In recent years, hematite's potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics, and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics.

Understanding the chemical nature and role of electrode surface states is crucial for improved electrochemical cell operation. For iron(III) oxide (α-Fe 2 O 3 ), which is one of the most widely studied anode electrodes used for water splitting, surface states were related to the appearance of a dominant absorption peak during water splitting. The chemical origin of this signature is still unclear, and this open question has provoked tremendous debate. In order to pin down the origin and role of surface states, we perform first-principles calculations with density functional theory + U on several possible adsorbates at the α-Fe 2 O 3 (0001) surface. We rule out the existence of a stable peroxo Fe−O−O−Fe adsorbate and show that the origin of the surface absorption peak could be a Fe−O• type bond that functions as an essential intermediate of water oxidation.

Understanding the role of an overlayer material on a catalyst is crucial for improving catalytic activity. Iron­(III) oxide (α-Fe2O3) is a widely studied catalyst commonly used for solar water splitting. Recently, the water splitting efficiency with α-Fe2O3 was enhanced by deposition of an α-Al2O3 overlayer. In order to understand the origin of this improvement, we perform first-principles calculations with density functional theory + U on the α-Fe2O3(0001) surface with an α-Al2O3 surface overlayer. We find catalysis is unfavorable directly over α-Al2O3 and rather takes place over α-Fe2O3 exposed areas. In agreement with experiment, we find that α-Al2O3 coverage decreases the overpotential required for water oxidation on α-Fe2O3. We explain this improvement through the decrease in the work function of α-Fe2O3 upon α-Al2O3 coverage that aids in extracting electrons during the water oxidation reaction. We suggest that selecting an overlayer with a smaller work function than that of the catalyst as a strategy for future development of better catalysts.

Hematite's (α-FeO) major limitation to efficiently splitting water using sunlight is the low rate of the oxygen evolution reaction (OER). Thus, identifying the OER rate limiting step is a cornerstone to enhancing the current under low applied potential. Different measurement techniques showed similar absorption difference spectra during a change in applied potential on the hematite anode below and above the onset of the OER in the dark and under light. This absorption change was shown to result from surface modification during the OER, but the specific surface species could not be resolved. On the basis of ab initio calculations, we analyze the calculated absorption spectra in relation to previous measurements. We provide for the first time solid evidence to specify HO + *O → *OOH + H + e as the rate limiting step and *O as the bottleneck intermediate of the hematite OER.

Abstract:The challenge of improving the efficiency of photo-electrochemical devices is often addressed through doping. However, this strategy could harm performance. Specifically, as demonstrated in a recent experiment, doping one of the most widely used materials for water splitting, iron(III) oxide (Fe 2 O 3 ), with niobium (Nb) can still result in limited efficiency. In order to better understand the hazardous effect of doping, we use Density Functional Theory (DFT)+U for the case of Nb-doped Fe 2 O 3 . We find a direct correlation between the charge of the dopant, the charge on the surface of the Fe 2 O 3 material, and the overpotential required for water oxidation reaction. We believe that this work contributes to advancing our understanding of how to select effective dopants for materials.

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Improving water splitting efficiency has been the holy grail of hydrogen fuel production. Major efforts have been invested in an attempt to enhance efficiency of a common water oxidation catalyst, α-Fe2O3, through doping and alloying. Recent experiments show that higher efficiency is achieved when niobium (Nb) is added beyond the solubility limit to generate a mixture of two phases: Nb-doped and Nb-alloyed α-Fe2O3. In order to understand why adding high concentrations of Nb is beneficial, we provide a thorough first principles study of the bulk and the surface of pure, Nb-doped, and Nb-alloyed α-Fe2O3 with several surface facets and terminations. We find that the addition of Nb changes the band edge and Fermi level positions. Therefore, we propose a mechanism by which having different Nb doping levels within and above the solubility limit has an advantage: electrons and holes could separate better between doped and alloyed regions that have different band edge positions or between regions with different doping concentrations. Furthermore, the holes' driving force to oxidize water can be increased by placing on the surface the undoped or alloyed phases, since they have a lower valence band maximum. We suggest that obtaining two material phases or gradual doping can be used as a design strategy for next generation catalysts.

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