Size-Dependent Underpotential Deposition of Copper on Palladium Nanoparticles

The underpotential deposition (UPD) of copper on palladium nanoparticles (NPs) with sizes in the range 1.6-98 nm is described. A dependence of the UPD shift on size of the nanoparticle is observed, with the UPD shift decreasing as the particle size decreases. This size dependence is consistent with the known dependence of UPD shift on work function difference between the substrate metal (Pd) and the depositing metal (Cu). The shift suggests the work function of the NPs decreases with decreasing size as expected (i.e., the smaller nanoparticles are more easily oxidized and therefore have lower work functions than larger NPs). For the smallest nanoparticles, the UPD shift does not follow the expected trend based solely on predictions of work function changes with size. On the basis of preliminary competitive anion adsorption experiments, it is speculated that strong chloride absorption on the smallest nanoparticles may be responsible for this deviation.