UC Merced

Metallic nanoparticles have tunable electrochemical properties that make them suitable for catalyzing redox reactions. Thus, understanding how their reduction potentials respond to changes in particle size and light excitation is important for predicting their chemical stability and catalytic activity. However, establishing their standard reduction potentials through voltammetry has been met with technical challenges, specifically those introduced by the support. To get around this issue, an alternative approach based on Galvanic equilibration was adapted to determine the standard reduction potentials of colloidal NPs, thus avoiding the substrate effects associated with the support. This approach opens the door to exploring various factors that cause the nanoparticle reduction potentials to shift. Factors such as light absorption, changes in particle size, and local environmental effects will be discussed using theoretical models and experimental evidence to help us understand the magnitude of their impacts. Lastly, this approach can be adapted to investigate other factors important to catalysis, including crystallographic shape and atomic arrangement at the surface.First, Chapter 1 provides a historical context of metallic nanoparticles, from their early uses in decorative artifacts to modern chemical catalysis. Their early uses in art pieces stemmed from their unique optical properties. These optical properties were later determined to result from localized surface plasmon resonance, which became the foundation of metal nanoparticle-driven photocatalysis. Improvements in synthetic and characterization methods made it possible to study in depth their optical, catalytic, and chemical and physical properties concerning particle size. Their electrochemical properties are especially important for understanding their chemical stability and catalytic activity. With great success, such properties have been quantified for substrate-supported metallic nanoparticles through electrochemical methods such as anodic stripping voltammetry. Towards the end of the chapter, we discuss the limitations of voltammetry and how a galvanic equilibration approach can address such issues. Chapter 2 explains the principles behind the Galvanic equilibration approach in greater detail. This approach allows measuring the standard reduction potentials of spherical AuNPs freely suspended in solution to better understand how they respond to a change in size and the stabilizer ligands without the influence of the substrate supports. As the NPs decrease in size, their reduction potentials shift towards negative values that can be observed by a shift in their equilibrium constant when coupled to a redox probe. Furthermore, the ligand stabilizers such as quaternary ammonium surfactants are shown to also influence the reduction potentials. These surfactants participate in the redox reactions by interacting with the gold complexes formed during Galvanic equilibration. As a result, surfactants like CTAB significantly lower the standard reduction potentials of the gold nanoparticles, making them more electrochemically reactive. At the end of this chapter, the advantages of using the galvanic equilibration method for measuring the reduction potentials are summarized, and an outlook is given. Chapter 3 then takes the galvanic equilibration approach to investigate how light absorption induces a shift in the reduction potential. When metallic nanoparticles absorb light, they generate high-energy (hot) electrons and holes (carriers) that can be used to drive chemical transformations on the surface. The efficiency and selectivity of these chemical transformations can be tuned by parameters such as photon flux and energy. For noble metal nanoparticles, how carrier generation is also dependent on the mode of excitation, which include interband and localized surface plasmon resonance. For instance, exciting the NPs at the localized surface plasmon resonance region generates hot electrons relative to the hot holes. On the other hand, exciting at the interband region of AuNPs generates very hot holes relative to the hot electrons. By exciting at the interband region, for example, we can use the high-energy hot holes to catalyze the main oxidation reactions while scavenging the “warmer” electrons using sacrificial compounds. This process can also induce photocharging by selectively harvesting the hot holes and allowing the hot electrons to relax to the ground state and accumulate in the nanoparticle. This photocharging process raises the NP's Fermi level, which can be measured through Galvanic equilibration. The results show that shorter excitation wavelengths induce the largest photocharging and shift the reduction potentials more negatively than longer wavelengths. This observation implies that catalytic activity in metallic nanoparticles can be tuned by light, especially for redox transformations. Finally, Chapter 4 summarizes the work presented in this dissertation and discusses how galvanic equilibration can be used to study other physical and chemical phenomena, such as the shape effects on the electrochemical properties of NPs. For crystalline nanoparticles, different shapes are dominated by one or more kinds of facets with different surface tension. Because the surface Gibbs free energy depends on the surface tension, their electrochemical properties should vary amongst nanoparticles dominated by different facets. Thus, along with excitation wavelength, size, and stabilizer ligands, the NP shape can also be used to tune metallic NPs’ electrochemical stability and catalytic activity.