An investigation of the morphology evolution in early-stage nucleation and growth of carboxyl and thiol capped gold nanoparticles

Yiu, Pak Leung Aidan (2025). An investigation of the morphology evolution in early-stage nucleation and growth of carboxyl and thiol capped gold nanoparticles. University of Birmingham. Ph.D.

Yiu2025PhD.pdf
Text
Available under License All rights reserved.
Download (133MB)

Abstract

This thesis explores the growth and stability of sub-10 nm gold nanoparticles (AuNPs) synthesized in aqueous media using various carboxyl and thiol based capping agents. An in-depth investigation into early nucleation stages and initial growth dynamics was conducted using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and reactive molecular dynamics simulations (r-MD), with a focus on combining classical and non- classical nucleation pathways and the role of pre-nucleation clusters (PNCs).

The first research chapter examined the size and shape evolution of AuNPs synthesized with sodium borohydride as a reducing agent. Key factors such as pH, molar ratio of capping agents, and the chemical structure of carboxyl-based ligands were analysed. Findings revealed unexpected behaviours, such as the limited influence of higher molar ratios on particle size and the optimal stability at pH 6. Pre-nucleation clusters were identified as metastable intermediates across most conditions, indicating their potential role in influencing particle growth and morphology throughout the nucleation process.

The second research chapter compared the growth and morphology of AuNPs capped with L-cysteine and glutathione (both thiol group ligands) to those capped with carboxyl group ligands. Distinct ligand dependent behaviours were observed, with carboxyl group PNCs exhibiting tendency to aggregate, leading to structural transformations, while thiol-based PNCs displayed non aggregate behaviours, promoting stable but distinct growth pathways. The persistence of PNCs in solution for up to 28 days, their size range, and their influence on aging processes, including the formation of non-spherical morphologies, were characterized, suggesting their influencing role in deviating from classical growth pathways.

The third research chapter applied reactive molecular dynamics simulations to provide atomistic insights into ligand mediated nucleation and growth mechanisms. Carboxyl groups were found to stabilize AuNPs through coordination interactions, while thiol groups formed robust covalent bonds that hindered growth but occasionally induced destabilization. Simulations highlighted the interplay between carboxyl and thiol ligands in guiding nanoparticle nucleation and growth, offering a detailed understanding of the structural evolution of AuNPs.

Overall, this thesis demonstrates the critical role of PNCs as intermediates in nanoparticle formation and growth, demonstrating their persistence, metastable nature, and ligand dependent behaviours. These findings bridge the classical and non-classical nucleation pathways, revealing how ligand chemistry governs PNC- mediated interactions, which in turn, influence the stability, morphology and unpredictable evolution of nanoparticles. This work provides new insights into the formation of PNCs and the interaction with crystallised NPs into irreversible changes of morphology.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):