Hematite Reduction upon Ti Doping in Hydrothermally Synthesized Photoanodes

Several studies report on the use of hematite-based nanorods (α-Fe2O3) for solar water splitting (SWS) applications, following various synthesis pathways with strong dispersion of the resulting photoelectrochemical (PEC) performances. Herein, we investigate the origin of strong PEC response variability in a set of Ti-doped α-Fe2O3 samples synthesized following, a priori, strictly the same conditions. Our findings reveal that alteration of Ti oxidation state in the TiCl3 precursor, induced most probably by aging, leads to a strong reduction of Fe3+ in the final Ti:α-Fe2O3 photoanode by a charge compensation process. While this adversely impacts PEC efficiency by altering the electronic structure, there are no detectable changes in the crystalline structure. Subtle chemical changes at the nanoscale were thus studied using a combination of transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM) and are demonstrated to be driving water splitting efficiency. The obtained results highlight the importance of a very detailed photoanode characterization, both structural and chemical, allowing us to understand and optimize their PEC activity.