Size Control of Colloidal Pd Nanoparticles: Role of Ligand Binding Strength

Through a rapidly developing interest in nanotechnology, the application of Palladium nanoparticles has been realized in several areas. The catalytic properties of Pd nanoparticles are one of their most researched applications. They are commonly used in the early stages of pharmaceutical drug manufacturing, specifically for drugs that are synthesized through organic cross-coupling reactions. To synthesize particles of specific size, the particles must often be stabilized by ligands. Studies have demonstrated that nanoparticle size is directly controlled by the type and concentration of ligand, but it is still unclear if they prevent nanoparticle agglomeration or have a role in controlling growth via blocking of surface sites. Our hypothesis is that nucleation and growth kinetics are affected by the ligand-metal binding strength, which can be controlled by the type of ligand and solvent. In this investigation, we aim to determine the relationship between Pd nanoparticle size and the binding strength of phosphine and amine ligands to the Pd precursor (PdClâ??). Synthesis reactions were carried out with PdCl₂ in pyridine and NaBH₄ as the reducing agent, in addition to the various capping agents. Particles sizes were measured by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) and ranged from 1 nm â?? 4 nm in diameter. The binding energy between the precursor and the ligand was measured using isothermal titration calorimetry (ITC). Using ITC, we are able determine the stoichiometry, binding heat, and equilibrium constant for the binding between different ligands and the precursor, and in turn better understand their binding behavior. The results are used to correlate the thermodynamics of metal-ligand interaction with the final size of the nanoparticles to help predict the size using other ligands and ligand:Pd ratios.