Compositional inhomogeneity of Pd-Ag nanoparticles probed by hydrogen

Nanoparticles of Pd77Ag23 and Pd60Ag40 of sizes 6-7 nm were prepared by a w et reduction method. The surface compositions obtained by electron spectros copy for chemical analysis indicated that the particles were not homogeneou s. 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 hig her temperatures, the fraction of Ag on the surface was increased due to it s lower surface energy than palladium. Hydrogen was used as a probe atom to examine the variation of surface composition of the nanoparticles. The pre ssure-composition isotherms were different for different heatings, and were different from those of coarse-grained materials, indicating that the comp osition across the diameter was not homogeneous. It is proposed that this i nhomogeneity is an intrinsic property for binary nanocrystalline alloys pre pared by wet reduction. (C) 2001 Elsevier Science BN. All rights reserved.