TiO2 Photocatalysis: The coupling of electrons, plasmons, polarons, and molecules by ultrafast photoemission spectroscopy and theory

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Titanium dioxide (Ti02) is a model system for solar energy capture and transfer to drive chemical processes. The material has the ability to absorb ultraviolet (UV) light and use it to assist in, or catalyze, the decomposition of water (H2O) to hydrogen (H2) and oxygen (O2) gases, which can be stored and later combined to produce energy on demand. The material can also catalyze the reduction of carbon dioxide (CO2), a greenhouse gas that is the final product of burning fossil fuels, and transform this environmentally harmful gas into useful hydrocarbons for the chemical industry. Dr. Petek is engaged in fundamental studies of how this material absorbs UV light to create electron-hole pairs and how these electrons and holes are transported through the material and used to effect chemical change for sustainable solar energy conversion. The fast speeds of these reactions are measured in femtoseconds (1 femtosecond equals 0.000000000000001 second), and are studied by ultrafast laser spectroscopy. Dr. Petek and his collaborator, Dr. Zhao, combine advanced ultrafast laser spectroscopy experiments with theoretical calculations to understand these photo-catalytic events. In addition to the broader impacts of the research to contribute to the development of clean and efficient solar energy capture, there are substantial educational benefits for the students involved in the project. The collaborative arrangement that Dr. Petek has established with Dr. Zhao at the University of Science and Technology China and her co-worker Dr. Min Feng at Wuhan University provides excellent opportunities for students from the University of Pittsburgh to work in the Chinese laboratories, as well as for Chinese students to serve as summer interns in Dr. Petek's laboratory. To broaden student participation in research, Dr. Petek uses the Pittsburgh Quantum Initiative to recruit talented undergraduate students into research assistantships. He is an ardent promotor of photocatalytic research, working to organize of international symposia on the topic as well as serving in the role of Editor-in-Chief of the journal Progress in Surface Science. With funding from the Chemical Catalysis Program of the Chemistry Division, Dr. Petek and Dr. Zhao study the electron spectroscopy and dynamics related to photocatalytic processes by means of time-resolved two-photon photoemission (TR-2PP) experiments and advanced electronic structure theory. The 2PP spectra and time resolved measurements reveal the polaronic character of electrons introduced into the conduction band of TiO2, the interaction of chemisorbed molecules with the polarons, and the unoccupied resonances of chemisorbed molecules such as CO2. Dr. Petek conducts TR-2PP experiments on noble metal decorated TiO2 surfaces to reveal the mechanisms of plasmonically enhanced photocatalysis. In collaboration, Dr. Zhao performs electronic structure calculations to identify the electron and hole acceptor states, potentially involved in photocatalysis, for molecules adsorbed on TiO2 surface. In addition, nonadiabatic molecular dynamics calculations are used to describe the interfacial charge transfer dynamics and carrier energy relaxation rates for chemisorbed molecular overlayers at finite temperatures. The research has broader impacts as it brings together experimentalists and theorists from the University of Pittsburgh and University of Science and Technology of China (USTC) to work on the fundamental aspects of sustainable solar energy conversion via TiO2 photocatalysis. In addition to the broader impacts of the research, there are substantial educational benefits for students involved in the established collaborative project between the USTC and Wuhan University with the University of Pittsburgh, which provides opportunities for undergraduate and graduate student international exchange. To broaden student participation in research, Dr. Petek uses the Pittsburgh Quantum Initiative to recruit talented undergraduate students into research assistantships. He is an ardent promotor of photocatalytic research, working to organize of international symposia on the topic as well as serving in the role of Editor-in-Chief of the journal Progress in Surface Science