Browsing by Author "Li, Wenhui"
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- Connecting Thermodynamics and Kinetics of Ligand Controlled Colloidal Pd Nanoparticle SynthesisLi, Wenhui (Virginia Tech, 2019-04-24)Colloidal nanoparticles are widely used for industrial and scientific purposes in many fields, including catalysis, biosensing, drug delivery, and electrochemistry. It has been reported that most of the functional properties and performance of the nanoparticles are highly dependent on the particle size and morphology. Therefore, controlled synthesis of nanomaterials with desired size and structure is greatly beneficial to the application. This dissertation presents a systematic study on the effect of ligands on the colloidal Pd nanoparticle synthesis mechanism, kinetics, and final particle size. Specifically, the research is focused on investigating how the ligand bindings to different metal species, i.e., metal precursor and nanoparticle surface, affect the nucleation and growth pathways and rates and connecting the binding thermodynamics to the kinetics quantitatively. The first part of the work (Chapters 4 and 5) is establishing isothermal titration calorimetry (ITC) methodology for obtaining the thermodynamic values (Gibbs free energy, equilibrium constant, enthalpy and entropy) of the ligand-metal precursor binding reactions, and the simultaneous metal precursor trimer dissociation. In brief, the binding products and reactions were characterized by nuclear magnetic resonance (NMR), and an ITC model was developed to fit the unique ITC heat curve and extract the thermodynamic properties of the reactions above. Furthermore, in Chapter 6, the thermodynamic properties, especially the entropy trend changing with the ligand chain length was investigated on different metal precursors based on the established ITC methodology, showing that the entropic penalty plays a significant role in the binding equilibrium. The second part of the dissertation (Chapter 7 and 8) presents the kinetic and mechanistic study on size-tuning of the colloidal Pd nanoparticles only by changing different coordinating solvents as ligands together with the trioctylphosphine ligand. In-situ small angle X-ray scattering was applied to characterize the time evolutions of size, size distribution, and particle concentration using synthesis reactor connected to a capillary flow cell. From the real-time kinetic measurements, the nucleation and growth rates were calculated and correlated with the thermodynamics, i.e., Gibbs free energies of solvent-ligand-metal precursor reactivity and ligand-nanoparticle surface binding which were modified by the coordination of different solvents. Higher reactivity leads to faster nucleation and high nanoparticle concentration, and stronger solvent/ligand-particle coordination energy results in higher ligand capping density and slower growth. The interplay of both effects reduces the final particle size. Furthermore, because of the significance of the ligand-metal interactions, the synthesis temperature and ligand to metal precursor ratio were systematically to modify the relative binding between the ligand and precursor, and the ligand and nanoparticle, and determine the effect on the nucleation and growth rates. The results show that the relative rates of nucleation and growth is critical to the final size. A methodology for using the in-situ measurements to predict the final size by developing a kinetic model based is discussed.
- The role of nanoparticle size and ligand coverage in size focusing of colloidal metal nanoparticlesMozaffari, Saeed; Li, Wenhui; Dixit, Mudit; Seifert, Soenke; Lee, Byeongdu; Kovarik, Libor; Mpourmpakis, Giannis; Karim, Ayman M. (2019-10-01)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 BrOnsted-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.