High-yield growth of carbon nanotubes on composite Fe/Si/O nanoparticle catalysts: A car-parrinello molecular dynamics and experimental study

Single-walled carbon nanotubes (SWCNTs) have recently been synthesized at high catalyst yield (∼90%) using a composite iron/silicon-oxide nanoparticle catalyst in a gas-phase diffusion flame environment. Since catalyst yields without silicon are less than 10%, the role of silicon in improving catalyst yield must be studied to understand the molecular-scale factors that govern carbon nanotube nucleation and growth. In this work, Car-Parrinello molecular dynamics simulations are employed to investigate the structure of Fe/Si and Fe/Si/O nanoparticle catalysts at synthesis temperatures (1300 K). The simulations show that silicon is uniformly dispersed on the iron surface when oxygen is not present, but covers only one hemisphere of the particle surface when oxygen is present to form a silica "cap". These results are consistent with the results of substrate synthesis and the phase diagram of this Fe/Si/O system. The structure of the catalyst particle when oxygen and silicon are present thus facilitates the preferential decomposition of a carbon precursor on the Fe-rich side of the particle. On the basis of this finding, SWCNTs will nucleate preferentially on Fe/Si/O with segregated phases compared to catalyst particles with a uniform surface composition that typically become encapsulated in carbon before nucleation can occur. High catalyst yields are also demonstrated on Fe/Al/O catalysts, which indicate that high yields are not specific to the presence of silicon in the particle. The results of this study support the hypothesis that the addition of silicon or aluminum, in the presence of oxygen, to iron oxide-based catalysts results in a nonuniform surface composition that facilitates SWCNT nucleation.