Spectroelectrochemical, Separation and Surface Functionalisation Studies of Plasmonic Nanoparticles

Abstract

Plasmonic nanoparticles are of significant research interest due to their enhanced light-matter interactions, demonstrating great potential for a range of applications from sustainable sun-driven photochemistry to therapeutics and sensitive biological sensors. These properties arise from localised surface plasmon resonances (LSPR), which are resonant oscillations of the free electron cloud in metal nanoparticles of sizes less than the wavelength of incoming light. Understanding nanoparticle synthesis and structure is key towards controlling their optical properties and implementing the promise of their applications. In particular, the LSPR peak frequency and linewidth is highly dependent on nanoparticle composition, size, and shape. Hybrid or bimetallic nanostructures offer the potential to couple the properties of two or more different metals, expanding their functionalities. Electrochemical deposition was coupled with a newly developed hyperspectral technique adapted for spectroelectrochemical measurements in and ex situ, enabling improved synthetic control on an electrode at the push of a button. Cu was electrodeposited on Au with fixed currents as an initial proof of concept system, achieving controllable morphology and optical properties. During deposition, LSPR shifts were measured, allowing the optimisation of the system with respect to deposition current, charge transfer, metal ion concentration and nanoparticle surface chemistry. Time-resolved in situ measurements of single particle spectra were obtained, giving an insight into the kinetics of the deposition process, as well as oxidation after synthesis in air and aqueous solution. The principles acquired from this study were applied to the electrochemical synthesis of bimetallic Pd and Pt on Au nanoparticles, achieving core shell nanoparticles with good control over size and homogeneity. Controlling the charge transfer at the working electrode led to controllable shell thicknesses with narrow size distributions and a tuneable plasmonic response. Additionally, the photocatalytic performance of the Pd on Au NPs was investigated. In all of these studies, the changes in plasmonic behaviour upon deposition were confirmed by numerical and analytical simulations of the bimetallic systems. Mg shows much promise as a low-cost, sustainable, and biocompatible alternative to the more expensive and noble plasmonic metals. The synthesis of bimetallic nanoparticles based on a Mg core was approached by galvanic replacement, where more the noble metal ions of Au, Pd and Pt are reduced on the surface of Mg. Optical and spectroscopic approaches were used to establish the reaction kinetics on the single particle level, revealing rapid reaction preceded by a waiting period. This waiting time is dependent on reagent concentration and the presence of water, while the kinetics of oxidation in water were also studied. Mg NPs, typically synthesised with multimodal or polydisperse size distributions and thus exhibiting an undesirable inhomogeneous plasmonic response, were separated by size and shape using density gradient ultracentrifugation. Here, a new density gradient system was implemented in organic medium for these water-reactive nanoparticles, and the effects of speed, time, density gradient and initial nanoparticles were explored. Finally, the chemistry of Mg surfaces was studied computationally with density functional theory, looking at the adsorption of different capping agents with implications for colloidal stabilisation, nanoparticle shape control and future functionalisation. Overall, the approaches studied in this thesis demonstrate the successful control of the properties of plasmonic nanoparticles and support future research directions towards realising the rich potential of their applications.