Showing papers on "Hematite published in 2010"

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

Abstract: 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.

Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach

Abstract: Sustainable hydrogen production through photoelectrochemical water splitting using hematite (α-Fe2O3) is a promising approach for the chemical storage of solar energy, but is complicated by the material’s nonoptimal optoelectronic properties. Nanostructuring approaches have been shown to increase the performance of hematite, but the ideal nanostructure giving high efficiencies for all absorbed light wavelengths remains elusive. Here, we report for the first time mesoporous hematite photoelectodes prepared by a solution-based colloidal method which yield water-splitting photocurrents of 0.56 mA cm−2 under standard conditions (AM 1.5G 100 mW cm−2, 1.23 V vs reversible hydrogen electrode, RHE) and over 1.0 mA cm−2 before the dark current onset (1.55 V vs RHE). The sintering temperature is found to increase the average particle size, and have a drastic effect on the photoactivity. X-ray photoelectron spectroscopy and magnetic measurements using a SQUID magnetometer link this effect to the diffusion and incorp...

Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition

Abstract: Thin films of three iron oxide polymorphs, hematite, maghemite, and magnetite, were produced on KBr substrates using a conventional electron beam deposition technique coupled with thermal annealing. This method allowed for iron oxide thin films free from chemical precursor contaminants. The films were characterized using Fourier-transform infrared spectroscopy (FTIR), Raman microspectroscopy, and ellipsometry. These spectroscopic techniques allowed for a clear assignment of the phase of the iron oxide polymorph films produced along with an examination of the degree of crystallinity possessed by the films. The films produced were uniform in phase and exhibited decreasing crystallinity as the thickness increased from 40 to 250 nm.

Controlling Photoactivity in Ultrathin Hematite Films for Solar Water-Splitting

Abstract: A promising route to increase the performance of hematite (alpha-Fe2O3) photoelectrodes for solar hydrogen production through water-splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm(-2) at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiOx is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum-dot and extremely-thin-absorber (ETA)-type solar cells.

Decoupling feature size and functionality in solution-processed, porous hematite electrodes for solar water splitting.

Abstract: We introduce a simple solution-based strategy to decouple morphological and functional effects of annealing nanostructured, porous electrodes by encapsulation with a SiO2 confinement scaffold before high temperature treatment. We demonstrate the effectiveness of this approach using porous hematite (α-Fe2O3) photoanodes applied for the storage of solar energy via water splitting and show that the feature size and electrode functionality due to dopant activation can be independently controlled. This allows a significant increase in water oxidation photocurrent from 1.57 mA cm−2 (in the control case) to 2.34 mA cm−2 under standard illumination conditions in 1 M NaOH electrolyte—the highest reported for a solution-processed hematite photoanode. This increase is attributed to the improved quantum efficiency, especially with longer wavelength photons, due to a smaller particle size, which is afforded by our encapsulation strategy.

Continuous Shape- and Spectroscopy-Tuning of Hematite Nanocrystals

Abstract: Uniform hexagonal hematite (α-Fe2O3) nanoplates have been synthesized by a facile alcohol-thermal reaction, and a new nanostructure of α-Fe2O3 has been proposed. Each nanoplate is enclosed by (0001) basal planes and {1012} side surfaces. The phase, size, shape, and growth orientation of these nanocrystals were characterized by powder X-ray diffraction and electron microscopy. The thickness and diameter of these nanocrystals could be finely tuned by the selective use of alcohol solvent with increasing carbon atom number in the linear alkyl chain. A variety of nanocrystals with systemically changeable shapes from nanoplates to nanograins have been obtained. Specific adsorption of alcohol molecules on polar (0001) facets is proposed to be the main issue to modify the growth behavior of hematite nanocrystals. The presence of distilled water and the addition of sodium acetate have also been investigated. Either of them has a great influence on the growth of hematite nanocrystals, and shape-controlled growth c...

