Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

Summary

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.

Abstract

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution of capping ligands during the synthesis reaction, their role in regulating the nucleation and growth rates of colloidal nanoparticles is not well understood. In this work, we demonstrate a mechanistic investigation of the role of trioctylphosphine (TOP) in Pd nanoparticles in different solvents (toluene and pyridine) using in situ SAXS and ligand-based kinetic modeling. Our results under different synthetic conditions reveal the overlap of nucleation and growth of Pd nanoparticles during the reaction, which contradicts the LaMer-type nucleation and growth model. The model accounts for the kinetics of Pd-TOP binding for both, the precursor and the particle surface, which is essential to capture the size evolution as well as the concentration of particles in situ. In addition, we illustrate the predictive power of our ligand-based model through designing the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can be applied to other synthesis systems and therefore serves as an effective strategy for predictive synthesis of colloidal nanoparticles.

Introduction

Controlled synthesis of metallic nanoparticles is of great importance due to the large applications of nanostructured materials in catalysis, photovoltaic, photonics, sensors, and drug delivery^1,2,3,4,5. To synthesize the nanoparticles with specific sizes and size distribution, it is vital to understand the underlying mechanism for the particle nucleation and growth. Nevertheless, obtaining nanoparticles with such criteria has challenged the nano-synthesis community due to the slow progress in understanding the synthesis mechanisms and the lack of robust kinetic models available in the literature. In 1950s, LaMer proposed a model for the nucleation and growth of sulfur sols, where there is a burst of nucleation followed by a diffusion-controlled growth of nuclei^6,7. In this proposed model, it is postulated that the monomer concentration increases (due to the reduction or decomposition of the precursor) and once the level is above the critical supersaturation, the energy barrier for particle nucleation can be overcome, resulting in a burst nucleation (homogeneous nucleation). Owing to the proposed burst nucleation, the monomer concentration drops and when it falls below the critical supersaturation level, the nucleation stops. Next, the formed nuclei are postulated to grow via the diffusion of monomers towards the nanoparticles surface, while no additional nucleation events occur. This results in effectively separating the nucleation and growth in time and controlling the size distribution during the growth process⁸. This model was used to describe the formation of different nanoparticles including Ag⁹, Au¹⁰, CdSe¹¹, and Fe₃O₄¹². However, several studies illustrated that the classical nucleation theory (CNT) cannot describe the formation of colloidal nanoparticles, in particular for metallic nanoparticles where the overlap of the nucleation and growth is observed^{1,13,14,15,16,17}. In one of those studies, Watzky and Finke established a two-step mechanism for the formation of iridium nanoparticles¹³, in which a slow continuous nucleation overlaps with a fast nanoparticle surface growth (where growth is autocatalytic). The slow nucleation and fast autocatalytic growth were also observed for different types of metal nanoparticles, such as Pd^14,15,18, Pt^19,20, and Rh^21,22. Despite recent advances in developing nucleation and growth models^1,23,24,25, the role of the ligands is often ignored in the proposed models. Nevertheless, ligands are shown to affect the nanoparticles size^14,15,26 and morphology^19,27 as well as the catalytic activity and selectivity^28,29. For example, Yang et al.³⁰ controlled the Pd nanoparticle size ranging from 9.5 and 15 nm by varying the concentration of trioctylphosphine (TOP). In the synthesis of magnetic nanoparticles (Fe₃O₄), the size noticeably decreased from 11 to 5 nm when the ligand (octadecylamine) to metal precursor ratio increased from 1 to 60. Interestingly, the size of Pt nanoparticles was shown to be sensitive to the chain length of amine ligands (e.g., n-hexylamine and octadecylamine), where smaller nanoparticle size could be obtained using longer chain (i.e., octadecylamine)³¹.

The size alteration caused by different concentration and different types of the ligands is a clear evidence for the contribution of ligands in the nucleation and growth kinetics. Unfortunately, few studies accounted for the role of ligands, and in these studies, several assumptions were often made for the sake of simplicity, which in turn make these models applicable only for specific conditions^32,33. More specifically, Rempel and co-workers developed a kinetic model to describe the formation of quantum dots (CdSe) in the presence of capping ligands. However, in their study, the binding of the ligand with nanoparticle surface is assumed to be at equilibrium at any given time³². This assumption might hold true when the ligands are in large excess. Our group recently developed a new ligand-based model¹⁴ which accounted for the binding of capping ligands with both the precursor (metal complex) and the surface of nanoparticle as reversible reactions¹⁴. In addition, our ligand-based model could potentially be used in other metal nanoparticle systems, where the synthesis kinetics seem to be affected by the presence of the ligands.

