Charge and Energy Transfer in Plasmonic Nanostructures

The development of new materials capable of transforming solar to chemical energy has been the center of attention in materials science over the last 50 years, as the need for producing clean energy and reduce the consumption of non-renewable fossil fuels is urgent to stop climate change and avoid energy shortage worldwide. Since the observation of photocatalytic activity of a titania-based catalyst doped with transition metals for nitrogen fixation in the early 1980s, numerous large band gap semiconductors have demonstrated photocatalytic energy conversion for nitrogen fixation, hydrogen evolution and CO2 reduction. However, because these photocatalysts only utilize light in the UV region (due to their large band gap), makes them inefficient for harvesting solar energy (which is composed of 40% of visible light and 50% of Near Infrared light). Additionally, because photocatalysts need to fulfill other requirements, including: the capacity to generate charge-separated states with lifetimes and electrochemical potentials compatible with the targeted chemical reaction (e.g. water splitting, ammonia fixation, etc.), several new combinations of carbon-based materials, plasmonic nanostructures and bio-inspired substrates have been created to reach these properties making these systems more complex and efficiency dependent on elemental composition, size, shape and configuration. This thesis provides a comprehensive study of the effect of geometrical and compositional parameters on the efficiency of charge separation on metal and metal-semiconductor nanostructures at the single particle level as it reveals the potential of hot carries derived from the surface plasmon of metal nanostructures for photocatalytic reactions. Furthermore, it allows to evaluate the main parameters that needs to be addressed in order to improve charge transfer efficiency such as surface quality, contact between metal-semiconductor structures, shape and size effect. In Chapter 1 - We provide a brief introduction to plasmonics photocatalysts, excitation and relaxation of localized surface plasmon resonances (LSPRs). How plasmonic photocatalysts operate. The methodology used to characterize plasmonic reactions by single particle spectroscopy techniques and current state of LSPR studies using electron beam microscopy technology and the more important parameters to acquire and process electron energy loss spectroscopy data in a transmission electron microscope. Finally, an overview of synthetic methods used to obtain plasmonic nanostructures and hybrid metal-semiconductors. In Chapter 2 - we developed a synthetic protocol to break plasmonic symmetric of gold rods by overgrowing one of their caps resulting in a "matchstick like" new shaped nanostructure. We present how this new nano-structure changes the LSPR distribution. Finally, we demonstrate how asymmetric Au nanorods exhibit improved surface-enhanced Raman scattering response compared to the Au nanorod seeds, attributed to the breaking of longitudinal localized surface plasmon resonance symmetry and the presence of reactive plasmonic hot spots. In Chapter 3 - of this thesis we demonstrate the effects of breaking the symmetry of plasmonic Au rods on charge separation and transfer by modifying the shape and composition to hybrid Au rod- CdSe overgrowth tip. Moreover, we developed a standard protocol to analyse, at single particle level, the broadening of the longitudinal localized plasmon of the Au nanorod due to localized plasmon damping at the chemical interface (CID). We found one of the highest yields of plasmon quenching through CID reported to date (∼ 48%). Furthermore, we found the main variables that influence CID: the quality of the metal-semiconductor (CdSe) interface, the size of the Au rod and CdSe tip. Also, we evaluate in which part of the hybrid structures photoreduction reaction starts by using electron energy loss spectroscopy and electron diffraction spectroscopy at the single particle level. In Chapter 4 - we explore the effect of increasing the plasmonic - semiconductor interface area and quality by the assembly of a new type of metal-semiconductor configuration using silver cuboids (that contain large flat planes) dropcasted onto two-dimensional (2D) indium oxide antimony doped and tin oxide semiconductors, promising new types of 2D materials. Additionally, we test their photo-electrochemical performance for water splitting and found that the surface of the 2D semiconductors influence significantly the plasmonic-semiconductor interaction. Finally, In Chapter 5 we summarize the most significant contributions made by the author and provide an outlook for future investigations.