2025 Theses Doctoral

Surface Engineering in Quantum Dots: From Ligand Binding Affinity to Photocatalysis

Zekarias, Bereket Lulseged

This thesis examines the fundamental surface chemistry of quantum dots (QDs) and their application as photoredox catalysts, with a particular emphasis on the role of binding site heterogeneity in cadmium sulfide quantum dots and the enhancement of catalytic performance through manganese doping. The work is divided into three main chapters that systematically explore size-dependent surface phenomena, ligand binding behavior, and strategies for improving photocatalytic efficiency.

Chapter 1 provides a comprehensive introduction to semiconductor nanocrystals, covering their unique size-dependent optical properties arising from quantum confinement, surface chemistry fundamentals including the LXZ ligand classification system, synthetic methodologies ranging from classical hot-injection to modern precursor-controlled approaches, and emerging applications in solid-state lighting, bioimaging, and photoredox catalysis.

Chapter 2 presents a detailed investigation of size-dependent ligand binding site heterogeneity in CdS nanocrystals. Through systematic ligand displacement studies using ¹H NMR spectroscopy on two distinct CdS sizes (~2.4 nm and 6.0 nm), we demonstrate that both sizes exhibit heterogeneous surface binding behavior consistent with a two-site model comprising weakly-binding (B₁) and strongly-binding (B₂) sites. Larger nanocrystals possess higher ratio of strong-to-weak binding sites (B₂:B₁ = 1.8 vs 0.5), consistent with increased exposure of {100} facets that bind ligands more tightly than {111} facets. Treatment with diethyl zinc to remove oleic acid impurities reduced total binding sites by ~18-26% across both sizes and decreased trap emission intensity in large CdS nanocrystals, indicating effective surface passivation of sulfur vacancies. X-ray photoelectron spectroscopy confirmed persistent zinc presence on treated surfaces. These findings establish a direct structure-property relationship in CdS nanocrystals, demonstrating that nanocrystal size fundamentally controls surface chemistry through facet exposure.

Chapter 3 explores the enhancement of photoredox catalytic activity through manganese doping and strategic surface functionalization. Initial studies with CdS QDs reveal that reducing oleate coverage from 3.5 to 2.1 oleates/nm² increases reaction yields three-fold, while fluorinated CdS QDs achieve nearly four-fold enhancement. However, CdS QDs suffer from photodegradation during catalysis. To address this limitation, this work investigates Mn²⁺-doped CdS/ZnS QDs, which generate hot electrons through Auger upconversion processes, enabling reduction potentials sufficient for challenging transformations. Exchanging native stearate ligands with polar alternatives—3-mercaptopropionic acid (MPA), 3-phosphonopropionic acid (PPA), and tributylammonium formate—significantly improves yields in dchlorination reactions at extremely low catalyst loadings (0.0005 mol%). PPA-capped QDs exhibit improved photostability with retained Mn²⁺ emission post-reaction, while formate-capped QDs demonstrate the highest reducing power, achieving moderate to high yields for challenging substrates.

The appendices describe scaled-up synthetic protocols for various quantum dot heterostructures, including CdZnSSe/ZnS for solid-state lighting applications, ZnSe-based systems for low-toxicity applications in photoredox catalysis, and CdS/CdSe/CdS spherical quantum wells for bioimaging, demonstrating the practical scalability of these synthetic approaches.

This work provides insights into the relationship between nanocrystal size, surface chemistry, and doping in photoredox catalysis.