Electrochemically induced hydrogen adsorption at Au-Pd core shell (CS) nanostructures was investigated as a function of the Pd lattice strain. CS nanoparticles with shell thicknesses between 1 and 10 nm were prepared by selective reduction of Pd on Au nanoparticles in aqueous solution. High-resolution transmission electron microscopy images and selected-area electron diffraction patterns confirmed the formation of epitaxial Pd films on the Au cores. Examination of diffraction patterns allowed a quantitative analysis of the lattice strain as a function of the shell thickness based on Matthews theory. This analysis showed that complete strain relaxation was not achieved even for Pd layers of 10 nm. Detailed electrochemical studies of two-dimensional assemblies with controlled particle number density provided valuable information not only on the H adsorption charge in acid solution but also on the average surface roughness of individual CS nanoparticles. These results show that apparent changes in the H adsorption charge density with increasing Pd thickness above 3 nm are mainly brought about by increasing shell roughness. Comparison with literature values suggests that these roughening effects are present in extended surfaces composed of a few atomic layers. The picture emerging from these results is that the H adsorption charge (coverage) is effectively independent of the Pd lattice strain. The implications of these studies with regard to established theoretical models for predicting the reactivity of epitaxial thin films are briefly discussed.
- ELECTROCATALYTIC ACTIVITY
- GOLD NANOPARTICLES