4. Nanostructured α-Fe₂O₃ Photoanodes

Due to its abundance, stability, and ability to absorb solar irradiation, Hematite, α-Fe₂O₃, has been investigated for its application in solar hydrogen production via water splitting for more than three decades. However, the recent application of nanostructuring techniques has provided significant advances in the performance of hematite photoanodes. Here, the basic material properties, the attractive aspects, and the challenges presented by hematite for photoelectrochemical (PEC) water splitting are reviewed. Various methods of enhancing performance by nanometer morphology control are detailed and the resulting PEC performances are compared. These techniques, including solution-based routes for porous thin films and nanowire arrays, potentiostatic anodization for nanotube arrays, electrodeposition, ultrasonic spray pyrolysis, and atmospheric pressure chemical vapor deposition, have increased the understanding of the material parameters critical to the performance of hematite, and resulted in an increase of quantum efficiency to over 20% with 450 nm light (compared to 6% with optimized single crystals under similar conditions) and an overall solar-to-hydrogen (STH) conversion efficiency of 3.3% when used in a tandem device. In addition, the remaining limitations of morphology, carrier recombination, slow oxidation kinetics, and flatband potential are presented with the recent advances and approaches in overcoming them and realizing the full potential of hematite for solar hydrogen production.