Growth and Oxidation of Ni Nanostructures on Stepped Rh Surfaces

The growth and oxidation of Ni nanostructures on stepped Rh(15 15 13) and (553) surfaces, both vicinals of Rh(111), have been studied by scanning tunneling microscopy (STM), LEED, core level spectroscopies with use of synchrotron radiation (XPS and NEXAFS), and work function measurements. At low coverages, quasi-one-dimensional Ni rows decorate the Rh step edges. With increasing coverages, the Ni adlayers grow layer-by-layer in a restricted step-flow growth mode, pseudomorphically up to ∼2 monolayer (ML). Relaxation to the Ni bulk lattice occurs in the range 2−10 ML. The oxidation of the low-coverage Ni nanowires creates ordered quasi-one-dimensional Ni−O structures at the Rh step edges. In the Ni coverage range up to 2 ML, uniaxially ordered (2 × 1) and (6 × 1) oxide structures are observed. On thicker films, local NiO (100)-type and (111)-type reconstructions are both observed in the STM. The NEXAFS measurements indicate that the low-dimensional Ni−O nanophases are distinctly different from bulk-type NiO, and three chemically different oxygen species are identified in the O 1s XPS core level spectra. The chemical reactivity of the Ni−Rh surfaces toward oxygen seems to be highest for the one-dimensional Ni−Rh nanowires, where the Ni atoms are coupled directly with the Rh step atoms.