Date on Master's Thesis/Doctoral Dissertation

12-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Zamborini, Francis

Committee Member

Baldwin, Richard

Committee Member

Grapperhaus, Craig

Committee Member

Sumanesekera, Gamini

Author's Keywords

Au nanoparticles; electrochemical reactivity

Abstract

This dissertation describes the factors controlling metal nanoparticle (NP) oxidation, using Au oxidation in Br- and Ag oxidation in acid as model systems. We found that the oxidation potential depends on size, surface charge, and electrode material. Size has a dramatic effect, shifting about 200 mV from > 200 nm diameter down to 4 nm diameter. The shift is more dramatic below 4 nm. 2.3 nm average diameter Au NPs oxidized in the 300-500 mV range and 1.5 nm average diameter Au NPs displayed several sharp peaks from 200 mV to as low as -200 mV. These smallest Au NPs show catalytic activity for CO2 reduction. The size-dependent oxidation results followed thermodynamic theory based on surface energy fairly well. A positive surface electrode charge also shifts the oxidation potential for Au and Ag NPs negative 100-200 mV, depending on the amount of charge, for NPs greater than about 10-15 nm. Ag NPs attached to different electrode materials show slightly different oxidation potentials, which is likely due to different electrode/linker work function relative to Ag. The negative shift in oxidation potential with decreasing NP size leads to size-dependent galvanic exchange. The level of galvanic replacement between PtCl4 2- and citrate-coated Au NPs increases as the Au NP diameter decreases, another example of unique electrochemical reactivity of small NP sizes. Throughout these studies, smaller particles often show low coverages and high size dispersity on the electrode. A new electrochemical precipitation approach based on the release of protons by H2O2 oxidation allowed for size-dependent deposition of electrodes without linkers from a solution containing different sized Au NPs. This work shows unique electrochemical oxidation, galvanic replacement, and electrocatalysis of metal NPs of 4 nm diameter and below. The developed electrochemical precipitation of small Au NPs could lead to high coverages of bare metal NPs for a variety of applications.

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