Growth of Three-Shell Onionlike Bimetallic Nanoparticles

We show by molecular dynamics simulations on three systems ($B/A=\mathrm{P}\mathrm{d}/\mathrm{A}\mathrm{g}$, $\mathrm{C}\mathrm{u}/\mathrm{A}\mathrm{g}$, and $\mathrm{N}\mathrm{i}/\mathrm{A}\mathrm{g}$) that three-shell metallic nanoparticles made by a core of a metal $A$, an intermediate shell of metal $B$ and an external shell of metal $A$ ($A\mathrm{\text{−}}B\mathrm{\text{−}}A$ nanoparticles) can be grown by deposition of $B$ atoms onto an $A$ core. The growth of the intermediate $B$ shell is triggered by the fact that the most favorable positions for isolated $B$ impurities inside $A$ clusters are located just one layer below the cluster surface.

Figure 1

Growth sequence for the deposition of Cu above a ${\mathrm{T}\mathrm{O}}_{201}$ of Ag at $T=500\mathrm{K}$ and $\tau =7\mathrm{n}\mathrm{s}$. Snapshots are taken (from top to bottom) at the beginning of the simulation and after the deposition of 67, 108, 122, and 201 atoms. In the left column we show the cluster surface, in the right column a cross section of the cluster to show its internal arrangement. Ag and Cu atoms are represented in light and dark gray, respectively. Below the growth sequences we plot the radial distribution functions (in arbitrary units) at the end of the simulation: ${\rho }_{Ag}$ (full line) and ${\rho }_{\mathrm{C}\mathrm{u}}$ (dotted line) versus the distance $r$ (in $\AA$) from the geometrical center of the cluster.

Figure 2

The same as in Fig. 1, but for deposition above an ${\mathrm{I}\mathrm{h}}_{147}$. Snapshots are taken (from top to bottom) at the beginning of the simulation and after the deposition of 54, 92, 121, and 162 atoms.

Figure 3

Final structures after the deposition of 200 atoms of Ni at 300 K (top row) and of 200 atoms of Pd at 600 K (third row) above a ${\mathrm{T}\mathrm{O}}_{201}$ of Ag. Ag atoms are represented in light gray, Pd and Ni atoms in dark gray. The radial distribution functions are plotted with the full line corresponding to Ag and the dotted line to Pd (second row) or Ni (fourth row).

Figure 4

Energies $\Delta E^{*}$ (in eV) for impurities in inequivalent sites in four different clusters. $r$ is the distance (in $\AA$) of the impurity is placed from the geometrical center of the clusters. In all panels, circles, squares and triangles refer to Ni, Cu, and Pd impurities, respectively. Open symbols refer to subsurface sites, full symbols for all other sites.