IR-analysis of H-bonded H 2O on the pure TiO 2 surface

Abstract

The surface state of optically pure polydisperse TiO 2 (anatase and rutile) was determined by infra-red (IR) spectroscopy analysis in the temperature range of 100–453 K. Anatase A300 spectrum, contrary to rutile R300 one, has a broad three-component absorption band with peaks at 1048, 1137 and 1222 cm −1 in the spectral range of δ(Ti–O–H) deformation vibrations. For rutile R300 we observed a very weak band at 1047 cm −1, and for the thermal treated rutile R900 these bands were not appeared at all. The analysis of temperature dependencies for the mentioned absorption bands revealed the spectral shift of 1222 cm −1 band towards the high frequencies, when the temperature increased, but the spectral parameters of 1137 and 1048 cm −1 bands remained the same. The temperature of 1222 cm −1 band maximum shift was 373–393 K and correlated with DSC data. Obtained results allowed to assign 1222 cm −1 band to the deformation vibrations of OH-groups, bounded to the surface adsorbed water molecules by weak hydrogen bonds (∼5 kcal/mol). During the temperature growth these molecules desorbed, which also resulted in the intensity decreasing of stretching OH-groups vibration IR-bands at 3420 cm −1. The destruction and desorption of surface water complexes led to Ti–O–H bond strengthening. IR bands at 1137 and 1048 cm −1 were attributed to the stronger bounded adsorbed water molecules, which are also characterized with stretching OH-groups vibration bands at 3200 cm −1. These surface structure were additionally stabilized by hydrogen bonds with the neighbouring TiO 2 lattice anions and other OH-groups, and desorbed at higher temperatures.