METAL NANOPLATELET SYNTHESIS AND DISPERSION IN SUSPENSION: SILVER, COPPER, NICKEL AND RELATED ALLOYS
Open Access
- Author:
- Yuan, Ying
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 10, 2007
- Committee Members:
- James Hansell Adair, Committee Chair/Co-Chair
Kwadwo Osseo Asare, Committee Member
Darrell Velegol, Committee Member
James Patrick Runt, Committee Member - Keywords:
- SILVER
DISPERSION
SYNTHESIS
BILAYER
NANOPLATELET
COPPER
NICKEL - Abstract:
- The synthesis and dispersion of metal nanoplatelets were investigated in the current work, driven by the demands of miniaturization and low cost in the multilayer ceramic capacitor (MLCC) industry. The specific materials in this Ph.D. dissertation include silver (Ag), copper (Cu), nickel (Ni) and copper/nickel (Cu/Ni) metal platelets. Metal nanoplatelets were synthesized in the lamellar bilayer phase regime of the polyoxyethylene (POE) – water binary system. The main focus is on determining the processing parameters related to high yield platelet synthesis, laundering the nanoplatelets while maintaining well dispersed particles, and the dispersion approaches associated with interfacial and colloidal chemistry of specific metal systems. A novel approach has been developed for the first time to statistically evaluate the morphology of the tabular shape nanoparticles using AFM analysis based on a relatively large population (n>200). The novel approach uses color delineation in the 3rd dimension (i.e., thickness) to produce both thickness and face diameter in the semi-automated analysis of several hundred nanoplatelets to quantify physical dimensions as well as to assess the degree of dispersion. High yields of Ag nanoplatelets (20 g/liter) and Cu nanoplatelets (12 g/liter) were achieved in the POE-water bilayer system with controlled platelet morphology. The influence of higher concentrations of metal precursors on the presence of a lamellar phase in the POE-water binary phase diagram was investigated. Oxyethylene (EO)-water interactions decrease because of the influence of anion concentrations leading to dehydration of the POE head group. Phase separation was induced when the ion concentration reached the upper limit. Therefore, the [water]/[surfactant] molar ratio used in the synthesis was decreased to compensate for the dehydration of the POE head groups to maintain the bilayer phase structure. In the Cu nanoplatelet synthesis, an elevated temperature was needed to increase the reaction rate. The free energy (DGf) of the oxyethylene (EO) / water structure is proportional to the number of EO units. As the temperature increased, the DGf decreased, resulting in a favorable EO/EO interaction instead of an EO/water interaction. When the temperature reached the upper limit, phase separation occurred due to dehydration of water as the EO/water interactions become less favorable. Similarly as anion concentration increases, dehydration of the EO-head group causes collapse of the bilayer leading to phase separation. However, it is shown that so long as the bilayer structure is present even at relatively high anion concentrations, templated platelets are synthesized. Thus, the reasonably high yields are obtained for both the Ag-POE-water and Cu-POE-water syntheses. A two-step protection-dispersion approach was investigated to stabilize Ag nanoplatelets in ethanol solution using a polyelectrolyte, polyethylenimine (PEI). As a cationic polyelectrolyte, degree of ionization of PEI is low at a high pH, and increases as pH decreases. To launder and concentrate Ag nanoplatelets without irreversible particle flocculation, PEI was used in initial washing iterations to form a protective polymer layer with a low charge. For the final wash, the solution pH was adjusted to provide a positive charge and promote electrosteric dispersion. The optimum PEI concentration and solution pH were investigated in ethanol. PEI self-aggregation behavior as a function of concentration and pH were examined. The PEI adsorption isotherm on silver was established to quantitatively evaluate the adsorption between particle surface and the polyelectrolyte. It is shown that there is a range of pH values suitable for flocculation of the Ag nanoplatelets with PEI near the isoelectric point of the polymer, and a pH range for electrosteric dispersion of the Ag nanoplatelets defined at the lower pH, ~ pH 5.5, by PEI self-aggregation and at the higher limit, ~ pH 7.5, by PEI charge. Prior to the dispersion study, the dispersion issues of Cu nanoplatelets in ethanol-water (7:3, vol%) are discussed, including dissolution, oxidation and possible surface charge heterogeneity. A passivation-dispersion approach was used to disperse Cu nanoplatelets in an ethanol-water (7:3, vol%) co-solvent. In this approach, a complexing agent is applied to the particle surface to form a stable surface metal-ligand complex with negligible Cu solubility in the solution. The formation of the stable complex is designed to protect against degradation, screen the possible charge heterogeneity in the solution, and provide a surface charge to promote reliable dispersion. Oxalic acid (HOx) and citric acid (HCit) were evaluated as dual passivation-dispersion agents. Colloid chemistry and surface chemistry were involved in evaluating the dispersion effectiveness. It was shown that HCit is an effective passivation-dispersion agent for stabilizing Cu nanoplatelets in an ethanol-water (7:3, vol%) co-solvent. At 10-3 M HCit concentration with respect to the solid loading, citrate provided a zeta potential equal to -25 mV, which was sufficient for dispersion. Sedimentation experiments and the electrostatic deposition technique verified that well-dispersed Cu nanoplatelet suspensions were obtained above pH 8. Ni nanoplatelets were synthesized in POE-water bilayer system. To maintain POE-water self-assembled lamellar structure, synthesis temperature needs to be controlled below ~ 60°C. With the temperature constraint, synthesis of pure phase Ni was conducted by the application of Pd as the nucleating agent at a molar ratio of 5×10-2 Pd/Ni. Cu0.81/Ni0.19 bimetallic nanoplatelets have also been synthesized in the same system using Cu as the nucleating agent.