Fundamentals of Photocatalytic Splitting of Water at TiO₂ surfaces

Since Fujishima and Honda published in 1972 that hydrogen can be produced by photocatalytic splitting of water on a titanium dioxide (TiO2) electrode, it is one of the most promising energy carriers. The energy of the sun is stored in chemical energy. Since 1972 a lot of research has been performed aiming at improving the efficiency of the process. Our research focus here in Mainz is on unravelling the mechanism of the dissociation reaction at the interface at the molecular level.

In the first part of the project we are determine the structure and orientation of water at the TiO2 interface by applying the sum-frequency generation techniques. In sum-frequency generation (SFG) two laser pulses are combined at the interface and the sum-frequency of the two is detected. As this process is a second-order non-linear optical process, it is forbidden in media with centrosymmetry, like bulk water. At any interface the symmetry is broken, making SFG a surface sensitive technique. If, in addition, one of the two incoming beams is an infrared (IR) beam in resonance with a molecular vibration, the SFG signal is strongly enhanced. By tuning the IR frequency, one can record the vibrational spectrum of the outermost monolayer of molecules. In this way molecular specific information can be obtained of an interface or of species at an interface; one can look (probe) directly at an interfacial molecule.

As the surface of titanium dioxide seems to be complex and not straightforward to characterize, it is necessary to understand the vibrational SFG response of water at such surfaces on a fundamental level. With silica and calcium fluoride, we are studying two mineral surfaces which serve as useful model systems and also provide scientific input for how to access physical surface properties such as surface potential, surface charge density and Debye screening length by using SFG. Additionally, we can deduce how water binds to such surfaces and its intermolecular structure differs from layers further away.