Water film inside graphene nanosheets: Electron transfer reversal between water and graphene via tight nano-confinement
Based on quantum dynamics simulations using the density functional tight-binding (DFTB) method, we provide a detailed geometric and electronic structure characterization of a nano-confined water film within two parallel graphene sheets. We find that, when the distance between the graphene bilayer is reduced to 4.5 A˚, the O-H bonds of the water molecules become almost parallel to the bilayer; however, further reducing the distance to 4.0 A˚ induces an abnormal phenomenon characterized by several O-H bonds pointing to the graphene surface. Electronic structure analyses revealed that the charge transfers of these nano-confined water molecules are opposite in these two situations. In the former scenario, the electron loss of each water molecule in the confined aqueous monolayer is approximately 0.008 e, with electrons migrating to graphene from the p orbitals of water oxygen atoms; however, in the latter case, the electron transfer is reversed, with the water monolayer gaining electrons from graphene in excess of 0.017 e per water molecule. This reversed behavior arises as a result of the empty 1s orbitals of H atoms, which are disturbed by the delocalized π orbitals formed by the p electrons of the carbon atoms. Our current study highlights the importance of the nano-confinement on the electronic structures of interfacial water, which can be very sensitive to small changes in physical confinement such as a small reduction in the graphene interlayer distance, and may have implications in de novo design of graphene nano-channels with unique water transport properties for nanofluidic applications.