How oxygen influences the catalytic activity of iron during carbon nanotube nucleation

The catalytic activity of metal nanoparticles toward single-walled carbon nanotube (SWCNT) nucleation is fundamental to achieving structure-controlled growth using catalytic chemical vapor deposition. Despite the success of oxidized catalysts in SWCNT growth, there is a lack of understanding regarding how oxygen influences the catalyst and the process of nucleation. Density functional tight binding molecular dynamics simulations demonstrate that the kinetics of carbon nucleation on an iron nanoparticle catalyst can be tuned via oxygen loading. Increasing the oxygen content in the catalyst leads to activation of surface-bound carbon species and enhanced carbon chain growth due to respective weakening and strengthening of the C–C and Fe–C bonding. This is due to oxygen modulating the electronic structure of the iron catalyst, with the Fermi level of the catalyst increasing proportionally with oxygen content until the iron:oxygen stoichiometry reaches parity. The increase in Fe 3d states near the Fermi level promotes the donation of electron density into unoccupied C 2p states. This activates C–C bonds to facilitate carbon chain growth while slowing carbon ring condensation.