Two-Dimensional Icy Water Clusters between a Pair of Graphene-Like Molecules or Graphene Sheets

Abstract

To understand the 2-dimensional (2D) structural evolution of water molecules intercalated into a graphene bilayer, the geometries of water clusters up to tridecamer formed between a pair of graphene sheets or between graphene-like molecules (coronene and dodecabenzocoronene) are investigated. Due to their large sizes, the self-consistent-charge density-functional tight-binding (SCC-DFTB) method expanded into the third order and supplemented with a Slater-Kirkwood dispersion term was used. In this way both hydrogen bonding and H-π/π-π interactions are calculated in a balanced manner with the right magnitude of binding energies very close to the reference values based on the most accurate ab initio results. It should be noted that conventional density functional theory (DFT) calculations underestimate the H-π/π-π interaction, while dispersion-corrected DFT calculations overestimate hydrogen bonding. The latter method is also employed for comparison and to confirm the reliability of the SCC-DFTB results. For (H2O)6, the fused bitetragonal hexamer is nearly isoenergetic to the most stable planar hexagonal ring structure, and it is more frequently found. In (H2O)10 and (H2O)13 clusters, a tetragon is the most frequent geometry followed by a pentagon, while the hexagon is less frequent. These results certainly provide evidence of the recent planar tetragonal ice structure found inside a graphene bilayer (Nature 2015, 519, 443) which is in contrast to the well-known hexagonal pattern of the bulk ice. This structural change from hexagonal to tetragonal network on the graphene surface is attributed mainly to the inherent nature of 2D water.