Effects of Water Vapor on Diameter Distribution of SWNTs Grown over Fe/MgO-Based Catalysts

With an interest in the H₂O-assisted chemical vapor deposition (CVD) process, roles of H2O in tuning the diameter and chirality distributions of single-walled carbon nanotubes (SWNTs) were studied using Fe/MgObased catalysts. A controlled amount of H₂O was found to narrow the diameter distribution of SWNTs by reducing the populations of small- and large-diameter nanotubes. The addition timing of H₂O vapor was investigated to understand the interaction between H₂O and growing nanotubes or catalyst nanoparticles. Also, the addition of a small amount of Mo in the Fe/MgO catalyst was observed to reduce the diameter distribution of SWNTs, and H₂O further promoted the formation of smaller-diameter SWNTs over the Fe-Mo/ MgO catalyst. These results elucidate that the surface chemistry of catalyst nanoparticles exerted significant effects on the behaviors of H₂O in the CVD process. Consequently, it is proposed that H ₂O first reacts with the surface of catalyst nanoparticle, and successively plays two aspects of roles, inhibiting the agglomeration of nanoparticles and preferential etching to small-diameter nanotubes, which determine the net diameter distribution of as-grown SWNTs.