Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Mayer, James

Abstract

Nanoparticles have wide utility due to their large reactive surface areas and their unique optical, electronic, and chemical properties relative to bulk materials. Titanium dioxide (TiO2) is a widely used photocatalyst and redox mediator due to its earth abundance, chemical stability, and ability to hold and transport multiple redox equivalents. In contrast, gold is an expensive coinage metal that is inert at the bulk scale; and yet, gold nanoparticles (AuNPs) exemplify the nano paradigm through their unique reactivity and vibrant colors. Despite their differences, catalysis with TiO2 NPs and AuNPs has traditionally been described using only electron transfer (ET) processes. The studies herein evaluate reduced AuNP and TiO2 colloids from a proton-coupled electron transfer (PCET) view. These studies demonstrate the importance of protons on nanoparticle reactivity and seek to draw broader connections between metal and metal oxide nanomaterials.Part I explores the redox chemistry of AuNPs and the interactions it has with various hydrogen species. Chapter 2 uses the AuNP surface plasmon resonance as a probe to monitor redox processes and the reversible hydrogen addition to these nanoparticles. The effects of pH and reduction on AuNPs are optically mapped out. Chapter 3 explores an intriguing phenomenon where the physical shaking of a reaction vessel with AuNPs reduced with hydrogen gas leads to the oxidation of the nanoparticles and uses kinetic studies to gain insights into different hydrogen species. These studies collectively give good support for gold interactions with hydrogen. Part II studies the proton coupling of trapped electrons on TiO2 nanoparticles. Chapter 4 uses thermochemical equilibrations to determine the reduction potential of reduced TiO2 across a span of 10 pH units. It shows PCET is the thermochemically preferred method of reactivity for TiO2. Chapter 5 builds on the discovery of two spectroscopically distinct types of trapped electrons and evaluates their kinetic reactivity. One type of trap was shown to react ~5x’s faster than the other. Finally, Chapter 6 determines that the two spectroscopically distinct trapped electrons on TiO2 differ in composition by one proton. The equilibrium can be systematically modified with stoichiometric amounts of proton additions.

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