Abstract
The structure-electroactivity relationship of graphene has been studied using coronene (Cor), polyaromatic hydrocarbon (PAH), and a subunit of graphene as a model system by chemically modified electrode approach. In general, graphene and PAH do not show any redox activity in their native form. Herein, we report a simple electrochemical approach for the conversion of electro-inactive coronene to a highly redox-active molecule (Cor-Redox; E°' = 0.235 ± 0.005 V vs Ag/AgCl) after being adsorbed on graphitic carbon nanomaterial and preconditioned at an applied potential, 1.2 V vs Ag/AgCl, wherein, the water molecule oxidizes to dioxygen via hydroxyl radical (•OH) intermediate, in acidic solution (pH 2 KCl-HCl). When the same coronene electrochemical experiment was carried out on an unmodified glassy carbon electrode, there was no sign of faradic signal, revealing the unique electrochemical behavior of the coronene molecule on graphitic nanomaterial. The Cor-Redox peak is found to be highly symmetrical (peak-to-peak potential separation of ∼0 V tested by cyclic voltammetry (CV)) and surface-confined (ΓCor-Redox = 10.1 × 10-9 mol cm-2) and has proton-coupled electron-transfer (∂E°'/∂pH = -56 mV pH-1) character. Initially, it was speculated that Cor is converted to a hydroxy group-functionalized Cor molecule (dihydroxy benzene derivative) on the graphitic surface and showed the electrochemical redox activity. However, physicochemical characterization studies including Raman, IR, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), redox-site selective oxidation probe, cysteine (for dihydroxy benzene), radical scavenger ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO), and scanning electrochemical microscopy (SECM) using ferricyanide redox couple have revealed that coronene cationic radical species like electroactive molecule is formed on graphitic material upon the electrochemical oxidation reaction at a high anodic potential. It has been proposed that •OH generated as an intermediate species from the water oxidation reaction is involved in the coronene cationic radical species. Studies on coronene electrochemical reaction at various carbon nanomaterials like multiwalled carbon, single-walled carbon, graphite, graphene oxide, and carbon nanofiber revealed that graphitic structure (without any oxygen functional groups) and its π-π bonding are key factors for the success of the electrochemical reaction. The coronene molecular redox peak showed an unusual electrocatalytic reduction of hydrogen peroxide similar to the peroxidase enzyme-biocatalyzed reduction reaction in physiological solution.