Nanotechnology

Carbon nanofiber

Organic chemistry

Nucleation

Deposition (geology)

Carbon nanotube supported catalyst

Biology

Etching (microfabrication)

Engineering

Catalysis

Paleontology

Chemical physics

Chemistry

Materials science

Sediment

Layer (electronics)

Chemical vapor deposition

Chemical engineering

Carbon nanotube

The Journal of Physical Chemistry C

Recent experiments have shown that single-walled carbon nanotube (SWCNT) chirality and diameter can be modulated by adjusting the carbon/hydrogen chemical potentials at the catalyst surface and also by adding chemical etchants, such as ammonia, to the chemical vapor deposition (CVD) feedstock. Here, we present nonequilibrium quantum chemical molecular dynamics simulations showing how these factors control the SWCNT nucleation mechanism in different ways on Fe and Ni catalyst nanoparticles. Polygonal carbon rings form on Ni catalysts via the collapse of extended, partially saturated carbon chains that are adsorbed weakly to the catalyst surface, while SWCNT nucleation on Fe is strongly mediated by the catalyst surface. The weaker Ni–C interaction here means that the carbon desorption rate, and hence carbon chemical potential, on Ni is consistently lower than it is on Fe. Ni also activates adsorbed C–H and N–H bonds more effectively than Fe. Nevertheless, the hydrogen chemical potential is consistently lower on Ni because of the relative strengths of the Ni–H and Fe–H interactions. Consequently, carbon chain growth and SWCNT nucleation on Ni catalysts is considerably faster than it is on Fe. Finally, we show that ammonia's capacity to etch carbon during SWCNT nucleation and growth is much greater on Ni catalysts compared to that on Fe catalysts. In the absence of ammonia, Fe and Ni catalysts yield comparable SWCNT cap structures from methane CVD. Interestingly however, coadsorption of ammonia during methane CVD on Ni catalysts yields hexagon-rich SWCNT cap structures, whereas on Fe catalysts, it yields pentagon-rich cap structures. These results demonstrate that the capacity of a chemical etchant to influence SWCNT chirality during CVD is therefore also a function of the CVD catalyst itself.

Catalyst- and Etchant-Dependent Mechanisms of Single-Walled Carbon Nanotube Nucleation during Chemical Vapor Deposition