Mantalidi, A; (2016) Understanding the formation of gold and iron based nanomaterials using X-ray absorption spectroscopy. Doctoral thesis , UCL (University College London).

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Abstract

In this thesis, X-ray Absorption Spectroscopy (XAS) is used to understand the speciation of molecular precursors in various reaction mixtures, elucidate potential effects on their structure from the presence of solvents and other reagents present, and monitor in situ their thermal decomposition leading to nanoparticle formation. XAS is a prominent technique for determining the local structure and oxidation state of an element of choice. Since long range order is not a requirement for XAS to be applied, it constitutes an ideal technique to study materials in solution phase. Chapter 1 provides a brief background of the key points in history that marked the commencement of the science of nanotechnology, as well as some of their important properties. An introduction on the general synthetic strategies of nanoparticles follows, focusing on the two main chemical methods that were employed in this thesis; the chemical reduction and the thermal decomposition. A background of the evolution of the Au nanoparticle syntheses is provided, followed by the latest developments in the field that involves the synthesis of anisotropic Au nanoparticles. Similarly to Au nanoparticles, a detailed literature survey on the synthesis methods of iron oxide nanoparticles is presented, focusing on the thermal decomposition route which is the synthesis of choice for the work undertaken in Chapter 6, accompanied by a small section devoted to the stabilisation methods of these nanomaterials. Chapter 2 discusses the basic theory of the laboratory and synchrotron based characterisation techniques that were utilised in this thesis. Special emphasis is given to XAS, as that is the key technique of this work. As a result, XANES and EXAFS are discussed in detail, and the data analysis procedure is also presented, due to its importance and extensive use in this thesis. The studies of Chapter 3 are focused on the speciation of [AuCl4]- in aqueous growth solutions, that upon addition of Au nanoparticle seeds, leads to the formation of Au nanoparticles with different morphologies. Since these growth solutions contain several reagents, this study addresses the effect of each reagent on the ligand environment and oxidation state of Au under realistic reaction conditions. For that purpose, ex situ studies were performed at the Au L3-edge upon stepwise addition of the reagents, and at the Ag k-edge whenever Ag+ was added to the growth solution. The studies at the Ag K-edge probed the effect of the growth solutions on the silver environment too. Ex situ XAS characterisation of the Au nanoparticles at the Au L3-edge and Ag K-edge was also performed, providing valuable information of the coordination and oxidation state of Ag at the final nanoparticles, which is a highly researched topic nowadays. Chapter 4 presents the results from the in situ XAS studies on Au nanoparticle formation in ethylene glycol, in the presence and absence of the particle stabilizer polyvinylpyrrolidone. The results revealed that the particle stabiliser has a retarding effect on the nucleation of the Au nanoparticles but also affects the final particle size. It was also illustrated that beam effects alter the specific decomposition process through interaction of the X-ray beam with the solvent. The studies illustrated in Chapter 5 investigate the structure and synthesis of Au-Pd bimetallic nanoparticles, and the speciation of the starting precursors. Initially, the ex situ characterisation of Au-Pd nanoparticles synthesised by two different syntheses is illustrated. The syntheses are performed in oleylamine/xylene, through the concomitant thermal decomposition of Au and Pd containing molecular precursors. The nanoparticles in the first case were prepared using Au(ethynyl-1-cyclohexanol) and [Pd(acac)2], while in the latter case, phase transferred [AuCl4]- and [Pd(acac)2] were used. Notably, the Au(ethynyl-1-cyclohexanol) precursor was used for the first time in the synthesis of Au-Pd bimetallic nanoparticles. The Au-Pd nanoparticle syntheses were investigated by in situ XAS to address the impact of the change of the Au precursor on the synthesis. Regarding the speciation of the starting materials, the results revealed that the structure of [Pd(acac)2] is dependent on the molar ratio of Pd to oleylamine, while the [AuCl4]- undergoes two structural changes prior to being reduced to the metallic state. The thermal decomposition of [Fe(acac)3] to iron oxide nanoparticles was investigated by in situ XAS for the first time, and the results are presented in Chapter 6. The decomposition of [Fe(acac)3] was studied in oleylamine, and in triethylene glycol in the presence and absence of polyvinylpyrrolidone. The role of the solvent was probed through XANES and LCF analysis, and was proven to be crucial since the decomposition profile of the precursor in these reactions varied considerably. In addition, the speciation was probed by EXAFS, and revealed that oleylamine induces changes to the precursor’s structure.

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