Transient spectroscopic studies of disordered semiconductors for solar-driven fuel synthesis
Creators: name: Sachs, Michael
Publisher: name: Imperial College London
Year: 2020

description: In this thesis, time-resolved spectroscopic techniques are used to link the activity of materials for solar-driven fuel generation with their excited state dynamics. Chapter 1 provides an introduction to the wider scope of this work, discussing global warming and the opportunities of solar energy in its mitigation, leading on to artificial photosynthesis as a means of storing solar energy at large scale. Chapter 2 covers the fundamentals of semiconducting photocatalysts and their photophysics before discussing the properties of metal oxide and polymer photocatalysts in particular. The aims and objectives of this thesis are presented. Chapter 3 describes the spectroscopic techniques used to characterise the photocatalyst materials in this thesis and elaborates on the experimental challenges which arise from applying these techniques to the materials studied herein. Chapter 4, the first results chapter, investigates a series of conjugated polymers as photocatalysts for hydrogen evolution. These polymers have sulfone groups embedded into their backbone and represent some of the most active materials discovered in the class of conjugated polymers so far. Photogenerated reaction intermediates are monitored on timescales of femtoseconds to seconds after excitation, and the yield of long-lived electrons is found to qualitatively correlate with the photocatalytic activity in this polymer series. Photocatalysts prepared via palladium-catalysed coupling reactions typically contain considerable amounts of palladium impurities, which have been shown to act as co-catalysts for hydrogen evolution in F8BT nanoparticles. Chapter 5 studies the effect of these palladium impurities in such F8BT nanoparticles on the excited state of the polymer, and demonstrates that the palladium centres within these particles quench photogenerated excitons about twice as fast as the electron donor diethylamine in the solution phase. A comparison to one of the sulfone polymers characterised in the previous chapter is made under charge accumulation conditions. Chapter 6 switches the focus to metal oxides and investigates the photophysical differences between near-stoichiometric and highly oxygen-deficient oxides using WO3 as a model material. Highly oxygen-deficient WO3 exhibits a strong blue colouration due to a large density of states within the bandgap, which is found to also give rise to rapid trapping of photogenerated holes. This rapid trapping process prolongs the lifetime of photogenerated charges by several orders of magnitude, but also leads to a severe reduction in oxidative driving force, thus compromising the efficiency of demanding oxidation reactions such as water oxidation. In Chapter 7, a series of 11 different metal oxides is investigated in order to test whether common activity-related photophysical characteristics arise from similarities in their electronic configurations. It is found that metal oxides with empty (d0) or closed d-shells (d10) exhibit delocalised electrons with generally longer lifetimes compared to oxides with open d-shells. This stark lifetime difference is due to a rapid sub-picosecond localisation process in the latter materials, which is attributed to the formation of small polarons. Finally, Chapter 8 presents the overall conclusions of this thesis and discusses the different chapters in context to each other. Directions for future work are suggested.