Abstract
Controlling the size distribution of nanoparticles is important for many applications and typically involves the use of ligands during synthesis. In this study, we show that the mechanism of size focusing involves a dependence of the growth rate on the size of the nanoparticles and the ligand coverage on the surface of the nanoparticles. To demonstrate these effects, we used in situ small angle X-ray scattering (SAXS) and population balance kinetic modeling (PBM) to investigate the evolution of size distribution during the synthesis of colloidal Pd metal nanoparticles. Despite temporal overlap of nucleation and growth, our in situ SAXS show size focusing of the distribution under different synthetic conditions (different concentrations of metal and ligand as well as solvent type). To understand the mechanism of size focusing using PBM, we systematically studied how the evolution of the nanoparticle size distribution is affected by nucleation rate, and dependence of the growth rate constant on ligand surface coverage, and size of the nanoparticles. We show that continuous nucleation contributes to size defocusing. However, continuous nucleation results in different reaction times for the nanoparticle population leading to time and size-dependent ligand surface coverage. Using density functional theory (DFT) calculations and Brønsted-Evans-Polanyi relations, we show that as the population grows, larger nanoparticles grow more slowly than smaller ones due to lower intrinsic activity and higher ligand coverage on the surface. Therefore, despite continuous nucleation, the faster growth of smaller nanoparticles in the population leads to size focusing. The size focusing behaviour (due to faster growth of smaller nanoparticles) was found to be model independent and similar results were demonstrated under different nucleation and growth pathways (e.g. growth via ion reduction on the surface and/or monomer addition). Our results provide a microscopic connection between kinetics and thermodynamics of nanoparticle growth and metal-ligand binding, and their effect on the size distribution of colloidal nanoparticles.
| Original language | English |
|---|---|
| Pages (from-to) | 4052-4066 |
| Number of pages | 15 |
| Journal | Nanoscale Advances |
| Volume | 1 |
| Issue number | 10 |
| DOIs | |
| State | Published - 9 Oct 2019 |
| Externally published | Yes |
Bibliographical note
Publisher Copyright:© The Royal Society of Chemistry.
Funding
We acknowledge funding from the National Science Foundation (NSF), Chemistry Division (CHE) under Grant No. 1507370 and CBET Division (CAREER program) under Grant No. 1652694. Ayman M. Karim would like to acknowledge partial nancial support provided by 3M Non-Tenured Faculty Award. This work 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. A portion of the research was performed at EMSL, a national scientic user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multiprogram national laboratory operated for the U.S. DOE by Battelle. Computational support was provided by the University of Pittsburgh Center for Research Computing. We thank Michael G. Taylor for the binding energy calculations shown in Fig. S23.† We are also very grateful to Prof. Richard Finke at Colorado State University (CSU) for fruitful discussions regarding the possible effect of nanoparticle size on surface growth.
| Funders | Funder number |
|---|---|
| National Science Foundation | |
| U.S. Department of Energy | |
| Division of Chemistry | 1507370, 1652694, GUP-45774 |
| Office of Science | |
| Biological and Environmental Research | |
| Argonne National Laboratory | DE-AC02-06CH11357 |
| University of Pittsburgh | |
| Pacific Northwest National Laboratory |