(499a) Colloidal Pd Nanoparticle Synthesis: The Effect of Ligand-Metal-Solvent Thermodynamics on Kinetics and Final Size

Metal nanoparticles are of great scientific interest due to their unique electronic, catalytic and optical properties. Colloidal metal nanoparticles of different sizes have been successfully synthesized by careful choice of precursors, solvents, and ligands. The roles of ligands and solvents in controlling the size are still not well understood. We have recently reported that surface capping by ligands as well as ligand-precursor coordination can affect the final size of Pd nanoparticles by modulating the surface growth and nucleation rates, respectively. This critical ligand role provides an opportunity for controlling the particle size by tuning the ligand interactions with both the metal precursor and nanoparticle surface. Herein, we report a study on how the thermodynamics of Pd coordination with phosphine and amine ligands in different solvents affect the kinetics of nanoparticles nucleation and growth, and consequently the final particle size. The kinetics were measured by in-situ small-angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) was used to characterize the solvent-ligand coordination with the Pd precursors. The possible structures and free energies of ligand-solvent exchange reactions in solution were calculated using Density function theory (DFT). Additionally, we used isothermal titration calorimetry (ITC) to quantitatively compare the thermodynamic interactions between different ligands and palladium precursor and nanoparticle surface atoms by measuring the binding equilibrium constant, heat of binding and stoichiometry of binding. We show how the nucleation and growth rates, and final size are correlated with ligand-Pd(0) and ligand-Pd(II) binding equilibrium. The results give a better understanding of the ligand effect on synthesis kinetics and show the potential for tailoring the solvent-ligand-metal interactions to move closer towards predictive synthesis of metal colloidal nanoparticles.