Pt0.3-0.6nm/Ni0.3-0.6nm/Pt(1 1 1) and Pt0.3-0.6nm/Ni0.3-0.6nm/Pt(1 1 0) atomic sandwich structures were prepared through alternating vacuum depositions of Ni followed by Pt onto clean Pt(1 1 1) and (1 1 0) substrates at room temperature under ultra-high-vacuum (UHV) conditions. After the samples were transferred from UHV to a 1-atm N-2 atmosphere, their oxygen reduction reaction (ORR) activities were evaluated in O-2-saturated 0.1 M HClO4 at 0.9V vs. reversible hydrogen electrode. Pt-0.6nm/Ni-0.6nm/Pt(1 1 1) and Pt-0.6nm/Ni-0.6nm/Pt(1 1 0) were most active among the respective Pt/Ni/Pt(1 1 1) and Pt/Ni/Pt(1 1 0) sandwich series: the activities of the former and latter sandwich structures were approximately five- and threefold greater than those of the corresponding clean Pt(1 1 1) and (1 1 0) substrate surfaces. Scanning tunneling microscopy images of the as-preparedPt(0.6nm)/Ni-0.6nm/Pt(1 1 1) and Pt-0.6nm/Ni-0.6nm/Pt(1 1 0) surfaces revealed three-dimensionally grown hexagonal-shaped small domains of Pt(1 1 1) (approximately 2 nm in size) and parallelogram-shape (1 1 0) terrace islands oriented along (1 1 0), respectively. The results indicate that not only the atomic arrangements of the topmost Pt layers but also the nanoscale morphologies of Pt-Ni in the surface vicinities determine the enhancement of the ORR activity of Pt-M alloy catalysts. (C) 2014 Elsevier B.V. All rights reserved.