A Unified Picture of Water Oxidation on Bare and Gallium Oxide-Covered Hematite from Density Functional Theory

Passivation of Hematite by a Semiconducting Overlayer Reduces Charge Recombination: An Insight from Nonadiabatic Molecular Dynamics

Hematite (α-Fe₂O₃) is a promising photoanode material for photoelectrochemical water splitting. Surface-passivating layers are effective in improving water oxidation kinetics; however, the passivation mechanism is not fully understood due to the complexity of interfacial reactions. Focusing on the Fe-terminated Fe₂O₃ (0001) surface that exhibits surface states in the band gap, we perform ab initio quantum dynamics simulations to study the effect of an α-Ga₂O₃ overlayer on charge recombination. The overlayer eliminates surface states and suppresses charge recombination 4-fold. This explains in part the observed cathodic shift in the onset potential for water oxidation. The increased charge carrier lifetime is an outcome of two factors, energy gap and electron-vibrational coupling, with a positive contribution from the former but a negative contribution from the latter. This work presents an advance in the atomistic time-domain understanding of the influence of surface passivation on charge recombination dynamics and provides guidance for designing novel α-Fe₂O₃ photoanodes.

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

Hematite has been considered as a promising photoanode candidate for photoelectrochemical (PEC) water oxidation and has attracted numerous interests in the past decades. However, intrinsic drawbacks drastically lower its photocatalytic activity. Ti-based modifications including Ti-doping, Fe₂O₃/Fe₂TiO₅ heterostructures, TiO₂ passivation layers, and Ti-containing underlayers have shown great potential in enhancing the PEC conversion efficiency of hematite. Moreover, the combination of Ti-based modifications with various strategies towards more efficient hematite photoanodes has been widely investigated. Nevertheless, a corresponding comprehensive overview, especially with the most recent working mechanisms, is still lacking, limiting further improvement. In this respect, by summarizing the recent progress in Ti-modified hematite photoanodes, this review aims to demonstrate how the integration of titanium and iron atoms into hematite influences the PEC properties by tuning the carrier behaviours. It will provide more cues for the rational design of high-performance hematite photoanodes towards future practical applications.

First-principles calculations of the structural, energetic, electronic, optical, and photocatalytic properties of BaTaO2N low-index surfaces

We present here the influence of different surface terminations on the electronic, optical, and photocatalytic properties of trans and cis BaTaO2N using density functional theory calculations.

Ultrathin space charge layer in hematite photoelectrodes: A theoretical investigation

The space charge layer in hematite photoelectrodes has been analyzed by means of Poisson-Boltzmann equations, the Stern model, and density functional theory, in view of its application for photoelectrochemical water oxidation. The width of the space charge layer can be smaller than ∼10 Å under experimental conditions. In this regime, a substantial part of the potential drop takes place in the Helmholtz layer, leading to important corrections to the Mott-Schottky behavior of the space charge layer capacitance. These results shed light on an unexpected regime of high photoelectrocatalytic efficiency, different from the classical picture of the electrochemical interface of a semiconducting photocatalyst, which is also amenable to direct study by quantum-mechanical atomistic simulations. Density functional theory has been used to calculate the band bending (BB) in the space charge layer in atomistic models of pristine stoichiometric and hydroxylated surfaces. These surface terminations display BBs of 0.14 and 0.49 eV, respectively, with an increasing width of the space charge layer, however still in the sub-nanometer regime. This work shows that, at high doping, the width of the space charge layer of a hematite photoelectrode can become comparable with interatomic distances.

Tuning the optoelectronic properties of hematite with rhodium doping for photoelectrochemical water splitting using density functional theory approach

Hematite (Fe₂O₃) is one of the best candidates for photoelectrochemical water splitting due to its abundance and suitable bandgap. However, its efficiency is mostly impeded due to the intrinsically low conductivity and poor light absorption. In this study, we targeted this intrinsic behavior to investigate the thermodynamic stability, photoconductivity and optical properties of rhodium doped hematite using density functional theory. The calculated formation energy of pristine and rhodium doped hematite was − 4.47 eV and − 5.34 eV respectively, suggesting that the doped material is thermodynamically more stable. The DFT results established that the bandgap of doped hematite narrowed down to the lower edge (1.61 eV) in the visible region which enhanced the optical absorption and photoconductivity of the material. Moreover, doped hematite has the ability to absorb a broad spectrum (250–800) nm. The enhanced optical absorption boosted the photocurrent and incident photon to current efficiency. The calculated results also showed that the incorporation of rhodium in hematite induced a redshift in optical properties.

