Room-temperature ice growth on graphite seeded by nano-graphene oxide.

Water wetting on a hydrophobic surface at ambient conditions is disallowed by the non-polar nature of the surface and high vapor pressure of water. However, the presence of sub-millimeter sized hydrophilic patches allows the waxy wings of desert beetles to become wettable by morning mist. Herein, we show that a sprinkle of graphene oxide nanoflakes (nanoGOs) is effective in condensing water nanodroplets and seeding ice epitaxy on graphite at ambient conditions. By controlling the relative humidity and nanoGO density, we are able to study the formation of a complete ice wetting layer on a time scale of 20 h. This presents an unprecedented opportunity to visualize ice nucleation and growth in real time using non-contact atomic force microscopy. The stages of crystallization, as proposed by Ostwald in 1897, are fully unfolded at a microscopic level for the first time. We obtain real-time imaging of the sequential phase transition from amorphous ice to a transient cubic ice Ic stage, and finally to the stable hexagonal ice Ih. Most interestingly, we discover that ice nucleation and growth can be influenced by modifying the functional groups of nanoGO and by intermolecular hydrogen-bonding between nanoGOs. This affords a strategy to control heterogeneous ice nucleation and snow crystal formation. The interaction of water with solid surfaces is one of the most pervasive natural phenomena which underpins, for example, rain precipitation, snow formation, rock erosion. The wetting of surfaces by ambient water is also of crucial importance in many processes, such as heterogeneous catalysis, photocatalysis, microelectronics, and drug development. In bulk ice which is formed below 0 8C, water molecules pack in a hexagonal ice Ih structure with four hydrogen bonds arranged in tetrahedral geometry. Above 0 8C, vibrational lattice instability leads to the liquid phase. A new idea emerged recently, that the ice-toliquid phase transition crossing the freezing point may not apply in the two dimensional (2D) limit. Recent research revealed that at a hydrophilic interface, or by nanoconfinement, ice Ih wetting layers can persist even at ambient conditions. To date, insights into the hydration structure and wetting dynamics of ambient water on solids is derived mainly from studies on single-crystal metal surfaces carried out at cryogenic temperatures in vacuum conditions. Molecular level studies of water/ice nucleation and growth on solid surfaces at ambient conditions remain a formidable challenge owing to the highly mobile nature of water molecules and its high vapor pressure. In our ice growth experiments, we use nanoGOs as ice nucleation seeds. These nanoGOs were heated in base and had a higher degree of restored sp conjugation than the assynthesized nanoGOs as well as carboxylate (COO ) groups on their periphery, as shown by vibrational spectroscopy (Figure S1 in the Supporting Information). After spin coating nanoGOs on a freshly peeled highly ordered pyrolytic graphite (HOPG), the samples were dried at 80 8C for 15 min before amplitude-modulated non-contact atomic force microscopy (NCAFM) imaging. The typical NCAFM amplitude setpoint for direct imaging of ice epitaxial domains is 2–3 nm. Using higher setpoints results in a perturbation of the wetting layer or even penetration of the water films (see Figure S2 for setpoints of 6 nm and > 10 nm). Penetration allows us to directly image the underlying substrate and determine the physical dimensions of nanoGOs, which have heights varying from 0.5 nm to several nm (Figure S2 and S3). Figure 1a illustrates the dynamic processes involved in room-temperature ice formation on graphite seeded by superhydrophilic nanoGOs. We modify the graphite substrate with 1–10% surface coverage of nanoGOs. The advantages of using the graphite surface as a template include its atomic flatness which does not disrupt the fragile hydrogen bonding in ice structures, and its ability to conduct the latent heat produced by ice condensation rapidly. The triangular sublattice of graphite (2.46 ) matches the natural ice structure very well. All these factors favor the epitaxial growth of a commensurate ffiffiffi