Palladium Nanoparticles Embedded in a Layer-by-Layer Nanoreactor Built with Poly(Acrylic Acid) Using “Electro-Click Chemistry”

Palladium nanoparticles (Pd NPs) were formed by electrochemical reduction of Pd­(NH₃)₄³⁺ ions entrapped by ion exchange in poly­(acrylic acid) (PAA) multilayer films grown by the Sharpless “click reaction.” The alkyne (PAA_alk) and azide (PAA_az) groups were covalently bound to the PAA, and the catalyzed buildup of the multilayer film was performed by electrochemical reduction of Cu²⁺ to Cu⁺. The size of the Pd NPs formed in Au/(PAA_alk)₃(PAA_az)₂ multilayer films by the click reaction, that is, 50 nm, is larger than that of similar Pd NPs formed in electrostatically bound Au/(PAA)₃(PAH)₂ nanoreactors, that is, 6–9 nm, under similar conditions. A combination of electrochemical methods and electrochemical quartz crystal microbalance, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), ellipsometry, and scanning electron microscopy has been used to follow these processes. Cyclic voltammetry of the resulting Pd NPs in a 0.1 M H₂SO₄ solution at 0.1 V·s^–1 shows the PdO reduction peak at the same potential as that on the clean Pd surface unlike the NPs formed in electrostatically self-assembled Au/(PAA)₃(PAH)₂ nanoreactors with a 0.2 V shift in the cathodic direction most probably because of the strong adsorption of amino groups on the Pd NP surfaces