Talk by Olaf M. Magnussen
Institute of Experimental and Applied Physics, Kiel University, Germany
Abstract
Design and control of functional surfaces requires insight into their structure and properties on the atomic scale. This is possible using modern in situ and operando techniques that allow direct observations, even during interface processes. In this talk, I will show examples from electrocatalysis and surface photochemistry, which provide detailed data on the surface structural changes at the molecular level.
First, I will discuss cobalt oxide electrodes, specifically Co3O4 and CoOOH, which are among the best precious-metal-free catalyst for the oxygen evolution reaction (OER) in alkaline media and thus of great interest for large-scale electrochemical water splitting into O2 and H2. Using ultrathin epitaxial films and surface x-ray diffraction methods, we investigated the oxide surface structure during massive oxygen evolution (up to 150 mA/cm2).
The two oxide materials show very different behavior: Whereas the CoOOH(001) samples are smooth and completely stable up to the OER regime, Co3O4(111) exhibited characteristic changes in the strain and film thickness with potential, which indicate a reversible structural transformation of an ≈1 nm thick surface layer under OER conditions. Nevertheless, the electrocatalytic reactivity of Co3O4 and CoOOH is very similar, suggesting that the importance of defects has been overestimated in previous studies of these materials.
In the second part, the functionalization of metal surfaces by photoswitchable organic molecules is discussed. Using the “platform concept”, freestanding groups of switchable azobenzene derivatives can be mounted in a defined perpendicular orientation to the surface. These groups exhibit facile photoswitching between the trans and the cis state, which we monitor by UV/VIS and IR spectroscopy as well as on the molecular scale by STM.
Interestingly, the thermal cis-trans backswitching is strongly affected by the electronic coupling to the surface. Thus, the residence time in the cis state can be controlled over several orders of magnitude by introducing electronically conducting or isolating vertical spacer groups.