(290b) Role and Stability of a Metal Carbide Catalyst in the Growth of Single-Walled Carbon Nanotubes
Achieving a high degree of selectivity in the synthesis of single-walled carbon nanotubes in a systematic and efficient way is one major barrier to be overcome before the exceptional properties of these materials can be exploited commercially. Many studies seem to indicate that the state and structure of the catalyst in CCVD play a fundamental role in determining structural features of the nanotubes such as chirality. Selective growth has been reported in both monometallic and carbide nanoparticles; however the contribution that either of them brings to this process has not been completely elucidated. The stability and the influence of the latter in the growing nanotube structure is the focus of this study. We determine the stability and relevant features of carbide nanoparticles and the interactions between carbide facets and the nascent carbon structure using ab initio molecular dynamics simulations. We use 55-metal-atom Ni and Co clusters to model the catalyst and run our simulations at three different temperatures to study the stability of the clusters. Nanotube caps of metallic and semiconducting nanotubes are placed in contact with the equilibrated nanoparticles to observe changes in the carbide due to the presence of the growing nanotube. Analyses of the electronic and geometric structure allow us to elucidate the nature of the carbide-nanotube interactions. We observe that the catalyst accommodates its overall shape to the nanotube cap according to an “inverse template effect” observed for unsupported nanoparticles. Although the original carbide structure is not completely maintained due to fluctuations in the structure of the solid, we observed that C atoms remained without forming bonds among themselves throughout the simulation time. There is also an apparent migration of the C atoms to the subsurface but no segregation is observed. We also performed simulations with surface slabs resembling different facets of the carbide catalyst placed in contact with a graphene fragment. Estimations of the work of adhesion of grahene and structural analysis reveal that the differences in the affinity of the graphene and the catalyst determine the mechanism of anchoring and liftoff of the nascent nanotube to the catalyst in accordance with electron microscopy observations.