Role of H2O for forming O-containing groups during the secondary oxidation stage of graphite

To investigate the impact of H₂O on the formation of O-containing functional groups during the secondary oxidation stage in the oxidation process of graphite, graphite oxide (GO) with varying types and quantities of O-containing functional groups is prepared by precisely controlling the reaction temperature. In the initial oxidation stage, Mn₂O₇ serves as the primary oxidizing agent. However, the diffusion of Mn₂O₇ between graphite layers is hindered by its significant volume, thereby complicating the oxidation of carbon atoms within the graphite layers. The initial oxidation stage primarily involves the intercalation of graphite, during which carbon atoms located at the edges of the atomic layers or surface defects undergo oxidation, resulting in the formation of carboxyl, carbonyl and other O-containing functional groups. The secondary oxidation stage is characterized by the introduction of a large number of O-containing functional groups in the GO. The introduction of H₂O transforms the main oxidizing substance into MnO₄⁺, which facilitates its penetration into the graphite layer and rapidly diffuse, thereby enabling the oxidation of carbon atoms within the graphite layers to form hydroxyl, epoxy and other O-containing functional groups. By controlling the reaction time and temperature during the secondary oxidation stage, it is possible to alter both type and content of O-containing functional groups in GO. Prolonging the reaction time or increasing the reaction temperature can reduce hydroxyl content while increasing epoxy and carbonyl groups in GO, consequently leading to an increase in structural defects as well as a decrease in cation exchange capacity. The results provide an experimental and theoretical foundation for customizing GO with specific functionalities at the chemical structure level.