Water-mediated NaNO₃ ultrathin flakes on highly oriented pyrolytic graphite at ambient conditions

Two-dimensional inorganic salt films have increasing potential in laboratory and industrial applications. However, the methods for fabricating these films are complicated and not appliable under ambient conditions, which extremely limits their applications. Here, we report a novel ultrathin NaNO₃ flake with a thickness of less than 1.0 nm on a highly oriented pyrolytic graphite (HOPG) surface mediated by water. We note that neither molten salt nor water is affinitive to the aged carbon-based surface so that they form droplets on the HOPG surfaces. Interestingly, the solution formed by the mixture of salt and water develops an affinity toward the HOPG. Further, the flakes had adjustable electric conductivity. Density functional computation showed that water substantially increased the ionic bond length between NO₃^- to Na⁺, enhancing the interaction between the NaNO₃ cluster and graphene via cation-π interaction, and rendering the mixture of water and NaNO₃ graphene-philic toward the graphitic surface. The findings provide a brand-new way to obtain thin films of inorganic salts.