Investigations of the structure of H2O clusters adsorbed on TiO2 surfaces by near-infrared absorption spectroscopy

The overtone and combination bands of the fundamental vibration modes (nu(1), symmetric stretching; nu(2), bending; and nu(3), asymmetric stretching) attributed to the H2O molecules adsorbed on a TiO2 surface could be observed in the near-infrared (NIR) region. Especially, two absorption bands attributed to the combination (nu(2) + nu(3)) and (nu(1) + nu(3)) modes of the H2O molecules adsorbed on the TiO2 surface were observed at around 1940 and 1450 nm, respectively. From detailed investigations on the (nu(2) + nu(3)) combination band, it was found that H2O molecules absorbed on a TiO2 surface aggregate to form clusters due to the high surface tension of H2O arising from the intermolecular hydrogen bonds, and the hydrogen-bonded H2O in the bulk part of the cluster and the hydrogen-bond-free H2O in the outside spherical part of the cluster could be easily distinguished. Furthermore, it was quantitatively confirmed that the relaxation of the surface energy accompanying the adsorption of H2O on the TiO2 surface stabilized the adsorption states of the hydrogen-bonded H2O molecules, while on the other hand, the hydrogen-bond-free H2O molecules became unstable as compared to the liquid-phase H2O molecules.