Carbon nanotube formation and growth via particle-particle interaction

Carbon nanotubes are observed to form under a wide range of temperatures, pressures, reactive agents, and catalyst metals. In this paper we attempt to rationalize this body of observations reported in the literature in terms of fundamental processes driving nanotube formation. Many of the observed effects can be attributed to the interaction of three key processes: surface catalysis and deposition of carbon, diffusive transport of carbon, and precipitation effects. A new nanotube formation mechanism is proposed that describes the nanotube structures observed experimentally in a premixed flame and can account for certain shortcomings of the prevailing mechanism that has been repeatedly applied to explain nanotube formation in nonflame environments. The interacting particle model (IPM) attributes the initiation of nanotube growth to the physical interaction between catalyst particles. Coalescence of two (or more) catalyst particles leads to partial blocking of the particle surface, causing a disparity in carbon deposition over the particle surface. The resulting concentration gradient generates a net diffusive flux toward the interparticle contact point. Dimers that separate in this condition can support continuous nanotube growth between the particles. The model can also be extended to multiple particles to account for more complex morphologies. The IPM is consistent with many of the structures observed in the flame-produced material. The validity of the model is evaluated through analysis of diffusion dynamics and a force analysis of particle binding and separation. The IPM is also discussed in relation to identifying the requirements and best conditions to support nanotube growth in the premixed flame. The formation of nanotubes between particles as described by the IPM indicates that a single mechanism cannot completely describe nanotube synthesis; more likely, multiple pathways exist with varying rates that depend on specific process conditions.