Characterization of a colloidal Pd(II)-based catalyst dispersion for electroless metal deposition

An aqueous Pd(II) dispersion, useful as a catalyst for the selective electroless deposition of nickel metal at ligand-bearing surfaces, is prepared by the hydrolysis of PdCl42- at pH 5 in an approximately 0.01 mol dm(-3) NaCl solution. The catalyst dispersion is characterized by W-visible absorption spectroscopy, electroless metallization, ultracentrifugation, and electrophoresis. The dispersion is found to consist of a distribution of anionic and uncharged Pd(II) species ranging in type from monomeric to colloidal. The species responsible for the initiation of electroless metal deposition at the ligand surface are identified as colloidal. Atomic force microscopy indicates that the colloidal catalysts are bound at the surface and range in diameter from approximately 4 to 53 nm with an average size of 30 +/- 12 nm. The behavior of the catalyst dispersion is consistent with a model in which colloid formation is initiated by polymerization of monomeric precursors generated by successive hydrolytic Cl- loss from PdCl42-, and deprotonation of the corresponding aquo-Pd(II) complex(es).