On the goethite to hematite phase transformation

Abstract: This study deals with some microstructural and crystallographic aspects of the thermally induced transformation of goethite (α-FeOOH) into hematite (α-Fe2O3), occurring at about 300 °C. Powder specimens of goethite have been annealed in air at different temperatures, ranging from 200 °C up to 1,000 °C. The resulting products have been analyzed for a complete characterization of the changes brought about by the thermal treatments, using a multianalytical approach, based on: thermogravimetry, differential thermal analysis, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. At lower temperatures, the transition to hematite produces no important changes in size and shape of the original goethite grains. Recrystallization, and partial sintering, occurs only at temperatures in excess of 800 °C. The relevant evolution of pores present in both phases has been also considered, as it may provide important indications on the actual formation mechanism of hematite.

Synthesis, Properties, and Applications of Magnetic Iron Oxide Nanoparticles

Abstract: Magnetic nanoparticles exhibit many interesting properties that can be exploited in a variety of applications such as catalysis and in biomedicine. This review discusses the properties, applications, and syntheses of three magnetic iron oxides – hematite, magnetite, and maghemite – and outlines methods of preparation that allow control over the size, morphology, surface treatment and magnetic properties of their nanoparticles. Some challenges to further development of these materials and methods are also presented.

Large-Scale and Controlled Synthesis of Iron Oxide Magnetic Short Nanotubes: Shape Evolution, Growth Mechanism, and Magnetic Properties

Abstract: We present a facile approach to the production of magnetic iron oxide short nanotubes (SNTs) employing an anion-assisted hydrothermal route by simultaneously using phosphate and sulfate ions. The size, morphology, shape, and surface architecture control of the iron oxide SNTs are achieved by simple adjustments of ferric ions concentration without any surfactant assistance. The result of a formation mechanism investigation reveals that the ferric ions concentrations, the amount of anion additive, and the reaction time make significant contributions to SNT growth. The shape of the SNTs is mainly regulated by the adsorption of phosphate ions on faces parallel to the long dimension of elongated α-Fe2O3 nanoparticles (c axis) during nanocrystal growth, and the hollow structure is given by the preferential dissolution along the c axis due to the strong coordination of the sulfate ions. Moreover, the as-synthesized hematite (α-Fe2O3) SNTs can be converted to magnetite (Fe3O4) and maghemite (γ-Fe2O3) ferromagneti...

Connecting Observations of Hematite (α-Fe2O3) Growth Catalyzed by Fe(II)

Abstract: Electron exchange between aqueous Fe(II) and structural Fe(III) in iron oxides and oxyhydroxides is important for understanding degradation of environmental pollutants through its apparent constitutive role underlying highly reactive “sorbed Fe(II)” and by catalyzing phase interconversion among these minerals. Although a mechanistic understanding of relationships between interfacial Fe(II)ads−Fe(III)oxide electron transfer, bulk electron conduction, Fe(II) release, and phase transformation behavior is emerging, much remains unclear, in part due to poorly interconnected investigations. The focus of this study is on reconciling two mutually similar observations of Fe(II)-catalyzed hematite growth documented spectroscopically and microscopically under substantially different chemical conditions. Here, we employ iron isotopic labeling to demonstrate that hematite grown on the (001) surface in Fe(II)−oxalate solution at pH 2.10 and 348 K has magnetic properties that closely correspond to those of hematite grow...

Biomineralization by Gallionella

Abstract: A new environmental scanning electron microscopic (ESEM) technique at low vacuum (5 torr) and 99% humidity, where the sample never has been exposed to high vacuum and coating of carbon or gold, has revealed a new insight into the nature of iron mineralization that develops in association with the stalked bacteria Gallionella. The stalk fibers contain minute flaky iron precipitates. The size of the crystallites is 0.1–0.5 micron and some of them exhibit a hexagonal feature. EDAX analyses on individual crystallites give an atomic ratio between Fe and O very close to 0.67. The stoichiometric formula would thus be Fe2O3. Stoichiometry and crystallinity are in accordance with the mineral hematite. The mineralization seems to take place inside the fibers of the stalk. With time the Gallionella stalk is covered with iron oxihydroxides of different kinds that probably are controlled by inorganic processes more than by the organic chemistry of the stalk. From a thermodynamic point of view, oxygen as well as carbon...

