  The contact of water with graphene is of fundamental importance and of great
interest for numerous promising applications, but how graphene interacts with
water remains unclear. Here we used atomic force microscopy to investigate
hydrophilic mica substrates with some regions covered by mechanically
exfoliated graphene layers in water. In water containing air gas close to the
saturation concentration (within ~40%), cap-shaped nanostructures and ordered
stripe domains were observed on graphene-covered regions, but not on pure mica
regions. These structures did not appear on graphene when samples were immersed
in highly degassed water, indicating that their formation was caused by
adsorption of gas dissolved in water. Thus, atomically thin graphene, even at a
narrow width of 20 nm, changes the local surface chemistry of a highly
hydrophilic substrate. Further, surface hydrophobicity significantly affects
gas adsorption, which has broad implications for diverse phenomena in water.
