Recent advances in probing surface/interfacial water by scanning probe microscopy

Surface and interfacial water is ubiquitous in nature and modern technology.It plays vital roles in an extremely wide range of basic and applied fields including physics,chemistry,environmental science,material science,biology,geology, etc.Therefore,the studies of surface/interfacial water lies at the heart of water science.When water molecules are brought into contact with various materials,a variety of phenomena can show up,such as wetting,corrosion,lubrication, nanofluidics,ice nucleation,to name just a few.Due to the complexity of hydrogen-bonding interactions between water molecules and the competition between water-water interaction and water-solid interaction,surface/interfacial water is very sensitive to local environment,which makes it necessary to study the structure and dynamics of water at the molecular level.In recent years,the development of new scanning probe techniques allows detailed real-space research on surface/interfacial water at single-molecule or even submolecular scale.In Section 2,several representative scanning probe techniques and their applications in surface/interfacial water are reviewed.The first one is ultra-high vacuum scanning tunneling microscopy,which allows molecular imaging of single water molecules,water clusters,wetting layers,and even water multilayers on metal surfaces as well as ultrathin insulating films.Based on scanning tunneling microscopy,the single-molecule vibrational spectroscopy can be further developed to probe the vibration and movement of individual water molecules,which assist us in understanding water diffusion,dissociation and quantum nature of hydrogen bonds.As a versatile tool at liquid/solid interfaces,electrochemical scanning tunneling microscopy opens up the unique possibility of probing the double electric layer and identifying water dynamics during electrochemical reactions. Moreover,non-contact atomic force microscopy yields higher resolution than scanning tunneling microscopy,such that the topology of hydrogen-bonding skeleton of surface/interfacial water and even the degree of freedom of hydrogen atoms can be discerned.To conclude this review,the challenges and future directions of this field are discussed in Section 3, focusing on non-invasive imaging under ambient conditions,ultrafast molecular dynamics,and novel structures under high pressures.