School of Chemistry - Theses

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    Surface plasmon spectroscopy of redox processes on single gold nanocrystals
    Collins, Sean ( 2016)
    Gold nanocrystals (Au NCs) are important materials for catalysis, sensing and photonics. Au NCs exhibit strong scattering signals in the visible and near infrared range due to localised surface plasmon resonances. Due to statistical averaging, when measurements are made on ensembles of particles, the precise determination of the effect of gold nanocrystal size, shape and local environment on specific application performance is not feasible. This ensemble problem is overcome by applying a combination of dark field imaging with surface plasmon spectroscopy, enabling the scattering spectra of individual nanocrystals to be measured. This approach allows changes in the electron concentration of a single Au NC to be observed via localised surface plasmon resonance shifts. In this thesis, the dark field microscopy technique has been expanded to study single gold nanocrystal electrodeposition, gas-phase adsorption, photoreduction, and solid-state charging. The first key objective was to understand how nanocrystal morphology and surface properties influence underpotential deposition. The second key objective was to understand how metal oxide supports influence charge transport during hydrogen adsorption and photoexcitation. These studies clearly demonstrate that the above parameters are crucial to the electrochemical and catalytic properties of Au NCs. By performing the measurements on single nanocrystals, the chemical kinetics and charging rates could be uncovered with detail never before achieved at this scale. In addition to using standard dark field microscopy techniques in this work, an upgraded laser illuminated dark field system was developed to optically reveal single particle charging rates by measuring electron transfer in real-time. The added sensitivity of this new approach has enabled the optical detection of fewer than 150 electrons as they are transferred to a single gold nanorod. In order to characterise the performance of the laser system, a reliable and reproducible method to rapidly charge single gold nanocrystals was developed. Au NCs were integrated in an ion gel capacitor, enabling them to be charged in a solid, transparent and highly capacitive device, ideal for transmission microscopy.