Electronic structures of alpha -Fe2O3 and Fe3O4 from O K-edge absorption and emission spectroscopy

Excitation-Wavelength-Dependent Charge-Carrier Lifetime in Hematite: An Insight from Nonadiabatic Molecular Dynamics

Experiments have reported that the photoexcited carrier lifetime in α-Fe₂O₃ has a significant excitation-wavelength dependence but leave the physical mechanism unresolved. In this work, we rationalize the puzzling excitation-wavelength dependence of the photoexcited carrier dynamics in Fe₂O₃ by performing nonadiabatic molecular dynamics simulation based on the strongly constrained and appropriately normed functional, which accurately describes the electronic structure of Fe₂O₃. Photogenerated electrons with lower-energy excitation relax fast in the t_2g conduction band within about 100 fs, while the photogenerated electrons with higher-energy excitation undergo first a slower interband relaxation from the e_g lower state to the t_2g upper state on a time scale of 135 ps, followed by the much faster t_2g intraband relaxation. This study provides insight into the experimentally reported excitation-wavelength dependence of the carrier lifetime in Fe₂O₃ and a reference for regulating photogenerated carrier dynamics in transition-metal oxides through the light excitation wavelength.

First-principles calculations of hematite (α-Fe2O3) by self-consistent DFT+U+V

Owing to the confined Fe-3d orbitals and self-interaction error of exchange-correlation functionals, approximate DFT fails to describe iron oxides electronic structure and magnetic properties accurately. Hybrid DFT or DFT + U can solve these problems, but the former is expensive, and the latter only considers on-site interactions. Here, we used DFT + U + V, a DFT + U extension including inter-site interactions, to simulate the structural, magnetic, and electronic properties, along with Fe and O K-edge XAS spectra of α-Fe₂O₃. Two types of atomic orbital projectors were studied, orthogonalized and non-orthogonalized. DFT + U + V improves the description of the structural, magnetic, and electronic properties of α-Fe₂O₃ compared to approximate DFT. The accuracy of the correction depends on the orbital projector used. DFT + U + V with orthogonalized projectors achieves the best experimental agreement at a fraction of hybrid DFT cost. This work emphasizes the importance of inter-site interactions and the type of atomic orbital projectors used in the theoretical research of α-Fe₂O₃.

Cation-Exchange-Induced Metal and Alloy Dual-Exsolution in Perovskite Ferrite Oxides Boosting the Performance of Li-O2 Battery

A temperature-controlled cation-exchange approach is introduced to achieve a unique dual-exsolution in perovskite La_0.8 Fe_0.9 Co_0.1 O_3-δ where both CoFe alloy and Co metal are simultaneously exsolved from the parent perovskite, forming an alloy and metal co-decorated perovskite oxide. Mossbauer spectra show that cation exchange of Fe atoms in CoFe alloy and Co cations in the perovskite is the key to the co-existence of Co metal and CoFe alloy. The obtained composite exhibits an enhanced catalytic activity as Li-O₂ battery cathode catalysts with a specific discharge capacity of 6549.7 mAh g^-1 and a cycling performance of 215 cycles without noticeable degradation. Calculations show that the combination of decorated CoFe alloy and Co metal synergistically modulated the discharge reaction pathway that improves the performance of Li-O₂ battery.

The band structure and optical absorption of hematite (α-Fe2O3): a first-principles GW-BSE study

Hematite (α-Fe2O3) is a widely investigated photocatalyst material for the oxygen evolution reaction, a key step in photoelectrochemical water splitting. Having a suitable band gap for light absorption, being chemically stable and based on earth abundant elements, hematite is a promising candidate for the fabrication of devices able to split water using sunlight. What limits its performance is the high rate of bulk recombination following the optical excitation, so that few charge carriers are able to reach the surface to perform the redox reactions. In an effort to better understand the light absorption properties of hematite, in this work we perform a theoretical first-principles investigation of its band structure and optical absorption properties. We use state-of-the-art many-body perturbation theory, including the effects of electron-hole interaction by solving the Bethe-Salpeter equation. Our approach provides good agreement with the available photoemission and absorption measurements and shows that the onset of light absorption is mostly due to ligand-to-metal charge transfer excitations, giving rise to fairly localized excitons. This is at variance with the previously accepted view that these excitations are due to Fe d-d transitions.

