Abstract: Hydrogen energy is a totally clean energy. The primary scientific and technical issue of the utilization of hydrogen energy is the development of efficient, clean, sustainable and low-cost hydrogen production technologies. Photoelectrochemical (PEC) water splitting is a preferred technology, which can directly achieve water splitting and hydrogen/oxygen separation at room temperature. PEC devices are not seriously limited by the cyclical fluctuations of sunlight, and can be made entirely of inorganic materials. Thus, PEC devices usually present high chemical activity and long lifetime. However, the efficiency of PEC devices is still not able to meet the requirements for practical applications. Moreover, the performance of PEC devices would decay with time and cannot provide long stable operations up to date. Among various photoelectrode materials, hematite (α-Fe2O3) is an important and promising one with excellent stability, low cost, abundant reserve, excellent solar spectrum response and high efficiency, and has become a research hotspot in recent years. However, its drawbacks of poor charge transport, high charge recombination and sluggish kinetics greatly limit its practical applications. In recent years, various approaches including doping, nanostructuring, heterojunction and surface modification/treatment have been reported. The doping of α-Fe2O3 with a variety of metallic and nonmetallic elements such as Ti, Sn, Si and S shows that the incorporated heteroatoms could reduce the effective electron mass, thereby increasing the conductivity, while inducing the crystal distortion of α-Fe2O3 for creating more active sites. The α-Fe2O3 nanomaterials with 0D, 1D, 2D, 3D and hierarchical structures have been successfully synthesized. Furthermore, nanostructuring arts are developed to fabricate highly conductive substrate with regular array patterns. The nanostructured α-Fe2O3 can enhance the generation and utilization of holes, which is an important way to improve the PEC performance. Various α-Fe2O3-based n-n and p-n heterojunctions such as α-Fe2O3/ZnFe2O4 and p-Si/α-Fe2O3 have also been developed. These heterojunctions greatly promote the light absorption, charge separation and kinetics of α-Fe2O3 photoanodes. The surface treatment of α-Fe2O3, such as catalyst modification, passivation layer modification, chemical/electrochemical etching and atmosphere treatment, could significantly improve its charge transfer, oxygen evolution kinetics and inhibit its charge recombination. Herein, we provide an overview mainly focusing on four approaches mentioned above, in particular the rational designs of materials and corresponding charge carrier dynamics. The effects of the different approaches on the α-Fe2O3 based photoelectrochemical water splitting are discussed. The structure-activity relationship including nanostructure and materials composition is analyzed. Further, we thoughtfully analyze the charge carrier dynamics during the processes of photoelectrochemical water splitting. We offer clear physical insights to show the relation of the performance improvement of α-Fe2O3 versus the charge carriers. Moreover, a brief introduction of the basic principles and processes of photoelectrochemical water splitting is presented. It is expected that this article could offer theoretical guidance and practical methods for rational designs of α-Fe2O3 for stable and high-efficient solar hydrogen conversion.
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