Largely enhanced oxidation of graphite flakes via ammonium persulfate-assisted gas expansion for the preparation of graphene oxide sheets

Abstract

The production of large graphene oxide (GO) sheets is often limited by the huge diffusion resistance for oxidizing agents caused by the structural uniformity of large raw graphite flakes. Previous methods often rely on the thermal or microwave, hydrogen peroxide (H2O2) expansion to overcome the large diffusion resistance. However, the pretreatment is usually energy consuming and costs extra acid which may cause more environmental risks. Here, we discovered that adding ammonium persulfate ((NH4)2S2O8) to concentrated sulfuric acid as gas expansion agent for the pretreatment of graphite flakes can lead the graphite to form a porous structure and effectively improve the specific surface area of graphite, providing more contact area for graphite with oxidizing agents due to the rapid gas release. Thus, the diffusion resistance for oxidizing agents is remarkably reduced. As a result, graphite oxide with pretreatment shows higher oxidation degree (C/O ratio: 1.7) compared with graphite oxide without pretreatment (C/O ratio: 2.3). Besides, without any exfoliation method, the as obtained uniformly oxidized graphite oxide can self-exfoliate into monolayer GO sheets with a weight average size of 1263 μm2. Moreover, the vacuum filtrated GO films exhibit superior mechanical properties (Tensile strength: 80.9 MPa).