Master of Science / Department of Chemistry / Kenneth J. Klabunde / The motivation for the preparation of gold nanoparticles includes their potential utility in sensors, nanoelectronics, and the vast basic knowledge we can gain from these novel materials. Colloids of gold nanoparticles are also one of the most stable and easiest to manipulate. Synthesizing gold nanoparticles with narrow size distribution, uniform shape, and good crystalline nature represents a significant challenge. Thiols were found to be very efficient capping ligands for the digestive-ripening process in our research group, during which a colloidal suspension in a solvent is refluxed at the solvent boiling temperature in the presence of a capping ligand to convert a highly polydispersed colloid into a nearly monodispersed one. The current thesis research focuses on using amines instead of thiols as the capping ligands, which were also found to have similar efficiency for this purpose. The major part of the work is devoted to understanding the digestive ripening of gold-amine colloids system, and the effect of the nature of the amine ligands. A noteworthy achievement of the current work is the ability to synthesize stable gold colloids with different sizes by using different amine ligands. A diverse set of instrumental techniques is used for the characterization of the gold nanoparticles.

Aufgrund des großen Verhältnisses von Oberfläche zu Volumen zeigen Nanopartikel interessante, größenabhängige Eigenschaften, die man im ausgedehnten Festkörper nicht beobachtet. Sie sind daher von großem wissenschaftlichem und technologischem Interesse. Die Herstellung kleinster Partikel ist aus diesem Grund überaus wünschenswert. Dieses Ziel kann mit Hilfe von Mikroemulsionen als Templatphasen bei der Herstellung von Nanopartikeln erreicht werden. Mikroemulsionen sind thermodynamisch stabile, transparente und isotrope Mischungen von Wasser und Öl, die durch einen Emulgator stabilisiert sind. Sie können eine Vielzahl verschiedener Mikrostrukturen bilden. Die Kenntnis der einer Mikroemulsion zugrunde liegenden Struktur und Dynamik ist daher von außerordentlicher Bedeutung, um ein gewähltes System potentiell als Templatphase zur Nanopartikelherstellung einsetzen zu können.<br><br> In der vorliegenden Arbeit wurden komplexe Mehrkomponentensysteme auf der Basis einer natürlich vorkommenden Sojabohnenlecithin-Mischung, eines gereinigten Lecithins und eines Sulfobetains als Emulgatoren mit Hilfe der diffusionsgewichteten 1H-NMR-Spektroskopie unter Verwendung gepulster Feldgradienten (PFG) in Abhängigkeit des Zusatzes des Polykations Poly-(diallyl-dimethyl-ammoniumchlorid) (PDADMAC) untersucht. Der zentrale Gegenstand dieser Untersuchungen war die strukturelle und dynamische Charakterisierung der verwendeten Mikroemulsionen hinsichtlich ihrer potentiellen Anwendbarkeit als Templatphasen für die Herstellung möglichst kleiner Nanopartikel.<br><br> Die konzentrations- und zeit-abhängige NMR-Diffusionsmessung stellte sich dabei als hervorragend geeignete und genaue Methode zur Untersuchung der Mikrostruktur und Dynamik in den vorliegenden Systemen heraus. Die beobachtete geschlossene Wasser-in-Öl- (W/O-) Mikrostruktur der Mikroemulsionen zeigt deutlich deren potentielle Anwendbarkeit in der Nanopartikelsynthese. Das Gesamtdiffusionsverhalten des Tensides wird durch variierende Anteile aus der Verschiebung gesamter Aggregate, der Monomerdiffusion im Medium bzw. der medium-vermittelten Oberflächendiffusion bestimmt. Dies resultierte in einigen Fällen in einer anormalen Diffusionscharakteristik. In allen Systemen liegen hydrodynamische und direkte Wechselwirkungen zwischen den Tensidaggregaten vor.<br><br> Der Zusatz von PDADMAC zu den Mikroemulsionen resultiert in einer Stabilisierung der flüssigen Grenzfläche der Tensidaggregate aufgrund der Adsorption des Polykations auf den entgegengesetzt geladenen Tensidfilm und kann potentiell zu Nanopartikeln mit kleineren Dimensionen und schmaleren Größenverteilungen führen. / Owing to their large surface-to-volume ratio nanoparticles show interesting size-dependent properties that are not observable in bulk materials. Thus, they are of great scientific and technological interest. Thereby, the highly desirable preparation of as small particles as possible might be easily achieved using microemulsions as template phases. Microemulsions are thermodynamically stable, transparent and isotropic mixtures of water and oil stabilized by an emulsifying agent. However, microemulsions may form a great variety of different microstructures. Thus, it is of utmost importance to know the underlying microstructure and microdynamics of a chosen microemulsion system in order to use it as a template phase for nanoparticle formation.<br><br> In the present study complex multi-component microemulsion systems based on a naturally occurring soybean lecithin mixture, purified lecithin and sulfobetaine as emulsifiers were investigated by diffusion-weighted pulsed field gradient (PFG) 1H NMR spectroscopy in the presence and absence of the polycation poly-(diallyldimethylammonium chloride) (PDADMAC). The central topic of this study was to structurally and dynamically characterize the present microemulsions with respect to their potential use in nanoparticle formation.<br><br> The concentration- and time-dependent NMR diffusion measurements turned out to be a suitable and accurate tool to investigate the microstructure and microdynamics of the systems under investigation. They reveal closed water-in-oil (W/O) microemulsion microstructures which prove the potential suitability of the respective systems as template phases for the preparation of nano-sized particles. The overall diffusion behavior of surfactants were found to be governed by varying contributions from displacements of entire aggregates, monomer diffusion in the medium and bulk-mediated surface diffusion, respectively. In some cases this led to a marked anomalous diffusion characteristics. In all systems interactions between aggregates are dominated by hydrodynamic and direct forces.<br><br> The addition of PDADMAC to the microemulsion systems results in a stabilization of the liquid interface of surfactant aggregates due to the adsorption of the polycation at the oppositely charged surfactant film and may potentially lead to nanoparticles of smaller dimensions and narrower size distributions.

