Mechanism of the graphene oxide formation. The role of water, “reversibility” of the oxidation, and mobility of the C–O bonds

Abstract

© 2020 Elsevier Ltd Despite recent progress, the mechanism of the graphene oxide (GO) formation remains largely unknown. In this work, we report new findings, which add crucial details to the previously envisioned mechanism of the Hummers’ method reaction. The rate of the chemical reaction increases significantly with water content in the acid, and reaches the maximum in the acid concentration range 88%–92%. Graphite cannot be oxidized in the anhydrous sulfuric acid. We propose that the species, attacking carbon atoms, are not the Mn(VII) oxygen derivatives, as it is commonly believed, but water molecules. The withdraw of electrons from carbon atoms by Mn(VII) species, and the nucleophilic attack by water molecules occur simultaneously as a concerted process. The Mn(VII) species do not penetrate graphite interlayer galleries, but water molecules do. The formation of the covalent C–O bonds on the graphene plane is reversible, as long as the graphite sample remains intercalated with sulfuric acid. The as-formed bonds can be easily cleaved by the laser irradiation, converting GO back to stage-1 GIC in small local areas of the graphite flake. We interpret this “reversibility” as the “migration” of the newly formed C–O bonds on the graphene plane. The pattern of the C–O bond migration is largely controlled by the local graphite structure. The discovered phenomena and proposed reaction mechanism provide rationale for a range of the well-known but yet poorly understood experimental observations in the graphene chemistry. Among them is the existence of the oxidized and graphenic domains in the GO structure. The new findings and conclusions create the holistic view on the driving forces of the transformations that occur with graphite in the oxidative acidic media.