Cation Segregation of A-Site Deficiency Perovskite La0.85FeO3-δ Nanoparticles toward High-Performance Cathode Catalysts for Rechargeable Li-O2 Battery

Cation segregation of perovskite oxide is crucial to develop high-performance catalysts. Herein, we achieved the exsolution of α-Fe₂O₃ from parent La_0.85FeO_3-δ by a simple heat treatment. Compared to α-Fe₂O₃ and La_0.85FeO_3-δ, α-Fe₂O₃-LaFeO_{3- x} achieved a significant improvement of lithium-oxygen battery performance in terms of discharge specific capacity and cycling stability. The promotion can be attributed to the interaction between α-Fe₂O₃ and LaFeO_{3- x}. During the cycling test, α-Fe₂O₃-LaFeO_{3- x} can be stably cycled for 108 cycles at a limited discharge capacity of 500 mAh g^-1 at a current density of 100 mA g^-1, which is remarkably longer than those of La_0.85FeO_3-δ (51 cycles), α-Fe₂O₃ (21 cycles), and mechanical mixing of LaFeO₃ and α-Fe₂O₃ (26 cycles). In general, these results suggest a promising method to develop efficient lithium-oxygen battery catalysts via segregation.

Atomic Iron Catalysis of Polysulfide Conversion in Lithium-Sulfur Batteries

Lithium-sulfur batteries have been regarded as promising candidates for energy storage because of their high energy density and low cost. It is a main challenge to develop long-term cycling stability battery. Here, a catalytic strategy is presented to accelerate reversible transformation of sulfur and its discharge products in lithium-sulfur batteries. This is achieved with single-atomic iron active sites in porous nitrogen-doped carbon, prepared by polymerizing and carbonizing diphenylamine in the presence of iron phthalocyanine and a hard template. The Fe-PNC/S composite electrode exhibited a high discharge capacity (427 mAh g^-1) at a 0.1 C rate after 300 cycles with the Columbic efficiency of above 95.6%. Besides, the electrode delivers much higher capacity of 557.4 mAh g^-1 at 0.5 C over 300 cycles. Importantly, the Fe-PCN/S has a smaller phase nucleation overpotential of polysulfides than nitrogen-doped carbon alone for the formation of nanoscale of Li₂S as revealed by ex situ SEM, which enhance lithium-ion diffusion in Li₂S, and therefore a high rate performance and remarkable cycle life of Li-sulfur batteries were achieved. Our strategy paves a new way for polysulfide conversion with atomic iron catalysis to exploit high-performance lithium-sulfur batteries.

Level Alignment as Descriptor for Semiconductor/Catalyst Systems in Water Splitting: The Case of Hematite/Cobalt Hexacyanoferrate Photoanodes

The realization of artificial photosynthesis may depend on the efficient integration of photoactive semiconductors and catalysts to promote photoelectrochemical water splitting. Many efforts are currently devoted to the processing of multicomponent anodes and cathodes in the search for appropriate synergy between light absorbers and active catalysts. No single material appears to combine both features. Many experimental parameters are key to achieve the needed synergy between both systems, without clear protocols for success. Herein, we show how computational chemistry can shed some light on this cumbersome problem. DFT calculations are useful to predict adequate energy-level alignment for thermodynamically favored hole transfer. As proof of concept, we experimentally confirmed the limited performance enhancement in hematite photoanodes decorated with cobalt hexacyanoferrate as a competent water-oxidation catalyst. Computational methods describe the misalignment of their energy levels, which is the origin of this mismatch. Photoelectrochemical studies indicate that the catalyst exclusively shifts the hematite surface state to lower potentials, which therefore reduces the onset for water oxidation. Although kinetics will still depend on interface architecture, our simple theoretical approach may identify and predict plausible semiconductor/catalyst combinations, which will speed up experimental work towards promising photoelectrocatalytic systems.

Photoanodes based on TiO2and α-Fe2O3for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Solar driven photoelectrochemical water splitting represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as H₂.

Solar water splitting: progress using hematite (α-Fe(2) O(3) ) photoelectrodes

Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.

Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach

Sustainable hydrogen production through photoelectrochemical water splitting using hematite (alpha-Fe(2)O(3)) is a promising approach for the chemical storage of solar energy, but is complicated by the material's nonoptimal optoelectronic properties. Nanostructuring approaches have been shown to increase the performance of hematite, but the ideal nanostructure giving high efficiencies for all absorbed light wavelengths remains elusive. Here, we report for the first time mesoporous hematite photoelectodes prepared by a solution-based colloidal method which yield water-splitting photocurrents of 0.56 mA cm(-2) under standard conditions (AM 1.5G 100 mW cm(-2), 1.23 V vs reversible hydrogen electrode, RHE) and over 1.0 mA cm(-2) before the dark current onset (1.55 V vs RHE). The sintering temperature is found to increase the average particle size, and have a drastic effect on the photoactivity. X-ray photoelectron spectroscopy and magnetic measurements using a SQUID magnetometer link this effect to the diffusion and incorporation of dopant atoms from the transparent conducting substrate. In addition, examining the optical properties of the films reveals a considerable change in the absorption coefficient and onset properties, critical aspects for hematite as a solar energy converter, as a function of the sintering temperature. A detailed investigation into hematite's crystal structure using powder X-ray diffraction with Rietveld refinement to account for these effects correlates an increase in a C(3v)-type crystal lattice distortion to the improved optical properties.