Controlling Post-Synthetic Modifications of Nanoparticle Reagents Towards Increasing Material Complexity
Open Access
- Author:
- Fagan, Abigail
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 13, 2021
- Committee Members:
- Benjamin Lear, Major Field Member
Kristen Fichthorn, Outside Unit & Field Member
Raymond Schaak, Chair & Dissertation Advisor
Lauren Zarzar, Major Field Member
Philip Bevilacqua, Program Head/Chair - Keywords:
- nanoparticles
post-synthetic modifications
hybrid nanoparticles
heterostructures
nanoparticle synthesis
cation exchange
seeded growth
phase transformation - Abstract:
- Nanoparticle syntheses have been explored for several decades, and a vast body of literature exists detailing mechanistic insights into the nucleation and growth of nanoscale materials, controlling their property-defining features, and, through these insights, the development of applications. However, technologies are rapidly and constantly advancing, and therefore, so are the demands for materials that can deliver targeted results and keep pace with new applications. To be considered for potential devices or other purposes, a material must be designed to possess the desired property or set of properties, but it must also be made in sufficiently high quality in order to deliver on those promises. Nanoparticle features such as morphology (size and shape), crystal structure, and elemental composition can all significantly affect the properties of a material and are often programmed in at early stages of the synthesis. It is therefore critical to understand and be able to control these features, in addition to creating materials that can offer synergistic or emergent properties. Post-synthetic modifications of preformed nanoparticle reactants are a means to develop materials that have desired or emergent properties by controlling and manipulating property- defining features. In this dissertation, I introduce some of these modification techniques including compositional transformations, seeded growth, and cation exchange and how each of these techniques accesses and controls a different property-defining feature, and offers a means to understanding new and complex nanomaterials. Through three collaborative works, I investigate reaction intricacies that affect materials on the nanoscale and highlight the need for more fundamental studies of simple nanoparticle reagents towards the development of materials with desired applications and compositional complexity. I begin by demonstrating that preformed colloidal metal nanoparticles can be used as reagents to form a corresponding metal telluride material using a low-temperature, generalizable reaction scheme. In collaboration with Dr. Julie Fenton, I show that nanoparticles of Pd, Pt, and Ni are converted to nanoscale materials of the layered transition metal dichalcogenides PdTe2, PtTe2, and NiTe2 upon reaction with bis(trimethylsilyl)telluride. Interestingly, when Pd nanoparticles react with a trioctylphosphine-Te complex that is less reactive than bis(trimethylsilyl)telluride, PdTe forms. This illustrates that the metal telluride phase that forms can be controlled by the reactivity of the tellurium reagent. Other nanoparticles, including Cu, Ag, Au, and Rh, also form crystalline metal tellurides upon reaction with bis(trimethylsilyl)telluride, which indicates that this approach to synthesizing nanoscale transition metal tellurides is general, and could potentially access a wider library of metal telluride materials using simple colloidal techniques. Next, I introduce seeded growth as a means to create heterostructured, or multicomponent, nanoparticles that exhibit emergent plasmonic properties. Using Pt cube seed particles, spherical Au domains are nucleated on the surface of Pt, and the resulting domain size is controlled by both reaction time and precursor concentration. During early stages of the growth process, the Au domain first forms on the higher energy corners of the Pt seeds, then migrates to a more energetically favorable position at the faces of the Pt cubes. By tuning the reaction conditions (time and Au concentration), Au–Pt hybrid nanoparticles from 4.4 to 16 nm are accessible within a very narrow standard deviation in diameter. This level of synthetic control in this system is unprecedented, and significantly affects the plasmon thermalization process upon excitation using a laser source. The Au–Pt hybrids were probed using 2-dimensional (2D) ultrafast spectroscopy and line width analysis in collaboration with Dr. Ken Knappenberger and William Jeffries. As the Au domain becomes smaller, the thermalization of the Au plasmon resonance becomes more efficient. We propose that this is due to the presence of an epitaxial Au–Pt interface and highlights a size-tunable optical property of Au in a size regime that otherwise does not have size-dependent plasmonic properties in single component Au nanoparticles. I then highlight a third and final post-synthetic technique, nanoscale cation exchange, as another means of creating and understanding heterostructures from simple nanoparticle reagents. In collaboration with Dr. Benjamin Steimle, a small library of two-component metal sulfide nanorods was synthesized via cation exchange from a Cu1.8S nanorod template. Using these two- component nanorod reagents consisting of combinations of ZnS, CdS, MnS, and CoS, which all contain cations of similar chemical hardness that is much different than Cu+, we observe Cu+ cation exchange selectivity for the CdS domain in ZnS–CdS nanorods, for MnS in ZnS–MnS, for ZnS in ZnS–CoS, and preliminary results for MnS selectivity in a CdS–MnS nanorod reagent. Additional studies show similar selectivity using Ag+ and ZnS–CdS and ZnS–MnS reagents. These results do not follow the trend of reactivity that is proposed based on hard-soft acid-base (HSAB) theory alone, which would be expected to be selective based on the greatest difference in hardness values between metal cations. Therefore, we need additional insights into these reactions to fully understand and rationalize the observed behavior. We hypothesize a tiered requirements system to rationalize the results of this work and one that could be used in future studies to predict selective cation exchange behavior in new systems. The first requirement for orthogonal selectivity is a difference in chemical hardness between the metal cations in the nanoparticle reagent and the exchanging cation (here, Cu+ and Ag+). The next requirement is a compression in the overall hexagonal unit cell volume during the transformation, or minimal expansion if compression is not possible. After these first two requirements are met, we propose that favorable solubility of the material(s) will then drive the cation exchange selectivity. However, additional work needs to be done in this area to fully understand the roles that these two parameters play in exchanges performed with cations other than Cu+. The results of the work discussed in this dissertation provide a platform for understanding fundamental and subtle reaction parameters that significantly affect nanoparticle systems and subsequent property-based applications.