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Electron paramagnetic resonance (EPR) study of air-physisorbed defectivecarbon nano-onions evidences in favor of microwave assisted formation ofweakly-bound paramagnetic complexes comprising negatively-charged O2- ions andedge carbon atoms carrying pi-electronic spins. These complexes being locatedon the graphene edges are stable at low temperatures but irreversiblydissociate at temperatures above 50-60 K. These EPR findings are justified bydensity functional theory (DFT) calculations demonstrating transfer of anelectron from the zigzag edge of graphene-like material to oxygen moleculephysisorbed on the graphene sheet edge. This charge transfer causes changingthe spin state of the adsorbed oxygen molecule from S = 1 to S = 1/2 one. DFTcalculations show significant changes of adsorption energy of oxygen moleculeand robustness of the charge transfer to variations of the graphene-likesubstrate morphology (flat and corrugated mono- and bi-layered graphene) aswell as edges passivation. The presence of H- and COOH- terminated edge carbonsites with such corrugated substrate morphology allows formation of ZE-O2-paramagnetic complexes characterized by small (<50 meV) binding energies andalso explains their irreversible dissociation as revealed by EPR.

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