Linking growth mode to lengths of single-walled carbon nanotubes

Maoshuai He,Yann Magnin,Hakim Amara,Hua Jiang,Hongzhi Cui,Frédéric Fossard,Alice Castan,Esko Kauppinen,Annick Loiseau,Christophe Bichara

doi:10.1016/j.carbon.2016.11.057

Abstract

Elucidating the key factors that determine the lengths of single-walled carbon nanotubes (SWCNTs) is of great importance for understanding the origin of chiral selectivity. We use transmission electron microscopy to thoroughly investigate as-grown SWCNTs. The lengths and growth modes of SWCNTs were decided by bright-field imaging. Their respective chiral angles were calculated on the basis of nanobeam diffraction patterns. Systematic investigations reveal that there is no correlation between the SWCNT length and its chiral angle. Instead, it shows that SWCNT lengths depend more on their growth mode, i.e. the link between SWCNT and its seeding catalyst particle. Atomistic computer simulations demonstrate that low carbon fractions in the catalyst lead to so-called tangential growth, with a partial wetting of the metal in the tube, where metal catalyst tends to be deactivated by graphite layer encapsulation and results in short SWCNTs. In contrast, a high carbon concentration inside metal particle favors perpendicular growth modes, where only the tube lip is in contact with the catalyst. Catalysts adopting perpendicular mode could have a longer lifetime, thus catalyze the growth of long SWCNTs. Finally, the carbon concentration related growth mode was applied to interpret diverse SWCNT growth results.

Highlights

Searching for chiral selectivity during the chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNTs) is a much coveted goal that has been partially reached in a number of experiments [1,2,3,4,5,6]
In order to clarify possible factors that affect SWCNT lengths, a prominent screw-dislocation model has been proposed [7], with the main result that the nanotube growth rate should be proportional to its chiral angle
The tube length and chiral angle were determined, with the result that we found no correlation between the SWCNT length and its chiral angle

Summary

Introduction

Searching for chiral selectivity during the chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNTs) is a much coveted goal that has been partially reached in a number of experiments [1,2,3,4,5,6]. In order to clarify possible factors that affect SWCNT lengths, a prominent screw-dislocation model has been proposed [7], with the main result that the nanotube growth rate should be proportional to its chiral angle. This model was extended by further taking the nanotube/catalyst interface thermodynamics into account [8]. Based on this approach, SWCNT growth rate and possibly length should be correlated with its chiral angle. Computer simulation was applied to show that catalyst particles growing SWCNTs by a tangential mode, tend to be deactivated by fully wetting the inner part of the tube, accounting for the shorter SWCNTs obtained in these CVD conditions

Preparation of Fe catalyst

Growth of carbon nanotubes by CVD

Characterizations of carbon nanotubes and catalyst particles by TEM

Findings

Calculations of interactions between SWCNTs and metal nanoparticles