Formation mechanism of CNT and related material

Abstract

Carbon nanotube (CNT), a cylinder rolled from graphene, has many potential applications in composite materials, sensors, electronics, energy conversion and storage, etc., due to its intrigue properties, such as superb strength, large surface area, chirality dependence of the electronic properties. To achieve the full employment of these properties, controlled synthesis of CNTs is highly desired. In CNT synthesis, the precise control of the structure, or chirality-selective CNT growth, has long been a daunting problem. The failure of chirality control is attributed mainly to the lack of understanding of the CNT formation mechanism. In this study, we present theoretical explorations on formation mechanisms on two CNT syntheses: i) peapod-derived double-walled carbon nanotube (DWCNTs) and ii) catalytic growth of single-walled carbon nanotubes (SWCNTs). i) An energy driven kinetic Monte Carlo (EDKMC) method was developed for the atomic simulation of peapod-derived DWCNTs. The EDKMC method was used to explore the formation process of the inner tubes, such as the coalescence of fullerenes, the fusion of fullerene with short tube and the fusion of two inner tubes of different lengths. During the research, two new findings emerged: (a) the abundance of large chiral angle inner tubes and (b) the formation of intra-tube junctions. The finding (a) indicates that the chirality control in peapod-derived DWCNTs is possible and finding (b) shows that inner tubes with large concentration of intra-tube junctions can be synthesized in such a process for many applications. ii) To perform the theoretic study of the catalytic growth of SWCNT, an atomic simulation of defect-free SWCNT is essential. In the history, the simulation of defect-free SWCNTs has been a great challenge for more than 10 years, mainly due to the lack of a precise potential energy surface (PES) of carbon-catalyst system and the very limited time scale in atomic simulation (i.e., from ps to ns). To overcome the first problem, a new generation of PES for carbon-nickel system is developed. To overcome the second problem, a hybridized molecular dynamics and energy driven kinetic Monte Carlo method (MD-EDKMC) atomic simulation strategy was proposed. With these improvements, we were able to simulate defect-free SWCNTs for the first time. Beyond, deep insights into the growth mechanism can be achieved. Particularly, the even chirality distribution of the simulated SWCNTs with liquid nickel catalyst showed good consistency with most of the experiments. Based on our study, two means of achieving chirality control, 1) by using solid catalyst particles and 2) by changing CNT's chirality during the growth, were proposed.