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RESEARCH

Our work for this project began with the study of the edge effects on the interaction between Li atoms and graphene, a single layer of graphite. To this end, we have studied the adsorption of Li atoms at the hollow sites of fullerenes, graphene, and graphene nanoribbons (GNRs), zigzag and armchair by means of density functional theory within the LSDA and GGA. Li interacts with armchair nanoribbons and two-dimensional graphene through the same charge transfer mechanism, with binding energies per adatom of about 1.70 and 1.55 eV (LSDA) and 1.04 and 1.20 eV (PBE- GGA), respectively. Li interacts with zigzag nanoribbons in a much stronger way. The binding energies of Li at the edges of zigzag nanoribbons are 2.27 (LSDA) and 1.70 eV (PBE), more than 50% stronger than in graphene. As seen in Figure 1, the binding energy progressively decreases as the adsorption position is shifted toward the center of the nanoribbon suggesting that narrow zigzag GNRs will be most ideal to attain larger ion concentrations. While the charge transfer between the Li adatom and the zigzag nanoribbon affects significantly the magnetic properties of the latter providing an additional interaction mechanism that is not present in graphene or armchair nanoribbons, we find that the morphology of the edges, rather than magnetization, is responsible for the enhanced Li adsorption. These results illustrate the importance of controlling the edges of GNRs with atomic precision in order to maximize their potential for technological applications.

 Besides graphitic materials, we decided to consider novel non-graphitic systems for Li adsorption. Therefore, we have studied the electronic properties and relative stability of the biphenylene sheet, composed of alternating eight-, six-, and four-carbon rings, and its one-dimensional derivatives, including ribbons and tubes of different widths and chiralities. Our DFT calculations show that the two-dimensional sheet presents a metallic character that is also present in the planar strips with zigzag-type edges. Armchair-edged strips develop a band gap that decreases monotonically with the ribbon width. The narrow armchair ribbon (w =0.62 nm) presents a large band gap of 1.71 eV, while a 2.14 nm wide armchair strip exhibits a band gap of 0.08 eV. Tubes made by seamlessly rolling these ribbons are all metallic, independent of their chirality. While planar strips present a relative stability comparable to that of C₆₀, tubes exhibit a more pronounced metastable nature with a δG that is at least 0.2 eV per carbon higher than in C₆₀. It is expected that the metallic nature of this materials will enhance their Li intake capacity with respect to graphites.

WORKFORCE DEVELOPMENT

 This project involved the participation of two MS students and three undergradute 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 at an average of 2 undergraduates for 8 months (up to 10 hours per week each). The graduate students were working independently in different aspects of the project and presented results in scientific meetings. As planned, two undergraduate students, one with a Chemistry background and the other with a Physics background, were chosen to work together in the properties of the biphenylene sheet. A peer reviewed paper published in ACS Nano has been the outcome of this combined effort.