Date: Friday, April 17, 2015
Time: 2:00 pm
Speaker: Artur Braun, Empa. Swiss Federal Laboratories for Materials Science and Technology
Title: Changes in photoanodes during solar water oxidation, the wet part of artificial photosnythesis
We know photosynthesis as the natural, biological process where sunlight is used to oxidize water molecules and reduce carbon dioxide molecules and convert those into carbohydrates. One can mimic this process with inorganic components in photoelectrochemical cells (PEC) . Water oxidation takes place at the photoanode, for which we use iron oxide as electrode material. We have looked into the structural changes that happen in photoelectrodes during synthesis, processing, function and operation, and also during ageing. We have particularly focused on soft Xray spectroscopy and were able to detect photoexcited holes while the PEC was operating . It is interesting to compare these observations with charge carrier dynamics measured with electroanalytic methods. Surface states and defect states are here relevant keywords, but it is not always clear what is exactly meant by that. We made also interesting observations ex situ, such as that the iron oxide surface is substoichiometric after preparation , but PEC operation oxidizes the reduced Fe2+ species towards Fe3+ in a parabolic film growth law . These metal oxides are not as stable as they are typically perceived.
In addition to looking at the inorganic components, we looked at bioorganic components which we used to functionalize the iron oxide photoanodes. One relevant question that comes up is about how is the charge transfer between, for example, proteins and metal oxides. We looked into the light harvesting antenna protein phycocyanin, which comes from cyanobacteria. Cyclic voltametry and impedance spectroscopy are established methods for the study of biointerfaces, such as in bio- sensors. We employ the same for our bioPEC electrodes, but I also want to know how one can measure them with xray and electron spectroscopy. So we tried some initial valence band photoemission studies and, in the end, went even as far as looking into biofilms grown from anabaena algae on iron oxide electrodes – while the biofilm was electrically biased and subject to 150 mTorr water vapor pressure, and illuminated: photoelectrophysiological conditions. We did Fe2presonant photoemission in order to be sensitive to the interface between biofilm and electrode . For sure it is likely to make a lot of mistakes in such experiments. But the goal is to correlate electric transport properties with electronic structure properties, at best in an element specific and orbitl specific way.