Etchant-induced shaping of nanoparticle catalysts during chemical vapour growth of carbon nanofibres

Carbon nanofibres (CNFs) obtained by plasma-enhanced chemical vapour deposition are made of cone-shaped graphene layers, the opening angle of which has a significant influence on their properties: the smaller the angle, the closer the properties to those of carbon nanotubes. That angle is determined by the shape of the metal nanoparticle used to catalyse the growth. We show in this paper that the shape of Ni nanoparticle catalysts, and in turn the CNF properties, can be tuned during plasma-enhanced chemical vapour deposition, by the choice of the etchant gas. We show in particular that a water-containing etchant (H2O or H2O + H2) increases the growth rate by an order of magnitude at 600 °C compared to an ammonia-containing etchant (NH3 or NH3 + H2), and leaves more elongated Ni particles with a cone angle three times smaller. We conclude that the cone angle and the growth rate are directly related, and propose a mechanism to explain that large difference between the two etchants

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► From carbon nanofibres toward carbon nanotubes using PECVD water based process.
► Water increase of PE-CVD growth velocity of C-nanofibres.
► Water keeps catalyst surface free of C-deposits during PE-CVD growth.
► Ammonia leaves catalyst top surface with protecting graphene layers after PE-CVD growth.