Colloidal Semiconductor Nanocrystals for Challenging Photocatalytic Organic Transformations

Abstract

Colloidal quantum dots (QDs) have emerged as a versatile photocatalyst for a wide range of photocatalytic transformations owing to its extraordinary properties, including high absorption coefficient for light harvesting, large surface-to-volume ratio, efficient charge and energy transfer dynamics, and high stability. The past decades have witnessed a rapid development of QDs for artificial photocatalysis. To explore unique capabilities of QDs for photocatalysis, my research focuses on designing, synthesizing and engineering QDs, then employing QDs for challenging organic transformations enabled by their exceptional photophysical properties.

In this thesis work, I have successfully adopted doping and ligand exchange strategies to functionalize various kinds of QDs including CsPbBr₃, CdS, and InP. Systematic optical spectroscopy techniques were applied to reveal the photophysical properties of as-prepared QDs. I demonstrated that the composition tunability of QDs allow us to construct a “two in one” photocatalyst for the oxidation of diaryl hydrazine, accompanied by H₂ evolution. In addition, the incorporation of Mn²⁺ ions into QDs significantly enhanced hot electron generation efficiency, facilitating the activation of inert organic substrates under mild conditions. The chirality transfer mechanism between organic molecules and inorganic perovskite nanocrystals was also studied in this thesis. The successful fabrication of chiral perovskites opens up promising avenues for enantioselective photocatalysis based on chiral QDs.

Chapter 1 gives a general review of photocatalysis based on QDs. The unique characteristics of QDs that can be utilized for photocatalytic transformations are introduced firstly. Then recent advancements in photocatalytic organic transformations enabled by QDs photocatalysts are summarized.

In Chapter 2, I synthesize atomically thin inorganic perovskite NPLs with precise control of layer thickness and functionalize them by chiral surface ligands, serving as a unique platform to probe the chirality transfer mechanism at the organic/perovskite interface. It is found that chirality is successfully imprinted into mono-, bi-, and tri-layer inorganic perovskite NPLs, exhibiting tunable circular dichroism (CD) and CPL responses. However, chirality transfer decreases in thicker NPLs, resulting in decreased CD and CPL dissymmetry factors for thicker NPLs. Aided by large-scale first-principles calculations, I propose that chirality transfer is mainly mediated through a surface distortion rather than a hybridization of electronic states, giving rise to symmetry breaking in the perovskite lattice and spin-split conduction bands.

In Chapter 3, acceptorless photocatalytic dehydrogenation of diaryl hydrazines was realized by employing Ni²⁺-dopedCsPbBr₃ Nanoplatelets (NPls). It is showed that surface Ni²⁺ ions can act as a “electron shuttle” to reduce protons generating clean energy H₂ gas, while the photogenerated holes can be used to oxidize diaryl hydrazines to afford valuable azobenzene and derivatives. Compared to previous photocatalytic demonstrations, our catalysts show excellent reaction yields with a wide substrate scope, unity atomic efficiency, and enhanced stability and recyclability.

In Chapter 4, an efficient hot-electron generation system facilitated by the spin-exchange Auger process in Mn²⁺-doped CdS/ZnS quantum dots is presented. These hot electrons can be effectively utilized in a wide range of organic reactions, such as the Birch reduction and reductive cleavage of C-Cl, C-Br, C-I, C-O, C-C, and N-S bonds. Notably, these reactions accommodate substrate reduction potentials as low as −3.4 V versus the saturated calomel electrode. Through two-photon excitation, I have achieved the generation of a ""super"" photoreductant using visible-light irradiation power that is only 1% of that previously reported for molecular and quantum dot systems. By modulating the intensity of light output, the spin-exchange Auger process enables the on/off generation of hot electrons, allowing for programmable assembly-point cross-coupling cascades.

Appendix shows a partially completed project. In this project, I incorporated Mn cations into III-V semiconductor nanocrystals InP and studied improvement of the hot electron generation efficiency induced by the spin-exchange interaction between Mn²⁺ dopants and InP host. Furthermore, the InP/ZnS:Mn²⁺ QDs were tentatively applied in the activation of alkyl chlorides with stronger bond dissociation energy than aryl chlorides. The extraordinary performance of InP/ZnS:Mn²⁺ is discussed.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Haipeng LU (Supervisor)