Size-selective synthesis of cubooctahedral palladium particles mediated by metallomicelles

The reduction of [PdCl4](2-) by hydrazine in the postmicellar region of even-numbered alkyltrimethyl-ammonium bromides C(n)TABr (with n = 8 to 16) yielded fairly monodisperse Pd particles with mean diameters in the range 1.6-6.8 nm. Transmission electron microscopy (TEM) measurements indicated that the particle size decreases with increasing length of the alkyl chain, decreasing precursor concentration, and decreasing surfactant concentration. Kinetic measurements demonstrated that the nucleation is a fast process and well separated from the growth. TEM and high-resolution TEM micrographs of the particles suggested that the dominant morphology is cubooctahedral. Composition analysis (H-1 NMR, total organic carbon, and inductively coupled plasma atomic emission spectroscopy), titration microcalorimetry, and spectroscopic methods (IR, Raman, and UV-vis) revealed that [PdCl4](2-) is not the real precursor species in the reduction process. In the presence of a large excess of Br-, [PdCl4](2-) transforms to [PdBr4](2-). The ligand-exchange reaction is accompanied by the formation of stoichiometric complex-surfactant aggregates [C(n)TA](2)[PdBr4], and followed by a slow, higher-order aggregation to microcrystals. The organic salt precipitates below the critical micelle concentration but undergoes solubilization above it, leading to the formation of palladate-surfactant metallomicelles. In the postmicellar region, the close proximity of the surfactant molecules to the reduction centers ensures rapid adsorption of the amphiphiles on the surface of the nascent particles, and a protective bilayer is formed, ultimately leading to long-term stability of the Pd hydrosols.