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.

Solar Water Splitting: Progress Using Hematite (α‐Fe2O3) Photoelectrodes

The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting

Hydrogen from solar-driven water splitting has the potential to provide clean energy. Current progress towards artificial photosynthetic devices is reviewed, with particular focus on visible light active nanostructures. A vision for a future sustainable hydrogen fuel community based on artificial photosynthesis is outlined.

Artificial photosynthesis for solar water-splitting

We report on the synthesis and characterization of Sn-doped hematite nanowires and nanocorals as well as their implementation as photoanodes for photoelectrochemical water splitting. The hematite nanowires were prepared on a fluorine-doped tin oxide (FTO) substrate by a hydrothermal method, followed by high temperature sintering in air to incorporate Sn, diffused from the FTO substrate, as a dopant. Sn-doped hematite nanocorals were prepared by the same method, by adding tin(IV) chloride as the Sn precursor. X-ray photoelectron spectroscopy analysis confirms Sn4+ substitution at Fe3+ sites in hematite, and Sn-dopant levels increase with sintering temperature. Sn dopant serves as an electron donor and increases the carrier density of hematite nanostructures. The hematite nanowires sintered at 800 °C yielded a pronounced photocurrent density of 1.24 mA/cm2 at 1.23 V vs RHE, which is the highest value observed for hematite nanowires. In comparison to nanowires, Sn-doped hematite nanocorals exhibit smaller fe...

Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting

Revved-up rust! Light-induced water splitting over iron oxide (hematite) has been achieved by using a particle-assisted deposition technique and IrO2-based surface catalysis. Photocurrents in excess of 3 mA cm-2 were obtained at +1.23 V versus the reversible hydrogen electrode under AM 1.5 G 100 mW cm-2 simulated sunlight. These photocurrents are unmatched by any other oxide-based photoanode. FTO=fluorine-doped tin oxide. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis†

Iron(III) oxide has been extensively studied as a possible n-type semiconductor for use in solar photoelectrolysis cells However, its properties have remained curiously elusive; even such fundamental properties as bandgap and flat-band potential are still controversial, and this uncertainty has hindered any rational evaluation of the use of the material in solar cells In this paper, an extensive study of the surface and bulk properties of both single-crystal and polycrystalline Fe2O3 is reported Surface pretreatment is found to have a major effect on the photoelectrochemical properties Even in properly treated samples, however, the photocurrent onset is found to be delayed due to the small value of the faradaic rate constant for the oxidation of water, and a semi-quantitative treatment of this case is provided Improper surface treatment is shown to lead to a substantial conversion of the surface of Fe2O3 to Fe3O4; this introduces a large fraction of recombination sites at the surface that not only delay dc photocurrent onset to very anodic potentials but also reduce the observed efficiency to remarkably low values

Electrochemistry and photoelectrochemistry of iron(III) oxide

Iron(III) oxide has been extensively studied as a possible n-type semiconductor for use in solar photoelectrolysis cells. However, its properties have remained curiously elusive; even such fundamental properties as bandgap and flat-band potential are still controversial, and this uncertainty has hindered any rational evaluation of the use of the material in solar cells. In this paper, an extensive study of the surface and bulk properties of both single-crystal and polycrystalline Fe2O3 is reported. Surface pretreatment is found to have a major effect on the photoelectrochemical properties. Even in properly treated samples, however, the photocurrent onset is found to be delayed due to the small value of the faradaic rate constant for the oxidation of water, and a semi-quantitative treatment of this case is provided. Improper surface treatment is shown to lead to a substantial conversion of the surface of Fe2O3 to Fe3O4; this introduces a large fraction of recombination sites at the surface that not only delay d.c. photocurrent onset to very anodic potentials but also reduce the observed efficiency to remarkably low values.

Electrochemistry and photoelectrochemistry of iron(iii) oxide

A new theory of alternating-current effects in semiconductors is presented. The equivalent circuit used by a large number of workers to describe the effects of electroactive surface states is shown to have a firm physical basis at the molecular level, and the connection between the empirically derived circuit elements and such microscopic parameters as rate constants and surface-state densities is given. The implementation of programs designed to obtain such circuit elements from the raw experimental data is described and some examples of the theory are given.

Peter R. Trevellick

Papers

Electrochemistry and photoelectrochemistry of iron(III) oxide

Electrochemistry and photoelectrochemistry of iron(iii) oxide

Alternating-current techniques in semiconductor electrochemistry