Effect of pH on the Water/α-Al₂O₃ (1102) Interface Structure Studied by Sum-Frequency Vibrational Spectroscopy

Abstract

Sum frequency vibrational spectroscopy was used to study the structure of water/α-Al2O3 (11̅02) interfaces at different pH values. The OH stretch spectra are dominated by interfacial water contributions at lower frequencies and by bonded hydroxyls on the Al2O3 surface at higher frequencies. Protonation and deprotonation of various oxide functional groups at the α-Al2O3 surface as pH varies can be monitored quantitatively by changes of the spectrum, allowing extraction of their pK reaction values. The point of zero charge of the interface is found to be at pH ∼6.7.