Topotactic conversion of calcium carbide to highly crystalline few-layer graphene in water

Abstract

The reaction of calcium carbide (CaC₂) with water to produce acetylene is common in industrial production, but its side reaction, removal of calcium from CaC₂ (also termed de-Ca) to fabricate highly graphitic carbon, is highly overlooked. Herein, we report the synthesis of highly crystalline few-layered graphene by controlling the reaction of tetragonal-phased CaC₂ with water at room temperature (20–25 °C). The resulting carbon materials were revealed to be highly graphitic, with ∼3 nm thickness, containing >93 at% carbon. Raman spectroscopy evidenced their low defect content with a defect (D)/graphitization (G) ratio of ∼0.07. HRTEM further verified their high graphitization degree. A formation mechanism was proposed: the C₂²⁻ dumbbells donate their electrons to nearby oxidative species, e.g. H⁺ in water, followed by topotactic cross-linking to form a conjugated sp²-carbon network. Furthermore, the capability of CaC₂ reduction and re-assembly into graphitic carbon was clearly evidenced by reaction with Ag⁺ in non-aqueous solvent, which resulted in a larger quantity of graphene materials and small-sized Ag nanoparticles.