Atomic-Scale Mapping of Interfacial Water on Oxide Surfaces via Proton-Resolved NMR and <i>Ab Initio</i> Simulations

Understanding the molecular structure of water at solid-liquid interfaces is essential for advancing catalysis, energy conversion, and environmental technologies. However, directly characterizing interfacial water species in excess liquid water remains a major experimental challenge. Here, we introduce a new strategy that combines high-resolution ¹H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy with first-principles molecular dynamics simulations to resolve and assign the chemical environments of interfacial water and hydroxyl species on hydrated titanium dioxide (TiO₂) nanoparticles. Using partial proton-deuteron exchange and fast MAS techniques, we achieve site-specific detection of surface-bound H₂O and OH groups at the solid-liquid interface. This enables a detailed atomistic assessment of surface hydration states under ambient conditions. Our results reveal that the fully hydrated anatase (101) TiO₂ surfaces are positively protonated and exhibit hydrophobic behavior, a counterintuitive finding with significant implications for interfacial reactivity. The approach developed in this work is widely applicable for unraveling complex hydration structures at oxide-water interfaces with molecular resolution.