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

TL;DR

This study uses EPR and DFT to investigate charge transfer between oxygen molecules and graphene edges, revealing weakly-bound paramagnetic complexes that dissociate above 50-60 K, with implications for graphene's surface chemistry.

Contribution

It demonstrates the formation and stability of charge-transfer complexes between oxygen and graphene edges, supported by combined experimental and theoretical analysis.

Findings

Weakly-bound paramagnetic complexes form at graphene edges.

Charge transfer alters oxygen's spin state from S=1 to S=1/2.

Complexes dissociate irreversibly above 50-60 K.

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 O2- 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 corrugated substrate morphology allows formation of ZE-O2- paramagnetic complexes characterized by small (<50 meV) binding energies and also explains their irreversible dissociation as revealed by EPR.