A Spectroscopic Investigation of Photoelectrochemical Reactions at the Semiconductor - Electrolyte Interface for Artificial Photosynthesis

The mechanism of water oxidation to molecular oxygen at the surface of strontiumtitanate, a representative transition-metal oxide photocatalyst, was investigatedusing ultrafast pump-probe spectroscopy and accompanying characterization methods.The dependence of the rate of initial charge transfer on applied potential was investigatedwith transient absorption spectroscopy, revealing a phenomenological transfer coefficientfor this single step of the multi-step water oxidation reaction. Time resolvedinfrared spectroscopy provided the first direct evidence of the molecular character of thefirst intermediate to be a titanium-bound oxyl radical, and revealed interfacial Fano couplingbetween the electrolyte, surface species, and catalyst electronic continuum. Furtherwork combining both transient absorption and time-resolved mid-infrared spectroscopyexposed a minimum of three distinct surface states at the strontium titanate surface,providing insight into the rich chemistry mediated at the catalyst surface. These resultshighlight the intricacies of charge transfer at the surface of these promising materials,provide insight into an important but difficult reaction necessary for any artificialphotosynthetic system, and offer a generalizable paradigm for considering heterogeneousphotochemistry. Finally, preliminary groundwork was done to uncover the mechanismof the carbon dioxide reduction half reaction in a novel inorganic-biological bacterialsystem with potential to produce green chemicals such as carbon-neutral liquid fuels.