GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday, July 21, 2020

10:00 AM
via

BlueJeans Video Conferencing

https://bluejeans.com/816591420

DISSERTATION DEFENSE

for

Yeu-Wei Harn

“Nonlinear Block Copolymer Nanoreactor-Enabled Precision Synthesis of Metal Oxide Nanoparticles with Controlled Dimensions and Compositions and Investigation into Their Electrocatalytic and Magnetic Properties”

Committee Members:

Prof. Zhiqun Lin, Advisor, MSE

Prof. Zhong Lin Wang, Co-Advisor, MSE

Prof. Seung Soon Jang, MSE

Prof. Vladimir Tsukruk, MSE

Prof. Younan Xia, BME

Prof. Lei Zhu, School of Macromolecular Science and Engineering, CWRU

Abstract:

Colloidal nanocrystals carry a wide array of intriguing features, thereby rendering a rich diversity of applications in optics, electronics, optoelectronics, catalysis, sensors, energy conversion and storage, nanotechnology, biotechnology, among other areas. Despite copious past works on development of synthetic approaches, the ability to craft functional nanocrystals with precisely controlled compositions and dimensions under mild reaction condition and facile purification process is comparatively few and limited in scope. In this thesis, we developed a general and robust star-like block copolymer nanoreactor strategy for precision synthesis of an assortment of metal oxide nanoparticles controlled dimensions, compositions and surface chemistry, followed by investigation into their size- and composition-dependent electrocatalytic and magnetic properties.

We first successfully synthesize both amphiphilic and hydrophilic star-like diblock and triblock copolymers, which can subsequently function as nanoreactors for templating the precision synthesis of inorganic nanoparticles. In particular, the molecular weights and molecular weight distribution of star-like block copolymers can be readily controlled with the addition of linear initiator as living radical polymerization is achieved. Furthermore, the dual pH-responsive behavior of double hydrophilic star-like diblock copolymer is found, suggesting the potential application as polymer nanocarriers for control-release of drugs.

By capitalizing on amphiphilic star-like diblock copolymer as nanoreactors, perovskite oxide (i.e., BaTiO₃, PbTiO₃ and doped BaTiO₃), La-based perovskite and layered perovskite (i.e., LaFeO₃, LaMnO₃ and La₂CoO₄) and magnetic spinel ferrite (i.e., CoFe₂O₄, MnFe₂O₄ and NiFe₂O₄) nanoparticles with controllable size can be obtained. Due to the unique compositional versatility and structural stability of perovskites, investigation into electrocatalytic performance of all the as-synthesized nanoparticles with perovskite and layered perovskite structure is conducted. First, we report the scrutiny of size- and dopant-dependent oxygen reduction reaction (ORR) activities of an array of pristine BaTiO₃ and La- or Co-doped BaTiO₃ nanoparticles. The ORR activities are found to progressively decrease with the increasing size of BaTiO₃ NPs because of increased active sites and electroconductivity. On the other hand, La- and Co-doped BaTiO₃ NPs display markedly improved ORR performance over the pristine counterpart, which can be attributed to the reduced limiting barrier and the possibly-increased conductivity that are verified by density functional theory-based first principle calculations. Second, the bifunctional electrocatalytic activity of LaFeO₃, LaMnO₃ and La₂CoO₄ nanoparticles for both ORR and oxygen evolution reaction (OER) have been revealed. Among all, layered La₂CoO₄ perovskite nanoparticles exhibit remarkable activity and excellent stability (i.e., with the oxygen activity (ΔE) of 0.88 V when D = 8.5 nm), which is greater than that of the previously reported perovskite and perovskite derivative electrocatalysts. The superior catalytic performances of layered La₂CoO₄ perovskite NPs may be resulted from highly active lattice oxygen and an increased concentration of hydroxyl groups, corroborated by the pH-dependent OER behavior and XPS of O 1s results, respectively. Finally, three representative spinel ferrites (i.e., MnFe₂O₄, NiFe₂O₄, CoFe₂O₄) nanoparticles with tunable size are prepared. The size- and composition-dependent magnetic properties are explored and discussed. Moreover, by changing the outer block of the star-like diblock copolymer, hydrophilic poly(ethylene glycol)-ligated spinel ferrite nanoparticles can be acquired, leading to possible biomedical applications. By extension, amphiphilic star-like block copolymer nanoreactor strategy is anticipated to enable the crafting of an exciting variety of other functional nanomaterials with tailored sizes, compositions and surface chemistry. As such, the fundamental correlations between morphologies, compositions and properties of judiciously designed nanomaterials can be elucidated, rendering the optimized materials performance.