Electrostatic Surface Charge at Aqueous/α-Al2O3Single-Crystal Interfaces as Probed by Optical Second-Harmonic Generation

Abstract

Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (11̄02) crystallographic surfaces of corundum (α-Al₂O₃) single-crystal substrates. The pH value of the point of zero charge (pH_pzc) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO₃ carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH_pzc, occurs at pH 4.1 ± 0.4 for the (0001) surface and pH 5.2 ± 0.4 for the (11̄02) surface. SHG measurements that were recorded as a function of NaNO₃ concentration at fixed pH values were found to corroborate the pH_pzc values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH_pzc values for the α-Al₂O₃ single-crystal surfaces are significantly more acidic than published values for Al−(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al−(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups.