Abstract
Nanoparticles of Pd 77 Ag 23 and Pd 60 Ag 40 of sizes 6-7 nm were prepared by a wet reduction method. The surface compositions obtained by electron spectroscopy for chemical analysis indicated that the particles were not homogeneous. The surface was Pd-rich, and oxidation of Pd on the surface was observed for the as-prepared samples. When the samples were heated in vacuum at higher temperatures, the fraction of Ag on the surface was increased due to its lower surface energy than palladium. Hydrogen was used as a probe atom to examine the variation of surface composition of the nanoparticles. The pressure-composition isotherms were different for different heatings, and were different from those of coarse-grained materials, indicating that the composition across the diameter was not homogeneous. It is proposed that this inhomogeneity is an intrinsic property for binary nanocrystalline alloys prepared by wet reduction. © 2001 Elsevier Science B.V.