DOI
https://doi.org/10.25772/342T-2986
Author ORCID Identifier
0000-0001-8324-4833
Defense Date
2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Katharine Moore Tibbetts
Abstract
Significant attention has been focused on metal nanoparticles (MNPs) due to their unique optical, catalytic, and electronic properties. In the last two decades, laser synthesis techniques have emerged as versatile routes to MNPs that enable control over particle size, shape, and surface chemistry without the use of chemical reducing agents or surface-blocking capping ligands. A method gaining increasing attention is the direct laser-driven reduction of metal salts, called Laser Reduction in Liquid (LRL). LRL typically involves the use of intense laser pulses of picosecond or femtosecond duration to ionize and dissociate solvent molecules, generating plasmas with reactive chemical species such as electrons, hydrogen radicals, and hydroxyl radicals that can drive the photochemical reduction of metal ions to produce MNPs. This work focuses on the synthesis and characterization of MNPs generated by LRL and discusses the relationship between the precursor solution composition, the MNPs sizes and stability, as well as the catalytic activity toward a model para-nitrophenol (PNP) reduction reaction and Suzuki cross-coupling reaction.
First, we discuss the kinetic control of gold nanoparticle (AuNPs) sizes during LRL of [AuCl4]-. The femtosecond laser-induced plasma generates hydrated electrons and hydroxyl radicals that recombine into hydrogen peroxide, which reduce Au(III) to form AuNPs . The reduction kinetics of Au(III) follow the Finke-Watsky autocatalytic rate law governed by rate constants k1 (dependent on electrons) and k2 (dependent on hydrogen peroxide). Control of hydroxyl radical chemistry was achieved through the addition of radical scavengers: isopropyl alcohol (IPA) and sodium acetate (Ac). Higher scavenger concentrations both lowered k2 values and produced AuNPs with smaller sizes and lower polydispersity. Second, uncapped Palladium nanoparticles (PdNPs) were synthesized by LRL with K2PdCl4, and Pd(NO3)2 as precursors. The reduction of each precursor yielded large anisotropic PdNPs (nanoflowers in chloride samples and nanopopcorn in nitrate samples). Addition of acid (HCl or HNO3) to the precursor solutions hindered precursor oxidative degradation, enabling the synthesis of ultrasmall spherical NPs in addition to the larger anisotropic NPs. These PdNPs exhibited high catalytic activity towards the synthesis of biaryl compounds by Suzuki cross-coupling reaction and reduction of PNP. Third, we discuss the mechanism for synthesis of AuNPs and AgNPs involving ns-LRL of [AuCl4]- and Ag+} below the plasma-formation threshold and compared it to the well-established plasma-driven reactions in fs-LRL. The formation of Cl-based oxidizing species during [AuCl4]- reduction provides further evidence for direct photodissociation of the Au-Cl bond. Also, the detection of the same products (hydrogen, methane, propane, and acetone) after ns-LRL and fs-LRL of AgClO4 in water/ alcohol solutions demonstrated that metal ions can activate redox reactions in isopropyl alcohol that can drive LRL of metal salts to metal nanoparticles. Lastly, we investigated the reaction pathways that take place upon laser-induced decomposition of four common solvents (acetone, hexane, ethanol and toluene) with fs and ps pulses. Ablation of the organic solvents with ps pulses was found to increase overall reactivity for all solvents and resulted in the production of multiple $sp$-hybridized compounds, which were not present in fs samples.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-10-2022