Fabrication and characterization of Ag/Pd colloidal nanoparticles as stable platforms for SERS and catalytic applications

We prepared stable Ag/Pd colloids to be used as both SERS-active and catalytic platforms by combining pulsed laser ablation of Ag in pure water and galvanic replacement with a successive addition of Pd(II) nitrate. Beyond UV–vis absorption and microscopic characterization, we also performed XPS tests that, by utilizing different Ar sputtering times, permitted to evaluate the Pd/Ag ratio in the nanoparticles and to confirm the results of the TEM analysis concerning their structure; Raman measurements performed in Ag/Pd colloidal suspensions allowed evaluating their SERS activity. We assessed both the Pd/Ag ratio and the timing that are required to obtain stable bimetallic colloids. The influence of the Ag colloid ageing on its reactivity with Pd(II) ions has been explained by the effects induced by the adsorbed charged species on the surface of the silver particles. This process is described by the observed temporal evolution of the ζ-potential of the Ag colloid. Lastly, we demonstrated the double functionality of our Ag/Pd nanostructures by SERS monitoring the Pd-induced catalytic reduction of the nitro group of 2-amino, 5-nitropyridine with subsequent formation of 2,5-diaminopyridine. These Ag/Pd colloidal particles allowed observing reactions induced by nanosized palladium by SERS measurements, without interference of residual reducing agents or by-products, as instead could occur for metal colloids obtained by chemical routes.