In the current study, we use our newly developed ligand-based model to predict the formation and growth of Pd nanoparticles in different solvents including toluene and pyridine. For our model input, in situ SAXS was utilized to obtain the concentration of nanoparticles and size distribution during the synthesis. Measuring both the size and concentration of particles, complemented by kinetic modeling, allows us to extract more precise information on the nucleation and growth rates. We further demonstrate that our ligand-based model, which explicitly accounts for the ligand-metal binding, is highly predictive and can be used to design the synthesis procedures to obtain nanoparticles with desired sizes.

Protocol

1. Pd Acetate Recrystallization

CAUTION: This protocol involves hands-on operations with high temperature glassware and solution. Use personal protective equipment including goggles and heat-resistant gloves. All the operations involving solution handling should be conducted in a fume hood and avoid other heating sources nearby due to the corrosive and flammable properties of anhydrous acetic acid.

1. Add 40 mL of anhydrous acetic acid into a 50 mL three neck round bottom flask with 0.75 g of Pd acetate and a stir bar. Attach the condenser to the middle neck, cap the other two openings and fix the flask on the stirring hotplate.
2. Open the condensing water valve slowly and let the water flow through the condenser. Stir the solution for 10-15 min at 300 rpm at room temperature until no more Pd acetate can dissolve.
3. Set the hotplate temperature at 100 °C. After the temperature reaches 100 °C, wait for around 30 min until the Pd acetate completely dissolves.
4. During this time, pre-heat two 20 mL glass vials and all the filtration parts at 90 °C in a drying oven. Also, heat some water in a 500 mL beaker until it approaches the boiling point.
5. Quickly assemble the filtration parts and place the filter flask on a pre-heated hotplate (at 100 °C). Connect the vacuum pump to the filter flask. Quickly remove the three-neck round bottom flask from the hotplate and filter the Pd acetate solution under vacuum.
6. After the filtration, quickly pour the liquid into two 20 mL vials. Cap the vials and immerse them into the hot water in the beaker.
7. Put the beaker on a hotplate at 80 °C and slowly decrease the temperature to room temperature by decreasing the hotplate temperature by 20 °C every hour.
8. Turn off the hotplate after 3 h. Leave the beaker overnight for crystallization.
9. Pour the acetic acid out of the vials. Leave the Pd acetate trimer crystals in the vial. Wash the crystals for 3 times to remove the acetic acid residual by dispensing 2 mL of hexane evenly onto the crystals and then draining the solution.
10. Cover the vials with aluminum foil to avoid light. Dry the crystals under N₂ flow at room temperature overnight. Store the crystals in inert atmosphere.

2. Preparation for Pd Acetate – TOP Synthesis Solution¹⁴

1. Degas each solvent (pyridine, toluene or 1-hexanol) under N₂ flow at 10 mL/min for 30 min.
2. Weigh 0.0112 g of recrystallized Pd acetate for 2.5 mL of 20 mM solution in a 7 mL vial. Cap the vial, then purge and fill it with N₂ through the inlet on the septum with an inserted needle outlet.
3. Transfer the solvents and the Pd acetate vial into an N₂ glovebox. Add 2.5 mL of pyridine or toluene into the Pd acetate vial. Sonicate the vial for 40 min to dissolve all Pd acetate.
4. For each sample, transfer 1 mL of 20 mM Pd acetate solution into a 7 mL vial with a micro stir bar in the glovebox. Add 8.9 μL of trioctylphosphine (TOP:Pd molar ratio = 2) into the solution. Shake the vial for 30 s with hands to mix the agents well. Then, add 1 mL of 1-hexanol into each sample vial (solvent:hexanol = 50:50 in volume).

3. Colloidal Pd Nanoparticle Synthesis¹⁴

1. Pre-heat the hotplate with a heating insert at 100 °C. Purge the reaction vials with 10 mL/min of N₂ flowing above the solution level to create an inert atmosphere and a constant pressure.
2. Put the reaction vials in the pre-heated hotplate insert under 300 rpm stirring to start the reaction.
3. To terminate the reaction, remove the vials from the insert and cool the vials down to room temperature.

