Date of Award
Ph.D. in Chemistry
Chemistry and Biochemistry
Stephen J. Cutler
Nanotechnology is an emerging field of science with applications in digital electronics, medicine, catalysis and energy. Gold nanoparticles are nanomaterials that have less than 100 nm at least in one dimension. The composition and the structure/geometry of the nanoparticles determine the chemical properties and reactivity. Modern research focus on method development in atomically monodisperse nanoparticle synthesis. This dissertation describes the method development for synthesis and isolation of atomically precise gold and gold-silver alloy nanoparticles and their comprehensive characterization and atomic structure investigation. Chapter one offers an introduction to the synthesis and isolation methods. These nanoparticles can be represented in the form of Aux(SR)y, where SR is the thiol ligand. The coreduction method was used for alloy nanoparticle synthesis using a fixed total metal molar ratio. After considering various possible elements, silver were selected to study the formation of alloys to atomically precise gold nanoparticles. Chapter two includes a discussion of the characterization methods used in the nanoparticles community including scanning transmission electron microscopy, X-ray techniques, UV-visible spectroscopy. Under X-ray techniques, powder X-ray diffraction, single crystal X-ray diffraction, small angle X-ray scattering and hard X-ray techniques were included. Mass spectrometry is commonly used for composition determination. Electron microscopy and small angle X-ray scattering experiments provide information on the size, shape and dispersity. Alloying provides a way to tune the properties of materials which is very different from those of their monometallic counterpart. Chapter 3 describes such an effect on Au25(SR)18 gold nanomolecules by alloying with silver. The atomic arrangement of Au and Ag atoms in Au25−xAgx(SR)18 was determined by X-ray crystallography and it was found that Ag atoms were specifically localized in the 12 vertices of the icosahedral core. Among ultra-small nanomolecules, Au38(SR)24 is one of highly attractive nanomolecules due to high stability, availability of single crystal X-ray structure, unique spectroscopic features and intrinsic chirality. Chapter 4 highlights alloying effect on Au38(SR)24 nanomolecules. Series of bimetallic Au38−xAgx(SR)24 nanomolecules were synthesized and its composition was determined to atomic precision. Incorporation of Ag atoms appears to smear out the distinct UVvisible features of Au38(SR)24. However, silver substitution decreased the stability of alloy nanoparticle when compared to the monometallic Au38(SR)24. The synthesized nanoalloy was subjected to a synchrotron based single crystal X-ray analysis. The substituted silver atom were concentrated in the core and preferentially occupy nine selected locations out of 38 possible sites. Furthermore, the independent compositional assignments were done using mass spectrometry. Au144(SR)60 is one of the ultra-stable gold nanomolecule that occur within the size regime of molecules to plasmonic transition. Chapter 5 describes the modulation of chemical and physical property of Au144(SR)60 by silver doping. UV-visible spectroscopy shows the Ag incorporation affects the electronic structure of the nanomolecules. The maximum number of Ag atoms substitute found to be 60. Extensive efforts have been made to synthesize and isolate monodisperse atomically precise nanomolecules larger than Au144(SR)60. In general, size dispersity of the larger nanoparticle can be corroborated by TEM, SAXS and other commonly used nanoparticle analysis methods. However, electron microscopic analysis on ultra-small scale often alter the size, composition and atomic structure due to the beam induced aggregation and sintering. Chapter 6 describes the first composition determination of super-stable plasmonic nanoparticles in the 2 nm (or 76.3 kDa mass region) and its alloying. This atomically monodisperse plasmonic molecule contains exactly 329 gold atoms and 84 ligands. Apart from the mass spectrometric composition, further characterization was conducted using scanning transmission electron microscopy equipped with high angle annular dark field imaging (HAADF-STEM), high energy X-ray based atomic pair distribution function (PDF) analysis and small angle X-ray scattering (SAXS). Composition determination of large nanocrystals is challenging due to the lack of available techniques to quantify the number of atoms and literature protocols for high yields. Chapter 7 discusses the two largest nanocrystals produced at 2.4 and 2.9 nm with a composition of Au500±10(SR)120±3 and Au940±20(SR)160±4. Most importantly, we were able to successfully extend the mass spectrometric window up to 200 kDa for compositional determination and to study the molecular nature of nanocrystals. Furthermore the monodispersity of the nanocrystals was examined by SAXS and STEM. Possible atomic arrangements were investigated by HR-PDF. Mass spectrometry was used to determine size and composition with additional supporting data from STEM, PDF and SAXS analysis. These atomically defined nanocrystals are very stable under ambient conditions and thermochemical treatment at 80 oC for over several days. Chapter 8 highlight the contributions made from this dissertations toward the advancement of thiol protected nanoparticle research.
Kumara, Chanaka Kapila, "Gold And Silver Alloy Nanoparticles: Synthesis, Characterization And Atomic Structure" (2016). Electronic Theses and Dissertations. 411.