Static Density Functional Study of Graphene–Hexagonal Bilayer Ice Interaction

Periodic static ab initio studies
are conducted of hexagonal bilayer
ice (HBL) and basal layers of ice-1h adsorbed on graphene using the
model BLYP-D in CRYSTAL09. Eight high-symmetry periodic forms of HBL
are optimized, of which four have lower energy; their electronic binding
energy to graphene is ∼1.6 kcal/mol per abutting H2O. Optimized geometries have the property of maximizing the occurrence
of a certain O–H–C alignment motif. One lattice is selected
for more detailed study. Its 2-D shear translation potential energy
surface is found to have barrier heights in two zigzag directions
of ∼140 cal/mol per abutting H2O. A second hexagonal
bilayer can be added and the electronic binding energy drops from
∼1.7 to ∼1.0 kcal/mol per abutting H2O. For
ice-1h monolayer adsorbed on graphene, a proton-ordered form in which
half of the O’s nearest the graphene carry a proton pointing
toward graphene is preferred over proton-ordered forms in which either
all or none of those O’s have H’s pointing toward graphene.
Cohesive energy for two-layer ice-1h on graphene is 0.66 kcal/mol
of H2O higher than for HBL, supporting experimental evidence
that the graphene+HBL isomer is more stable. However, the HBL and
two-HBL structures are unstable or at best metastable with respect
to four layers of ice-1h.