Effects of processing and material parameters on synthesis of monolayer ultralarge graphene oxide sheets

Chemically-derived ultralarge graphene oxide (UL-GO) sheets are synthesized from natural graphite (NG) flakes based on the modified Hummers method. Three different approaches are adopted and the effects of ultrasonication, thermal shock expansion, degree of oxidation and precursor NG flake size are specifically studied on the quality and size of GO sheets produced. Results show that the use of large-size NG flakes as precursors does not necessarily produce large GO sheets. Optimal processing conditions are identified to be thermal shock exfoliation with the addition of moderate oxidation, i.e. with an expanded graphite to KMnO₄ weight ratio = 1:7 for 24 h, and avoiding ultrasonication during the oxidation process. The resulting UL-GO sheets have a maximum area over 10,000 μm² with a mean area 3400 μm² at a yield of 39.8% for GO sheets larger than 2500 μm², which are considered quite sufficient as precursors for many multifunctional applications, including transparent conductive films, optoelectronic devices and aligned graphene composites.