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Saumya Badoni

Development of DNP hyperpolarized NMR for structural determination of ligand-stabilized ZnO nanoparticles

Published on 22 July 2021
Thesis presented July 22, 2021

The nanoparticle-ligand interface plays a key role in the unique physicochemical properties of nanoparticles (NPs). Thus, a deep understanding of surface properties such as intimate interactions on NP surfaces, the surface ligand dynamics, ligand packing density, and their binding mode and affinity is required to fine-tune materials for desired applications. In this work, novel applications of dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance (ssNMR) spectroscopy for the characterization of ligand shells on zinc oxide (ZnO) NPs have been presented. The ability of DNP-enhanced ssNMR to characterize the ligand shell of NPs has been demonstrated in three key areas concerning ZnO NPs. These include a comparison between the ligand shell arrangement and stability of ZnO NPs synthesized using two different synthetic approaches, understanding the role of ligand in the growth and stabilization of hexagonal ZnO nanoplatelets, and studying the ligand shell morphology (namely, Janus, patchy, stripe-like, and random) of mixed ligand shell NPs.

Firstly, DNP enhanced ssNMR was used to study the surface ligand binding and arrangement on ZnO NPs prepared by a sol-gel and by an organometallic approach. It was shown that the ligand shell on NPs prepared by the one-pot self-supporting organometallic (OSSOM) approach was more stable and uniform compared to that produced through sol-gel methodology, which shows a time-dependent surface restructuring process with a displacement of the majority of the ligands from the surface. Moreover, the study revealed stable m2 and m3 coordination modes of ligands that pair up on polar surfaces.

In order to deepen our understanding of ZnO interfaces, size-controlled ZnO nanoplatelets (NPLs) produced by OSSOM-II, a modified version of the OSSOM approach, were studied. The crystal structure and morphology of the NPLs were analyzed using powder X-ray diffraction (PXRD) and high-resolution transmission electron microscopy (HRTEM) analysis. The multi-faceted system was investigated to understand the role of benzamidine ligands in its growth and stabilization. Therefore, binding modes and atomic-scale arrangements of ligands on the ZnO surfaces were analyzed using DNP-enhanced solid-state NMR. Bimodal binding of benzamidine was observed, with different binding strengths being related to the polar and non-polar facets of these NPLs. The assignments of NMR peaks were confirmed using density functional theory (DFT) calculations. Moreover, insights into the role of ligand in the growth of these NPLs were provided.

Lastly, DNP-enhanced ssNMR was used to investigate NPs capped with mixed ligands. Mixed ligand shells on NPs are known to improve NP properties such as solvability, catalysis, and cation capturing ability. Several recent reports have presented methods for the characterization of mixed ligand shell morphologies, namely Janus, random, and stripe-like, however, a generally applicable method is still lacking. In this work, a quantitative analysis of neighboring ligands based on the well-known transferred echo double resonance (TEDOR) experiment, which is used here for the measurement of hetero-ligand proximities, is presented. DNP enhancements allowed the acquisition of multiple data points for 13C{31P} TEDOR at natural isotopic abundance (NA) for ligands present on the NP surfaces. In this work, the enhancements provided by DNP have made possible the measurement of NMR spectra for nuclei with low NA and low gyromagnetic ratio, and inter-ligand long-range distance measurements on the surface of ZnO NPs. Using this information, detailed pictures of ligand shell morphologies on ZnO NP surfaces have been provided. The methods presented in this work for surface ligand characterization can be extended to other hybrid materials with an internal crystalline core and surface species.

materials, DNP, ZnO, DFT