Reactive Sites for Chiral Selective Growth of Single-Walled Carbon Nanotubes: A DFT Study of Ni₅₅–C_n Complexes

Abstract

The physical and electronic properties of single-walled carbon nanotubes (SWCNTs) are determined by their chirality. The chirality selection mechanism in SWCNT growth is not fully understood. In this study, the interaction between near-armchair (n,5), where n = 6, 7, 8, and 9, zigzag (9,0), and armchair (5,5) nanotubes and a fully relaxed Ni(55) metal cluster during the early stage of growth is studied by density functional theory calculations. We found that kink sites at the end edge of (n,5) nanotubes are more reactive than other sites based on the charge transfer analysis at the Ni-C interface. The frontier orbitals of the (6,5) and (7,5) caps are localized on their kink-step sites, which stretch outward from the carbon cap surface, having typical 2p(z) orbital feature of carbon atom with high reactivity. Such favorable frontier orbital spatial orientation and location is ideal to incorporate more carbon species. These reactive sites may lead to the faster growth rate, resulting in the chirality selectivity toward the (6,5) and (7,5) nanotubes. In contrast, the frontier orbitals of (8,5) and (9,5) caps spread over the entire carbon cap surface. Adding carbon species at these sites may lead to the chirality change or formation of other carbon structures. Our results showed that the spatial distribution and orientation of frontier orbitals is useful in explaining the chiral selectivity. Engineering catalyst clusters to control these reactive sites has high potential to further improve chirality control in SWCNT synthesis.

Atomic-scale mechanisms of plasma-assisted elimination of nascent base-grown carbon nanotubes

“…The clustering of hydrogen on the tube walls leads to sp 3 C atoms which have somewhat longer bond lengths than the sp 2 carbon atoms, thus leading to a local amorphization [24]. In agreement with DFT results, these amorphous sites are found in kink-steps of a chiral tube endedge, as well as on the cap [48]. In contrast, in achiral (armchair and zigzag) SWNTs, the circumference of the tube end-edge is smoother than that of chiral tubes.…”

“…Therefore, the clustertube interface is less reactive in the case of achiral tubes than their pentagon-containing cap region. Moreover, the tube end-edge is less stressed than the cap end-edges [48] and consequently H adatoms can be found on the entire tube surface, although their concentration is elevated in the cap region. The etching onset is characterized by the eventual breaking of the more elongated HC-CH ortho bonds, which is also considered as the rate-limiting step of the hydrogenation/etching process [29].…”

“…This effect leads to a local amorphization of the cap carbon network where H-adatoms coalesce. The H-adatoms on the highly curved tip of the cap [46] and stressed rim of the cap [48] can subsequently change the ring distribution in the carbon cap. Indeed, we observe that the pentagon rings gradually disappear due to a pentagon (R5) -to -hexagon (R6) conversion (i.e., C-R5 to R6 with a barrier of 3.2 eV in DFT calculations [49]).…”

Chiral-Selective Carbon Nanotube Etching with Ammonia: A Quantum Chemical Investigation

“…[28][29] Hence, the most likely stage during growth for a SWCNT to be etched and destroyed is immediately after, or during, nucleation. A number of previous theoretical investigations have considered the reactivity of SWCNT cap edges, both with 19,[30][31] and without [32][33][34] catalyst nanoparticles. However, we know of no study concerning the reactivity between a SWCNT cap and nitrogenous radicals.…”

Chiral‐selective etching effects on carbon nanotube growth at edge carbon atoms

“…In addition, we compare the OH etching effects with the H radical case . Our density functional theory (DFT) calculations demonstrate the chirality‐dependence of the reactivity on the edge of SWCNT caps . Furthermore, we document a consistency in the edge reactivities for both etchant as well as growth‐mediating species, such as C 2 H .…”