First-principles based kinetic modeling of effect of hydrogen on growth of carbon nanotubes

A study of the key parameters, including the crucial role of H2 for uniform graphene growth on Ni foil

“…We have chosen to use ambient pressure conditions, as they are technologically more attractive, and to feed methane, to not complicate the interpretation of the results. In fact, for the understanding of the graphene formation mechanism it is a necessity to give attention to the number of gas phase transformations of the precursor molecules [39] before they reach the catalyst surface: this parameter can lead to a very different gas composition with respect to the reactor inlet gas. A reaction temperature of 950 • C has been chosen as the best compromise between economic implications and the need to decompose the "stable" methane molecules, also resulting in high quality materials.…”

Insights into carbon nanotube growth using an automated gravimetric apparatus

“…The exact mechanism of deactivation is also unknown. It may be amorphous carbon formation [77,81], a cementite (Fe 3 C) to Hä gg carbide (Fe 5 C 2 ) transition [49], dangling bond growth site loses [42], catalyst particle morphology changes [97], adjustment of deposition control mechanism [26] (surface, adsorption, diffusion), declining CNT population density [107], or gas diffusion limitations [108]. The transition between sustained near linear growth and premature deactivation could be determined by weight measurements.…”

“…H 2 concentrations at these levels are not required for catalyst reduction, even accounting for gas flow considerations, so the benefits are typically explained by one of two mechanisms: (i) H 2 can increase yields by gasifying amorphous carbon deposits to retain catalytic activity [26,119]; however, at high H 2 concentrations and/or temperatures, this gasification process can become sufficiently aggressive to remove graphitised products and reduce CNTyields [78,81,109,123]; or (ii) H 2 stabilises the growing structure by saturating dangling bonds that would otherwise lead to closure and cessation of growth [42,97,102,119,124,125]. These may occur concurrently with H 2 effects on deposited carbon morphology [109,126,127], by catalyst structural modification [128,129], changes in surface bonding and chemistry [22], and altering carbon/metal diffusion kinetics [57,81].…”

“…The chemical system containing 227 species and 827 reactions that was developed and experimentally validated by Norinaga et al [24,25] for light hydrocarbon pyrolysis under CVD conditions was used rather than the plasma-or oxidation-derived mechanisms currently used in the CNT literature [23,[26][27][28][29][30][31][32][33][34]. Surface reactions were not considered because they are primarily a function of catalyst rather than apparatus.…”

From Nanotechnology to Nanoengineering

“…Since different hydrocarbons are characterized by different sticking coefficients and decomposition energies, the carbon supply rate onto the catalyst surface can be changed (Lebedeva et al 2011). It is generally accepted that the growth is more important to control and predetermine the CNT structure.…”