Structural Anomality of the Adsorbed Water on Al-Doped Silica Revealed by Heterodyne-Detected Vibrational Sum-Frequency Generation Spectroscopy

Water adsorption on glasses is known to change their macroscopic surface properties, such as wettability and friction coefficient; however, the mechanism by which it changes them is still poorly understood. Furthermore, the surface structure of water adsorbed on glasses has not been explored except for pure silica (SiO2). This is presumably because of the experimental difficulty involved in selective measurements under ambient conditions. Although SiO(2 )is the main constituent of glasses, most glasses contain various minor elements that affect the structure of adsorbed water. In this study, heterodyne-detected vibrational sum-frequency generation spectroscopy was applied to aluminum (Al)-doped (0-75%) SiO2 and the spectra were analyzed by using singular value decomposition. The results clearly indicated that the adsorbed water drastically changes its structure only on adding a small amount of Al (<13%), which is contrary to the previous notion that the water structures simply change from those on pure SiO(2 )to Al2O3. The anomaly was discussed from the viewpoint of a characteristic water structure around the boundaries of silicon and Al nanodomains in the materials.