Supercritical fluid conversion of graphene oxides

In the preparation of graphene sheets for various studies and applications, the indirect route through the reduction of graphene oxides (GOs) has been widely pursued. Exfoliated GOs are shown to be mostly single-layer sheets in aqueous solution, and they are also demonstrated for conversion to recover some of the properties intrinsic to graphene. Beyond the commonly used thermal annealing and chemical reduction methods, several environmentally friendly conversion strategies have been explored in the literature. Reported here is a method of annealing GOs in supercritical fluids (SCFs, including carbon dioxide and ethanol) at relatively lower temperatures (up to only 300°C) for their conversion to reduced GOs (rGOs). The characteristic properties of SCFs include low densities (thus low viscosity/high diffusivity) and diminished surface tension, which have found successful applications in extraction and the cleaning of fragile electronic devices, and also found to enable lower-temperature crystallization of amorphous nanomaterials in a fluid-assisted calcination process. In this study the same principles for lower-temperature calcination in SCFs were applied to the conversion of GOs. The rGOs thus obtained were characterized, with their electrical and thermal conductive properties evaluated and correlated with the different processing conditions. The benefits and shortcomings of the SCF processing method are discussed.