Kinetic- or Thermodynamic-Growth Formation of Pd Nanocrystals: a combined TEM-DLS Approach
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1087-V06-06
Kinetic- or Thermodynamic-Growth Formation of Pd Nanocrystals: a combined TEM-DLS Approach Gilles Berhault, and Marta Bausach IRCELYON, CNRS - University of Lyon, 02 Avenue Albert Einstein, Villeurbanne, 69100, France ABSTRACT The kinetics of formation of palladium nanoparticles with well-defined morphologies (rods, cubes, MTP icosahedra, bipyramids) was studied using a combination of TEM and dynamic light scattering (DLS). The present study was focused on the role of the concentration and the size of seeds on both the kinetics of growth and the morphology distribution. Results clearly emphasize the role of the seed concentration (and not the size) on the growth kinetics in agreement with a collision theory model.
INTRODUCTION Palladium nanoparticles with defined morphologies were successfully synthesized using a seed-mediated approach in the presence of CTAB (cetyltrimethylammonium bromide) [1]. Various types of morphologies were obtained going from isotropic cubes and icosahedral particles to nanorods or bipyramids. A systematic study of experimental conditions showed that the morphology was mainly governed by the crystal growth rate. The CTAB surfactant controls the rate of reduction of the Pd precursor by embedding it inside micelles leading to a high monomer concentration during the growth step and therefore to a kinetic-controlled regime. This growth regime favors the formation of anisotropic Pd nanoparticles like nanorods with a five-fold symmetry. Shifting the conditions to a thermodynamic-controlled regime selectively led to the formation of cubic (80%) or icosahedral (100%) particles. Cubes were formed by avoiding the embedding of Pd while icosahedral particles were obtained by capping the Pd seeds with citrate. To gain insight into the growth mechanism, the synthesis of the Pd nanocrystals was also followed combining TEM and DLS (dynamic light scattering). DLS was a technique of choice to in situ follow the growth kinetics from the injection of the Pd seeds to the final morphology.
EXPERIMENTAL
Synthesis of Pd seeds Pd nanoparticles (3-4 nm in diameter) were synthesized using Na2PdCl4, cetyltrimethylammonium bromide (CTAB), and NaBH4 as a strong reducing agent. Typically, 50
mL of an aqueous 0.5 mM Na2PdCl4 solution was mixed with 25 mL of an aqueous 0.3 M CTAB solution prepared at 303 K. 6 mL of an ice-cold aqueous 0.01 M NaBH4 solution was then added quickly under vigorous stirring. The solution turned dark immediately after the borohydride addition, indicating metallic palladium nanoparticle formation.
Synthesis of Pd nanostructured particles The as-formed Pd nanoparticles were used as “seeds” in a second step. The seeds were injected into a “growth” solution prepared using Na2PdCl4, CTAB, and a weak reducing agent, L-ascorbic acid. In a typical procedure of preparation, the growth solution was obtained by mixing 50 mL of an aqueous 1.0 mM Na2PdCl4 solution with 50 mL of an aqueous 0.08 M CTAB solution under gently stirring at 303 K. 0.7 mL of an aqueous 0.08 M sodium ascorbate solut
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