Surface structural evolution of α-Al2O3(11-20) from UHV to ambient conditions probed by vibrational sum frequency spectroscopy

Water can restructure oxide surfaces and thereby alter interfacial chemistry, yet determining the operative termination under realistic conditions remains challenging when multiple terminations are close in stability and when interfacial OH-stretch spectra are congested by physisorbed water contributions. Here we combine vibrational sum-frequency spectroscopy (VSFS) in the surface-phonon region with density functional theory (DFT) to resolve water-driven termination changes of α-Al₂O₃(11-20) from ultrahigh vacuum (UHV) to ambient conditions. VSFS spectra recorded between 900 and 1200 cm^-1 probe Al-O(H) phonon modes that directly report on surface coordination environments. For the UHV-prepared O-I surface (Al₂O:Al₃O = 1:2), two resonances near 970 and 1040 cm^-1 are observed and assigned-supported by DFT normal-mode analysis-to vibrations of Al₃O and Al₂O associated surface motifs. Low-coverage water dosing produces small red shifts and selectively perturbs the Al₂O related feature, consistent with dissociative adsorption forming Al₂OH at this site. In contrast, exposure to ambient water yields blue-shifted resonances (~980 and ~1049 cm^-1), indicating a distinct termination, which we attribute to additional sub-monolayer O atoms generated by H₂O dissociation. Comparison to DFT models supports assignment of the ambient surface to a fully protonated O-III termination characterized by AlOH/Al₂OH/Al₃OH motifs in an approximately 1:1:1 ratio. Interestingly, unlike the (0001) surface, exposure to bulk liquid water does not further alter the surface termination. These results establish surface-phonon VSFS as a sensitive route to track water-induced termination transitions at oxide/water interfaces. The atomic-level insights provided in this work are essential for applications employing α-Al₂O₃ as a substrate.