Ultrafast coherent spectroscopy of photocatalytic TiO2 surfaces

Among transition metal oxides, TiO2 serves as a model system for solar energy capture and to drive the consequent photocatalytic chemical processes. It is a widely investigated metal oxide because it is possible to prepare atomically defined single-crystal samples and probe their optical and electronic properties at the fundamental level. In this thesis, I explored the spectroscopy and dynamics of reduced rutile TiO2(110) single-crystal samples, both cleaned and covered with Au nanoclusters, using interferometric time resolved-multi photon photoemission spectroscopy (mPP). A thorough understanding of the electronic structure and dynamics of photoexcited electrons in TiO2 is essential to extend its potential technological and photocatalytic applications. Due to its wide-bandgap, TiO2 is predominantly known to become photoactive upon UV light illumination. Upon reducing TiO2, oxygen vacancy defects are introduced onto the surface, which impart notable surface electronic and optical responses in TiO2 extending its absorption to the infrared-visible range of the optical spectrum. Using mPP, where m=2-4, I studied the optical responses of TiO2 processes in the visible-near infrared spectral range. mPP in the visible-near infrared spectral range, reveals new resonant transitions from the Ti-3d defect band involving unoccupied states that are localized at defects. The interferometric time–resolved measurements at these resonances reveal measurable coherent polarization and population relaxation dynamics, which persists on several tens of femtosecond time scales. Strong optical resonances are introduced when plasmonic metal nanoparticles are integrated into semiconductor substrates, broadening the range of the material’s photocatalytic and photovoltaic properties. To understand how plasmonically generated hot electrons interact with the metal oxide support, I studied the noble metal (Au) covered TiO2(110) surface. mPP reveals new spectroscopic features of Au/TiO2 surface identifying new occupied and unoccupied states introduced by Au deposition. The atomic scale characterization of Au/TiO2 by STM imaging reveals aggregation of Au atoms into metallic clusters and relates the growth to the observed photoemission plasmonic response. Finally, I combine mPP and STM/STS techniques to correlate the optical responses, electronic structure, and morphology of Au clusters on TiO2