Hematite heterostructures for photoelectrochemical water splitting: Rational materials design and charge carrier dynamics

Hematite (α-FeO), with a bandgap suitable for absorption of the solar spectrum, is ideally suited for use as a photoanode material in photoelectrochemical (PEC) conversion of solar light into hydrogen fuel via water splitting. However, low hole mobility, short hole lifetime, high density of surface states, and slow kinetics for oxygen evolution at the α-FeO/electrolyte interface have limited the PEC performance of α-FeO photoanodes to date. Along with numerous reports on doping and nanostructuring of α-FeO, increased attention has been paid to α-FeO heterostructure design for improved PEC performance. This review article provides an overview of four main approaches to rational heterostructure design: coupling α-FeO with (1) an n- or p-type semiconductor for promoting charge separation; (2) a nanotextured conductive substrate for efficient charge collection; (3) a surface/interface passivation layer for reduced surface/interface charge recombination; (4) a catalyst for accelerated water oxidation kinetics. The achievements to date demonstrate that high PEC performance may be accessed with these designs. In addition, we review time-resolved laser techniques used to probe the charge carrier dynamics of these heterostructures. Dynamic studies have provided insight into the mechanisms responsible for the improved PEC performance in these materials and helped to guide continued design of α-FeO heterostructures for further enhancement of PEC water splitting. As summarized in this review article, rational heterostructure design is a promising strategy to push forward the application of α-FeO for potential low cost and high efficiency solar hydrogen conversion. A better fundamental understanding of the charge carrier dynamics in these structures in turn helps to guide and improve the heterostructure design.