Predicting the chiral enrichment of metallic swcnts on Ni-cu bimetallic surfaces

Previous modeling and experimental studies have suggested that Cu-based metal catalysts can preferentially produce metallic carbon nanotubes. Here, we explain this selectivity by separately modeling nanotube cap nucleation and nanotube growth on Cu and Ni_xCu_1-x surfaces. Cap nucleation is modeled by invoking an epitaxial lattice matching criterion and comparing the binding strengths of different cap chiralities. Nanotube growth on various catalyst surfaces was studied by calculating differences in armchair and zigzag dangling bond energies, relative chemical activity ratios, and nanotube growth rates of different nanotube chiralities. All energies associated with nanotube cap nucleation and armchair and zigzag dangling bond energies were obtained using density functional theory (DFT). We find that certain armchair and zigzag nanotube caps exhibit higher binding strengths than chiral caps, and the stability of the caps on the various surfaces decreases as Ni > Ni_0.5Cu_0.5 > Cu, in accordance with the respective carbon-metal adhesion strengths. Both the relative chemical activity ratios and the nanotube growth rates suggest that Ni_xCu_1-x bimetallic nanoparticles with increased bond length or lattice-strained surfaces are excellent candidates for preferentially producing metallic nanotubes.