Characterization of peroxo reaction intermediates in the water oxidation process on hematite surfaces

We use density functional theory-based calculations to study structural, electronic, and magnetic properties of two key reaction intermediates on a hematite, [Formula: see text]-Fe₂O₃, photoanode during the solar-driven water splitting reaction. Both intermediates contain an oxygen atom bonded to a surface iron atom. In one case, the adsorbed oxygen also forms a peroxo bond with a lattice oxygen from hematite; in the second case no such bond is formed. Both configurations are energetically equivalent and are related to the overpotential-determining step in the oxygen evolution reaction. The calculated reaction path for the breaking of the peroxo bond shows a barrier of about 0.86 eV for the transformation between the two intermediates. We explain this high barrier with the drastically different electronic and magnetic structure, which we also analyze using maximally localized Wannier functions. Photo-generated electron holes are shown to localize preferentially close to the reaction center at the surface in both configurations. In the case of the oxo species, this localization favors subsequent electron transfer steps during the oxygen evolution cycle. In the case of the peroxo configuration, this fact together with the high barrier for breaking the oxygen-oxygen bond indicates a possible loss mechanism due to hole trapping. Graphical Abstract Calculated spin density at a hematite surface with peroxo intermediate.

Magnetoelectric ϵ-Fe2O3: DFT study of a potential candidate for electrode material in photoelectrochemical cells

The metastable iron oxide ϵ-Fe₂O₃ is rare but known for its magnetoelectric properties. While the more common alpha phase has been recognized for a long time as a suitable material for photoelectrochemical cells, its use is limited because of the electron-hole recombination problem when exposed to light. The indirect bandgap of the epsilon phase with its spontaneous polarization may offer a better potential for the application in photoelectrochemistry. Here, we report a detailed study of the electronic and structural features of the epsilon phase of iron oxide, its stability in thin films, and possible water dissociation reactions. Our studies are performed using density functional theory with a Hubbard-U correction. We observe that the stable ϵ-Fe₂O₃ surfaces favor the dissociation of water. The average difference in the energies of the states when water is adsorbed and when it is dissociated is roughly found to be -0.40 eV. Our results compare with the available experimental results where the epsilon phase is reported to be more efficient for the release of hydrogen from renewable oxygenates when exposed to sunlight.

Designing a porous-crystalline structure of β-Ga2O3: a potential approach to tune its opto-electronic properties

β-Ga2O3 has drawn recent attention as a state-of-the-art electronic material due to its stability, optical transparency and appealing performance in power devices. However, it has also found a wider range of opto-electronic applications including photocatalysis, especially in its porous form. For such applications, a lower band gap must be obtained and an electron-hole spatial separation would be beneficial. Like many other metal oxides (e.g. Al2O3), Ga2O3 can also form various types of porous structure. In the present study, we investigate how its optical and electronic properties can be changed in a particular porous structure with stoichiometrically balanced and extended vacancy channels. We apply a set of first principles computational methods to investigate the formation and the structural, dynamic, and opto-electronic properties. We find that such an extended vacancy channel is mechanically stable and has relatively low formation energy. We also find that this results in a spatial separation of the electron and hole, forming a long-lived charge transfer state that has desirable characteristics for a photocatalyst. In addition, the electronic band gap reduces to the vis-region unlike the transparency in the pure β-Ga2O3 crystal. Thus, our systematic study is promising for the application of such a porous structure of β-Ga2O3 as a versatile electronic material.

Ab initio simulations of water splitting on hematite

In recent years, hematite has attracted great interest as a photocatalyst for water splitting, but many questions remain unanswered about the mechanisms and the main limiting factors. For this reason, density functional theory has been used to understand the optical, electronic and chemical properties of this material at an atomistic level. Bulk doping can be used to reduce the band gap, and to increase photoabsorption and charge mobility. Charge transport takes place through adiabatic polaron hopping. The stable (0 0 0 1) surface has a stoichiometric termination when exposed to oxygen, it becomes hydroxylated in water, and it has an oxygen-rich termination under illumination in a photoelectrochemical setup. On the oxygen-rich termination, surface states are present that might act as recombination centres for electrons and holes. On the contrary, on the hydroxylated termination surface states appear only on reaction intermediates. The intrinsic surface states disappear in the presence of an overlayer of gallium oxide. The reaction of water oxidation is assumed to proceed by four proton-coupled electron transfers and it is shown to involve a nucleophilic attack with the formation of an OOH group. Calculated overpotentials are in the range of 0.5-0.6 V. Open questions and future research directions are briefly discussed.

Surface Fe vacancy defects on haematite and their role in light-induced water splitting in artificial photosynthesis

A pathway for water dissociation near a surface Fe vacancy site on a hematite surface with photogenerated holes.