Trends in the Adsorption of 3d Transition Metal Atoms onto Graphene and Nanotube Surfaces: A DFT Study and Molecular Orbital Analysis
The functionalization of graphene and (8,0) single-walled carbon nanotubes (SWCNTs) with individual 3d transition metal (TM) atoms was modeled using density functional theory (DFT) calculations. The structural geometry, magnetism, and binding energies were analyzed in terms of the density of states (DOS), Bader charges, and organometallic M(η6-C6H6) orbital molecular models. Trends in the binding energies were explained by a model, which included several contributions from the chemisorbed atoms: Coulomb interaction, 3dn4sx → 3dn+x electronic promotion energy (EPE), and occupation of the 1e2(δ), 2e1(π), and 2a1(σ) metal orbitals. 4s occupation, which causes Pauli repulsion, explained the physisorption trends of Cr, Mn, and Cu. The model was successfully extrapolated to a convex surface, such as that of (8,0) SWCNTs. The potential energy surfaces for the adatoms adsorbed on graphene were determined to evaluate the diffusion energy barriers. We found that Sc, Ti, Fe, and Co metals could be isolated on the graphene surface, whereas all other 3d TM atoms diffused (with possible aggregate formation).