Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Brudvig, Gary
Abstract
To meet current goals for carbon neutral economies it is important to transition to renewable energy sources and storage solutions. Solar radiation is a cheap and abundant source of renewable energy, but it suffers in grid scale applications due to being intermittent and diffuse. Similar to how plants use sunlight to store energy in the form of sugar via photosynthesis, artificial systems can store solar energy in the form of chemical bonds as solar fuels. One such type of artificial system is a water splitting dye sensitized photoelectrochemical cell (WS-DSPEC), which will be discussed throughout this thesis. WS-DSPECs are introduced as well as the design principles for building an efficient and robust photoanode. An effective photoanode requires strong surface immobilization strategies for attachment of molecular dyes and catalysts to the semiconductor surface. Several different attachment methods are discussed. Additionally different photosensitizers are reviewed and with special attention paid to BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-sindacene).Covalent attachment of molecules to metal oxide surfaces typically demands the presence of an anchoring group that in turn requires synthetic steps to introduce. BODIPY chromophores have long been used in dye-sensitized solar cells, but carboxylic acid groups typically had to be installed to act as surface anchors. In this thesis we find that even without the introduction of such anchors, the unmodified BODIPY can bind to TiO2 surfaces via its BF2 group through boron–oxygen surface bonds. Dipyrrin, the parent molecule of BODIPY, is also capable of binding directly to TiO2 surfaces, likely through its chelating nitrogen atoms. These binding modes prove to be even more robust than that of an installed carboxylate and offer a new way to attach molecular complexes to surfaces for (photo)catalytic applications since, once bound, we show in this thesis that surface bound BODIPY and dipyrrin derivatives exhibit ultrafast photoinjection of electrons into the conduction band of TiO2. Advancement toward dye-sensitized photoelectrochemical cells to produce solar fuels by solar-driven water splitting requires a photosensitizer that is firmly attached to the semiconducting photoelectrodes. Covalent binding enhances the efficiency of electron injection from the photoexcited dye into the metal oxide. Optimization of charge transfer, efficient electron injection, and minimal electron-hole recombination are mandatory for achieving high efficiencies. In this thesis, a BODIPY-based dye exploiting an innovative surface-anchoring mode via boron is compared with a similar dye bound by a traditional carboxylic acid anchoring group. Through terahertz and transient absorption spectroscopic studies, along with GFN-xTB calculations, we find that, when compared to the traditional carboxylic acid anchoring group, electron injection of boron-bound BODIPY is faster into both TiO2 and SnO2. In addition, binding stability is improved over a wide range of pH. Subsequent photoelectrochemical studies using a sacrificial electron donor showed this combined dye and anchoring group maintained photocurrent with good stability over long-time irradiation. This recently discovered binding mode of BODIPY shows excellent electron injection and good stability over time, making it promising for future investigations. Finally, for a functional WS-DSPEC photoanode there needs to be both an effective photosensitizer and catalyst combined. There are various different designs for these photoanodes, which are discussed in detail in this thesis. Three different arrangements of the BODIPY photosensitizer and an iridium water oxidation catalyst were synthesized for photocatalysis. While none were effective for water oxidation, lessons were extracted regarding the charge transport properties of the different photoanodes and insights into to future designs.
Recommended Citation
Jayworth, Josephine Anne, "BODIPY as a Chromophore and an Anchor to Metal Oxides for Solar Fuel Applications" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1384.
https://elischolar.library.yale.edu/gsas_dissertations/1384