4. Pd Nanoparticle Characterization- Ex situ Small-angle X-ray Scattering (SAXS)³⁴

1. Mean size and size distribution characterization
   1. Initialize the SAXS instrument. Click on the commander window in the measurement software and adjust the voltage and current to 50 kV and 1000 µA, respectively.
   2. Load the background solution (1:1 mixture of the solvent (pyridine or toluene) and 1-hexanol) into the capillary holder. Seal the capillary and fix it to the holder parallel to the X direction. Mount the holder inside the instrument chamber.
   3. Start the vacuum pump and wait until the vacuum level in the chamber stabilizes (lower than 0.3 mbar).
   4. Fix the X axis (along the capillary) and scan in the Y direction (across the capillary) to find the middle position as the measurement position, at which the X-ray pathway length through the liquid sample reaches the maximum (the diameter of the capillary).
   5. Setup and run the wizard to conduct steps 4.1.5 – 4.1.8. Set the capillary position and mount the glassy carbon through the X-ray pathway so that the X-ray will go through the glassy carbon first and then the capillary. Take a measurement of 10 s and save the 2D scattering graph.
   6. Move the glassy carbon out of the pathway. Take a measurement of 1800 s on the background solution and save the background scattering graph.
   7. Move the capillary out of the pathway, mount the glassy carbon only and take a 10 s measurement.
   8. Move the glassy carbon out of the pathway. Take a 10 s measurement of the black current (vacuum chamber only).
   9. To measure the nanoparticle solution, load the sample into the capillary and follow the same procedures from 4.1.2 – 4.1.6.
   10. For data analysis, open SAXS analysis software via File | Import from file | Import the background and the sample files.
   11. Choose the 2D pattern of the background. Click Indirect transmission calculation in tool. Input the background with glassy carbon, glassy carbon and blank frame files and click on OK. Do the same operations on the sample pattern. The transmissions will be automatically calculated.
   12. Drag the circle ring cursor from the edge to the center of the 2D scattering pattern to integrate the background and sample 2D graph to 1D scattering curve.
   13. Choose the background curve in the list. Check it as background measurement in SAXS information.
   14. Choose the background and the sample curves together. Right click and choose Background correction to subtract the background from the sample.
   15. Right click on the curve after background correction. Choose SAXS modeling | Directly modeling | Sphere | Schultz | No interaction.
   16. Set the Q range between 0.02 to 0.3. Click on Initial guess to give an estimation on the fitting results. Then click on Fit to fit the 1D SAXS curve with Schultz polydisperse sphere model to obtain the mean diameter and standard deviation (corresponding to the size distribution of the nanoparticles).
2. Concentration of particles () extraction
   1. Use the absolute intensity (), which can be correlated to both the size and concentration of nanoparticles in the solution as follows^14,35:
      
      where is the scattering vector, N_p is the concentration of nanoparticles, is the nanoparticle volume, and is the single-particle form factor. Calculate the Schultz distribution factor³⁶ in the case of polydisperse spherical shape nanoparticles using the following expression:
      
      Here.
   2. Consider → 0, which is the extrapolation of the SAXS curve to the intercept to Y axis:
      
      ∆ is the scattering length density difference between metal and solvent and is the average square of the particle volume.
   3. Calculate using equation:
      
   4. To obtain , use water (as a standard) to calibrate the scattering intensity to absolute scale due to its well-known absolute differential scattering cross-section of 1.632×10^-2 cm^-1 at room temperature³⁴. Measure the empty capillary and water and subtract the empty capillary as a background for water following the procedures from 4.1.2 to 4.1.14.
   5. The 1D scattering curve for water is a straight line parallel to X-axis. Extrapolate the line to get the intercept intensity (cm^-1) on Y-axis. Calculate the calibration factor (CF) as
      .
   6. Find the extrapolation intensity for the nanoparticle curves. Calibrate to obtain at absolute scale using the CF:
      
   7. Extract the concentration of the particles from the following equation derived from (3):
      
3. Extraction of concentration of atoms in nanoparticles () from in situ and ex situ SAXS
   1. Use both the concentration of nanoparticles () and average value of number of atoms per nanoparticle (N_ave) to calculate the total concentration of atoms as discussed below.
   2. Calculate N_ave based on the following equation³⁷:
      
      where r is the nanoparticle radius, is the Avogadro’s number, ρ is the metal density, and is the metal molecular weight. For palladium, ρ = 12023 kg/m³ and = 0.1064 kg/mol.
   3. To account for the size distribution in estimating the total concentration of atoms in nanoparticles, calculate the using equation (7) along with the Schultz distribution factor:
      
