Godfrey, Ian; (2018) Synthesis, structure and catalytic applications of monometallic and bimetallic gold-silver nanomaterials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis consists of a structural investigation of a range of gold and silver nanomaterials and associated core-shell structures, using X-ray absorption spectroscopy (XAS) as the primary tool; other techniques, including UV-Vis spectroscopy and transmission electron microscopy (TEM), were also used, demonstrating the wider applicability of XAS and other methods to nanomaterials. Dilute, gold/silver core@shell colloids with nominal Au@Ag and Au@Ag@Au structures were prepared and then characterised by extended X-ray absorption fine structure (EXAFS) and TEM. Au@Ag and Au@Ag/Au structures, respectively, were assigned to each sample. Combining EXAFS and energy-dispersive X-ray spectroscopy (EDS) results for Au@Ag@Au, the presence of a concentration gradient through the shells, arising from interfacial alloying, is suggested. This extends the conclusions made from TEM alone. The reduction of AgNO₃ by Na₃C₆H₅O₇ to produce pure Ag and Au@Ag colloids was investigated using Ag K-edge X-ray absorption near-edge structure (XANES). This was found to be a first order process, which is consistent with the unimolecular decomposition proposed in the literature. The seeded reaction had a reduced rate constant which was attributed to the presence of Cl⁻ ions, leftover from HAuCl₄, forming AgCl. Similar studies on Au colloids were attempted at the Au L₃-edge, but beam induced reduction was found to be the dominant effect. Finally, a suite of metal-oxide supported gold and silver samples were prepared by impregnation and characterised by XAS. Electron donation from the support to the gold was observed. The stability of oxidised gold on the supports increased with metal cation electropositivity, following the trend MgO>TiO₂>Al₂O₃>SiO₂. Additionally, in the case of MgO, gold hydroxide was formed and found to be stable up to at least 300 °C, before its decomposition to gold metal. The opposite trend was observed for silver, and this was attributed to the formation of metal-surface bonds.

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