A team of researchers from Lund University in Sweden have succeeded in imaging the aggregation behaviour of palladium nanoparticles in solution, at different values of pH and ionic strength, by low-electron dose cryo energy-filtered transmission electron microscopy (cryo-EFTEM) (J.-O. Bovin, T. Huber, O. Balmes, J.-O. Malm and G. Karlsson, Chem. Eur. J ., 2000, 6, (1), 129–132).

Palladium colloids, covered by sodium sulfanilate protective ligands, were rapidly cooled by plunge-freezing to avoid particle rearrangement. Elemental mappings were taken at low energy and short exposures to prevent damage. Shapes, sizes, structural defects and distances between the agglomerated colloids were visible. A two-window method (jump ratio imaging) identified the palladium colloids. The colloids were always present as a mixture of single nuclei and aggregates in solution. The number of single particles in solution could be increased by lowering the ionic strength and raising the pH, but some agglomerates of two (or more) nuclei still remained.

This technique may be used to determine the best deposition conditions for the palladium/ligand and other metal/ligand systems and to study the chemistry of solids interacting with liquids.