Zinc oxide and its n-type semiconducting properties are well known and have already been discussed intensively. Illumination of zinc oxide with sufficiently energetic photons leads to an excitation of electrons from the valence to the conduction band and the formed exciton may be separated by the band bending present on the interface between semiconductor and electrolyte. The present work focuses on the photoelectrochemical properties of special galvanized steel samples from industry (ZM 120, Z 275, ZE 75 / 75, Galvalume) investigated by an in house developed flow-type photoelectrochemical scanning droplet cell microscope (FT-PE-SDCM). In this field the main goal was to obtain an increase of charge carrier densities within the mentioned sample materials and consequently to reduce the electrical resistivity to improve especially the weldability of galvanized steels. Besides that the photocorrosion properties were also of peculiar interest.
The experiments were performed under strongly alkaline conditions and the n-type semiconducting zinc oxide was formed electrochemically using anodic polarization as well as thermally through a special heat treatment. Illumination was done with a 365 nm high-power UV-LED attached to a coated PMMA photofiber, directing the light directly in the FT-PE-SDC. In general the photoelectrochemical investigations were performed potentiodynamically, using linear potential sweeps, during alternating illumination. These experiments showed a strong difference in the current densities comparing illuminated and dark conditions and were strongly dependent on the present electrical field in the system. Further experiments for better understanding of the occurring reactions were performed, including online atomic absorption spectroscopy (AAS) downstream analytics and Mott-Schottky analysis. AAS was used to correlate the measured currents under illuminated / dark conditions and the total amount of corroded zinc (oxide). Mott-Schottky analysis was performed to determine the flat band potentials and the charge carrier densities of the samples under illumination / without illumination. Interestingly enough the experiments showed at some points deviating results as the known literature proposes. Especially the photo-corrosion of zinc oxide seems to be more complicated as suggested so far. Furthermore the measured high anodic current densities under anodic polarization during illumination appear to be predominantly the result of water oxidation.