Date on Master's Thesis/Doctoral Dissertation

12-2010

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Zamborini, Francis Patrick

Author's Keywords

Size; Composition; Electrochemical oxidation; Underpotential deposition; Porous nanostructures; Metal nanoparticles; Metal polymer multilayers

Subject

Nanoparticles

Abstract

This dissertation describes (1) size-dependent electrochemical oxidation/stripping of gold and silver nanoparticles (NPs), (2) alloying of copper with gold nanoparticles at underpotential deposition potentials, (3) electrochemical characterization of Au/Ag core-shell structures, (4) characterization of metal nanoparticle alloys by stripping voltammetry, and (5) layer-by-layer assembly of metal nanoparticle/polymer structures. The motivation of this work is to better understand fundamental properties of metal nanostructures as a function of size, shape, and composition. We synthesized Au and Ag NPs with different size by electrochemical reduction of the metal salt directly on the electrode surface and by seed-mediated growth in solution followed by chemisorption on a silane functionalized electrode surface, respectively. Linear sweep voltammetry results demonstrated a negative shift in peak potential for oxidation with decrease in size. For Ag NPs, the oxidation potential is 275 mV and 382 mV for 8 and 50 nm particles, respectively. In the case of Au NPs, the peak potentials are 734 and 913 mV for 4 and 250 nm particles, respectively. This shift in oxidation potential with change in size of metal nanoparticles is consistent with Plieth theory. Underpotential deposition of copper on Au NPs of different size led to alloying of Au and Cu. Several peaks were observed on linear sweep voltammograms. We assigned these peaks to different copper locations in the alloy structure: (1) Cu UPD on the surface of Au NP, (2) outer-shell Cu-Au alloy, and (3) core of Cu-Au NP alloy. Au/Ag core-shell nanostructures were synthesized by seed-mediated growth directly on the electrode surface and characterized with electrochemical techniques. During electrochemical characterization, dealloying of Au from Au/Ag alloy structures occurred by cycling in bromide containing electrolyte solution. Composition analysis based on LSV showed that less than 3% of Au remained on the electrode surface. SEM images showed that the morphology of Au/Ag nanostructures changes after electrochemical oxidation. Particles become bigger and form hollow "bulbs", porous structures, and networks. We also synthesized Au/Ag alloy nanoparticles through a high temperature seed-mediated growth procedure and characterized them by UV-Vis and LSV at different stages of synthesis. LSV results provided information about the composition and atomic arrangements of alloy nanoparticles synthesized using 1:1 Au:Ag ratio, but a different synthesis method. After a 24-hour heating time, the (Au 4nm )Ag NPs did not show the oxidation peak for Ag, indicating that it stabilized during the alloy formation. In the case of (Ag 8nm )Au NPs, Ag oxidation peak appeared on LSVs regardless the heating time. Electrochemical characterization and UV-vis spectroscopy results for metal nanoparticle-polymer multilayer films showed that, with increase in the number of metal-polymer layers, absorbance and coverage increases due to an increase of the amount of metal assembled on the surface. A red shift in peak wavelength indicates an increase in size and aggregation of NPs on the electrode surface. SEM analysis shows that the morphology of the film depends on the nature of the metal deposited and the size of NPs. Films of Ag NPs consisted of large aggregated structures on the electrode surface, while films of Au NPs were uniform and porous. Experiments on the electron transfer through the polymer film to the metal NPs, demonstrated that electron transport depends on the number of polymer bilayers and the nature of the NPs. After deposition of 5 polymer bilayers, Au oxidation peak disappeared, while Ag oxidation peak was lower compared to 1 layer, but still observable. This dissertation describes a few sets of experiments on fundamental electrochemical properties of metal nanostructures. The results of these experiments are crucial for the application areas such as catalysis and sensing. It is important to study the stability of these nanoparticles, and also their recycling potential, since it can be affected by changes in the shape and size of the nanoparticles during the course of a reaction. This will not only provide information about electrochemical stability but may also prove useful as a method for analyzing nanoparticles and using them as labels for analytical applications by electrochemical stripping voltammetry.

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