Elucidating the structure of the interface between natural (reduced) anatase TiO2 (101) and water is an essential step toward understanding the associated photoassisted water splitting mechanism. Here we present surface X-ray diffraction results for the room temperature interface with ultrathin and bulk water, which we explain by reference to density functional theory calculations. We find that both interfaces contain a 25:75 mixture of molecular H2O and terminal OH bound to titanium atoms along with bridging OH species in the contact layer. This is in complete contrast to the inert character of room temperature anatase TiO2 (101) in ultrahigh vacuum. A key difference between the ultrathin and bulk water interfaces is that in the latter water in the second layer is also ordered. These molecules are hydrogen bonded to the contact layer, modifying the bond angles.

Despite the proven properties of the anatase phase of TiO2 related to photocatalysis, detailed mechanistic information regarding a photo-oxidation reaction has not yet been derived from single crystal studies. In this work, we have studied the photo-oxidation of ethanol (as a prototype hole scavenger organic molecule) adsorbed on an anatase TiO2 (101) surface by STM and on-line mass spectrometry to determine the adsorbate species in the dark and UV illumination in the presence of O2 and to extract kinetic reaction parameters under photo-excitation. The reaction rate for the photo-oxidation of ethanol to acetaldehyde is found to depend on the O2 partial pressure and surface coverage, where the order of the reaction with respect to O2 is close to 0.15. Carbon-carbon bond dissociation leading to CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of rutile TiO2 (110) single crystal. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to its molecular and dissociative modes. A mixed adsorption is also supported by our DFT calculations in which we determine similar Energies of adsorption (Eads) for molecular (1.11 eV) and dissociative modes (0.93 eV). Upon UV exposure at (and above) 3×10 −8 mbar O2, a third species is identified on the surface as a reaction product, which can be tentatively attributed to acetate/formate species on the basis of C1s XPS results. The kinetics of the initial oxidation steps are evaluated using the STM and mass spectrometry data.

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