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Hannover, Germany

Lovis F.,Universitaot Hanover | Imbihl R.,Universitaot Hanover
Journal of Physical Chemistry C

Self-organization of ultrathin vanadium oxide layers (θ V < 0.5 MLE) on a Rh(111) surface during the H 2 + O 2 reaction has been investigated in the 10 -6-10 -4 mbar range using photoemission electron microscopy (PEEM) as a spatially resolving method. We observe that the homogeneous state is transformed via reaction fronts into macroscopic quasi-stationary patterns. In these patterns a condensation of V and O into stripes or islands takes place (θ V < 0.5 MLE). The coarsening of the patterns with time follows a power-law dependence. The patterns are a nonequilibrium structure explained here tentatively as a result of reactive phase separation. Formation of the oxide distribution patterns is associated with a considerable memory effect. This memory effect can be exploited for microstructuring of the surface by applying a schedule for parameter variation. Decoration of such microstructures with nickel leads to channel-like structures through which pulses propagate during the H 2 + O 2 reaction. © 2011 American Chemical Society. Source

Lovis F.,Universitaot Hanover | Hesse M.,Universitaot Hanover | Locatelli A.,Elettra - Sincrotrone Trieste | Mentes T.O.,Elettra - Sincrotrone Trieste | And 5 more authors.
Journal of Physical Chemistry C

Upon exposure of ultrathin vanadium oxide layers on Rh(111) (θV ∼ 0.2 MLE) to the H2 + O2 reaction, we observe the reversible formation of stationary macroscopic stripe patterns caused by lateral redistribution of the vanadium oxide. These patterns are investigated in the 10-6 mbar range at around 750 K, employing LEEM (low energy electron microscopy), microprobe-LEED and microprobe-XPS. The stripe patterns evolve via several stages and structures: essentially, vanadium oxide nanoislands with a (√7 × √7)R19.1° structure are reduced from V+5 to V3+ and condense into a macroscopic (2 × - 2) oxide phase upon increasing p(H2). The orientation of the stripe patterns is determined by step bunches, which introduce a diffusional anisotropy. The stationary patterns are discussed within the concept of reactive phase separation. © 2011 American Chemical Society. Source

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