Electron-stimulated oxidation of thin water films adsorbed on TiO2(110)

Electron-stimulated reactions in thin (<3 monolayer, ML) water films adsorbed on TiO2(110) are investigated. For electron fluences less than similar to 1 x 10(16) e(-)/cm(2), irradiation with 100 eV electrons results in electron-stimulated desorption (ESD) of atomic hydrogen and small amounts of molecular hydrogen but no measurable O-2. The ESD results in oxygen enrichment ("oxidation") of the remaining film and oxidation of the oxygen vacancies originally present on the TiO2(110) surface. The postirradiation temperature-programmed desorption (TPD) spectra of the remaining water change in characteristic ways. The species remaining on the TiO2(110) after irradiation of adsorbed water films are similar to those produced by codosing water and O-2 without irradiation. Annealing above similar to 600 K reduces the oxidized surface, and water TPD spectra characteristic of a reduced (i.e., ion-sputtered and vacuum annealed) TiO2(110) surface are recovered. The rate of electron-stimulated "oxidation" of the water films is proportional to the coverage of water in the first layer for coverages less than 1 ML. However, higher coverages suppress this reaction. When thin water films are irradiated, the rate of electron-stimulated oxidation is independent of the initial oxygen vacancy concentration, as is the final state achieved at high electron fluences. To explain the results, we propose that electron excitation of water molecules adsorbed on Ti4+ sites leads to dissociation and desorption of hydrogen atoms while leaving OHs adsorbed at those sites. If hydroxyls are present in the bridging oxygen rows, these react with the OHs on the Ti4+ sites to re-form water and heal the oxygen vacancy associated with the bridging OH. Once the bridging hydroxyls have been eliminated, further irradiation increases the concentration of OHs in the Ti4+ rows leading to the creation of species which block sites in the Ti4+ rows, perhaps H2O2 and/or HO2. The results show that electron-stimulated oxidation of the adsorbed water films via H atom ESD is considerably more efficient than electron-stimulated reduction of the TiO2 substrate due to O+ and O ESD: the ratio of the dissociation cross section of adsorbed water to the cross section for ESD of substrate oxygen is estimated to be similar to 18:1 for 100 eV electrons.