   4. Estimate the concentration of atoms () through multiplying by the concentration of nanoparticles () at any given time as follows:
      

5. Obtaining Kinetic Data from in situ SAXS on Colloidal Pd Nanoparticle Synthesis at Synchrotron

1. Before starting the reaction, take SAXS measurements on the empty capillary, capillary filled with water, and capillary filled with solvent:hexanol at 50:50.
2. Consider that the agent preparation procedures for in situ SAXS are the same with steps 1 and 2, except that the total reaction solution volume is 6 mL (10 mM Pd(OAc)₂ in 3 mL of pyridine or toluene mixed with 3 mL of 1-hexanol, with TOP:Pd molar ratio = 2).
3. In the glovebox, transfer the reaction solution into a 25 mL round bottom flask with a stir bar inside. Purge the space above the solution with N₂ (10 mL/min).
4. Set the stirring rate at 300 rpm. Put the flask in the pre-heated hotplate insert to trigger the reaction.
5. Take 300 μL of reaction solution into the capillary mounted through the X-ray beam path every 8 s using a programmed syringe pump. Collect the scattering data by the detector.
   Note: The transmission of the sample is directly measured by an ionized chamber (without glassy carbon). After each measurement, the solution is pumped back to the bulk reactor.
6. Consider that the data can be automatically converted to 1D curve with the beamline program. The mean diameter and standard deviation are obtained by fitting the data with Schultz polydisperse sphere model. The extraction of concentration of particles follows the same procedures in step 4.2 using the synchrotron X-rays.

6. Modeling Approach and Simulation Procedures for Nucleation and Growth of Palladium (Pd) Metal Nanoparticles

1. Consider the reduction and nucleation as one first-order pseudo-elementary reactions (equation (10)).
   Note: A pseudo-elementary reaction is defined as the sum of one (or more) slow elementary reactions followed by fast elementary reactions (non-rate determining reactions). Herein, the pseudo-elementary reaction represents the kinetics of the slow reaction(s), but have reaction orders equal to the stoichiometry of the sum reaction (hence, the term pseudo-elementary)³⁸. For example, the corresponding reactions for Pd(OAc)₂ reduction and nucleation (TOP:Pd molar ratio=1) in the excess of 1-hexanol are presented below¹⁵:
   (i) Pd(TOP)(OAc)₂(Solv) + R'CH₂OH→Pd⁰ + TOP + R'CHO + 2AcOH + Solv (overall ligand dissociation and reduction), which can be split into steps (ii) and (iii):
   (ii) Pd(TOP)(OAc)₂(Solv) + Solv → Pd(OAc)₂(Solv)₂ + TOP (Ligand dissociation)
   (iii) Pd(OAc)₂(Solv)₂ + R'CH₂OH→Pd⁰ + R'CHO + 2AcOH + (Solv)₂ (reduction)
   (iv) n Pd⁰ →Pd⁰_n (nucleation)
   The reduction (iii) and nucleation (iv) reactions are combined and shown as one pseudo-elementary reduction-nucleation step (A→B). Note that A represents the kinetically active precursor, and while it is written as Pd(OAc)₂(Solv)₂ in reaction (iii), other Pd complexes could be present.
2. Consider the surface growth of nanoparticles to be autocatalytic. Autocatalytic growth is one mode of growth which occurs through the reduction of precursor on the nanoparticle surface (equation (11))³⁷.
3. Account for the binding of capping ligands (TOP) with the precursor (which alter the precursor reactivity) as well as the surface of the particle.
   Note: The dissociation of the ligands (reverse reaction 12) was shown to be important for the nucleation of Ir nanoparticles³⁹. Additionally, other studies have shown that the ligands affect the precursor reactivity (reaction 12) as well as the growth rate of colloidal nanoparticles^14,15,16. Include these reactions in the model (equations (12) and (13)) as two reversible reactions (neither is assumed to be equilibrated during the reaction)¹⁴. Note that our expansion of the FW mechanism¹³ (reactions 10 and 11) accounted for the first time for the reversible binding of the ligands with both the precursor (reaction 12) and the surface of the nanoparticles (reaction 13).¹⁴
4. Assume the following reactions are pseudo-elementary.
   
