Correlation between hot carrier generation and reactivity under photoelectrochemical and gas catalytic reactions

Abstract

Investigation of hot carrier generation from a photon or exothermic chemical energy is crucial in heterogeneous catalysis due to a correlation between hot carrier flow and catalytic activity enhancement. In the case of photons, a plasmonic nanostructure can lead to a localized surface plasmon resonance effect, which can then generate hot carriers. These plasmonic hot carriers can enhance the photocatalytic activity directly under photoelectrochemical (PEC) reactions. On the other hand, the exothermic catalytic reactions can induce hot carrier production at the metal catalysts. However, unlike the plasmonic hot carriers, these chemically-induced hot carriers are a sort of electrical signal that can estimate the reactivity and selectivity of catalytic reactions. Based on this difference between the plasmonic and chemically-induced hot carriers, I have studied research on both plasmonic hot hole generation and chemically-induced hot electron production to investigate the relationship between the catalytic activity and hot carrier flow. In this dissertation, Chapter 1 contains the research background of hot carrier generation, which consists of hot electron-hole pairs on metal catalysts induced by an external energy, such as a light irradiation and an exothermic catalytic reaction. In Chapter 2, I demonstrate direct photoelectrochemical (PEC) experimental proof that the injection of plasmonic hot holes depends on the size of the metallic nanostructures. PEC results clearly indicate that a plasmonic template with smaller Au nanoprisms exhibits higher external and internal quantum efficiency rates, significantly enhancing of both the oxygen evolution and hydrogen evolution reactions. We verified that these outcomes stemmed from the enhanced hot hole generation with higher energy and transfer efficiency driven by enhanced field confinement. In Chapter 3, I synthesized well-defined Co3O4 and CeO2 cubes with distinct facets and investigated their catalytic performance when deposited on a Pt-thin film, focusing on the influence of metal-oxide and oxide-oxide interfaces. Notably, the CeO2/Co3O4/Pt nanodevice exhibited improved partial oxidation selectivity, highlighting the role of the CeO2/Co3O4 interface in methyl formate production. I also conducted ambient pressure X-ray photoelectron spectroscopy (AP-XPS) analysis, revealing a higher concentration of Ce3+ ions and increased oxygen vacancies in the CeO2/Co3O4/Pt catalyst, suggesting oxygen migration from CeO2 to Co3O4. In Chapter 4, I demonstrate that well-aligned CeOx nanowire arrays can systematically control the interfaces between Pt catalysts as a model heterogeneous system. Independent modulation of CeOx properties with preservation of other variables enables quantitative analysis of their effects on partial oxidation selectivity, resulting in hot electron generation during methanol oxidation reaction. Consequently, these results prove how the catalytic performance improves with sophisticated control of metal oxides and establish how their role acts in heterogeneous catalysis.