Oxygen Reduction Reaction Activities for Pt/Au(hkl) Bimetallic Surfaces Prepared by Molecular Beam Epitaxy

We investigated electrochemical properties for the Pt-Au bimetallic surfaces prepared by 0.3-nm-thick Pt depositions on Au(111), (110), and (100) substrates (Pt_0.3nmAu(hkl)) by molecular beam epitaxy. The Pt_0.3nmAu(111) surface prepared at the substrate temperature of 300 K (300K-Pt/Au(111)) showed a corrugated Pt(111) epitaxial surface on which 3-5 nm size monoatomic height Pt islands are located. Oxygen reduction reaction (ORR) activity of the surface was enhanced by a factor of ca. 2 relative to that of the clean Pt(111) surface. In contrast, the topmost surfaces of 473K-Pt_0.3nmAu(111), 300K-Pt_0.3nm/Au(100), and 300K-Pt_0.3nm/Au(110) were composed of both Pt and Au atoms, and the ORR activities were lower than those for corresponding clean surfaces. The specific ORR activities for the Pt_0.3nm/Au(111) and Pt_0.3nm/Au(100) surfaces estimated after recording CV curves of 0.05-1.7 V were enhanced relative to the as-prepared samples, although the activity of the Pt_0.3nm/Au(110) surface was slightly decreased by the wide region CV recording. These results suggest that atomic arrangements of surface Pt atoms, particularly the most dense plane of Pt(111), determine the ORR activity enhancement for Pt-Au bimetallic surfaces.