Ultrafast Coherent Spectroscopy of TiO2 Photocatalytic Processes

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With support from the Chemical Structure Dynamics and Mechanisms-A (CSDM-A) Program of the Chemistry Division, Professor Hrvoje Petek of the University of Pittsburgh is performing time-resolved photoelectron spectroscopy studies of electron dynamics in titanium dioxide (TiO2) semiconductor on the femtosecond (10^-15 s) time scale. TiO2 is a material with well-known photocatalytic properties that can convert the solar to electrical or chemical energy, as well as induce decomposition of harmful chemicals, by light promoted transfer of electrons across the molecule/TiO2 interface. It is well documented that such light-induced processes occur in TiO2, but the dynamics of how light energy is used to perform useful chemical transformations, in competition with relaxation and degradation into heat, is poorly known. Such understanding exists for electronic semiconductors and metals, but is poorly understood for a large body of chemical materials that can be classified as metal oxides, such as TiO2. The characterization of electronic and chemical processes in TiO2 can advance the use of metal oxides in light-activated applications. The project supports the formal training of graduate students and post-doctoral associates. Informal education and outreach is being done via the virtual International Ultrafast Knowledge Coffee House ("IUNCH") that Professor Petek initiated during the COVID-19 pandemic lockdown.Photoinduced electron dynamics at single crystal TiO2 surfaces are being examined by ultrafast time-resolved photoelectron spectroscopy. Detailed interpretation of photoexcitation dynamics in the stoichiometric TiO2 is difficult, because the measurements probe electrons that are inhomogeneously distributed in space and momentum. Titanium dioxide has many atomic defect states, which give sharp photoelectron spectra, that enable the study of electron phase and energy relaxation. Ultrafast femtosecond (10-15 s) time scale measurements will be performed on such defect features to establish intrinsic energy and momentum relaxation processes, broadly, in metal oxide materials. Titanium dioxide also has well-documented plasmonically enhanced photocatalytic activity. Studies will be performed on how light excitation in a plasmonic metal activates ultrafast electronic processes at TiO2 surfaces. The students and post-doctoral associates involved in this project will gain experience in ultrafast laser techniques and computational modeling of ultrafast coherent dynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.