Carbon nanotube growth catalyzed by metal nanoparticles formed via the seed effect of metal clusters†

Abstract

Carbon nanotubes (CNTs) are useful nanomaterials owing to their distinct functions that depend on their structure and diameter; therefore, CNTs have recently attracted much attention. Catalytic chemical vapor deposition using metal nanoparticles as catalysts is one of the most useful methods for synthesizing CNTs. However, fine control of the CNT diameter has been technically difficult at the 1 nanometer level owing to the size distribution of metal nanoparticles and the difficulty in suppressing their aggregation during the CNT growth reaction. In this study, we developed a method for simultaneously conducting nanoparticle formation and CNT growth through the in situ preparation of nanoparticles using highly dispersed metal clusters on a support substrate as seeds. In this method, the migration and aggregation behavior of seed metal clusters on a substrate upon heating uniformly induces nanoparticle formation until nanoparticles are enlarged to suitable size for CNT growth, and then CNTs grow with a diameter corresponding to that of the nanoparticles. The nanoparticle size does not change afterward by spatial separation of the nanoparticles from the substrate due to the tip-growth mechanism of CNTs. Detailed analysis focusing on cobalt as a catalyst demonstrated that the diameters of the multi-walled CNTs could be simply tuned solely by modifying the reaction temperature and time. This result reveals that the synthetic concept using the seed clusters enables the control of the CNT size to a certain extent using only temperature control (thermal energy), without conventional separation processes for synthesizing nanoparticles and determining conditions to retain nanoparticle size.