Nanoscale Surface Roughness Induces Long-Range Ordering of Interfacial Water Over Tens of Molecular Layers

Understanding how far solid surfaces influence the structure of adjacent water remains a central challenge in interfacial science. Here, we demonstrate that nanoscale surface roughness can induce long-range ordering of interfacial water extending over tens of molecular layers under ambient conditions. We develop an attenuated total reflection infrared (ATR-FTIR) differential spectroscopy strategy that enables quantitative extraction of interfacial water spectra from the overwhelming bulk-water background and allows simultaneous determination of interfacial water thickness. Applied to TiO₂-water interfaces, this approach reveals a clear transition in hydrogen-bond structure: smooth single-crystal surfaces exhibit moderately strengthened hydrogen bonding similar to bulk water perturbation, whereas rough surfaces generate pronounced low-frequency O-H stretching bands characteristic of strongly hydrogen-bonded, ice-like networks. Cluster-based vibrational analysis indicates that these spectral features originate from highly coordinated hydrogen-bond motifs. Quantitative analysis further shows that interfacial water extends to 23-41 molecular layers on rough TiO₂ surfaces, significantly exceeding the 11-15 layers observed on smooth crystals. These results provide direct experimental evidence that nanoscale confinement at realistic solid-liquid interfaces can stabilize extended hydrogen-bond networks and establish a quantitative framework for probing interfacial water beyond the limits of conventional surface-sensitive techniques.