Self-assembly of polyoxornetalate macroanion-capped Pd0 nanoparticles in aqueous solution

The self-assembly behavior of polyoxometal ate (POM) macroanion-capped 3-nm-radius Pd-0 nanoparticles in aqueous solution is reported. Pd-0 nanoparticles are synthesized from reducing K2PdCl4 by using Dawson-type V-substituted POM K-9[(H4PVW17O62)-W-IV] (HPVIV) clusters as the reductant and stabilizer simultaneously in acidic aqueous solutions. The starting molar ratio of K2PdCl4 to HPVIV (R value) in solution is important to the formation of Pd nanoparticles. When R < 0.6, similar to 20-nm-radius Pd-0 colloidal nanocrystals are formed. When R >= 0.6, HPV-capped (and therefore negatively charged) 3-nm-radius Pd-0 nanoparticles are formed, which can further self-assemble into stable, hollow, spherical, 30-50-nm-radius supramolecular structures in solution without precipitation, as confirmed by light scattering and transmission electron microscopy studies. This structure resembles the unique supramolecular structure formed by hydrophilic POM macroanions in polar solvents, which we refer to as blackberry structures. It is the first evidence that the blackberry formation can occur in hydrophobic nanoparticle systems when the surface of nanoparticles is modified to be partially hydrophilic. Counterions play an important role in the self-assembly of Pd nanoparticles, possibly providing an attractive force for blackberry formation, which is the case for blackberry formation in POM macroanionic solutions. Our results suggest that the blackberry formation is not a specific property of POM macroions but most likely a general phenomenon for nanoparticles with relatively hydrophilic surfaces and suitable sizes and charges in a polar solvent.