The most conventional way for GO synthesis follows the
modified Hummer’s method,30 where the strong oxidant KMnO4 induces the oxygenated functional groups on well exfoliated graphite to produce highly dispersible graphene oxide sheets (GO-1). In our modified method GO-2 was prepared in the presence of excess KMnO4 with H3PO4 as expected; the excess KMnO4 would raise the level of oxidation and could lead to the overoxidation of the initially produced oxygenated functional groups. Furthermore, the excessive oxidation could result in hole formation and uncontrollable breaking of the edges and basal planes. The mild oxidizer H3PO4 reacts with a small amount of the surface hydroxyl groups via ester formation and
hence prevents the GO basal plane from further oxidation. Sequentially, the GO-3 preparation with excess H3PO4 improved the protection of the GO basal plane. A similar phenomenon has been reported by Higginbotham et al. while using the mild oxidizing agent H3PO4 to fabricate graphene oxide nanoribbons from multiwall carbon nanotubes.31