On the thermal stability of ligand-stabilised gold nanoparticles

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Gold nanoparticles possess many interesting and useful properties, which have made them the subject of extensive research. The most notable of these are their optical properties, which can be tuned to suit a variety of applications or monitored as conditions change for sensing applications. The thermal stability of gold nanoparticles is also of particular interest, as this affects their sintering behaviour and therefore their utility in applications such as printed electronics, catalysis and sensing. The bulk of the research on thermal stability has focussed on lowering their stability to facilitate the formation of continuous, electrically conducting films at moderate to low temperatures. However, relatively little is known about increasing their thermal stability for applications where it is necessary for their useful properties to be retained at higher temperatures. This thesis presents an investigation into the thermal stability of gold nanoparticles, with a focus on probing the upper temperature limits of stabilising the particles using organic compounds. Firstly, a new method was developed for synthesising gold chloride as a precursor to gold nanoparticle synthesis, using the known reaction of gold metal with chlorine gas. The resulting gold chloride solutions were of high purity and stability, and were used directly for synthesising the nanoparticles used in this project. For the thermal stability studies, a selection of compounds was tested for their ability to delay the onset of nanoparticle sintering upon heating at a constant rate. Samples were analysed using a range of techniques including electrical resistance measurements, SEM, TGA, and XRD. Comparisons were made between stabilisers that were bound to the particles and those that were mixed with the particles without being chemically attached. A number of compounds of high thermal stability and compatible solubility were identified as particularly effective stabilisers, such as a ruthenium phthalocyanine complex, oleylamine, 1-pyrenebutanethiol and a perylenedicarboximide derivative, with sintering of the particles not occurring until more than 300 °C with these stabilisers, up to an unprecedented 540 °C. Important insights were also gained into the interactions between nanoparticles and unbound stabilisers and the qualities required for an effective stabiliser. Some of the highly stable gold nanoparticles were then monitored for changes in their optical and structural properties with temperature using reflection spectroscopy and SEM, with the results having potential applications in high temperature optical sensing and thermal history indicators.
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