Photodeposition of Co-Based Oxygen Evolution Catalysts on α-Fe₂O₃ Photoanodes

Cobalt-based oxygen evolution catalysts contain-ing phosphates (Co-Pi OEC) were photochemically deposited onto the surface of n-type α-Fe2O3 electrodes to enhance solar O2 production. α-Fe2O3 films used in this study were prepared by electrodepositing Fe films followed by thermal oxidation at 500 °C. The use of a nonaqueous plating solution made it possible to deposit adherent and uniform Fe films, which is difficult to achieve in an aqueous medium. Photodeposition of Co-Pi OEC was carried out by using photogenerated holes in the valence band of α-Fe2O3 to oxidize Co2+ ions to Co3+ ions in a phosphate buffer solution, which resulted in the precipitation of Co-Pi OEC on the α-Fe2O3 surface. Two different deposition conditions, open circuit (OC) and short circuit (SC) conditions, were studied comparatively to understand their effect on the growth and composition of Co-Pi OEC deposits. The results showed that the SC condition where the photoreduction reaction is physically separated from the photo-oxidation reaction significantly increased the yield and nucleation density of Co-Pi OECs, resulting in a better coverage of the α-Fe2O3 surface with Co-Pi OEC nanoparticles. X-ray photoelectron spectroscopy showed that the OC condition resulted in a higher Co2+/Co3+ ratio in the Co-Pi OEC deposits than the SC condition. This difference in composition is due to the simultaneous photoreduction occurring on the α-Fe2O3 surface under OC conditions. Co-Pi OEC improved the photocurrent of α-Fe2O3 electrodes more than Co2+ ions simply adsorbed on the α-Fe2O3 surface and the Co-Pi OEC deposited under SC conditions resulted in the most pronounced photocurrent enhancement. These results demonstrate the advantages of creating a SC condition for photodeposition of Co-Pi OECs. O2 detection measurements show that the presence of photodeposited Co-Pi OEC on the α-Fe2O3 surface not only increases the total amount of photocurrent generated by facilitating electron−hole pair separation but also increases the photocurrent to O2 conversion efficiency by improving O2 evolution kinetics.