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  • 101.
    Önsten, Anneli
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Göthelid, Mats
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Karlsson, Ulf O.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Atomic structure of Cu2O(111)2009In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 603, no 2, p. 257-264Article in journal (Refereed)
    Abstract [en]

    Low-energy electron diffraction and scanning tunneling microscopy have been used to probe the surface atomic structure Of Cu2O(111) after various sample preparations. Annealing in oxygen gives a stoichiometric (1 x 1) oxygen terminated surface and further annealing in ultra-high vacuum results in a clear (root 3 x root 3)R30 degrees reconstruction and surface faceting. Tunneling from filled states in the reconstructed surface reveals a hexagonal pattern of large protrusions, which show an internal structure. The reconstruction is believed to be due to one-third of a monolayer of ordered oxygen vacancies. At areas on the surface where the large features are missing, another smaller type of protrusions is visible, which is associated with the ideal (1 x 1) surface. The relative position of the two types of features gives two possible models of the (111) surface. In the first model, the (1 x 1) surface is the ideal bulk terminated surface and coordinatively unsaturated oxygen ions are missing in the reconstructed surface. The second model agrees with the first model with the exception that coordinatively unsaturated copper ions in the outmost copper layer are missing in both the (1 x 1) and the reconstructed surface. The latter model is supported by previous surface free energy calculations. Since the undercoordinated copper ions have been suggested to be the catalytic active sites Of Cu2O(111), the presence or absence of these cations could be of great importance for the fundamental understanding of the surface reactivity Of Cu2O and of copper-based catalysts.

  • 102.
    Önsten, Anneli
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Stoltz, Dunja
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Palmgren, Pål
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Yu, Shun
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Claesson, Thomas
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Göthelid, Mats
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Karlsson, Ulf O.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    SO2 interaction with Zn(0001) and ZnO(0001) and the influence of water2013In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 608, p. 31-43Article in journal (Refereed)
    Abstract [en]

    Photoelectron spectroscopy has been used to study room temperature adsorption of sulfur dioxide on clean and water exposed (0001) surfaces of zinc and zinc oxide. Water has no significant effect either on clean or on SO2 exposed Zn(0001) at the low water pressures used (p < 10(-7) mbar). In the Case of the zinc-terminated ZnO(0001) surface, however, water adsorbs dissociatively and OH groups are shown to have a considerable effect on SO2 surface reactions. A strong oxidation reaction occurs between Zn(0001) and SO2 giving various sulfur containing species. On ZnO(0001), SO2 interacts mainly with oxygen sites giving SO3 or SO4 species. It is shown that the ZnO(0001) sample preparation procedure can have large effects on surface chemical and physical properties. Samples cleaned by four different preparation procedures are investigated, namely sputtering only and sputtering followed by annealing at 450 degrees C, 530 degrees C and 600-650 degrees C. Annealing at 600 degrees C leads to a transition from a partly OH-terminated surface to a triangularly structured surface free from OH groups. Adsorption of SO2 on the latter surface leads to a decreased surface conductivity, which hampers photoemission measurements. Water is shown to block SO2 adsorption sites on both 450 degrees C and 530 degrees C annealed samples. On the latter sample. SO2 reduction has been observed to a small extent on the clean surface and to a larger extent when the surface is prehydroxylated. Here, we speculate that water, similar to hydrogen, generates surface zinc clusters on ZnO(0001). Zinc clusters could enable charge transfer to the antibonding LUMO of the SO2 molecule and subsequent dissociation.

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