All-Electron DFT Modeling of SWCNT Growth Initiation by Iron Catalyst

Electronic and geometrical structures of Fe4Cn(CO)m (n+m≤6) and Fe4Cn (n=7—16) clusters along with their singly negatively and positively charged ions are computed using density functional theory with generalized gradient approximation (DFT-GGA). Isomers with CO bonded directly to the iron atoms and bonded to a carbon atom chemisorbed on the cluster surface are optimized for the Fe4C2CO, Fe4C2(CO)2, Fe4C3CO, and Fe4C4CO series. The computed total energies are used to estimate the energetics of the Boudouard disproportionation reactions Fe4Cn(CO)m + CO → Fe4Cn+1(CO)m-1 + CO2. Optimizations of the Fe4C4–Fe4C16 clusters have shown that dimers C2 are formed in the lowest energy states of Fe4C4,, trimers C3 – in Fe4C5 and Fe4C6, tetramers C4 –in Fe4C7 and Fe4C8, a pentamer C5 – in Fe4C9, and a hexamer C6 – in Fe4C10. Cn rings attached to a Fe3 face are formed in the lowest energy states of Fe4Cn beginning with n=11.