DOI
https://doi.org/10.25772/9K70-QB81
Author ORCID Identifier
0000-0002-6984-7010
Defense Date
2020
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Katharine M. Tibbetts
Abstract
The development of metal-oxide nanostructures is a growing area due to their applications in diverse fields spanning energy conversion and storage, chemical manufacturing, and en- vironmental technology. This interest in catalytically active nanomaterials has prompted the synthesis and investigation of highly functionalized nanoparticles (NPs), including core- shell, silicate-stabilized, and bi- and multi-metallic nanocomposites. While wet-chemical synthesis methods of metal-oxide nanostructures have led to several morphologies, compositions, and shapes, these syntheses often require high temperatures, toxic solvents or reducing agents, and long reaction times.
Laser synthesis and processing of colloids (LSPC) encompasses both ‘top down’ and ‘bottom up’ approaches to synthesize metal-oxide nanostructures. Pulsed laser ablation in liquid (PLAL) involves focusing laser pulses onto a solid target immersed in a liquid in which target atoms coalesce to form nanostructured materials once ejected into solution. Laser reduction in liquid (LRL) is a second laser-assisted approach to synthesizing nanomaterials, where photochemical reduction of metal salts is achieved by focusing the laser beam into solution. Both PLAL and LRL are able to generate metal and semiconductor NPs at room temperature in aqueous solutions without added surfactants or stabilizers, giving them an advantage over conventional wet-chemical methods.
Recently, these two approaches have been combined into a single step- referred to as reactive laser ablation in liquid (RLAL), in which laser ablation of a solid target is carried out in a metal salt solution. This work goes through the synthesis and characterization of femtosecond-RLAL (fs-RLAL)-generated silica-metal nanostructures, and discusses the relationship between the precursor solution composition, the product morphology, and the catalytic activity toward a model para-nitrophenol (PNP) reduction reaction. First, silica-Au NPs were synthesized, exhibiting two populations of product nanoparticles which resulted from reaction dynamics occurring on two distinct timescales. Next, silica-Cu NPs were synthesized under different pH conditions, yielding pH-dependent product morphology. The different morphologies resulted from the surface charge of ablated silica species, which repelled the Cu2+ ions in solution at low pH yielding core/shell morphology, and attracted the Cu2+ ions at high pH, forming well-dispersed ∼1.5 nm Cu clusters stabilized by a phyllosilicate matrix. This led to the investigation of pH-dependent dissolved silicate species generated from ablating the Si wafer in water and solutions of added Ni(NO3)2 over a range of pH conditions. When the solution was above pH 10, silicic acid was generated which was the key species leading to the formation of nickel-phyllosilicate (Ni-PS) when nickel nitrate was added to solution. When the solution was below pH 7, no silicic acid was generated from ablation, and consequently no Ni-PS was formed in the dried product. The mechanism of Ni-PS formation from fs-RLAL of a silicon wafer immersed in aqueous nickel nitrate solu- tions is discussed. Based on this mechanism, it is expected that the fs-RLAL method will be capable of generating a variety of metal-phyllosilicates from different metal salt precursors.
Rights
© Mallory G. John
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-20-2020