Sum-Frequency Scattering Spectroscopy Reveals the Charging Mechanism and Surface Structure of hBN Nanoflakes in Solution

A molecular understanding of the interactions between two-dimensional (2D) layered materials and liquids is crucial for nanofluidics, catalysis, and solution-based 2D material processing. Among 2D materials, hexagonal boron nitride (hBN) has a number of outstanding properties, but its interactions with liquids remain poorly characterized. Here, we investigate the interfacial structure of few-layer hBN nanoflakes suspensions in ethanol and ethanol-water mixtures. Electrophoretic light scattering suggests that the nanoflakes are effectively positively charged in ethanol and negatively charged in an ethanol-water mixture. Vibrational sum-frequency scattering spectroscopy reveals the surface structural changes underlying this charge reversal. Signatures of charge transfer of opposite direction are detected on both the flake lattice and in the liquid. The different (partial) charge distributions in ethanol and water explain the apparent charge reversal.