Spark-Discharge as a Nanoparticle Source to Study Size-Dependent Plasmonic Properties for Photo-electrochemical Water Splitting

Abstract

This work exploits the ability of the spark discharge particle generator (SDG) to produce metallic nanoparticles (NPs) with control over the size, shape and composition, to unravel the plasmonic mechanisms by which NPs can enhance the photoelectrochemical performance of semiconductor photoanodes. Chapter 1 gives an overview of the SDG and the aerosol technology used in this thesis to synthesize the NPs. Chapter 2 summarizes the different aerosol NP immobilization techniques (both on solids and in liquids) and introduces for the first time an electrospray technique to efficiently capture neutral NPs in liquids. In chapter 3, an extensive literature review on plasmonic photoelectrocatalysis is given to introduce the plasmonic mechanisms that are experimentally studied in Chapter 4, 5, 6 and 7. Chapter 4 and 5 are dedicated to study the hot electron injection (HEI) mechanism by which plasmonic NPs create light-induced “hot” charge carriers upon illumination that can drive photoelectrochemical reactions. Chapter 4 reveals that alloying Ag NPs with Au can be used to shift in a control way the absorption and utilization of light to longer wavelengths. However, due to the low interband energy of Au (i.e., 2.3 eV) compared to that of Ag (i.e., 3.6 eV), the alloy NPs exhibited more interband excitations when illuminated with visible light than pure Ag NPs. Such increase in interband excitations resulted in lower hot electron energies and HEI efficiencies in the alloy NPs than in pure Ag NPs. Chapter 5, reveals the HEI size dependency of Ag NPs. It is found that smaller NPs (< 10 nm) where the surface-induced excitations are prominent result in higher HEI efficiencies, while for larger light absorbing NPs (in the range 10-25 nm) a maximum in the performance is found that corresponds well with the size of the Ag NP with the largest nearfield enhancement. Chapter 6, studies the ability of Ag NPs to concentrate and scatter light into thin film semiconductors to enhance their absorption. It is found that most of the solar energy absorbed by pure 15 nm Ag NPs is lost through heat dissipation. However, larger NPs preferentially scatter the incoming light to the neighbour 6 semiconductor, improving its absorption above their band gap energy. Finally, two configurations of plasmonic NP/semiconductor composites were studied to enhance the semiconductor absorption. In the first configuration the NPs were placed at the semiconductor-electrolyte interface and in the second configuration, the NPs were embedded in the semiconductor at the back-contact/semiconductor interface. It was found that an absorption enhancement at the semiconductor/electrolyte interface was better utilized due to the ability of the surface charge layer to efficiently separate the extra electron holes induced by the plasmonic NPs.