Spontaneous hierarchical assembly of rhodium nanoparticles into spherical aggregates and superlattices

Reduction of aqueous RhCl3 with NaBH4 in the presence of poly(vinyl pyrrolidone) (PVP) yields dense spherical nanostructures. The spherical aggregates, which generally have diameters between 10 and 100 nm, are built from smaller 1-3 nm Rh particles. The dense nanostructures are thermally stable beyond 100 degreesC, and they have a tendency to form ordered superstructures upon drying. Combining sodium n-dodecyl sulfate (SDS) with PVP modifies the size and morphology of the primary 1-3 nm particles, but does not change the spherical shape of the aggregates except at high concentrations of SDS. Small-angle X-ray scattering measurements show that the large aggregates are formed directly in solution from small Rh particles, consistent with TEM and AFM results. Magnetic measurements indicate that the Rh nanoparticle aggregates are Pauli paramagnetic.