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)

Baldwin, Richard

Committee Member

Baldwin, Richard

Committee Member

Grapperhaus, Craig

Committee Member

Sumanasekera, Gamini

Author's Keywords

stability; reactivity; single metal; bimetallic; nanostructures; anodic stripping; voltammetry

Abstract

This dissertation has two main themes. The first involves studies aimed at developing anodic stripping voltammetry (ASV) as an analytical tool to characterize metal nanoparticles (NPs), with a focus on the size and aggregate structure of single metal NPs and the composition and atomic arrangement of bimetallic NPs. The second main theme involves studies that use ASV and electrochemical surface area-to-volume (SA/V) measurements to study the unique reactivity and transformations of single metal and bimetallic NPs. The transformations involve size and composition changes in response to ozone and electrochemical potential. Reactions involve size dependent galvanic exchange and electrocatalytic activity. The size of 1.6 nm diameter tetrakis(hydroxymethyl)phosphonium chloride (THPC)-stabilized and 0.9 nm diameter triphenylphosphine monosulfonate (TPPS)-stabilized Au NPs were characterized by ASV, which show Ep (oxidation peak potential) values of 0.45 and 0.20 V, respectively, vs. Ag/AgCl. The Ep values followed the trend of decreasing values as the size of the Au NPs decrease and fit well with theory and the size determined by electron microscopy. 15 nm diameter Au NPs aggregated by addition of THPC displayed an Ep value of 0.77 V, which is the same Ep as that of isolated, non-aggregated 15 nm diameter Au NPs. Transmission electron microscopy (TEM) measurements and SA/V measurements showed that the aggregates were highly linear and the NPs were not well connected with THPC, which led to no change in SA/V between aggregated and non-aggregated Au NPs. The pH-induced Au NP aggregates, in contrast, were fused together, 3D in nature, and had lower SA/V. This explains the larger Ep values for oxidation (~0.95 V). ASV also provides the composition of Cu and Au in bimetallic Cu1AuX (x = 0.1-1) NPs in one scan in KCl electrolyte. The ASV shows a different peak signature for different arrangements of Cu and Au in the NP, including Cu/Au core/shell, Au/Cu core/shell, and CuAu mixed alloy arrangements. The second aim of this work involves the use of ASV and SA/V measurements to monitor the size-dependent reactivity of Au NPs and reactivity of bimetallic AuCu NPs. 1.6 nm THPC-stabilized and 0.9 nm diameter TPPS-stabilized Au NPs increased in size to 4-10 nm diameter following 1-2 min of ozone treatment or one electrochemical oxidation-reduction cycle based on a positive shift in their Ep in ASV. 4 nm diameter Au NPs and larger are stable during these same treatments. 1.6 nm diameter Au NPs exchange with Ag+ ions by galvanic exchange, whereas 4 nm diameter Au NPs and larger show no exchange with Ag+ ions. Finally, 1.6 nm diameter Au NPs show higher activity than 0.9 or 4 nm diameter Au NPs for CO2 reduction or the hydrogen evolution reaction (HER). The activity is related to the number of corner and edge sites available for the different sized NPs.

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