IMAGING THE INCIPIENT ELECTROCHEMICAL OXIDATION OF HIGHLY ORIENTED PYROLYTIC-GRAPHITE

Oxidation of the first fractional carbon monolayer on highly oriented pyrolytic graphite (HOPG) electrodes is topographically manifested by the formation of well-defined surface blisters consisting of a solid skin covering a hollow interior. Atomic force microscopy (AFM), optical microscopy (OM), and scanning electron microscopy (SEM) show that the surface blisters formed by application of potentials from +1.5 to +1.63 V vs SSCE in 1.0 m KNO3 are from 20 to 1000 nm high and from 0.5 to 50 mum at the base. Surface analyses by AFM, X-ray microprobe, and Auger electron spectroscopy indicate that the outermost layer of the blister skin is essentially intact HOPG lattice (at the atomic scale) while the interior of the blister top contains a layer of graphite oxide (EGO). We propose that, following intercalation of electrolyte and water into the HOPG, blisters form as a result of electrolytic gas evolution at subsurface active sites (e.g., crystallite grain boundaries) with accompanying parallel electrolytic formation of EGO. The HOPG electrode kinetics of the Fe(CN)6(3-/4-) couple are only slightly enhanced by oxidation at +1.62 V, relative to the large changes in k-degrees caused by oxidation at +1.99 V, consistent with AFM images that show modest overall changes in the HOPG surface lattice in the former case and extensive lattice damage with exposure of edge plane sites in the latter case.