Regensburg 2007 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 56: Symposium: The Solid-Liquid Interface: A Challenge for Theoreticians I
O 56.3: Vortrag
Donnerstag, 29. März 2007, 16:45–17:00, H36
Sum-Frequency Vibrational Spectroscopy of Al2O3(0001)/ Water Interfaces — •Björn Braunschweig, Stephan Eißner, and Winfried Daum — Institut für Physik und Physikalische Technologien, TU Clausthal, Leibnizstrasse 4, D-38678 Clausthal-Zellerfeld
Sum-frequency generation (SFG) has been applied to study O-H stretching vibrations of water molecules and hydroxyl groups at Al2O3(0001)/water interfaces. The vibrational spectra of these interfaces strongly depend on the roughness of the Al2O3 surface which was characterized by AFM. O-H stretching bands at 3140 cm−1 and 3450 cm−1 are assigned to hydrogen-bonded water molecules at the interface with tetrahedral and non-tetrahedral coordination, respectively. A strong pH dependence observed for the intensity of the 3140 cm−1 band can be attributed to the interfacial electric field caused by protonation or deprotonation of surface hydroxyl groups. A marked minimum in the pH dependence of this band is related to the isoelectric point of the Al2O3(0001) surface. On rough Al2O3(0001) surfaces we observe an additional narrow band at 3700 cm−1 originating from O-H stretching vibrations of free surface hydroxyl groups (aluminol, AlOH). This band is completely absent on smooth Al2O3(0001) surfaces which have been prepared by annealing at 1100 ∘C and which exhibit atomically flat terraces. The 3700 cm−1 band is assigned to hydroxyl groups, presumably adsorbed at defect or step sites, that do not couple to the hydrogen-bonded network of the interfacial water molecules.