Elementary Steps in the photocatalytic Water Splitting over TiO2-based Model Electrode Systems

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Hydrogen production through photo-induced splitting of water is a promising avenue for sustainable energy economy, although no cost-effective photocatalytic system has been identified so far. It is believed that a successful search for a new and improved photocatalyst will be based on a molecular understanding of all elementary reaction steps in the photocatalytic water splitting. In the present project model systems for photocatalytic water splitting on the basis of single-crystalline TiO2(110) and TiO2(011) will be prepared and extensively characterized by surface sensitive methods (in-situ) as well as theoretical (ab-initio) photo-electrochemical methods. Single crystallinity of the model systems is required to allow for a direct comparison of ab-initio calculations with corresponding experiments. The choice of TiO2 as photocatalyst material is motivated by its activity, being non-toxic, omnipresent, inexpensive and stable under photochemical reaction conditions. These preconditions are indispensable for the development of a future hydrogen fuel economy. However, the quantum efficiency of TiO2 for sun light is quite low as the band gap is quite wide with more than 3eV. Besides band gap engineering by doping with impurities such as N and Cr, the recently discovered dopant-free TiO2 phase on TiO2(011) with a band gap of only 2.1eV will be in the focus of the present project. The rate determining part in the photocatalytic splitting of water is the oxygen evolution reaction (due to the 4-electron process: OER). Therefore co-catalysts, such as RuO2, the best catalyst known for OER, are frequently co-added. The combined system of single crystalline TiO2-based model photocatalysts and ultrathin RuO2 films provides atomic-scale control of the interface that in turn is highly beneficial for electronic and structural characterization as well as the identification of elementary reaction steps in the photo-induced OER.