Correlative angstrom-scale microscopy and spectroscopy of graphite-water interfaces

TL;DR

This study combines advanced microscopy and spectroscopy to reveal the complex, potential-dependent structures of water at graphite interfaces, resolving longstanding debates about interfacial water configurations.

Contribution

It introduces a correlative in situ microscopy-spectroscopy method to directly observe interfacial water structures under realistic conditions.

Findings

Identification of two interfacial configurations at open circuit potential.

Discovery of a transient state with strong hydrogen bond breaking.

Observation of a stable structure with pristine water at negative potentials.

Abstract

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 and interface-sensitive Raman spectroscopy 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 (HB) breaking effects, and a steady state with hydrocarbons dominating the interface and weak HB breaking in the surrounding water. At sufficiently negative potentials, both states transition into a stable structure featuring pristine water with a broader distribution of HB configurations. Our three-state model resolves many long-standing controversies on interfacial water structure.