Showing papers on "Hematite published in 2011"
TL;DR: Ultrafast spectroscopy studies revealed that there is significant electron-hole recombination within the first few picoseconds, while Sn doping and the change of surface morphology have no major effect on the ultrafast dynamics of the charge carriers on thepicosecond time scales.
Abstract: 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...
990 citations
TL;DR: In this paper, electron microprobe analyses of minor and trace elements in magnetite and hematite from a range of mineral deposit types (IOCG), Kiruna apatite, magnetite, chromite, and spinel series, and ulvospinel as a result of divalent, trivalent, and tetravalent cation substitutions) are used to construct discriminant diagrams that separate different styles of mineralization.
Abstract: Magnetite and hematite are common minerals in a range of mineral deposit types. These minerals form partial to complete solid solutions with magnetite, chromite, and spinel series, and ulvospinel as a result of divalent, trivalent, and tetravalent cation substitutions. Electron microprobe analyses of minor and trace elements in magnetite and hematite from a range of mineral deposit types (iron oxide-copper-gold (IOCG), Kiruna apatite–magnetite, banded iron formation (BIF), porphyry Cu, Fe-Cu skarn, Fe-Ti, V, Cr, Ni-Cu-PGE, Cu-Zn-Pb volcanogenic massive sulfide (VMS) and Archean Au-Cu porphyry and Opemiska Cu veins) show compositional differences that can be related to deposit types, and are used to construct discriminant diagrams that separate different styles of mineralization. The Ni + Cr vs. Si + Mg diagram can be used to isolate Ni-Cu-PGE, and Cr deposits from other deposit types. Similarly, the Al/(Zn + Ca) vs. Cu/(Si + Ca) diagram can be used to separate Cu-Zn-Pb VMS deposits from other deposit types. Samples plotting outside the Ni-Cu-PGE and Cu-Zn-Pb VMS fields are discriminated using the Ni/(Cr + Mn) vs. Ti + V or Ca + Al + Mn vs. Ti + V diagrams that discriminate for IOCG, Kiruna, porphyry Cu, BIF, skarn, Fe-Ti, and V deposits.
400 citations
TL;DR: A review of the work being carried out on synthesis of iron oxides in nano form and their various applications can be found in this paper, where the authors discuss the potential for their applications as catalytic materials, wastewater treatment adsorbents, pigments, flocculants, coatings, gas sensors, ion exchangers, magnetic recording devices, magnetic data storage devices, toners and inks for xerography, magnetic resonance imaging, bioseparation and medicine.
Abstract: The nano iron oxides have been synthesized by almost all the known wet chemical methods which include precipitation at ambient/elevated temperatures, surfactant mediation, emulsion/micro-emulsion, electro-deposition etc. Iron oxides in nano-scale have exhibited great potential for their applications as catalytic materials, wastewater treatment adsorbents, pigments, flocculants, coatings, gas sensors, ion exchangers, magnetic recording devices, magnetic data storage devices, toners and inks for xerography, magnetic resonance imaging, bioseparation and medicine. Nano sized magnetite Fe3O4, and maghemite γ-Fe2O3 exhibiting excellent magnetic properties find applications for biomedical purposes and as soft ferrites. Iron hydroxides and oxyhydroxides such as ferrihydrite, goethite, akaganeite, lepidocrocite are being evaluated for their applications in water purification for the removal of toxic ions. Hematite, α-Fe2O3 in the nano range has been used to obtain transparent paints. In catalysis both iron oxides and hydroxides find application in numerous synthesis processes. This review outlines the work being carried out on synthesis of iron oxides in nano form and their various applications.
293 citations
TL;DR: Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary.
Abstract: Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hema...
267 citations
TL;DR: This work uses quantum mechanics to understand how titanium, zirconium, silicon, or germanium n-type doping affects the electron transport mechanism in hematite and 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.
Abstract: 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.
266 citations
TL;DR: In this paper, the onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers.
Abstract: The onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers. It is proposed that the corundum-type overlayers released lattice strain of the ultrathin hematite layer and decreased the density of surface states. Particularly, a Ga2O3 overlayer exhibited an enhanced photocurrent attributed to stoichiometric water splitting near the onset potential. The photocurrent was sustained over a day, attesting to its outstanding performance and durability for water splitting.
256 citations
TL;DR: Transient absorption spectroscopy on the μs-s time scale is used to monitor the yield and decay dynamics of photogenerated holes in nanocrystalline hematite photoanodes and results in the generation of long-lived photoholes.
