Charge transfer and weak bonding between molecular oxygen and graphene zigzag edges at low temperatures

Abstract

Electron paramagnetic resonance (EPR) study of air-physisorbed defective carbon nano-onions evidences in favor of microwave assisted formation of weakly-bound paramagnetic complexes comprising negatively-charged O-2 ions and edge carbon atoms carrying pi-electronic spins. These complexes being located on the graphene edges are stable at low temperatures but irreversibly dissociate at temperatures above 50-60 K. These EPR findings are justified by density functional theory (DFT) calculations demonstrating transfer of an electron from the zigzag edge of graphene-like material to oxygen molecule physisorbed on the graphene sheet edge. This charge transfer causes changing the spin state of the adsorbed oxygen molecule from S = 1 to S = 1/2 one. DFT calculations show significant changes of adsorption energy of oxygen molecule and robustness of the charge transfer to variations of the graphene-like substrate morphology (flat and corrugated mono-and bi-layered graphene) as well as edges' passivation. The presence of H-and COOH- terminated edge carbon sites with such a corrugated substrate morphology allows formation of ZE-O-2(-) paramagnetic complexes characterized by small (˂ 50 meV) binding energies and also explains their irreversible dissociation as revealed by EPR. (C) 2016 Elsevier Ltd. All rights reserved.