Low-cost synthesis of large graphene oxide flakes by the total oxidation of large natural graphite flakes

Large graphene oxide (LGO) sheets have significant advantages over smaller ones in various applications. However, producing them by the Hummers-type oxidation of large natural graphite flakes is challenging. The inherent limiting factors are generally believed to be that large graphite flakes are both difficult to oxidize fully and prone to fragmentation during the process. By in-situ monitoring the graphite oxidation, we observed that, given sufficient time, large graphite flakes may be fully oxidized while still remaining largely intact. Graphite oxidation is governed by diffusion of the oxidizer between the layers, and is described by Fick’s law, where a high oxidizer concentration gradient increases the diffusion rate. We therefore increased the oxidizer concentration by minimizing the amount of solvent (concentrated H2SO4), achieving full oxidation of gram-scale large graphite flakes in a semi-solid state with significantly reduced reagent consumption. In addition, the reaction temperature was adjusted to balance graphite oxidation and Mn(VII) self-decomposition. Using this approach, gram-scale 200-, 100-, and 50-mesh natural graphite were all fully oxidized with a significantly reduced consumption of both H2SO4 and KMnO4. A reduction in size occurs during exfoliation, yielding LGO with average sizes of 27.3, 58.7, 116.2 μm, respectively. This study not only provides a scalable and cost-effective strategy for LGO production but also advances the understanding of Hummers-type methods.