Cubic Pt nanoparticles were prepared from a solution of K,PtCl containing sodium polyacrylate as a capping reagent. The effects of the Pt/polymer molar ratio, the average molecular weight (M,) of the polymer, and reaction temperature on the shape and size were investigated. When the polymer of M-w = 5100 was added at a molar ratio of Pt/polymer = 1/12, cubic platinum nanoparticles of an average size of 10.3 nm were predominantly formed (ca. 50% in number) at 25 degrees C. The electron diffraction pattern of the cubic nanoparticles revealed that they are single crystals with Pt {10 0} faces on the surface. The cubic nanoparticles were electrochemically active, and showed strong features of Pt {10 0} faces on cyclic voltammogram under argon atmosphere. After repeated potential cycling in the range 0.05-1.4 V, the features of Pt {100} were gradually lost, and changed to those of polycrystalline Pt. Rotating ring disk electrode measurements in O-2-saturated H2SO4 Solution revealed that the cubic nanoparticles had a high catalytic activity for oxygen reduction reaction (ORR). After polycrystallization by repeated potential cycling, the activity for ORR and hydrogen peroxide formation decreased slightly, which were attributed to the surface structural change from Pt {100} to polycrystal line. (c) 2006 Elsevier Ltd. All rights reserved.