Growth and Termination Dynamics of Multiwalled Carbon Nanotubes at Near Ambient Pressure

Advancement in nanotechnology has seen the rapid development and deployment of carbon nanotubes due to their excellent physical and chemical properties desirable for various industrial applications. This, however, requires more understanding of their growth and synthesis.

Among the available methods for the fabrication of carbon nanotubes, metal-catalyzed chemical vapor deposition is widely preferred due to its simplicity, controllability, and scalability. Consequently, the high performance of carbon nanotube-based devices depends on the diameter, length, and chirality of the used carbon nanotube. However, the limited atomic-scale information on the carbon nanotube growth mechanism poses a great challenge in achieving desirable controllability and understanding the impacts of external parameters such as temperature and pressure on the growth of carbon nanotubes.  Additionally, there is a need to determine the causes of growth termination of carbon nanotubes.

Recently, Fritz Haber Institute of Max Planck Society and ETH Zurich scientists: Dr. Xing Huang, Dr. Ramzi Farra, Prof. Robert Schlögl and Dr. Marc-Georg Willinger investigated the growth and termination dynamics of multiwalled carbon nanotubes at near ambient pressure, which is more relevant for most practical applications. Their experiments implemented the use of in situ transmission electron microscopy via iron catalysts in a gas mixture of hydrogen, methane and helium gases. The main objective was to demonstrate atomic-scale observations to enhance understanding of the carbon nanotube growth and termination. The work is currently published in the research journal, Nano Letters.

Continuous atomic-scale imaging was carried out to reveal the structure of the active catalyst and its dynamical behaviors. During the catalyzed growth, the catalyst which was identified as Fe₃C was constantly reshaped at its apex while at the base, it remained faceted with almost no morphological change. However, the carbon concentration in the catalyst particle fluctuated during the growth as indicated by the growth behavior and structural dynamics of the carbon nanotube.

From the atomic-scale observations, three different scenarios were observed for the growth termination of carbon nanotubes. First, the full coverage of the catalyst of carbon layers was observed to have the potential of deactivating the catalyst and stopping the growth of the carbon nanotube. Also, the catalyst underwent splitting before encapsulation resulting in significant shrinkage in the size. This further facilitates the carbon coverage and growth termination of the carbon nanotube. Secondly, Oswald ripening could initiate growth termination. It occurred through diffusion of catalytic particle from a growing tube into a nearby particle. Finally, the catalyst particle could diffuse from a growing carbon nanotube without combining it with other particles, giving rise to growth termination. The direct out-diffusion is attributed to the reduced adhesion strength between the carbon nanotube and the catalyst induced by the unbalanced carbon transportation in the catalyst.

The study by Dr. Xing Huang and colleagues provided detailed atomic-scale observations that may lead to better and more in-depth understanding of the growth and termination of multiwalled carbon nanotube at relevant conditions. Additionally, the study will provide the foundation for controlled synthesis of carbon nanotubes with improved controllability through optimization of both experimental conditions and catalysts design.