Synthesis, Characterization, and Stability of Dendrimer-Encapsulated Palladium Nanoparticles

Abstract

Here we report on the synthesis, physical and chemical properties, and stability of Pd nanoparticles encapsulated within poly(amidoamine) (PAMAM) dendrimers. Specifically, amine- and hydroxyl-terminated PAMAM dendrimers ranging in generation from 4 to 8 were studied. Under appropriate conditions, addition of K 2 PdCl 4 results in covalent attachment of the PdCl 3 - hydrolysis product of this complex to tertiary amines within the dendrimers. Reduction with NaBH 4 results in conversion of dendrimer-encapsulated PdCl 3 - to nearly size monodisperse, encapsulated, zerovalent Pd nanoparticles. Details regarding the Pd species present in solution and within the dendrimer prior to reduction are reported, as is the maximum Pd 2+ loading of the dendrimers. Dendrimer-encapsulated Pd nanoparticles undergo oxidation in air, but this process is slowed significantly when coordinating ions are removed from solution. In the absence of O 2, dendrimer-encapsulated Pd nanoparticles are stable indefinitely. The oxidation product is not PdO, but rather Pd ions coordinated to the dendrimer interior. Dendrimer generation does not affect the rate of Pd oxidation. The dendrimer itself undergoes irreversible oxidation in the presence of O 2 . Finally, the oxidation of dendrimer-encapsulated Pd nanoparticles is reversible. Specifically, H 2 gas can be used to re-reduce partially oxidized Pd nanoparticles without changing their average size.