Unravelling the nucleation mechanism of bimetallic nanoparticles with composition-tunable core-shell arrangement.

The structure and atomic ordering of Au-Ag nanoparticles grown in the gas phase are determined by a combination of HAADF-STEM, XPS and Refl-XAFS techniques as a function of composition. It is shown consistently from all the techniques that an inversion of chemical ordering takes place by going from Au-rich to Ag-rich compositions, with the minority element always occupying the nanoparticle core, and the majority element enriching the shell. With the aid of DFT calculations, this composition-tunable chemical arrangement is rationalized in terms of a four-step growth process in which the very first stage of cluster nucleation plays a crucial role. The four-step growth mechanism is based on mechanisms of a general character, likely to be applicable to a variety of binary systems besides Au-Ag.