Atomically Flat Pt Skin and Striking Enrichment of Co in Underlying Alloy at Pt(3)Co(111) Single Crystal with Unprecedented Activity for the Oxygen Reduction Reaction

[Image: see text] By the use of in situ scanning tunneling microscopy and surface X-ray scattering techniques, we have clarified the surface structure and the layer-by-layer compositions of a Pt skin/Pt(3)Co(111) single-crystal electrode, which exhibited extremely high activity for the oxygen reduction reaction. The topmost layer was found to be an atomically flat Pt skin with (1 × 1) structure. Cobalt was enriched in the second layer up to 98 atom %, whereas the Co content in the third and fourth layers was slightly smaller than that in the bulk. By X-ray photoelectron spectroscopy, the Co in the subsurface layers was found to be positively charged, which is consistent with an electronic modification of the Pt skin. The extremely high activity at the Pt skin/Pt(3)Co(111) single crystal is correlated with this specific surface structure.