AVS 46th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS-WeP

Paper SS-WeP13
The Interaction of Water with the Basal Plane of @alpha@-Cr@sub 2@O@sub 3@: Dissociation and Multiply Coordinated Adsorption Sites

Wednesday, October 27, 1999, 5:30 pm, Room 4C

Session: Poster Session
Presenter: M.A. Henderson, Pacific Northwest National Laboratory
Authors: M.A. Henderson, Pacific Northwest National Laboratory
S.A. Chambers, Pacific Northwest National Laboratory
C.L. Perkins, Pacific Northwest National Laboratory
Correspondent: Click to Email
We have examined the interaction of water with an @alpha@-Cr@sub 2@O@sub 3@(001) thin film surface using TPD, HREELS and LEED. The @alpha@-Cr@sub 2@O@sub 3@(001) film is the terminus of an alternating @alpha@-Cr@sub 2@O@sub 3@/@alpha@-Fe@sub 2@O@sub 3@ heterostructure grown on an @alpha@-Al@sub 2@O@sub 3@(001) substrate using molecular beam epitaxy. The @alpha@-Cr@sub 2@O@sub 3@ lattice is 2@super o@ expanded inplane due to lattice matching to the @alpha@-Fe@sub 2@O@sub 3@. Water interacts strongly with the @alpha@-Cr@sub 2@O@sub 3@(001) surface, desorbing in two prominent TPD states at 345 and 297 K, both of which exhibit first-order desorption behavior. Weaker binding TPD states are observed at 210 and 185 K. Using a calibrated doser, TPD results indicate that the coverage of water in the 345 and 297 K TPD states is twice that of the surface Cr@super 3+@ coverage suggesting that each surface cation site binds two water molecules. Although no irreversible decomposition is detected by TPD, HREELS (acquired at room temperature to avoid sample charging) shows prominent losses at 3605, 2890 and 915 cm@super -1@. Using D2O, the latter two losses shift to 2645 and 2135 cm@super -1@ and the former is undetected (shifted under the lattice phonon modes). The 3605/2645 cm@super -1@ and 915/(?) cm@super -1@ losses are indicative of the hydroxyl stretching and bending modes, respectively, of a terminal hydroxyl group, whereas the 2890/2135 cm@super -1@ losses are suggestive of a very weak hydrogen-bonding interaction, probably between the deposited hydrogen/deuterium atom and the terminal OH/OD group. Although this hydrogen-bond is weak, as suggested by its low O-H stretching frequency, the dynamic dipole of its O-H stretch is as strong as that of the terminal OH group. Although HREELS analysis of the 297 K TPD state is in progress, our tentitive model is that each surface Cr@super 3+@ site binds both a molecular and dissociative form of water. The absence of strong coverage dependence in the TPD suggests that these water species are only weakly interacting. The role of lattice expansion will also be discussed, in comparison with studies on an unstrained @alpha@-Cr@sub 2@O@sub 3@(001) film (to be conducted).@footnote 1@ @FootnoteText@ @footnote 1@ This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. The research reported here was performed in the William R. Wiley Environmental Molecular Science Laboratory, a Department of Energy user facility funded by the Office of Biological and Environmental Research.