57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

For this project, we study light metal absorption on the surface of graphene nanoribbons (GNRs) and their influence on the hydrogen storage capacity of GNRs using first-principles methods. The newly synthesized GNRs are stripes of graphene with an extremely large length/width ratio. The quantum confinement of electrons along their width gives rise to unique edge effects that dominate their electronic and magnetic behavior. Although their two-dimensional counterpart, graphene, has already been studied in terms of its capacity for absorbing light metals, it is expected that the symmetry breaking of the two-dimensional lattice (which is a consequence of the finite width of GNRs) will affect the absorption energy of alkali and alkaline metals with the potential of producing more efficient hydrogen storage materials. For instance, we find that the binding energy of a Li atom on armchair nanoribbons [of about 1.70 eV for local spin density approximation (LSDA) and 1.20 eV for Perdew–Burke–Ernzerhof (PBE)] is comparable to the corresponding value in graphene (1.55 and 1.04 eV for LSDA and PBE, respectively). Notably, the interaction between Li and zigzag nanoribbons is much stronger. The binding energy of Li at the edges of zigzag nanoribbons is about 50% stronger than in graphene for the functionals studied here. While the charge transfer between the Li adatom and the zigzag nanoribbon significantly affects the magnetic properties of the latter providing an additional interaction mechanism that is not present in a two-dimensional graphene or armchair nanoribbons, we find that the morphology of the edges, rather than magnetism, is responsible for the enhanced Li-nanoribbon interaction.

We have also studied metal adsorption at different concentrations. At low adsorbant densities, we observe a strong ionic interaction characterized by a substantial charge transfer from the adatoms to the substrate. In this low concentration regime, the electronic density around the Li adatoms is well localized and does not contribute to the electronic behavior in the vicinity of the Fermi level. For larger concentrations, we observe the formation of a chemically bound Li layer characterized by a stronger binding energy as well as a significant density of states above the Fermi level coming from both graphene and the two-dimensional Li sheet.

WORKFORCE DEVELOPMENT

This project involved the participation of two MS students and six undergraduate students. The graduate students are supported by the College of Science and Technology at CMU while the undergraduate research has been supported entirely by the PRF award.