254 citations
TL;DR: In this paper, the reversibility of the arsenate adsorption process was studied using chlorides and phosphates as competing ions, and it was shown that arsenate adaption is related to the iron content of adsorbents, and the rate increases in the following order: goethite
244 citations
TL;DR: Raman spectroscopy can be used for the rapid identification of a wide variety of minerals ranging from common iron oxy(hydroxides), such as ferrihydrite, to rare minerals, such as adelite.
220 citations
TL;DR: Coordination polymer nanorods are synthesized from the hexagonal 3D structure of Fe-MIL-88B and hematite and magnetites are selectively prepared by controlling the calcination conditions of coordination polymer Nanorods.
202 citations
TL;DR: In this paper, the possibility of making construction bricks by using the hematite tailings from western Hubei province of China was investigated, where the additives of clay and fly ash were added to the raw materials to improve the brick quality.
TL;DR: Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates and only charge carriers generated in the depletion region were found to contribute to the photocurrent.
Abstract: Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)6]3-/4- electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ∼20 nm. Mott−Schottky analysis of differential capacitance measurements indicated a depletion region of ∼17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.
TL;DR: This is the authors' version of the following article: "Activation of Hematite Nanorod Arrays for Photoelectrochemical Water Splitting" published in ChemSusChem.
Abstract: Title Activation of hematite nanorod arrays for photoelectrochemical water splitting Authors(s) Morrish, Rachel; Rahman, Mahfujur; MacElroy, J. M. Don; Wolden, Colin Andrew Publication date 2011-04-18 Publication information ChemSusChem, 4 (4): 474-479 Publisher Wiley Link to online version http://dx.doi.org/10.1002/cssc.201100066 Item record/more information http://hdl.handle.net/10197/2886 Publisher's statement This is the authors' version of the following article: \"Activation of Hematite Nanorod Arrays for Photoelectrochemical Water Splitting\" (2011) published in ChemSusChem. It is available in its final form at http://dx.doi.org/10.1002/cssc.201100066 Publisher's version (DOI) 10.1002/cssc.201100066
TL;DR: Initial pH optimization experiments indicated that at pH 6.0 the maximum hydrogen yield reached to 3.57 mol H(2)/mol sucrose and hydrogen content was 66.1%, and the slow release of hematite nanoparticles had been recorded by transmission electron microscopy.
TL;DR: In this paper, the properties of pure (crystal) and complex (product of corrosion) iron oxides, magnetite (Fe 3 O 4 ), hematite (α-Fe 2 O 3 ) and goethite (β-FeO·OH), were determined by means of molecular dynamics analysis (MDA) and instrumented indentation.
TL;DR: In this paper, the physical properties and photoelectrochemical characterization of aluminium doped hematite α-Fe2O3, synthesized by spray pyrolysis, have been investigated in regard to solar energy conversion.
Abstract: The physical properties and photoelectrochemical characterization of aluminium doped hematite α-Fe2O3, synthesized by spray pyrolysis, have been investigated in regard to solar energy conversion. Stable Al-doped iron (III) oxide thin films synthesized by a spray pyrolysis technique reveals an oxygen deficiency, and the oxide exhibits n-type conductivity confirmed by anodic photocurrent generation. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The deposited iron oxide thin films show the single hematite phase with polycrystalline rhombohedral crystal structure with crystallite size 20–40 nm. Optical analysis enabled to point out the increase in direct band-gap energy from 2.2 to 2.25 eV with doping concentration which is attributed to a blue shift. The dielectric constant and dielectric loss are studied as a function of frequency. To understand the conduction mechanism in the films, AC conductivity is measured. The conduction occurs by small polaron hopping through mixed valences Fe2+/3+ with an electron mobility 300 K of 1.08 cm2/(Vs). The α-Fe2O3 exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as a photoanode for various electrolytes using white light to obtain I — V characteristics. The Al-doped hematite exhibited a higher photocurrent response when compared with undoped films achieving a power conversion efficiency of 2.37% at 10 at % Al:Fe2O3 thin films along with fill factor 0.38 in NaOH electrolyte. The flat band potential Vfb (−0.87 VSCE) is determined by extrapolating the linear part to C−2 = 0 and the slope of the Mott-Schottky plot.
