The manganese oxides are important materials in many fundamental and technological respects, from magnetism to electrode materials for lithium batteries.Despite of this broad interest, the surface science of Mn-oxides is relatively little explored due to the difficulties in preparing oxide surfaces of bulk crystals with a high structural quality. Another problem arises from the insulating nature of most bulk-type manganese oxides.
Here we describe the formation of manganese oxide structures on a Pd(100) surface in thin film form and their characterisation by STM, AFM, SPA-LEED, XPS, XAS and DFT calculations. In the ultrathin film limit (1-2 monolayers, ML) a rich variety of ordered interface-stabilised oxide structures forms as the oxygen pressure varies between 5x10-8 and 1x10-6 mbar. These structures will be discussed in terms of common building units, as revealed by STM and XAS data.
In the bulk-type oxide limit (oxide coverage ≥ 20 ML) the Mn-oxide morphology and structure exhibit pronounced oxygen pressure dependence. At 2x10-7 mbar of oxygen an epitaxial MnO(100) layer is kinetically stabilised on the Pd(100) surface. The latter transforms under more reducing conditions (O2 pressure < 1x10-7 mbar, or annealing in UHV to 600°C) into a (√3x√3)R30° reconstructed MnO(111) surface, on top of which three-sided nanometer-sized pyramids with (100) facets are observed in SPA-LEED and NC-AFM. This transition is suggested to be driven by the reduced strain energy of the MnO(111)-Pd(100) interface, through better row matching. DFT calculations have demonstrated that the growth of MnO(111) layers is energetically preferred over MnO(100), because of the epitaxial stabilisation at the MnO(111)-Pd(100) interface. More oxidising conditions (O2 pressure > 1x10-6 mbar) cause a structural transformation of the MnO to a Mn3O4 phase. The latter grows in the form of rectangular-shaped nanoparticles, terminated by a (001) plane.
Supported by the Austrian Science Funds and by the EU STREP programme GSOMEN.
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