Electrochemical formation of graphite oxide from the mixture composed of sulfuric and nitric acids

Electrochemical overoxidation of graphite in different mixtures of H2SO4 and HNO3 was investigated. During the anodic treatment, beforehand formed graphite intercalation compound (GIC) was transformed into the graphite oxide (GtO) by reaction of co-intercalated water molecules with the surface of graphene layers. Depending on the volume ratio of H2SO4/HNO3, the synthesized materials exhibit different oxidation degrees. The highest oxygen concentrationwas reached for graphite being oxidized in electrolyte composed of concentrated H2SO4 and HNO3 with volume ratio equal to 3: 1 containing 26% of water by weight. For this sample the C/O ratio was equal to 2.09 which is very closed to that noted for GtO prepared by chemical methods. It should be emphasized that 26% admixture of water to the examined electrolytes enabled the entire transformation of graphite into GtO. This fact was proven by the results of Raman spectroscopy and X-ray diffraction (XRD) analysis. X-ray photoelectron spectroscopy (XPS) measurements reveals that during the electrochemical overoxidation of graphite in aqueous mixtures of H2SO4/HNO3 mainly alkoxy and/or epoxy groups are formed. Scanning electron microscopy (SEM) observations showed that deep anodic oxidation contributed to the enormous deformations of layered graphite structure. (C) 2019 Elsevier Ltd. All rights reserved.