   
   
   
   Here, is the reduction/nucleation rate constant, the surface growth rate constant, the forward reaction rate constant for reaction (12), the equilibrium constant for ligand-metal precursor binding (i.e. reaction 12),  the forward reaction rate constant for reaction (13), and the equilibrium constant for the binding of ligand with the nanoparticle surface (i.e. reaction 13).
   Note: In addition, A is representative of the kinetically active precursor, L the capping ligand (here TOP), AL the ligand–metal complex (here Pd(II)–TOP) which can be coordinated with different ligands (such as acetate, 1-hexanol or pyridine), B the uncapped Pd surface atom, and BL the Pd atom bound with ligand, Pd⁰ –TOP. In addition, see the complete list for model description and assumptions in previous publication¹⁴.
5. Calculate the concentration of Pd atoms () from the kinetic model based on the following equation.
   
6. Calculate the concentration of nanoparticles () from the model (if no evidence of agglomeration exists) as follows:
   
   Here, is reaction time, the active precursor concentration, Avogadro’s number (6.022 x 10²³) and the nucleus size (atoms/nucleus). is selected to be “4” based on the smallest size detected during the reaction.
7. Use the following differential equations and initial conditions (in MATLAB) to obtain the concentration profile of different species.
   Differential Equations:
   
   
   
   
   
   In addition, for the metal precursor and ligand concentrations (equations 21 and 22) at any given time “t”, the following relationships can be written as follows:
   
   
   
   Note: Reaction is considered to be at equilibrium at time=0. After the reaction proceeds, the reaction is no longer constrained to be at equilibrium.
   
8. Minimize the SR (i.e., sum of normalized squared errors) between the experiments and model for and using the MATLAB function fminsearch to extract the fitting parameters (rate constants shown in equations 10-13).
   
   Here is number of experimental data points.
9. Select similar distribution of the number of data points along the reaction time and Y-axis ( or ) to make sure the minimization function is not weighted toward data points at early or later reaction times.

7. Obtaining Nucleation and Growth Rates from Both the Experimental Data and Model

1. Calculate the nucleation and growth rates from the model using the followings equations.
   
   
   Here, [] represents the concentration of atoms that contributed only to the particle growth.
   Note: To make the unit of nucleation and growth rates the same (i.e., mol.L^-1.s^-1), it is required to multiply equation (26) by []. This allows us to make a comparison between the rates.
2. Estimate the nucleation rate from the experimentally measured number of particles using short time intervals.
   
3. Estimate the growth rate by subtracting the contribution of nucleation from the total concentration of atoms () or metal precursor consumption. “” quantifies both the formation of particles (nucleus) and particle growth.
   

Results

To systematically examine whether the capping ligands alter the kinetics of nucleation and growth, we took the two following approaches: (i) the binding of the ligand with the metal was not considered in the kinetic model similar to previous studies (i.e., the nucleation and autocatalytic growth) (ii) the reversible binding of capping ligand with the precursor and surface of the nanoparticle was taken into account in the model (i.e., ligand-based model described in Proto...

Discussion

In this study, we presented a powerful methodology to examine the effect of capping ligands on the nucleation and growth of metal nanoparticles. We synthesized Pd nanoparticles in different solvents (toluene and pyridine) using Pd acetate as the metal precursor and TOP as the ligand.We used in situ SAXS to extract the concentration of reduced atoms (nucleation and growth events) as well as the concentration of nanoparticles (nucleation event), where both experimental observables were used as the model inputs. In...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
palladium acetate (Pd(OAc)2)	ALDRICH	520764
anhydrous acetic acid	SIAL	338826
trioctylphosphine	ALDRICH	718165
pyridine	MilliporeSigma	PX2012-7
toluene	SIAL	244511
1-hexanol	SIAL	471402
N8 Horizon SAXS	Bruker	A32-X1
glovebox	Vaccum Atmospheres Co.	109035
MR HEI-TEC 115V Hotplate	Heidolph	5053000000
hotplate Monoblock insert	Heidolph	5058000800
heat-On 25-ml insert	Heidolph	5058006200
7 mL vials	SUPELCO	27518
micro stir bar PTFE	VWR	58948-353
egg-Shaped Bars	Fisherbrand™	14-512-121
25 mL round bottom flasks	ALDRICH	Z167495
quartz capillary	Hampton Research	HR6-148
MATLAB R2016b	MathWorks
Bruker SAXS 1.0v	Bruker
Diffrac Measurement Center 4.0v	Bruker