Light-to-Chemical Energy Conversion with Surface Engineered Quantum Dots

Abstract

The unique optoelectronic properties of three-dimensionally confined semiconductor nanocrystals or quantum dots (QDs) have rendered them a special place in light harvesting studies, including photocatalysis. As a result, QDs have emerged as potential photocatalysts towards various classes of chemical transformations. Interestingly, unlike molecular photocatalysts, along with size-tuneable core optoelectronic properties, QDs exhibit rich and highly modular surface properties as well. By strategically modifying the surface with suitable ligand chemistry, it’s possible to fine tune the interaction between the QD photocatalysts and the reactant molecules. Such precise regulation of the catalyst-reactant interactions results in the emergence of exceptional and unprecedented behaviours, particularly in terms of the efficiency of the photocatalytic processes. Moreover, besides the traditional photophysical processes, QDs also exhibit exciting non-trivial photophysical processes (such as triplet-triplet annihilation, two-photon absorption etc.), which in principle can be advantageous if employed into the field of photocatalysis to perform challenging chemical reactions under milder rection conditions. However, the aspects of both the surface directed as well as the non-trivial processes still remain elusive in the area of QD photocatalysis, and only dedicated studies in these directions will install the realisation of the vast importance of QDs in photocatalysis in its full potential. Thus, the present thesis focuses on developing a strategy to systematically modulating both the core optoelectronic as well as the surface ligand directed properties of the QDs to carry out a wide variety of chemical transformations under visible light. Furthermore, efforts were made to investigate the stated objectives with environmentally friendly QDs (e.g. indium phosphide (InP) QDs), wherever possible, thereby enhancing the sustainability aspect of the QD photocatalysis.

At first, the potency of InP/ZnS QDs was established in independently photocatalyzing two important classes of reactions, namely, metal-centred redox and organic C-C coupling reactions. The two types of reactions were carried out in aqueous and organic medium respectively via proper surface engineering of the QDs. This work allowed us to reveal the full potential of the InP QDs in devising an artificial photosynthetic system via nicotinamide cofactor regeneration. The photosynthetic light and dark cycles reactions were closely mimicked using our devised InP QD-enzyme nanobiohybrid system, which was finally applied for the continuous synthesis of useful chemicals under visible-light. We could enhance the catalytic efficiency of the reaction by introducing favourable catalyst-reactant interaction via precise surface engineering of the InP QDs. In another attempt, the decisive role of ligand chemistry in directing the charge transfer process from the QD photocatalyst to the reactant was established. Here, we could identify and establish the dual role of triphenylphosphine in acting both as a surface passivating agent as well as a reactant in a series of QD photocatalyzed olefination reactions. Finally, we could explore the intriguing non-trivial photophysical processes of QDs in developing a triplet-triplet annihilation based upconversion system to carry out photoredox, as well as photopolymerization reactions, that are beyond the redox potentials of the QDs alone.

In summary, the present thesis effectively demonstrates a practical design strategy for environmentally friendly QD based photocatalysts. The strategy involves a systematic modulation of both the core and the surface properties, enabling a wide variety of chemical as well as biochemical transformations occurring within, or even beyond the redox potentials of the QDs, under visible light irradiations.