Anisotropie des proprietes electriques de l'oxyde de fer Fe2O3α

Benjelloun, Driss; Bonnet, Jean-Pierre; Doumerc, Jean‐Pierre; Launay, Jean-Claude; Onillon, Marc; Hagenmuller, Paul

doi:10.1016/0254-0584(84)90001-4

Cited by 67 publications

(40 citation statements)

References 10 publications

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“…The data in Fig. 7 follow the optical absorption of a-Fe203 [43,51], and indicate a clear absorption edge at about 630 nm, along with a maximum around 520 nm. This maximum has also been observed in the photocurrent action spectra of titanium doped n-type a-Fe203 [18,19] electrodes.…”

Section: Spectral Sensitivitymentioning

confidence: 58%

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Tinnemans

Koster

Thewissen

et al. 1986

Ber Bunsenges Phys Chem

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Electrical Properties / Electrochemistry / Materials Properties / Photoelectrochemistry 1 Semiconductors Polycrystalline Mg-doped (0.1 -5 at.%) Fe203 photoelectrodes have been prepared by annealing at 1573 K followed by slow cooling in air to ambient temperature. These materials are essentially single-phase at Mg concentrations not exceeding 0.5 at. %. Disk-shaped (photo)-electrodes were prepared, and their electric and photoelectrochemical (PEC) properties were measured. The positive sign of the photovoltage and of the thermoelectric power 8, confirmed the materials to be p-type semiconducting. -Carrier concentrations calculated from 8, revealed each Mg2+ to introduce one electron-hole up to about 0.1 at.% Mg. The a.c. conductivities, showing conductivity activation enthalpies of about 0.5 eV in the temperature region 300-800 K, are affected by grain growth and porosity. -The currentvoltage characteristics in the dark and under illumination, the spectral response, and the flatband potential (Vh = +2.2 V vs. SCE pH = 10) have also been determined. -PEC photocurrents in the visible region correspond to very low (-0.lY0) monochromatic quantum efficiency which is due to the low electron-hole mobility. The electronic transition in the visible region is indirect, and evidence has been found for this transition to be Fe-Fe charge transfer. Surface pretreatment is found to have a major effect on the photoelectrochemical properties as deduced from current-voltage curves. The stability of the a-Fe203 electrode in alkaline solution is discussed. IntroductionThe cleavage of water into hydrogen and oxygen in a photoelectrochemical (PEC) cell using a semiconductor photoelectrode has been well established [l -41. Optimal solar energy conversion requires a chemically inert semiconducting electrode material which shows photoconductivity far into the visible, and operates with high energy-conversion efficiencies.Recently, we have studied impurity doping of the stable wide bandgap materials n-type Ti02 and SrTi03 in order to extend their photoresponse into the visible part of the solar spectrum [5, 61. It appeared that with transition metal ion dopants efficient power-conversion efficiencies could only be reached under near uv irradiation. Besides these titanates, the potential applicability of other oxides as photoelectrodes in PEC cells has been studied.The photoelectrochemical properties of iron oxide, a-Fe2O3, have attracted wide-spread attention [7 -301 due to its relatively low bandgap energy of about 2.2 eV [18,31] which is close to the ideal value for optical solar energy absorption, its stability against chemical, and photochemical corrosion [7-9, 16, 191, its vast abundance, and its low cost of preparation.Aliovalent doping is required to render a-Fe2O3 semiconducting [15 -181. However, the flat-band potential of n-type iron oxide lies far below the hydrogen reduction potential [20,27].Hence, a PEC cell with this electrode requires an external bias of about 0.6 eV. While photoassisted electrolysis of water has been dem...

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Section: Spectral Sensitivitymentioning

confidence: 58%

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Tinnemans

Koster

Thewissen

et al. 1986

Ber Bunsenges Phys Chem

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“…This is plausible, as SiO 2 can be transported at temperatures greater than 900 1C using various halogens, 24 but is at odds with several reports in the literature using identical or similar synthesis conditions, who reported insulating (resistivity 410 5 O cm) undoped a-Fe 2 O 3 . 13,36,37 As silica is ever-present in these syntheses, we do not understand this discrepancy at present. Interestingly, when Ti metal was added to the reaction vessel, only trace quantities (o20 ppm) were present in the crystals, indicating very inefficient Ti-incorporation by this method.…”

Section: Phase and Compositionmentioning

confidence: 73%

Synthesis, electronic transport and optical properties of Si:α-Fe₂O₃ single crystals

et al. 2016

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We report the synthesis of silicon-doped hematite (Si:a-Fe 2 O 3 ) single crystals via chemical vapor transport, with Si incorporation on the order of 10 19 cm À3. The conductivity, Seebeck and Hall effect were measured in the basal plane between 200 and 400 K. Distinct differences in electron transport were observed above and below the magnetic transition temperature of hematite at B265 K (the Morin transition, T M ). Above 265 K, transport was found to agree with the adiabatic small-polaron model, the conductivity was characterized by an activation energy of B100 meV and the Hall effect was dominated by the weak ferromagnetism of the material. A room temperature electron drift mobility of B10 À2 cm 2 V À1 s À1was estimated. Below T M , the activation energy increased to B160 meV and a conventional Hall coefficient could be determined. In this regime, the Hall coefficient was negative and the corresponding Hall mobility was temperature-independent with a value of B10 À1 cm 2 V À1 s À1. Seebeck coefficient measurements indicated that the silicon donors were fully ionized in the temperature range studied. Finally, we observed a broad infrared absorption upon doping and tentatively assign the feature at B0.8 eV to photon-assisted small-polaron hops. These results are discussed in the context of existing hematite transport studies.

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“…[43] Further studies of hematite single crystals identified a highly anisotropic electron transport with conduction along the iron bilayer (001) basal plane up to four orders of magnitude greater than perpendicular directions (parallel to [001]). [44][45] This discrepancy cannot be explained by the proximity of iron cations alone, as the shortest Fe-Fe distances are actually along the [001] direction. [46] However the anisotropic conductivity can be classically explained considering Hund's rule and the magnetic structure of hematite.…”

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confidence: 83%

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

2011

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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.

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Anisotropie des proprietes electriques de l'oxyde de fer Fe2O3α

Cited by 67 publications

References 10 publications

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Synthesis, electronic transport and optical properties of Si:α-Fe₂O₃ single crystals

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

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Anisotropie des proprietes electriques de l'oxyde de fer Fe2O3α

Cited by 67 publications

References 10 publications

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

Synthesis, electronic transport and optical properties of Si:α-Fe2O3 single crystals

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

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Product

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Synthesis, electronic transport and optical properties of Si:α-Fe₂O₃ single crystals

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