Understanding charge transfer, defects and surface states at hematite photoanodes

Abstract

Hematite (α-Fe₂O₃) has been widely investigated as a promising photoanode candidate in photoelectrochemical cells for solar water splitting. Although significant advances have been made to improve bulk charge properties as well as surface catalytic activity for oxygen evolution reaction, it still remains challenging to meet the standards for practical applications. As such, deeper understanding and analysis is necessary to guide efforts to achieve higher activities. This perspective reviews and analyzes the important progress on hematite photoanodes from multiple angles. We highlight the critical role of defect chemistry in terms of bulk properties and surface reaction kinetics. Careful manipulation of the quantity of oxygen vacancies and majority/minority charge carriers is shown to be essential for higher activity. One major type of surface recombination site, which can be readily removed, is identified to be an Fe²⁺ species based on multiple photoelectrochemical and spectroscopic observations. Analyzing X-ray absorption spectroscopy and electrochemical energy diagrams, we present a clear picture of water oxidation dynamics at different operating conditions, revealing the relationship between photo-generated holes and surface recombination states. Finally, we conclude that to make hematite photoanodes commercially viable, tuning the minority charge transport properties should be regarded as the priority.