Insights on Interfacial Structure, Dynamics, and Proton Transfer from Ultrafast Vibrational Sum Frequency Generation Spectroscopy of the Alumina(0001)/Water Interface

Abstract

Steady-state and time-resolved vibrational sum frequency generation (vSFG) were used to investigate the structure and dynamics of water at the α-Al 2 O 3 (0001) surface. The vSFG spectra of the O—H stretch of water next to the Al 2 O 3 (0001) surface are blue-shifted compared to the Al 2 O 3 (112̅0) surface, indicating its weaker hydrogen bonding network. Consequently, the vibrational dynamics of the O—H stretch of the neutral Al 2 O 3 (0001) surface is two times slower than the neutral Al 2 O 3 (112̅0) surface. Furthermore, the vibrational dynamics of the O—H stretch of water next to charged Al 2 O 3 surfaces is observed to be faster than that in bulk water and at charged SiO 2 surfaces, which could be due to (a) fast proton transfer dominating the vibrational relaxation and/or, (b) efficient coupling between the O—H stretch and the bend overtone via the presence of low frequency (∼3000 cm –1 ) O—H stretching modes. Lastly, the addition of excess ions (0.1 M NaCl) seems to have little to no effect on the time scale of vibrational dynamics, which is in contrast with the behavior observed at the silica surface, where addition of excess ions was observed to change the time scale of vibrational relaxation of interfacial water.