Low-cost synthesis of large graphene oxide flakes by the total oxidation of large natural graphite flakes
Large graphene oxide (LGO) sheets offer significant advantages over smaller ones in various applications, yet their production via Hummers-type oxidation of large natural graphite flakes remains challenging due to difficulties in achieving full oxidation and avoiding fragmentation. This study provides the first direct evidence that large graphite flakes (up to 1 mm) can be completely oxidized without fragmentation under static conditions, as revealed by in-situ monitoring. The oxidation process is governed by diffusion of the oxidizer between layers, described by Fick's law, where a high oxidizer concentration gradient increases the diffusion rate. By minimizing the amount of concentrated H2SO4 solvent, we achieved a semi-solid state that elevates oxidizer concentration, facilitating Mn(VII) diffusion and enabling complete oxidation of gram-scale large flakes with significantly reduced reagent consumption. Reaction temperature was optimized to balance graphite oxidation and Mn(VII) self-decomposition. Using this approach, 200-, 100-, and 50-mesh natural graphite were fully oxidized with reduced H2SO4 and KMnO4 usage. After exfoliation, LGO with average lateral sizes of 27.3, 58.7, and 116.2 μm were obtained, respectively, with 100% conversion and yield over 165%. This work not only provides a scalable, cost-effective strategy for LGO production but also advances the fundamental understanding of Hummers-type oxidation.