An HREELS and TPD study of water on TiO 2(110): the extent of molecular versus dissociative adsorption

The interaction of water with the (110) crystal face of TiO ₂ (rutile) was examined with high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). Water desorption occurred in three sequential TPD features as a function of exposure at 270, 175 and 155 K, ascribed to monolayer, second layer and multilayer states, respectively. Estimation of the extent to which water molecularly versus dissociatively adsorbed on the TiO ₂(110) surface was attempted by comparing HREELS and TPD data. Very low water exposures (below 7 × 10 ¹³molecules/cm ²) dissociatively adsorbed at 135 K to surface hydroxyl groups ( v(OH) = 3690 cm ^{- 1}), but molecular adsorption ( v(OH) = 3420 - 3505 cm ^{- 1} and δ(HOH) = 1625 cm ^{- 1}) resulted from additional water exposure. The appearance of molecular water in HREELS coincided with a red-shift in the OH stretching frequency of the hydroxyl, presumably due to the formation of hydrogen-bonding interactions with the adsorbed water molecules. Little or no additional water dissociation occurred during heating of the monolayer as inferred from HREELS. Second layer water on TiO ₂(110) was evident in HREELS by OH stretching intensity below 3400 cm ^{- 1} indicative of hydrogen-bonding. However, the OH stretching mode of monolayer water molecules was not red-shifted by the presence of second layer water suggesting that protons on monolayer water molecules were not involved in hydrogen-bonding second layer water molecules. The influence of second layer water on subsequent layers was evident in both HREELS and TPD.