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

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/PtCo(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.