The oxidation mechanism of highly ordered pyrolytic graphite in a nitric acid/sulfuric acid mixture

We describe the oxidation mechanism of a highly crystalline graphitic carbon in a nitric acid/sulfuric acid mixture, which is the most commonly used medium for the purification and chemical modification of carbon-based materials. Highly ordered pyrolytic graphite (HOPG) specimens embedded in acid-proof high-density polyethylene were treated in the acid mixture at 100 °C for between one to four days. The degree of HOPG damage could be seen by transmission and scanning electron microscopy, X-ray photoelectron and Raman spectroscopy, and X-ray diffraction. In addition to “interlayer-acid-penetration” (acid-intercalation), the results suggest that a new corrosion process could be “direct-acid-penetration” from the outer to inner graphitic layers with nitration and sulfonation. This observation shows that the most commonly used acid mixture is not only introducing oxygenated groups together with nitrogen- and sulfur-containing groups, but also creating structural defects in the graphitic carbon lattice.