Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

The advantage of this SAXS technique is that it can obtain the time evolution of particle size distribution and the actual concentration of particles at the same time. Extracting the kinetic rates from SAXS is crucial for understanding the effects of capping ligands on the nucleation growth mechanism of colloidal metal non-particles. Chemically accurate kinetic models enable predictive synthesis of palladium nanoparticles of specific sizes.

Similar methods can be used for other metal and metal oxides, minimizing trial and error testing of synthetic conditions. First add 40 milliliters of glacial acetic acid to a 50 milliliter three neck round bottom flask containing 0.75 grams of palladium acetate and a stir bar. Equip the flask with a condenser, stopper the other necks and fix the flask in a heating insert on a stirring hot plate.

Slowly open the condenser water valve and allow the water to flow through the condenser. Stir the solution at 300 rpm at room temperature until no more palladium acetate dissolves. Which usually takes 10 to 15 minutes.

Then, set the hotplate to 100 degrees celsius. Continue stirring the mixture at 100 degrees celsius until the palladium acetate has dissolved completely which usually takes about 30 minutes. During this time, preheat two 20 milliliter glass vials, vacuum filtration glassware and filter paper in a drying oven at 90 degrees celsius.

Heat about 80 milliliters of water in a 250 milliliter beaker to 80 to 90 degrees celsius. Preheat another hotplate to 100 degrees celsius. Once the palladium acetate has dissolved, quickly assemble the filtration components and fix the filter flask on the preheated hotplate.

Remove the glass vials from the oven. Connect a vacuum pump to the filter flask, start the vacuum pump, and quickly filter the palladium acetate solution under vacuum. Quickly transfer the filtrate to the two preheated 20 milliliter vials.

Cap the vials with polypropylene caps with PTFE silicone septa. Seal the vials with plastic paraffin film and immerse them in the hot water in the beaker. Cover the beaker with aluminum foil and place the beaker on the hotplate used for the filter flask.

Set the hotplate temperature to 80 degrees celsius. Decrease the temperature by 20 degrees celsius every hour to cool the solutions to room temperature. Then turn off the hotplate and leave the beaker undisturbed overnight to allow crystallization.

The next day, remove the acetic acid from the vials leaving the palladium acetate trimer crystals in the vial. Wash the crystals three times with two milliliter portions of hexane. Wrap the vials in aluminum foil to exclude light and dry the crystals under a flow of nitrogen gas at room temperature overnight.

Store the crystals under an inert atmosphere. To begin the synthesis procedure, degas about five milliliters each of one hexanol and pyridine by bubbling nitrogen gas through each solvent at about 10 milliliters per minute for 30 minutes. Then, weigh 0.112 grams of recrystallized palladium acetate into a seven milliliter vial.

Cap the vial with a polypropylene cap with a PTFE silicone septum. Insert a needle through the septum as a vent and purge the vial atmosphere with nitrogen gas for five minutes. Transfer the solvents and the vial of palladium acetate into a nitrogen filled glove box and add 2.5 milliliters of pyridine to the palladium acetate.

Seal the vial with plastic paraffin film remove the vial from the glove box and sonicate the vial for 40 minutes to dissolve the palladium acetate. Begin preheating a hotplate with a vial heating insert to 125 degrees celsius so that the solution will reach 100 degrees celsius. Once the palladium acetate has dissolved, return the vial to the glove box.

Transfer one milliliter of this 20 millimolar palladium acetate solution to a seven milliliter vial equipped with a micro stir bar. Add 8.9 microliters of trioctylphosphine to the solution and shake the vial for 30 seconds by hand. Add one milliliter of one hexanol to the reaction mixture seal the vial, and remove the reaction mixture from the glove box.

Flow nitrogen gas above the solution level at a low flow rate to maintain an inert atmosphere in the vial at a slight positive pressure. Place the vial in the hotplate insert and begin stirring the reaction mixture at 300 rpm. Allow the reaction to proceed for the desired duration.

Then remove the vial from the insert and allow the mixture to cool to room temperature. Initialize the SAXS software and click on the command window in the measurement software. Set the voltage and current to 50 kilovolts and 1000 micro amperes respectively.

Load a one to one by volume mixture of pyridine and one hexanol into a quarts capillary and seal the capillary. Fix the capillary to the capillary holder parallel to the X direction, which is perpendicular to the beam. Mount the holder in the instrument chamber and close the chamber.

Start the vacuum pump and wait for the chamber pressure to stabilize at less than 0.3 millibars. Fix the X axis within the capillary sample range. Then drag the Y axis slider to move the capillary close to the beam.

Select scan type Y fill in the start and stop positions, and set the increment to 0.05 millimeters. Start the scan across the Y axis. Once the scan has finished, identify the middle position across the capillary at which the X ray path length through the liquid sample is at a maximum.

Which is the measurement position. In the wizard, set the capillary to the measurement position and select transmission of sample to measure the sample transmission using glassy carbon as the reference standard. Apply the new settings and move the glassy carbon into the beam path to take a 10 second measurement of the sample with and without glassy carbon in the beam path.

Still scan and save the 2D scattering graph. Then, set up the wizard to take a 1800 second measurement of the solvent background alone. Then set up the wizard to measure glassy carbon only.

Move the capillary out of the xray path by setting it to a different position. Place the glassy carbon in the path and take a 10 second measurement of glassy carbon alone. Save the wizard and run the wizard program to take the measurements specified in the text protocol.

When finished, vent the instrument chamber and mount a sealed capillary containing the palladium nanoparticle suspension in the instrument. After that, repeat the same procedure with an empty capillary and with a capillary filled with water for later use in calibrating the scattering intensity to an absolute scale. The absolute scaling of SAXS intensity by using water or another standard sample allows the extraction of the actual particle concentration of the solution which is directly related to the nucleation events in the synthesis reaction.

When the synthesis of palladium nanoparticles in toluene was modeled without accounting for ligand metal binding, the model did not reflect the time evolution of the concentration of nano particles or of the concentration of palladium atoms. When association and dissociation of the capping ligands was incorporated into the model, the model closely followed the experimental data indicating that the capping ligands affected the nucleation and growth kinetics of palladium nano particles. Estimation of the rate kinetics indicated that nucleation was slow and growth was fast which agrees with previous studies.

The binding of ligands to the nano particle surface reduced the concentration of active sites expanding the time window for nucleation. The model also accurately captured the nucleation and growth kinetics of palladium nano particles in pyridine despite the significant difference in kinetics between the toluene and pyridine systems. Further, the model accurately predicted nano particle sizes in pyridine from different precursor concentrations using the estimated rate constants.

We first had the idea for this method when we found that despite the significant contribution of capping ligands in altering the size colloidal nano particle, their exact role in controlling nano particle nucleation growth is poorly understood. Our SAXS and kinetic modeling methodology can pave the way to design synthetic procedures to obtain colloidal nano particles with the desired sizes for potential applications in catalysis and drug delivery.