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

da Trindade L, Oliveira RCD, D. Costa L, C. de Oliveira M, de Lazaro SR, Sambrano JR, Mendonça CR, de Boni L, L. Pontes FM, de Oliveira AJA, Leite ER, Longo E. Unconventional Disorder by Femtosecond Laser Irradiation in Fe2O3

This paper demonstrates that femtosecond laser-irradiated Fe₂O₃ materials containing a mixture of α-Fe₂O₃ and ε-Fe₂O₃ phases showed significant improvement in their photoelectrochemical performance and magnetic and optical properties. The absence of Raman-active vibrational modes in the irradiated samples and the changes in charge carrier emission observed in the photocurrent density results indicate an increase in the density of defects and distortions in the crystalline lattice when compared to the nonirradiated ones. The magnetization measurements at room temperature for the nonirradiated samples revealed a weak ferromagnetic behavior, whereas the irradiated samples exhibited a strong one. The optical properties showed a reduction in the band gap energy and a higher conductivity for the irradiated materials, causing a higher current density. Due to the high performance observed, it can be applied in dye-sensitized solar cells and water splitting processes. Quantum mechanical calculations based on density functional theory are in accordance with the experimental results, contributing to the elucidation of the changes caused by femtosecond laser irradiation at the molecular level, evaluating structural, energetic, and vibrational frequency parameters. The surface simulations enable the construction of a diagram that elucidates the changes in nanoparticle morphologies.

Heterostructures of ε-Fe2O3 and α-Fe2O3: insights from density functional theory

Many materials used in energy devices or applications suffer from the problem of electron-hole pair recombination. One promising way to overcome this problem is the use of heterostructures in place of a single material. If an electric dipole forms at the interface, such a structure can lead to a more efficient electron-hole pair separation and thus prevent recombination. Here we model and study a heterostructure comprised of two polymorphs of Fe₂O₃. Each one of the two polymorphs, α-Fe₂O₃ and ε-Fe₂O₃, individually shows promise for applications in photoelectrochemical cells. The heterostructure of these two materials is modeled by means of density functional theory. We consider both ferromagnetic as well as anti-ferromagnetic couplings at the interface between the two systems. Both individual oxides are insulating in nature and have an anti-ferromagnetic spin arrangement in their ground state. The same properties are found also in their heterostructure. The highest occupied electronic orbitals of the combined system are localized at the interface between the two iron-oxides. The localization of charges at the interface is characterized by electrons residing close to the oxygen atoms of ε-Fe₂O₃ and electron-holes localized on the iron atoms of α-Fe₂O₃, just around the interface. The band alignment at the interface of the two oxides shows a type-III broken band-gap heterostructure. The band edges of α-Fe₂O₃ are higher in energy than those of ε-Fe₂O₃. This band alignment favours a spontaneous transfer of excited photo-electrons from the conduction band of α- to the conduction band of ε-Fe₂O₃. Similarly, photo-generated holes are transferred from the valence band of ε- to the valence band of α-Fe₂O₃. Thus, the interface favours a spontaneous separation of electrons and holes in space. The conduction band of ε-Fe₂O₃, lying close to the valence band of α-Fe₂O₃, can result in band-to-band tunneling of electrons which is a characteristic property of such type-III broken band-gap heterostructures and has potential applications in tunnel field-effect transistors.