Structure and Magnetic Properties of Superoxide Radical Anion Complexes with Low Binding Energy at the Graphene Edges
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cture and Magnetic Properties of Superoxide Radical Anion Complexes with Low Binding Energy at the Graphene Edges V. Yu. Osipova, *, D. W. Boukhvalovb, c, and K. Takaid aIoffe
Institute, St. Petersburg, 194021 Russia of Physics and Technology, Ural Federal University, Yekaterinburg, 620002 Russia c College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing, 210037 P.R. China dDepartment of Chemical Science and Technology, Hosei University, Koganei, Tokyo, 184-8584 Japan *e-mail: [email protected] bInstitute
Received April 19, 2020; revised May 6, 2020; accepted May 8, 2020
Abstract—The complexes of superoxide radical anions formed at the zigzag edges of curved graphene sheets upon microwave excitation were studied by electron spin resonance and quantum chemical density functional theory. The binding energy of the complex decreases from 700 to 42 meV as the distance between the oxygen atom and the zigzag graphene edge increases from 1.46 to 1.64 Å. The configurations and binding energies of the complexes depend both on the topology of the underlying graphene layer (single- or double-layer, flat, or curved) and on the type of atomic groups terminating the edge carbon atoms via σ-bonds. Complexes with low dissociation energy ( 50 K, the complex irreversibly dissociates and the O2 molecule is detached from the edge. The existence of complexes with a low dissociation energy expands the views on the ionic chemical bond with a large distance between the interacting species. Keywords: superoxide radical anion, graphene, edge states, paramagnetic susceptibility, ESR spectroscopy DOI: 10.1134/S107032842011007X
INTRODUCTION Graphenes and materials based on nanographene sheets are materials with extended π-electron system and unusual electronic properties [1–3]. These properties include both orbital diamagnetism [4–6], which substantially exceeds that observed for large planar aromatic molecules, and unusual paramagnetism caused by the edge electronic states at the zigzag edges [7–10]. The paramagnetism is related to the peak in the density of electron states at the contact point between graphene π- and π*-areas, while in terms of the Clar’s aromatic sextet rule, it is associated with unpaired π-electrons located at every third zigzag along the zigzag edge [2, 11, 12] (Fig. 1). It is assumed that the external σ-bonds of the graphene sheets are saturated by functional groups: hydrogen and hydroxyl or carboxyl groups. The unusual π-electron states of the graphene edges account for their unusual chemical properties, in particular catalytic properties. The edges of specially perforated graphene efficiently catalyze the nitrobenzene reduction to aniline [13]. This is partly due to the fact that the edges act as π-electron donors for molecular agents occurring in the close
vicinity and thus facilitate specific chemical reactions of these molecular agents. Carbon nanomaterials, including fullerenes and multiwalled nanotubes, have been actively studied since the 1990s. In 1996, Mitsuta
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