Two-dimensional Na–Cl crystals of unconventional stoichiometries on graphene surface from dilute solution at ambient conditions

NaCl in a 1:1 stoichiometry is the only known stable form of the Na-Cl crystal under ambient conditions, and non-1:1 Na-Cl species can only form under extreme conditions, such as high pressures. Here we report the direct observation, under ambient conditions, of Na2Cl and Na3Cl as two-dimensional (2D) Na-Cl crystals, together with regular NaCl, on reduced graphene oxide membranes and on the surfaces of natural graphite powders from salt solutions far below the saturated concentration. Molecular dynamics and density functional theory calculations suggest that this unconventional crystallization process originates from the cation-π interaction between the ions and the π-conjugated system in the graphitic surface, which promotes the ion-surface adsorption. The strong Na+-π interaction and charge transfer lead to stoichiometries with an excess of Na+. With unique electron and spin distributions and bonding, the resulting 2D crystals may have unusual electronic, magnetic, optical and mechanical properties.