Dynamics of compositional evolution of Pd-Cu alloy nanoclusters upon heating in selected atmospheres

We have investigated the dynamics of the compositional and structural evolution of solid solution Pd-Cu metal nanoclusters obtained by sequential ion implantation in silica, upon heating in selected atmospheres as a function of the annealing time. A correlated diffusion of the dopants is evidenced for both reducing or oxidizing atmospheres, mostly triggered by the Cu atom diffusion (such migration is absent in silica implanted only with Pd). In particular, upon annealing in reducing atmosphere a preferential migration of Pd towards the sample surface is observed and aggregation of large bimetallic clusters is favored near the implantation range, as also found in the case of Au-Cu alloy clusters in silica. On the other hand, upon annealing in air, the Pd-Cu alloy nanoclusters undergo a progressive oxidation process in which the Cu atoms are oxidized mostly as CuO oxide partial shell around a Pd-rich core. This behavior is different from the one observed for Au-Cu clusters in silica under the same annealing conditions: in that case, Cu atoms were extracted through the surface of the alloy clusters as Cu2O aggregates with their crystallographic axes coherently oriented with those of the fee Au-rich alloy nanocrystals. In the present Pd-Cu case this process is not energetically favorable due to the large lattice mismatch between Pd-Cu solid solution alloy and the cubic Cu2O, therefore indicating that the structural constraint given by the lattice mismatch between the core and oxide shell is crucial for controlling the stoichiometry of the oxide.