Core-and-shell nanoparticles of Ru-Ni in the size range of 5-10 nm were synthesized by a single-step spray-pyrolysis process. Ruthenium chloride (RC) and nickel chloride (NC) were dissolved in water and used as a feed stream through an ultrasonic atomizer. The ruthenium chloride to nickel chloride (RC/NC) ratio was varied from 1: 1 to 5:1 at various operating temperatures ranging from 500 degrees C to 1000 degrees C. Smooth core-and-shell nanoparticles were formed when the RC/NC ratio was 3:1 and 5:1 at 800 degrees C and 600 degrees C. A mechanism leading to the formation of such morphologies was proposed and was compared with the experimental results. It was found that as the RC/NC ratio in the precursor increased, temperature can be decreased to obtain core-and-shell nanoparticles. RC/NC ratio of 1: 1 at low temperatures of 500 degrees C generated core-and-shell nanoparticles which are concentric rings. No core-and-shell morphologies were observed at temperatures higher than 800 degrees C. Once the core-and-shell particles are formed, the temperature can be varied in the range of 200 degrees C to cause a geometry change from core-and-shell to spherical narroparticles. (c) 2007 Elsevier B.V. All rights reserved.