|M.Sc Student||Peer Imanuel|
|Subject||Anisotropic Hybrid Metal-Insulator-Semiconductor|
Nanostructures for Plasmon Enhanced Photocatalysis
|Department||Department of Chemistry||Supervisor||Professor Lilac Amirav|
|Full Thesis text|
Rising global energy demands along with environmental considerations require the development of a sustainable energy system. Solar energy is a potential natural and renewable source. By using sunlight to split water, this energy can be stored within hydrogen as a solar fuel. This can be realized by photocatalysis, whereby a photon of sufficient energy excites electrons and holes in a semiconductor, which promote reduction and oxidation half-reactions, respectively. However, photocatalytic redox reactions such as water splitting that require the injection of multiple photoinduced charge carriers often include highly reactive intermediates, with back reaction routes that hinder the overall photocatalytic efficiency. This work postulates that these back reactions can potentially be minimized, rendering the photocatalytic reactions more efficient, if multiple carriers are excited simultaneously (as multi-excitons).
Multi-exciton generation rates are naturally small relative to single excitons. Localized Surface Plasmon Resonances and intense electromagnetic fields generated near sharp structural features in metallic nanostructures are known to enhance by orders of magnitude various light-matter interactions. We aim to employ this phenomenon for the enhancement of inherently weak multi photon absorption in the semiconductor photocatalysts. Our strategy relies on the synthesis of novel hybrid metal-insulator-semiconductor nanostructure, which allows coupling of the plasmonic and excitonic properties.
The hybrid nanostructure of choice is comprised of a plasmonic metal nanoprism core, encapsulated by an insulating silica layer, which prevents undesired charge transfer, onto which semiconductor quantum dots are attached. The thesis details the development and elucidation of a pathway of reactions leading to a well-dispersed colloidal solution of the desired hybrid nanostructure. In particular, the synthesis steps include: (1) Nanoprism synthesis, i.e. choice of the metal and morphological control. (2) Silica coating, while avoiding etching of the metal core and agglomeration. (3) Functionalization of the silica surface with a coupling agent, while retaining colloidal stability. (4) Attachment of the semiconductor quantum dots.
The experimental work commenced with the synthesis of silver (Ag) nanoprisms, followed by their coating with silica, surface functionalization with an aminosilane coupling agent and quantum dots attachment. The silver core was completely etched away due to amine-catalyzed oxidation. In search for improved stability, silver nanoprisms with gold frames (Ag@Au) were then prepared and silica coated, however these too exhibited oxidative instability. Next, gold (Au) nanoprisms were synthesized via two different methods: seed-mediated growth and seedless-growth through oxidative etching. Their subsequent coating and functionalization resulted in aggregation and precipitation. The problem was solved by changing the silica precursor and the silane coupling agent, ultimately leading to the desired nanostructure.
Additionally, Pt photodeposition was applied on the quantum dots, and demonstrated as a two-electron reduction photochemical staining technique for further assessment of the plasmonic effect.
These novel hybrid nanostructures can become generalized systems for exploring and exploiting plasmon-enhanced photophysical processes, relevant for photocatalytic reactions.