PhD Defense by Mingyue Zhang
THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Friday, September 23, 2022
Zoom Video Conferencing
will be held the
"Polymer-Ligated Nanocrystals with Tunable Dimensions, Compositions, and Architectures"
Prof. Zhiqun Lin, Advisor, MSE
Prof. Vladimir Tsukruk, Advisor, MSE
Prof. Seung Soon Jang, MSE
Prof. Zhitao Kang, GTRI/MSE
Prof. Xing Xie, CEE
The ability to produce monodisperse nanocrystals with stable and tunable surface chemistry is of key importance to render investigation into their size- and shape-dependent physical properties and thus an array of applications including electronics, photonics, catalysis, sensors, energy storage, information technology, bionanotechnology, etc. In this context, nonlinear block copolymer nanoreactor has emerged as a general and robust route to synthesis of a gallery of nanocrystals with precisely controlled sizes, shapes, compositions, and surface chemistry. In this thesis, I capitalized on a set of rationally designed star-like and bottlebrush-like block copolymer to template the growth of a host of functional 0D and 1D nanocrystals with controlled dimensions, compositions, and architectures, and scrutinize the dependence of physical properties and energy-related applications on their size, shape, and surface chemistry.
First, a series of star-like copolymers were synthesized via sequential atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) and styrene from star-like macroinitiators, brominated β-cyclodextrin (β-CD). Due to the living nature of ATRP, the molecular weight of each polymer block can be precisely controlled by simply tuning polymerization time and a low polydispersity index (PDI) can be achieved. The inner hydrophobic poly(tert-butyl acrylate) (PtBA) blocks were then converted into hydrophilic poly(acrylic acid) (PAA), which strongly coordinates with the metal moieties of precursors of targeted nanocrystals, leading to the nucleation and growth of nanocrystals confined within the space occupied by the PAA blocks. As a result, the size and shape of nanocrystals can be readily controlled by the molecular weight of PAA blocks (i.e., diameter of nanoparticles). Moreover, the outer PS blocks, originally covalently linked to the inner PAA blocks, form a layer of permanently anchored ligands on the nanocrystal surface to enable stable surface chemistry. This synthetic strategy was successfully applied for preparing a diversity of functional nanoparticles for the investigation into their physical properties and applications. Specifically, this judiciously designed nanoreactor was utilized to craft monodispersed magnetic spinel CoFe2O4 nanoparticles, which was studied for their magnetic and surface chemistry related electrocatalytic activity. It was the first systematic scrutiny of the influence of spin-pinning effect in spinel nanoparticles realized via surface reconstruction on the oxygen evolution reaction (OER).
Second, using the same chemistry, 1D bottlebrush-like PAA-b-PS templates can be realized by employing brominated cellulose (Cell-Br) as macroinitiators. Due to the larger number of side chains on one Cell-Br macroinitiator (ranging from about 40 chains to more than 150 chains), high quality bottlebrush-like block copolymers are more challenging to synthesize than star-like block copolymers, which only have 21 arms. Systematic scrutiny was made to investigate the reaction conditions (e.g., catalyst ratio, ligand ratio, reaction concentration, degassing method, etc.) that affect the uniform growth of the highly dense block copolymer side chains, which has a determining effect on the quality of the bottlebrush-like templates and their application as nanoreactors for the synthesis of 1D nanocrystals.
In addition to focusing on the precise synthesis of 0D and 1D nanocrystals via nanoreactor strategy, this thesis also covers the practical application of multi-functional nanocomposites. In this work, a ternary nanocomposite consisting of antibacterial silver (Ag) NPs, photocatalytic titania oxide (TiO2) NPs, and upconverting NPs are prepared, manifesting a greatly enhanced biocidal performance under ambient environment. It was found that the visible light (blue) and ultraviolet (UV) light which were converted from near infrared (NIR) radiation by the NaYF4@Yb:Tm upconverting NPs can be effectively absorbed by Ag and TiO2 NPs to generate electrons and electron-hole pairs, respectively. Reactive oxygen species (ROS) could then be produced from the reactions between environment and the electrons and holes to terminate bacteria. The outstanding antibacterial performance of this nanocomposite system renders it the potential to be used in food packaging industry.
Moreover, reversible photo responsive Ruddlesden-Popper 2D perovskite nanoplatelets were explored in this thesis. Colloidal two-dimensional RP perovskite nanoplatelets with a general formula L2(ABX3)n-1BX4 are a rapidly emerging type of semiconductor materials with excellent optical and electronic properties. Research on the functional organic spacers (L) has become a popular direction in the past several years. Inspired by our previous research about reversible photo-crosslinkable nanoparticles realized by capitalizing on star-like nanoreactors with photo responsive coumarin containing repeat units in the outer block, the preparation of RP lead halide perovskite nanoplatelets with coumarin containing ammonium as organic spacers was attempted. Molecular modification and mixed organic spacer strategies were adopted to overcome the solubility limitation of coumarin containing molecules in non-polar solvents. Such coumarin containing 2D perovskite nanoplatelets will undergo controllable and reversible layer-by-layer crosslinking and de-crosslinking under radiation of certain wavelength, leading to many intriguing and tunable optical and electronic properties.