Structures and charging of α-alumina (0001)/water interfaces studied by sum-frequency vibrational spectroscopy

Sum-frequency vibrational spectroscopy in the OH stretch region was employed to study structures of water/alpha-Al(2)O(3) (0001) interfaces at different pH values. Observed spectra indicate that protonation and deprotonation of the alumina surface dominate at low and high pH, respectively, with the interface positively and negatively charged accordingly. The point of zero charge (pzc) appears at pH approximate to 6.3, which is close to the values obtained from streaming potential and second-harmonic generation studies. It is significantly lower than the pzc of alumina powder. The result can be understood from the pK values of protonation and deprotonation at the water/alpha-Al(2)O(3) (0001) interface. The pzc of amorphous alumina was found to be similar to that of powder alumina.