Heterogeneous catalysis model of growth mechanisms of carbon nanotubes, graphene and silicon nanowires

The catalytic chemical vapour deposition of carbon nanotubes on transition metal catalysts from hydrocarbons is analysed in terms of concepts of heterogeneous catalysis, which leads to a 'volcano plot' of the catalyst efficiency vs. the metal d orbital energy. In this plot, noble metals are inefficient because they dissociate the feedstock poorly, the early transition metals are inefficient because their surface sites become blocked by absorbed carbon and they do not release the nanotube, while the optimum catalyst dissociates the feedstock readily and releases the nanotube. The model is extended to explain why graphene CVD works best with non-optimum catalysts. For silicon nanowires, the rather high stability of metal silicides limits the nanowire growth rates and may explain why there are few alternatives to gold as a catalyst.