Free energy surface of initial cap formation in carbon nanotube growth.

Satoru Fukuhara,Yasushi Shibuta

doi:10.1039/d1na00377a

Abstract

Initial cap formation is an important process of carbon nanotube (CNT) growth where a hexagonal carbon network is lifted off from the catalyst surface. In this study, free energy surface (FES) of initial cap formation in the CNT growth is investigated by metadynamics simulation. A two-dimensional collective variable (CV) space is newly developed to examine the complicated formation process of the cap structure, which consists of the formation of a hexagonal carbon network and lift-off of the network from the catalyst surface. States before and after the lift-off of the carbon network are clearly distinguished in the two-dimensional FES. Therefore, free energy difference before and after the lift-off can be directly derived from the two-dimensional FES. It was revealed that the cap structure is stable at a high temperature due to the entropy effect, while the carbon network covering the catalyst surface is energetically stable. The new insight in this study is achieved owing to metadynamics simulation in conjunction with a newly developed two-dimensional CV space since it is impossible to explore FES for such complicated processes in the framework of conventional molecular dynamics simulation.

Highlights

Carbon nanotubes (CNTs)[1] can potentially be used in a variety of applications.[2]
We previously developed a collective variable (CV) based on the coordination number of carbon atoms to discuss a part of the initial formation process of CNTs and we sampled the potential energy surface of carbon segregation from a nickel nanoparticle by metadynamics simulation.[18]
Initial cap formation process of the CNT growth is investigated by metadynamics simulation on a two-dimensional CV space

Summary

Introduction

Carbon nanotubes (CNTs)[1] can potentially be used in a variety of applications.[2]. It is essential to control the diameter and chirality of CNTs during the synthesis for better application. The catalytic chemical vapor deposition (CCVD) method[2] is widely used to synthesize CNTs, in which CNTs grow from dispersed nanoparticles.[3] Diameter and chirality of CNTs are strongly correlated with the properties of catalytic nanoparticles at the initial stage of the synthesis process. It is important to understand the role of catalytic nanoparticles at the initial stage of CNT growth. In conjunction with in situ transmission electron microscopy (TEM) observations,[10,11] the initial growth process of CNTs by CCVD is considered as follows:[12] aggregation of carbon atoms in/on the catalytic nanoparticle, formation of the hexagonal carbon network on the surface of catalytic nanoparticles and the li -off of the carbon network to form a cap structure

Methods

Results

Conclusion