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Anal. Chem. 1993, 65, 1378-1389
Imaging the Incipient Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite Charles A. Goss, Jay C. Brumfield, Eugene A. Irene, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
Oxidation of the first fractional carbon monolayer 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 pm 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)63_/4_ couple are only slightly enhanced by oxidation at +1.62 V, relative to the large changes in k° 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.
others5-7 have focused on HOPG electrodes, which offer a
well-defined, layered structure with large crystallites and atomically flat, low defect density basal plane surfaces. McCreery and co-workers4 have used Raman, SEM, doublelayer capacitance (Cdl)> electrode kinetics (k°), and redox molecule adsorption coverage (r) measurements to probe HOPG microstructural changes accompanying electrolytic oxidation in aqueous KNO3. From the correlation of the electrochemical parameters with graphite edge plane density, it was proposed that HOPG oxidation leads to delamination followed by lattice strain induced fracturing, resulting in smaller microcrystallite size.5b Recent Raman evidence6 indicating that in aqueous 1 M H2SO4,1M HC104, and 1 M HNO3 graphite lattice damage is always preceded or accompanied by intercalation was interpreted with a model where the graphite intercalation compound initially formed subsequently oxidizes water or carbon to form graphite oxides. In situ scanning tunneling microscopy (STM) was applied to the HOPG oxidation problem by Gewirth and Bard,7 who proposed that graphite oxide layer formation follows a nucleation and growth mechanism. The 60 X 60 nm HOPG area described was initially atomically flat with a unform ca. 0.9-eV barrier height. Electrolytic oxidation in aqueous 0.1 M H2SO4 by potential cycling between 0 and +1.8 V vs Ag wire produced roughened, apparently oxidized, regions with lowered barrier heights (
