The bonded interfacial layer structure of α-Al₂O₃ (0001)/water at different pH values studied by sum frequency vibrational spectroscopy.

This article focuses on the molecular-level structure of the bonded interfacial layer (BIL) at the α-Al₂O₃ (0001)/water interface across different pH values, investigated using sum-frequency vibrational spectroscopy (SFVS) with suppression of the diffuse layer (DL) contribution by adding salts. The study finds that variations in pH, which alter surface charge through protonation/deprotonation of surface aluminols, have minimal impact on the BIL structure, indicating that the interfacial hydrogen-bonded network is primarily governed by the surface atom configuration rather than surface charge. The SFVS spectra reveal dominant features at ∼3700 cm⁻¹ (dangling OH groups) and ∼3430 cm⁻¹ (hydrogen-bonded OH), with negligible presence of the ∼3200 cm⁻¹ band, suggesting relatively weak alumina-water interactions. Additionally, the ∼3430 cm⁻¹ band exhibits a threefold anisotropy consistent with the α-Al₂O₃ (0001) surface symmetry, demonstrating a templating effect of the solid surface on the interfacial hydrogen-bonded network.