The Synthesis and Tunable Processing of Metallic and Magnetic Nanoparticles in a Functional Solvent System
Type of Degreedissertation
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The concepts presented in this dissertation describe techniques that have been developed to produce metallic and magnetic nanoparticles using dimethyl sulfoxide (DMSO). The unique and size-dependent properties of nanoparticles require development of sustainable production techniques that allow for fine control over particle size and morphology. Typical methods that are used to produce specifically-sized nanoparticles involve either tedious synthesis methods or extensive post-synthesis processing. In addition, these methods often require the use of expensive solvents and reagents, produce significant amounts of waste, and are time-intensive. One way to simplify and improve nanoparticle synthesis is to develop a method that uses a single molecule “functional solvent” for both nanoparticle solvation and nanoparticle stabilization. Previously, it has been shown that DMSO can provide sufficient surface coverage and solvation of noble metal palladium nanoparticles via interactions involving the sulfur and oxygen moieties of the DMSO molecule. The studies outlined in this dissertation explore the production of magnetic cobalt nanoparticles and metallic gold nanoparticles using simple and sustainable synthesis techniques which employ the use of a DMSO functional solvent. Magnetic nanoparticles can become overly oxidized and unstable during synthesis, ultimately leading to nanoparticle agglomeration and degradation. Popular methods typically employed to prevent excessive oxidation and agglomeration involve adding ligands during nanoparticle synthesis that attach directly to the particle surface to provide particle surface coverage and improve nanoparticle dispersibility in solution. However, this dissertation contains a technique for the synthesis of amorphous 3.7 ± 1.5 nm magnetic cobalt nanoparticles using DMSO as a functional solvent via a quick, solvent-based reduction of Co2+ with NaBH4 in a DMSO solvent. The DMSO functional solvent adsorbs to the cobalt nanoparticle surface via the sulfoxide functional group (particularly the oxygen component), thereby protecting the particle from excessive oxidation. Furthermore, the effects of synthesis temperature on cobalt nanoparticle size and stability are investigated using DMSO as a functional solvent. High synthesis temperature was found to induce the aggregation of 3.7 nm cobalt nanoparticles into 20 nm clusters, likely by weakening the interaction between DMSO and the cobalt nanoparticles. Co-solvent addition to the clusters was found to liberate the individual cobalt nanoparticles within the clusters and allow them to re-disperse into the solution. The thermal oxidation of the aforementioned amorphous cobalt nanoparticles are also investigated by annealing the particles in air up to 800°C. Upon annealing, the amorphous cobalt nanoparticles transition into ordered structures of spinel-Co3O4 nanoparticles. These Co3O4 particles exhibit very unique magnetic properties, such as an increase in coercivity as a function of the annealing temperature. Previous work has demonstrated that gas expanded liquid technology can be utilized for the controlled precipitation of non-polar, aliphatic, ligand-stabilized gold nanoparticles from conventional non-polar organic solvents (e.g., aliphatic hydrocarbons) by exploiting the pressure-tunable, physico-chemical properties of the mixture. Central to the research presented in this dissertation, CO2 can diffuse into DMSO to create a CO2-gas expanded liquid. The dispersibility of gold nanoparticles in various CO2-gas expanded DMSO solvent systems is investigated and described in this dissertation. Contrary to the aliphatic ligand-stabilized nanoparticles in organic solvents, the dispersibility of gold nanoparticles in CO2-gas expanded DMSO is drastically different. Gold nanoparticles can be clustered within the DMSO+CO2 mixture as a function of pressure, and, after a certain transition pressure is reached, the nanoparticle clusters become destabilized and precipitate from the solution as a function of time. In essence, the research presented in this dissertation demonstrates that noble metal and magnetic nanoparticles can be synthesized and subsequently processed using a single molecule, DMSO, to function concomitantly as the solvent and the stabilizing ligand. Ultimately, the development of these simple, fast, and efficient nanoparticle production techniques is an important step in the evolution of nanoparticle engineering.