Hot carrier extraction in plasmonic Au-Pt heterostructures

Abstract

The coherent collective oscillation of free electrons in metal nanoparticles under the external illumination is referred to as localised surface plasmon resonance. They may decay either radiatively in light or non-radiatively, producing high energy electron-hole pairs, termed hot carriers. These hot carriers may relax via electron-phonon coupling locally heating the particle or reach the particle surface and transition into unoccupied levels of acceptor adsorbates and trigger chemical reactions. Photochemistry represents one of the most promising fields of study in hot carrier technologies, where hot carriers can catalyse chemical reactions by interacting with external molecules at the particle surface. This includes degradation of organic pollutants from wastewaters, hydrogen generation by solar water splitting and reduction of CO₂, amongst others. The photocatalytic efficiency depends on various factors, such as hot carrier generation rate, hot carrier energy distribution, rate of adsorption of molecules and chemical stability of the photocatalyst. In some cases, plasmonic metals are paired with semiconductors in order to improve the efficiency by extending hot carrier lifetimes, improve optical absorption and promote molecular adsorption. In this work, silica nanoparticles decorated with plasmonic Au nanoparticles and Pt as co-catalyst were designed, characterised and optimised for the enhancement of hot electron generation and extraction. The resulting heterostructure was systematically investigated by varying Au and Pt loading, allowing assessment of the hot carrier extraction via observation of photodegradation of methylene blue, an organic molecule extensively used by the textile industry and that is usually found in wastewaters. An optimal heterostructure design was found while studying the response of the photocatalyst to diffeerent metal loadings, maximising hot carrier generation from Au, and also maximising the catalytic effects. In the case of optimised heterostructures, Pt layers formed on the Au nanoparticles. Since the Pt layers have a rough surface with small Pt clusters which locally support electric field enhancement, the catalytic efficiency was improved due to the efficient hot-carrier generation and extraction. It was found that hot electrons are responsible for triggering the degradation of methylene blue by interacting with oxygen molecules and exciting them to generate superoxide ion species, which in turn chemically react and degrade methylene blue molecules. The combination of plasmonic/catalytic metals in nano-heterostructured devices provide an important impact on photocatalytic processes, and may be of significant interest for future clean technologies.

Date of Award	1 Feb 2020
Original language	English
Awarding Institution	King's College London
Supervisor	Anatoly Zayats (Supervisor)