Water oxidation with holes: what we learn from operando "synchrotron" studies(Conference Presentation)

Photoelectron holes are key players in photoelectrochemical water oxidation. They provide the basis for direct solar fuel production in photoelectrochemical cells. Physics and chemistry in semiconductor photoelectrochemistry are probably the most complex known in physical chemistry. Therefore it is not surprising that the science of solar water splitting rests still on some speculative elements. Thanks to progress in synchrotron instrumentation, x-rays and electrons as probes for chemical and physical processes arenow used in complex experiments during device operation. The studies which were impossible until recently are spectacular. We show how we assess with x-ray based ligand and valence band NEXAFS and AP-XPS spectroscopy the density of hole states in photoelectrodes as a function of electrochemical parameters and at the same time find quantitative information on surface intermediates. We can resolve the interaction of the photoelectrode with the electrolyte down to the Fe3d and O2p orbitals with bias parametrized energetic and spatial depth resolution, including the charge carrier accumulation layer, the electrode surface and the Helmholtz layer. The x-ray electronic structure data are in full alignment with the charge carrier dynamics probed with electroanalytical methods. Noteworthy is that we verify and confirm a historically speculated second electron hole, which corresponds to a transition into the charge transfer band, which precedes the water splitting and coincides with the formation of a hydroxyl intermediate. Latter disappears when water oxidation sets on.