Molecular hydrogen uptake by zigzag graphene nanoribbons doped with early 3d transition-metal atoms

We performed ab initio density-functional calculations to investigate the structural, electronic and magnetic properties of nanostructures comprising single-adatoms of Sc, Ti or V adsorbed on a hydrogen-passivated zigzag graphene nanoribbon (GNR). We also investigated the affinity of the resulting doped nanostructures for molecular hydrogen. In all cases, the most stable structures featured the adatom at positions near one of the edges of the GNR. However, whereas in the most stable structures of the systems Sc/GNR and V/GNR the adatom was located above a bay of the zigzag edge, Ti/GNR was found to be most stable when the adatom was at a first-row hole site. Adsorption at sites near one of the ribbon edges reduced drastically the average magnetic moment of the carbon atoms at that edge. On the other hand, the magnetic moments of the adatoms on the GNR, as the electronic character of the doped nanostructures, depended on the adsorption site and on the adatom species, but their absolute values were in all cases, except when Sc was at an edge bay site, greater than those of the corresponding free atoms. Our results showed that, of the three systems investigated in this paper, Ti/GNR (except when Ti is adsorbed at an edge bay site) and V/GNR appear to satisfy the criterion specified by the U. S. Department of Energy for efficient H₂ storage, as far as binding energy is concerned. We discussed in detail the differences between the adsorption of H₂ on the system Ti/GNR and the adsorption of H₂ on Ti-adsorbed carbon nanotubes, which have been proposed as a high-capacity hydrogen storage media.