H2O interaction with bare and Li-precovered TiO2:: studies with electron spectroscopies (MIES and UPS(HeI and II))

TiO2(1 1 0) single crystals and titania thin films on Mo(1 1 0), bare and precovered by Li atoms, were exposed to water at 120 K. During the adsorption process metastable impact electron spectroscopy (MIES) and ultraviolet photoemission spectroscopy (UPS)(Hel and II) spectra were collected in situ. Spectroscopic and temperature-programmed desorption data were obtained during the annealing of water multilayers grown at 120 K. For the adsorption on the bare TiO2 surface we conclude that an initial dissociative adsorption at defect sites is followed by molecular adsorption up to multilayer formation. From the exposure dependence of the MIES and UPS signals we conclude that, even before the completion of the first adlayer, no isolated water molecules are sampled, and strong lateral interaction between adjacent adsorbates stabilizes molecular water. On TiO2(1 1 0), precovered by about 1 ML of Li atoms, water is adsorbed dissociatively both at 120 K and at room temperature until the first, mixed layer consisting of Li and OH species, is completed. On top of the first layer additional water is adsorbed molecularly. Annealing experiments show that the first layer remains intact when the second layer forms. (C) 2001 Elsevier Science B.V. All rights reserved.