TL;DR: In this paper, the morphology, mineralogy, and magnetic properties of technogenic magnetic particles (TMPs) were analyzed in four kinds of industrial dust produced during high temperature technological processes of different branches of industry (lignite and hard coal burning, cement production, coke production).
TL;DR: The photoelectrochemical characterization shows that the colloidal approach used to process an undoped hematite photoanode produced a high-performance electrode for water photooxidation with an onset potential as low as 0.8 V(RHE), comparable to the best results reported in the literature.
Abstract: In this study, we demonstrate an alternative and promising way to produce hematite photoanodes with high performance and without the addition of doping or catalytic coating. In this approach, we pr...
TL;DR: In this paper, α-Fe 2 O 3 nano-particles were prepared by direct thermaldecomposition of γ-Fe O 3, and the precursors were subsequently calcined in air for 1h at 500°C.
TL;DR: It is found that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties.
Abstract: We present a hybrid density functional theory (DFT) study of doping effects in α-Fe(2)O(3), hematite. Standard DFT underestimates the band gap by roughly 75% and incorrectly identifies hematite as a Mott-Hubbard insulator. Hybrid DFT accurately predicts the proper structural, magnetic, and electronic properties of hematite and, unlike the DFT+U method, does not contain d-electron specific empirical parameters. We find that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties. We then show that the antiferromagnetic symmetry in the pure α-Fe(2)O(3) crystal is broken by all dopants and that the ligand field theory correctly predicts local magnetic moments on the dopants. We characterize the resulting band gaps for hematite doped by transition metals and the p-block post-transition metals. The specific case of Pd doping is investigated in order to correlate calculated doping energies and optical properties with experimentally observed photocatalytic behavior.
01 Jan 2011
TL;DR: In this article, the physical properties and photoelectrochemical characterization of aluminium doped hematite -Fe2O3, synthesized by spray pyrolysis, have been investigated in regard to solar energy conversion.
Abstract: The physical properties and photoelectrochemical characterization of aluminium doped hematite - Fe2O3, synthesized by spray pyrolysis, have been investigated in regard to solar energy conversion. Stable Al-doped iron (III) oxide thin films synthesized by a spray pyrolysis technique reveals an oxygen deficiency, and the oxide exhibits n-type conductivity confirmed by anodic photocurrent generation. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The deposited iron oxide thin films show the single hematite phase with polycrystalline rhombohedral crystal structure with crystallite size 20-40 nm. Optical analysis enabled to point out the increase in direct band-gap energy from 2.2 to 2.25 eV with doping concentration which is attributed to a blue shift. The dielectric constant and dielectric loss are studied as a function of frequency. To understand the conduction mechanism in the films, AC conductivity is measured. The conduction occurs by small polaron hopping through mixed valences Fe 2C=3C with an electron mobility 300 K of 1.08 cm 2 /(V s). The -Fe2O3 exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as a photoanode for various electrolytes using white light to obtain I - V characteristics. The Al-doped hematite exhibited a higher photocurrent response when compared with undoped films achieving a power conversion efficiency of 2.37% at 10 at% Al:Fe2O3 thin films along with fill factor 0.38 in NaOH electrolyte. The flat band potential Vfb (-0.87 VSCE/ is determined by extrapolating the linear part to C 2 D 0 and the slope of the Mott-Schottky plot.
TL;DR: In this paper, the size of the iron oxide nanoparticles was estimated to be significantly smaller than 19nm by using transmission electron microscopy, X-ray powder diffraction, and thermogravimetric and differential thermal analyses.
TL;DR: Sintering temperature and particle size dependent structural and magnetic properties of lithium ferrite (Li 0.5 Fe 2.5 O 4 ) were synthesized and sintered at four different temperatures ranging from 875 to 1475 K in the step of 200 K as discussed by the authors.
TL;DR: In this article, shape and size controlled single-phase α-Fe 2 O 3 (hematite) nanoparticles have been successfully synthesized via a simple and facile thermal treatment route using modified precursors.
TL;DR: In this article, the photoanodes were measured as a function of titanium concentration and the incident photon-to-current conversion efficiency (IPCE) to hydrogen was measured in alkaline electrolyte.
