Name: ligand-mediated nucleation and growth of palladium metal nanoparticles
Uploaded: 2018-06-25T13:00:00
Description: Watch this Scientific Journal Video about Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles at JoVE.com

The work was primarily funded by the National Science Foundation (NSF), Chemistry Division (award number CHE-1507370) is acknowledged. Ayman M. Karim and Wenhui Li acknowledge partial financial support by 3M Non-Tenured Faculty Award. This research used resources of the Advanced Photon Source (beamline 12-ID-C, user proposal GUP-45774), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors would like to thank Yubing Lu, a Ph.D. candidate in the Chemical Engineering Department at Virginia Tech for his kind help with the SAXS measurements. The presented work was partly executed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396.

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.
<ol>
	<li>Add 40 mL of anhydrous acetic acid into a 50 mL three neck round bottom flask with 0...

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...

Controlled synthesis of metallic nanoparticles is of great importance due to the large applications of nanostructured materials in catalysis, photovoltaic, photonics, sensors, and drug delivery1,2,3,4,5. 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 nuclei6,7. 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 observed1,13,14,15,16,17. 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 Pd14,15,18, Pt19,20, and Rh21,22. Despite recent advances in developing nucleation and growth models1,23,24,25, the role of the ligands is often ignored in the proposed models. Nevertheless, ligands are shown to affect the nanoparticles size14,15,26 and morphology19,27 as well as the catalytic activity and selectivity28,29. 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 conditions32,33. 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.

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			<td height="21" style="height:21px;width:291px;">palladium acetate (Pd(OAc)2)</td>
			<td style="width:203px;">ALDRICH</td>
			<td style="width:119px;">520764</td>
			<td style="width:167px;"></td>
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			<td height="21" style="height:21px;width:291px;">anhydrous acetic acid</td>
			<td style="width:203px;">SIAL</td>
			<td style="width:119px;">338826</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">trioctylphosphine</td>
			<td style="width:203px;">ALDRICH</td>
			<td style="width:119px;">718165</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">pyridine</td>
			<td style="width:203px;">MilliporeSigma</td>
			<td style="width:119px;">PX2012-7</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">toluene</td>
			<td style="width:203px;">SIAL</td>
			<td style="width:119px;">244511</td>
			<td></td>
		</tr>
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			<td height="21" style="height:21px;width:291px;">1-hexanol</td>
			<td style="width:203px;">SIAL</td>
			<td style="width:119px;">471402</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:291px;">N8 Horizon SAXS</td>
			<td style="width:203px;">Bruker</td>
			<td style="width:119px;">A32-X1</td>
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			<td height="23" style="height:23px;width:291px;">glovebox</td>
			<td style="width:203px;">Vaccum Atmospheres Co.</td>
			<td style="width:119px;">109035</td>
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			<td height="21" style="height:21px;width:291px;">MR HEI-TEC 115V Hotplate</td>
			<td style="width:203px;">Heidolph</td>
			<td style="width:119px;">5053000000</td>
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			<td height="21" style="height:21px;width:291px;">hotplate Monoblock insert</td>
			<td style="width:203px;">Heidolph</td>
			<td style="width:119px;">5058000800</td>
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			<td height="21" style="height:21px;width:291px;">heat-On 25-ml insert</td>
			<td style="width:203px;">Heidolph</td>
			<td style="width:119px;">5058006200</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:291px;">7 mL vials</td>
			<td style="width:203px;">SUPELCO</td>
			<td style="width:119px;">27518</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:291px;">micro stir bar PTFE&#160;</td>
			<td style="width:203px;">VWR</td>
			<td style="width:119px;">58948-353</td>
			<td></td>
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			<td height="21" style="height:21px;width:291px;">egg-Shaped Bars&#160;</td>
			<td style="width:203px;">Fisherbrand&#8482;&#160;</td>
			<td style="width:119px;">14-512-121</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:291px;">25 mL round bottom flasks</td>
			<td style="width:203px;">ALDRICH</td>
			<td style="width:119px;">Z167495</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">quartz capillary</td>
			<td style="width:203px;">Hampton Research</td>
			<td style="width:119px;">HR6-148</td>
			<td></td>
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			<td height="21" style="height:21px;width:291px;">MATLAB R2016b</td>
			<td style="width:203px;">MathWorks</td>
			<td style="width:119px;"></td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:291px;">Bruker SAXS 1.0v</td>
			<td style="width:203px;">Bruker</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">Diffrac Measurement Center 4.0v</td>
			<td style="width:203px;">Bruker</td>
			<td style="width:119px;"></td>
			<td></td>
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ligand-mediated nucleation and growth of palladium metal nanoparticles

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.

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...

Watch this Scientific Journal Video about Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles at JoVE.com

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 size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution ...

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