Surfactant-free single-layer graphene in water

Dispersing graphite in water to obtain true (single-layer) graphene in bulk quantity in a liquid has been an unreachable goal for materials scientists in the past decade. Similarly, a diagnostic tool to identify solubilized graphene in situ has been long awaited. Here we show that homogeneous stable dispersions of single-layer graphene (SLG) in water can be obtained by mixing graphenide (negatively charged graphene) solutions in tetrahydrofuran with degassed water and evaporating the organic solvent. In situ Raman spectroscopy of these aqueous dispersions shows all the expected characteristics of SLG. Transmission electron and atomic force microscopies on deposits confirm the single-layer character. The resulting additive-free stable water dispersions contain 400 m 2 l –1 of developed graphene surface. Films prepared from these dispersions exhibit a conductivity of up to 32 kS m –1 . Aggregation usually prevents dissolution of graphene in water. Now, hydroxide ion adsorption has been shown to allow the stabilization of true single-layer graphene in water — with no surfactant required — so long as the liquid is degassed beforehand. The resulting aqueous dispersions can contain high concentrations of exfoliated graphene that are stable for several months.