Date on Master's Thesis/Doctoral Dissertation

12-2019

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Zamborini, Francis

Committee Co-Chair (if applicable)

Yappert, M. Cecilia

Committee Member

Yappert, M. Cecilia

Committee Member

Ramezanipour, Farshid

Committee Member

Sumanasekera, Gamini

Author's Keywords

nanoparticles; voltammetry; oxidation; surface area-to-volume ratio; stability

Abstract

This dissertation describes an electrochemical approach for measuring the surface area-to-volume ratio (SA/V) of electrode-attached metal nanoparticles (NPs), which was used to analyze their size, aggregation state, and porosity. This dissertation further describes the effect of the metal NP electrode assembly method on the SA/V, which is related to metal NP catalytic activity and stability. Cyclic voltammetry (CV) in acid electrolyte followed by anodic stripping voltammetry (ASV) in KBr electrolyte allows the electrochemical measurement of the SA/V of electrode-attached Au nanospheres (NSs). In CV, the forward scan produces a thin surface Au2O3 layer on the Au NSs. Measuring the Coulombs of charge passed during the reduction of the Au2O3 on the reverse scan allows a measurement of the total surface area (SA) of all NSs on the electrode. Subsequent measurement of the Coulombs of charge passed during oxidative dissolution of all of the Au NSs in KBr electrolyte provides a measurement of the total V of the Au NSs. Since the radius of a sphere is equal to 3/(SA/V), the SA/V provides a direct measure of the Au NS size, which matches very closely to the size of several Au NSs ranging from 4 nm to 70 nm in diameter based on scanning electron microscopy (SEM) size measurements.. Electrochemical SA/V measurements provide information about the aggregation state of electrode-attached Au NSs. The SA/V decreases as 4 nm and 15 nm diameter Au NSs go from isolated NSs to aggregated structures, caused by the neutralization of the citrate stabilizer at low pH. The decreased SA/V for aggregated Au NSs accounts for the positive shift in the peak oxidation potential (Ep) of aggregated NSs compared to isolated ones. The SA/V of distorted, flattened, and porous Au NSs formed by dealloying Ag from AuAg alloy NSs increases with an increasing percentage of Ag in the initial alloy NSs. Greater removal of Ag leads to greater porosity and a higher SA/V. The increased SA/V for porous NSs resulted in a negative shift in Ep relative to similar diameter non-porous Au NSs. The SA/V of similar sized NSs assembled onto indium-tin-oxide-coated glass electrodes (glass/ITO) by different assembly methods, including electrostatic attachment to an amine-functionalized silane linker, electrophoretic deposition (EPD), direct drop-cast deposition, and drop-cast deposition after mixing with carbon black, can be dramatically different. Au NSs with higher SA/V generally exhibit higher electrocatalytic activity, as demonstrated for the oxygen reduction reaction (ORR), but also oxidize at lower potentials (lower Ep). The SA/V follow the order of electrostatic attachment > EPD > drop-cast with carbon black > drop-cast. Since different assembly methods lead to different SA/V for the same size Au NSs, the SA/V is a better predictor of Ep, or the oxidation properties of the NSs, compared to the size (or curvature) of the Au NSs. This is generally true for all of the different variables studied in this dissertation, including NSs of different size, aggregation state, porosity, and assembled by different methods. The SA/V provides important information about both activity and stability of metal NPs.

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