Probing the molecular structure at graphite-water interfaces by correlating 3D-AFM and SHINERS.

Water at solid surfaces is key for many processes ranging from biological signal transduction to membrane separation and renewable energy conversion. However, under realistic conditions, which often include environmental and surface charge variations, the interfacial water structure remains elusive. Here we overcome this limit by combining three-dimensional atomic force microscopy (3D-AFM) and interface-sensitive shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) to characterize the graphite-water interfacial structure in situ. Through correlative analysis of the spatial liquid density maps and vibrational peaks within ≈2 nm of the graphite surface, we find the existence of two interfacial configurations at open circuit potential, a transient state where pristine water exhibits strong hydrogen bond (H-bond) breaking effects, and a steady state with hydrocarbons dominating the interface and weak H-bond breaking in the surrounding water. At sufficiently negative potentials, both states transition into a stable structure featuring pristine water with a broader distribution of H-bond configurations. Our three-state model resolves many long-standing controversies on interfacial water structure.