Abstract: To improve the optoelectronic properties of iron oxide as a photoelectrode, hematite (α-Fe2O3) thin films were doped with titanium using atmospheric pressure chemical vapor deposition (APCVD) for synthesis. The films were prepared by pyrolysis of Fe(CO)5 and TiCl4 precursors on fluorine-doped tin oxide (FTO) substrates and found to have a polycrystalline morphology with faceted particulates ∼20 to 50 nm in size with a preferred crystallographic growth along the [110] direction. The performance of the photoanodes was measured as a function of titanium concentration. A maximum efficiency was observed at ∼0.8 atom% Ti in hematite. The Incident Photon-to-current Conversion Efficiency (IPCE) to hydrogen was measured in alkaline electrolyte. Under an applied bias of 0.6 V vs.Ag/AgCl at 400 nm the IPCE for water splitting in alkaline solution was found to be 27.2%, the highest efficiency reported for Ti doped hematite photoanodes. The IPCEs of the photoanode thin films at lower applied bias were further increased by calcination at 500 °C and by use of glucose as an anolyte.
TL;DR: An oriented attachment growth process was found for the morphology and shape control of various inorganic materials in solution-based systems and the as-prepared products exhibit excellent gas sensing selectivity to ethanol vapor.
Abstract: An oriented attachment growth process was found for the morphology and shape control of various inorganic materials in solution-based systems. In this Article, a well-defined iron oxide polyhedron was prepared via a simple hydrothermal method. Detailed investigations revealed that the single-crystalline polyhedron evolved from polycrystalline aggregates. The shape and size control of the iron oxide crystals were achieved by simply adjusting the synthesis parameters. The as-prepared products exhibit excellent gas sensing selectivity to ethanol vapor. The gas-sensing ability is closely related to the exposure of high-index facets.
TL;DR: In this article, the influence of reduction conditions on the metallization degree of the samples, such as reduction temperature, C/Fe molar ratio, pre-oxidation and additives were studied.
TL;DR: In this article, a tuneable dye laser was used to enhance the Raman spectra for the detection of iron oxide and iron oxyhydroxide compounds using an excitation wavelength of 636.4 nm.
Abstract: Raman spectroscopy, in principle, is an excellent technique for the study of molecular species developed on metal surfaces during electrochemical investigations. However, the use of the more common laser wavelengths such as the 514.5-nm line results in spectra of less than optimal intensity, particularly for iron oxide compounds. In the present work, near-resonance enhancement of the Raman spectra was investigated for the iron oxide and iron oxyhydroxide compounds previously reported to be present in the passive film on iron, using a tuneable dye laser producing excitation wavelengths between 560 and 637 nm. These compounds were hematite (α-Fe2O3), maghemite (γ-Fe2O3), magnetite (Fe3O4), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH) and feroxyhyte (δ-FeOOH). Optimum enhancement, when compared to that with the 514.5-nm line, was obtained for all the iron oxide and oxyhydroxide standard samples in the low wavenumber region (<1000 cm−1) using an excitation wavelength of 636.4 nm. Particularly significant enhancement was obtained for lepidocrocite, hematite and goethite. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory and the contribution of magnetic dispersion interactions for particle sizes in the superparamagnetic regime was found to be negligible.
Abstract: The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory. Expressions for the dielectric responses of three iron oxide phases (magnetite, maghe...
TL;DR: In this article, the relationship between size, shape, and thermodynamic stability of unpassivated hematite (α-Fe2O3) and goethite nanoparticles, using a robust thermodynamic morphology model with input parameters from reliable first-principles calculations and thermochemical data, was investigated.
Abstract: Iron oxide and oxyhydroxide nanoparticles are among the most important mobile and catalytic agents in a variety of biogeochemical environments, and are being increasingly synthesized for energy, electronic, catalyst, environmental and medical applications. The morphologies at nanoscale are relevant to the control of shapes and sizes, surface chemistry, and performance of these nanoparticles, as well as our understanding of naturally occurring processes. Therefore, we have begun to develop this understanding by studying the relationship between size, shape, and thermodynamic stability of unpassivated hematite (α-Fe2O3) and goethite (α-FeOOH) nanoparticles, using a robust thermodynamic morphology model with input parameters from reliable first-principles calculations and thermochemical data. The results revealed the thermodynamic stable shapes of hematite and goethite nanoparticles, and demonstrated that the phase transformation from goethite to hematite is highly dependent on the particle size and temperature. Goethite nanoparticles are thermodynamically stable with small sizes, compared to hematite, but the equilibrium transformation temperature increases rapidly with decreasing particle size. The morphology sensitive phase transformation predicted by our model is a step further towards a nanophase diagram of iron oxides and oxyhydroxides.