nanoparticle and the nanotube increases with nanoparticle size. However, the heterogeneity of the nanoparticle structure increases with size, reducing the structural correlation. These results suggest that an appropriate combination of nanoparticle size and strength of the catalyst/support interaction may enhance the desired template effect and bias formation of specific nanotube chiralities.

Determination of local chirality in irregular single-walled carbon nanotubes based on individual hexagons

We develop a robust theoretical method for the determination of local chirality based on individual hexagons that compose single-walled carbon nanotubes (SWCNTs). The method and code are applied to various SWCNT-like irregular structures that are formed during the growth process. The local chiral index and its distribution are well defined for irregular structures as well as ideal structures of SWCNTs, and can be used to characterize any SWCNT-like irregular structures in terms of local chirality. The present method also permits monitoring the chirality of nanostructures during the growth process.

Threshold barrier of carbon nanotube growth

A previously overlooked step of carbon nanotube (CNT) growth, incorporating C atoms into the CNT wall through the CNT-catalyst interface, is studied by density functional theory calculations. A significant barrier for incorporating C atoms into the CNT wall (∼2  eV for most used catalysts, Fe, Co, and Ni) is revealed and the incorporation can be the threshold step of CNT growth in most experiments. In addition, the temperature dependent CNT growth rate is calculated and our calculation demonstrates that growing 0.1-1 m long CNTs in 1 h is theoretically possible.

Changing chirality during single-walled carbon nanotube growth: a reactive molecular dynamics/Monte Carlo study

The growth mechanism and chirality formation of a single-walled carbon nanotube (SWNT) on a surface-bound nickel nanocluster are investigated by hybrid reactive molecular dynamics/force-biased Monte Carlo simulations. The validity of the interatomic potential used, the so-called ReaxFF potential, for simulating catalytic SWNT growth is demonstrated. The SWNT growth process was found to be in agreement with previous studies and observed to proceed through a number of distinct steps, viz., the dissolution of carbon in the metallic particle, the surface segregation of carbon with the formation of aggregated carbon clusters on the surface, the formation of graphitic islands that grow into SWNT caps, and finally continued growth of the SWNT. Moreover, it is clearly illustrated in the present study that during the growth process, the carbon network is continuously restructured by a metal-mediated process, thereby healing many topological defects. It is also found that a cap can nucleate and disappear again, which was not observed in previous simulations. Encapsulation of the nanoparticle is observed to be prevented by the carbon network migrating as a whole over the cluster surface. Finally, for the first time, the chirality of the growing SWNT cap is observed to change from (11,0) over (9,3) to (7,7). It is demonstrated that this change in chirality is due to the metal-mediated restructuring process.

Recent progress on the growth mechanism of carbon nanotubes: a review

Tremendous progress has been achieved during the past 20 years on not only improving the yields of carbon nanotubes and move progressively towards their mass production, but also on gaining a profound fundamental understanding of the nucleation and the growth processes. Parameters that influence the yield but also the quality (e.g., microstructure, homogeneity within a batch) are better understood. The influence of the carbon precursor, the reaction conditions, the presence of a catalyst, the chemical and physical status of the latter, and other factors have been extensively studied. The purpose of the present Review is not to list all the experiments reported in the literature, but rather to identify trends and provide a comprehensive summary on the role of selected parameters. The role of the catalyst occupies a central place in this Review as a careful control of the metal particle size, particle dispersion on the support, the metastable phase formed under reaction conditions, its possible reconstruction, and faceting strongly influence the diameter of the carbon nanotubes, their structure (number of walls, graphene sheet orientation, chirality), their alignment, and the yield. The identified trends will be compared with recent observations on the growth of graphene. Recent results on metal-free catalysts will be analyzed from a different perspective.