The Photodynamic and Structural Analyses of Advanced Materials for Solar Fuel Conversion
Mitigating the current and future climate and pollution issues that have been brought on by the combustion of fossil fuels is of utmost importance and will rely on, in part, the availability of renewable fuel sources. Of the possible sources of energy, solar is abundant, but must be harnessed efficiently and stored as a solar fuel to overcome the current storage issues that limit photovoltaic cells. One such fuel, H2(g), represents a carbon-neutral source of energy if it can be efficiently liberated from water via the water splitting reaction. Thus, much attention is focused on designing materials to perform the water splitting reaction efficiently as well as fundamentally understanding the complex dynamics that occur during the coupled light-harvesting and catalytic events that comprise photocatalysis. Herein, these concepts are applied to two classes of materials with a focus on their application to solar fuel photocatalysis. One class of materials that has shown promise as an oxygen evolution reaction (OER) catalyst is bismuth vanadate (BiVO4) due to its visible light absorption, stability, and photocatalytic ability. However, fundamental understanding of the factors that limit the photocatalytic efficiency of BiVO4 toward OER are poorly understood. Chapter 3 focuses on both revealing and resolving the limiting attributes of BiVO4, thereby significantly enhancing its photocatalytic OER efficiency. A second class of emerging materials investigated are porous zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs). The excited state dynamics of ZIF-67 are characterized in chapter 4, demonstrating a long-lived charge separated (CS) state in the material after photoexcitation. The understanding of the nature of this CS state is then extended by optical studies that reveal metal-to-metal charge transfer (MMCT) as a contributing mechanism to charge separation in ZIFs. A further study then shows that the electron in this CS state can be extracted through interfacial electron transfer from excited ZIF to an organic dye species providing crucial insights into the ability of ZIFs as intrinsic photocatalyst materials. Following these fundamental studies, ZIF-67 is applied as an efficient hydrogen evolution reaction (HER) photocatalyst in conjunction with an auxiliary photosensitizer in chapter 5.