Application of supercritical fluid in the synthesis of graphene materials: a review

The studies on the utilisation of supercritical fluids (SCFs) in processing chemicals and materials have garnered significant attention in the past two decades. SCFs possess both gas- and liquid-like properties that are tunable, rendering them as superior solvents for reactions and processes, for example in the delamination of graphite. SCF technologies are deemed to be potential alternatives to existing technologies for graphene production that are yet to be industrially scalable. This review features recent works on the production of graphene facilitated by SCFs, with emphasis on the conversion of graphite to graphene through exfoliation and reduction. The exfoliation processes report the yield of 6 to 27% of monolayer graphene and 3 to 25% of ≤ 5 layers of graphene, whilst the carbon-to-oxygen (C/O) ratio of graphene produced via different reduction processes ranges from 0.37 to 28.2 with interlayer spacing of 0.35 to 0.38 nm. Recent applications of gas-expanded solvents for the synthesis of graphene and the fabrication of functionalised graphene materials via SCF-aided processes are also described. In addition, a summary of the properties of common SCFs as well as the characterisation of graphene materials, such as the number of layers, C/O ratio, interlayer spacing, pore size and surface area, is included to provide insights on the process efficiency.