Recent advances in nanoscience and technology have provided the scientific community with new exciting opportunities to rationally design and fabricate materials at the nanometer scale with drastically different properties as compared to their bulk counterparts. A variety of challenges related to nanoparticle (NP) synthesis and materials characterization have been tackled , allowing us to make more homogenous, well defined, size- and shape-selected NPs, and to probe deeper and more comprehensively into their distinct properties. In this dissertation, a variety of phenomena relevant to nanosized materials are investigated, including the thermal stability of NPs and coarsening phenomena in different environments, the experimental determination of NP shapes, gaining insight into NP-support interactions, epitaxial relationships, and unusual thermodynamic and electronic properties of NPs, including the effect of adsorbates on the electron density of states of small clusters, and the chemical, and structural evolution of NPs under reaction conditions. In chapter 2, a general description of different characterization tools that are used in this dissertation is provided. In chapter 3, the details of two different methods used for NP synthesis, namely inverse micelle encapsulation and physical vapor deposition (PVD) are described. Chapter 4 describes the thermal stability and coarsening behavior of Pt NPs supported on TiO2(110) and γ-Al2O3 as a function of the synthesis method, support pretreatment, and annealing environment. For the Pt/TiO2(110) system, micellesynthesized NPs showed remarkable stability against coarsening for annealing temperatures up to 1060°C in vacuum, in contrast to PVD-grown NPs. When comparing v different annealing environments (H2, O2, H2O), Pt NPs on γ-Al2O3 annealed in O2 were found to be the least affected by coarsening, followed by those heated in H2O vapor. The largest NP growth was observed for the sample annealed in H2. The role of the PtOx species formed under oxidizing conditions will be discussed. In chapter 5, the shape of Pt and Au NPs and their epitaxial relationship with the TiO2(110) support was extracted from scanning tunneling microscopy (STM) measurements. Three main categories of NP shapes were identified, and through shape modeling, the contribution of facets with different orientations was obtained as a function of the number of atoms in each NP. It was also shown that the micellesynthesized Pt and Au NPs have an epitaxial relationship with the support, which is evident from the fact that they always have one symmetry axis parallel to TiO2(110) atomic rows in [001] directions. Chapter 6 describes how the presence of NPs on TiO2(110) surface affects its reconstruction upon high temperature annealing in vacuum. In contrast to NP-free TiO2(110) substrates, long and narrow TiO2 stripes are observed for Pt NP-decorated surfaces. This phenomenon is explained based on the stabilization of TiO2, induced by Pt NPs, which hinders the desorption of oxygen atoms in TiO2 to vacuum. In chapter 7, a systematic investigation of the thermodynamic properties of γ- Al2O3-supported Pt NPs and their evolution with decreasing NP size is presented. A combination of in situ extended x-ray absorption fine structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling is used to obtain the NPs shape, thermal expansion coefficient, and Debye vi temperature. The unusual thermodynamic behavior of these NPs such as their negative thermal expansion and enhanced Debye temperature are discussed in detail. Chapter 8 presents an investigation of the electronic properties of size-controlled γ-Al2O3-supported Pt NPs and their evolution with decreasing NP size and adsorbate (H2) coverage. The hydrogen coverage of Pt NPs at different temperatures was estimated based on XANES data and was found to be influenced by the NP size, and shape. In addition, correlations between the shift in the center of the unoccupied d-band density of states (theory) and energy shifts of the XANES spectra (experiment) upon hydrogen chemisorption as well as upon modification of the NP structure were established. Chapter 9 is dedicated to an operando study, describing the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts during the in-situ selective reduction of NO in H2 via EXAFS and XANES measurements.