Transport of conduction electrons through basal planes of the hematite lattice was modeled as a valence alternation of iron cations using ab initio molecular orbital calculations and electron transfer theory. A cluster approach was successfully implemented to compute electron-transfer rate-controlling quantities such as the reorganization energy and electronic coupling matrix element. Localization of a conduction electron at an iron lattice site is accompanied by large iron–oxygen bond length increases that give rise to a large internal component of the reorganization energy (1.03 eV). The internal reorganization energy calculated directly is shown to differ from Nelsen’s four-point method due to the short-range covalent bridge interaction between the Fe–Fe electron transfer pair in the hematite structure. The external reorganization energy arising from modification of the lattice polarization surrounding the localization site is predicted to contribute significantly to the total reorganization energy. Th...

An ab initio model of electron transport in hematite (α-Fe2O3) basal planes

Hematite (α-Fe2O3) is a promising candidate for photoelectrochemical splitting of water. However, its intrinsically poor conductivity is a major drawback. Doping hematite to make it either p-type or n-type enhances its measured conductivity. We use quantum mechanics to understand how titanium, zirconium, silicon, or germanium n-type doping affects the electron transport mechanism in hematite. Our results suggest that zirconium, silicon, or germanium doping is superior to titanium doping because the former dopants do not act as electron trapping sites due to the higher instability of Zr(III) compared to Ti(III) and the more covalent interactions between silicon (germanium) and oxygen. This suggests that use of n-type dopants that easily ionize completely or promote covalent bonds to oxygen can provide more charge carriers while not inhibiting transport.

Electron Transport in Pure and Doped Hematite

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10–4 and 10–2 cm2 V–1 s–1, respectively. Future directions to build ...

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes

The structure of hematite (α-Fe2O3) (001) surfaces in aqueous media: scanning tunneling microscopy and resonant tunneling calculations of coexisting O and Fe terminations

Natural hematite ore is used as a novel electrolyte material for advanced solid oxide fuel cells (SOFCs). This hematite-based system exhibits a maximum power density of 225 mW cm−2 at 600 °C and reaches 467 mW cm−2 when the hematite is mixed with perovskite-structured La0.6Sr0.4Co0.2Fe0.8O3–δ. These results demonstrate that natural materials for next-generation SOFCs can influence the multiutilization of natural resources, thereby affecting the environment and energy sustainability.

Natural Hematite for Next‐Generation Solid Oxide Fuel Cells

Electrical properties of pure and titanium-doped hematite single crystals, in the basal plane, at low oxygen pressure

The electrical conductivity has been measured at oxygen pressure varying from 0.21 atm. to the equilibrium with magnetite and farther. In the temperature range 200-900°C, three domains are observed. The Seebeck coefficient shows that the Ti donor centres are ionized at 300 K. This allows knowing the electrons concentration and describing the transport by the Small Polaron model

Electrical Properties of Pure and Titanium-Doped Hematite Single Crystals, in the Basal Plane, at Low Oxygen Pressure.

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Electrical properties of pure and titanium-doped hematite single crystals, in the basal plane, at low oxygen pressure

Electrical Properties of Pure and Titanium-Doped Hematite Single Crystals, in the Basal Plane, at Low Oxygen Pressure.