Coupling Hydrogen Evolution to Clean and Sustainable Oxidations in Dye-Sensitised Photocatalysis

The holy grail of solar fuels is the ability to use solar energy to split water into its elements: hydrogen and oxygen. One of the newest approaches is dye sensitised photocatalysis which combines the fields of molecular catalysis and nanomaterials. In this approach, molecular dyes and catalysts are immobilised on a semiconductor nanoparticle such as titanium dioxide. When light is irradiated, the dye is excited and the photoelectrons are transferred to the conduction band of the semiconductor leading to efficient charge separation. They are later transferred to the molecular catalyst, where hydrogen evolution occurs. In a 'dream' system, water oxidation would allow for the regeneration of the dye. However, water oxidation is a kinetically slow process which is difficult to couple to hydrogen evolution. Furthermore, the production of oxygen is undesired as it damages the system, evolution of both hydrogen and oxygen in the same cell introduces a risk of explosion and oxygen itself has little market value. Until now, to study the hydrogen half reaction, sacrificial electron donors such triethanolamine are used as a substitute for water oxidation to allow for dye regeneration. However, sacrificial electron donors are expensive and their oxidation produces highly reactive radicals which decompose the catalyst. The aim of this PhD is to explore different alternatives to substitute the use of sacrificial reagents for an oxidation of an organic compound with faster kinetics and higher value than oxygen production. Different possible approaches might include direct one-electron radical substrate oxidation, oxidation by an organic radical or inorganic complex, or introduction of a redox mediator and coupling to another photocatalyst, forming a Z-scheme.