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

Abstract

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.

1. Introduction

The properties and applications of two-dimensional (2D) materials are strongly correlated to their lateral dimensions[1–2]. Graphene oxide (GO) is a crucial precursor for scalable graphene production and has great potential for numerous applications in nanocomposites[3–5], corrosion protection, biomedicine[6], energy storage and conversion devices[7], and thermal management[8–9]. The large-area GO (LGO) is particularly desirable for assembling dense coatings, and fibers or membranes with high mechanical strength, electrical and thermal conductivities[3,9–17]. However, producing LGO through Hummers-type oxidation of large-area natural graphite flakes has been considered to be challenging especially in large-scale industrial production, which results from 2 aspects: (1) The oxidation is typically considered to be controlled by diffusion of oxidizer from the edges of the graphite to the cores[18–19]. When the size of the graphite flakes is large, the diffusion path for oxidizer is long, often resulting in flakes with oxidized edges but insufficiently oxidized cores[15,19–28]. (2) The oxidation process involves the insertion of oxygen-containing functional groups into the graphite lattice, which causes stress build-up and disrupts the planar structure and leads to the fragmentation of large flakes into smaller pieces[2,29].

A number of studies have focused on the producing of LGO with the Hummers-type oxidation method, as summarized in Fig. S1. In many works, the excessive oxidizer (e.g. 6-12∶1 for the mass ratio of KMnO4 to graphite) was used to enhance the oxidation[23,25,30]. Furthermore, researchers employed large expandable graphite or expanded graphite as raw materials since highly accessible structures facilitate the mass diffusion of oxidizer into graphene layers[11,31–33]. However, most strategies are not welcome in large-scale production due to complex processes and heavy consumption of oxidizer, intercalating agents, solvent, thermal or electrical power.

In a typical Hummers-type method, graphite oxidation consists of three basic stages[18–19,34].