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.

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.

Size-induced chemical and magnetic ordering in individual Fe-Au nanoparticles.

Formation of chemically ordered compounds of Fe and Au is inhibited in bulk materials due to their limited mutual solubility. However, here we report the formation of chemically ordered L12-type Fe3Au and FeAu3 compounds in Fe-Au sub-10 nm nanoparticles, suggesting that they are equilibrium structures in size-constrained systems. The stability of these L12-ordered Fe3Au and FeAu3 compounds along with a previously discovered L10-ordered FeAu has been explained by a size-dependent equilibrium thermodynamic model. Furthermore, the spin ordering of these three compounds has been computed using ab initio first-principle calculations. All ordered compounds exhibit a substantial magnetization at room temperature. The Fe3Au had a high saturation magnetization of about 143.6 emu/g with a ferromagnetic spin structure. The FeAu3 nanoparticles displayed a low saturation magnetization of about 11 emu/g. This suggests a antiferromagnetic spin structure, with the net magnetization arising from uncompensated surface spins. First-principle calculations using the Vienna ab initio simulation package (VASP) indicate that ferromagnetic ordering is energetically most stable in Fe3Au, while antiferromagnetic order is predicted in FeAu and FeAu3, consistent with the experimental results.