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Abstract
The development of synthetic processes that provide precise control of nanoparticle composition, size, shape, and surface functionality are important for understanding and utilizing these materials. Limited insight into the mechanisms underlying nanoparticle formation and growth hampers fundamental investigation and practical application of these new materials. To determine critical variables and processes involved in the reactions, the formation and interconversion reactions of discrete metal clusters possessing less than 20 core atoms were investigated. By studying these clusters, it was possible to observe differences of only one atom in the core composition and to identify different numbers of ligands in the shell. Identification and monitoring of discrete intermediates provided opportunities to understand the early stages of nanoparticle synthesis.
Observation of intermediates during the synthesis of gold clusters permitted the investigation of reaction pathways and facilitated the development of controlled routes to atomically precise materials. Specifically, the synthesis of triphenylphosphine-stabilized undecagold (Au11) was studied in batch reactions and continuous flow microcapillary reactors. Determination of product distributions during the early stages of reactions clarified the impacts of reaction parameters. Reaction chemistry was developed that selectively produced Au11(PPh3)7Cl3, a material previously available in low yields through isolation from mixtures of products. Synthetic selectivity was achieved through identification of reaction conditions that enhanced the stability of Au11(PPh3)7Cl 3 and simultaneously suppressed formation of other clusters.
In situ monitoring of gold clusters also led to the discovery that the clusters' reactivities depend markedly on small differences in ligand shell composition. Single crystal x-ray structure determination of Au11(PPh 3)7Cl3 and Au11(PPh3) 8Cl3 identified the only set of undecagold clusters that exhibits different bonding patterns for the same ligands. NMR and UV-visible spectroscopy investigations established that accelerated loss of ligands from Au11(PPh3)7Cl3 leads to decomposition and conversion to the more stable Au11(PPh3)8Cl 3.
To probe the evolution of gold clusters in real time, a microscale flow reactor approach was combined with simultaneous synchrotron-based small-angle x-ray scattering and optical absorbance spectroscopy. In situ characterization of steady-state populations of clusters and nanoparticles at points along the reactor enabled quantitative determination of core size and dispersity during reactions.
This dissertation includes my co-authored materials.
