Hematite Nanowire and Nanoflake-Decorated Photoelectrodes: Implications for Photoelectrochemical Water Splitting

Hematite, a low-cost, nontoxic, and earth-abundant n-type semiconductor, is still an intriguing photoanode material for photoelectrochemical (PEC) water splitting. Nevertheless, the PEC performance of hematite is still hindered by ultrafast recombination rates or short diffusion lengths of charge carriers. Therefore, nanostructure implementation has been proposed in this and other cases to overcome this limitation, while simultaneously improving the photon harvesting efficiency. However, this approach must be critically reviewed. We show that both, hematite nanowire- and nanoflake-decorated photoelectrodes, show a low PEC performance in a NaOH (1 M) electrolyte. Reproducible nanostructure synthesis was achieved by the thermal oxidation of low-cost steel foils using only ambient air. Full absolute-energy reconstruction under ambient conditions of the electronic surface band structure of these nanostructured surfaces showed distinct Fermi-level pinning, resulting in high recombination rates. Based on our results, we can conclude that unmodified nanostructures hardly improve the performance but suffer from the lack of internal electrical splitting fields, which suppresses the electron–hole pair separation and can thus actually decrease the performance of PEC electrodes.