High-resolution imaging of interfacial water: from water monomer to two-dimensional ice

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Recently, we made a breakthrough in achieving submolecular-resolution imaging of individual water molecules using a scanning tunneling microscope (STM) [1]. Such a technique opens up the possibility of determining the detailed topology of H-bonded networks at water/solid interfaces with atomic precision. Thanks to the high-resolution STM imaging, we discover a new type of two-dimensional (2D) ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond the conventional bilayer picture [2]. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a 2D extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. [1] J. Guo, X. Z. Meng, J. Chen, J. B. Peng, J. M. Sheng, X. Z. Li, L. M. Xu, J. R. Shi, E. G. Wang*, and Y. Jiang*, Nature Materials 13, 184 (2014). [2] J. Chen, J. Guo, X. Z. Meng, J. B. Peng, J. M. Sheng, L. M. Xu, Y. Jiang*, X. Z. Li*, E. G. Wang, Nature Communications 5, 4056 (2014).