Surfaces

Lead by Prof. Jürg Osterwalder

The heterogeneous route to a solar-light-driven water splitting process, where catalyst molecules and photosensitizers are immobilized on a suitable solid substrate, is technologically compelling because the water reduction and oxidation steps can be spatially separated, thus avoiding unwanted recombination reactions and facilitating separation of the produced gaseous hydrogen and oxygen. Since catalytic activity and selectivity are strongly related to the molecular structure and the accessibility of the active sites to the reactant molecules, structural investigations of adsorbed catalyst molecules are of interest for understanding the mechanism. We apply rigorous surface science methods, including low-energy-electron diffraction (LEED), scanning tunneling microscopy (STM) and x-ray photoelectron diffraction (XPD) in order to study the bonding geometry of adsorbed molecules in vacuo. Fig. 1 shows the multitechnique instrument available in our laboratory.

 

Figure 1: (left) Ultrahigh vacuum system in the Surface Physics Laboratory, comprising facilities for preparing atomically clean single-crystal surfaces and molecular monolayers, as well as various surface sensitive characterization techniques. (right) Sample stage inside the preparation chamber, showing the cylindric sample holder with a TiO2(110) crystal mounted on top.

 

Several other factors influence strongly the catalyst performance upon attachment to a surface, like energy shifts and rehybridization of molecular orbitals and charge redistribution and charge dynamics upon photoexcitation. Such effects are studied by means of ultraviolet photoelectron spectroscopy (UPS) and its angle-dependent variant (ARPES), as well a by time-resolved photoemission and two-photon photoemission. Such fundamental studies are complemented by measurements of the activity for water splitting, where such surfaces, prepared under highly controlled conditions, are brought into water and exposed to visible light. These studies are carried out with novel water reduction and water oxidation catalysts, developed within the URPP LightChEC, and eventually also including photosensitizers. In close collaboration with the synthetic chemists, strategies will be developed for obtaining stable surface bonding of the catalyst molecules and maximum activity and photostability.