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 oxidiz-ing atmospheres, mostly triggered by the Cu atom diffusion ssuch migration is absent in silica implanted only
with Pdd. 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 fcc
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 Cu 2 O, 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.