Iron Oxidation at Low Temperature (260-500 C) in Air and the Effect of Water Vapor

Abstract: The oxidation of iron has been studied at low temperatures (between 260 and 500 °C) in dry air or air with 2 vol% H2O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. 18O tracer experiments performed at 400 °C allowed to propose a growth mechanism of the scale.

Nanosized Iron Oxide Colloids Strongly Enhance Microbial Iron Reduction

Abstract: Microbial iron reduction is considered to be a significant subsurface process. The rate-limiting bioavailability of the insoluble iron oxyhydroxides, however, is a topic for debate. Surface area and mineral structure are recognized as crucial parameters for microbial reduction rates of bulk, macroaggregate iron minerals. However, a significant fraction of iron oxide minerals in the subsurface is supposed to be present as nanosized colloids. We therefore studied the role of colloidal iron oxides in microbial iron reduction. In batch growth experiments with Geobacter sulfurreducens, colloids of ferrihydrite (hydrodynamic diameter, 336 nm), hematite (123 nm), goethite (157 nm), and akaganeite (64 nm) were added as electron acceptors. The colloidal iron oxides were reduced up to 2 orders of magnitude more rapidly (up to 1,255 pmol h 1 cell 1 ) than bulk macroaggregates of the same iron phases (6 to 70 pmol h 1 cell 1 ). The increased reactivity was not only due to the large surface areas of the colloidal aggregates but also was due to a higher reactivity per unit surface. We hypothesize that this can be attributed to the high bioavailability of the nanosized aggregates and their colloidal suspension. Furthermore, a strong enhancement of reduction rates of bulk ferrihydrite was observed when nanosized ferrihydrite aggregates were added. Dissimilatory iron reduction is an important anaerobic respiration process in anoxic subsurface environments. However, the reactivity of ferric iron is mostly limited by the reduction kinetics of the poorly soluble, extracellular iron minerals. Electron transfer from microorganisms to iron oxides can occur via direct contact or by electron shuttling compounds (46). Trans

Photoreductive dissolution of iron oxides trapped in ice and its environmental implications.

Abstract: The availability of iron has been thought to be a main limiting factor for the productivity of phytoplankton and related with the uptake of atmospheric CO_2 and algal blooms in fresh and sea waters. In this work, the formation of bioavailable iron (Fe(II)_(aq)) from the dissolution of iron oxide particles was investigated in the ice phase under both UV and visible light irradiation. The photoreductive dissolution of iron oxides proceeded slowly in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice, subsequently releasing more bioavailable ferrous iron upon thawing. The enhanced photogeneration of Fe(II)_(aq) in ice was confirmed regardless of the type of iron oxides [hematite, maghemite (γ-Fe_2O_3), goethite (α-FeOOH)] and the kind of electron donors. The ice-enhanced dissolution of iron oxides was also observed under visible light irradiation, although the dissolution rate was much slower compared with the case of UV radiation. The iron oxide particles and organic electron donors (if any) in ice are concentrated and aggregated in the liquid-like grain boundary region (freeze concentration effect) where protons are also highly concentrated (lower pH). The enhanced photodissolution of iron oxides should occur in this confined boundary region. We hypothesized that electron hopping through the interconnected grain boundaries of iron oxide particles facilitates the separation of photoinduced charge pairs. The outdoor experiments carried out under ambient solar radiation of Ny-Alesund (Svalbard, 78°55′N) also showed that the generation of dissolved Fe(II)_(aq) via photoreductive dissolution is enhanced when iron oxides are trapped in ice. Our results imply that the ice(snow)-covered surfaces and ice-cloud particles containing iron-rich mineral dusts in the polar and cold environments provide a source of bioavailable iron when they thaw.

Iron hydroxides in soils: A review of publications

Abstract: Iron hydroxides are subdivided into thermodynamically unstable (ferrihydrite, feroxyhyte, and lepidocrocite) and stable (goethite) minerals. Hydroxides are formed either from Fe3+ (as ferrihydrite) or Fe2+ (as feroxyhyte and lepidocrocite). The high amount of feroxyhyte in ferromanganic concretions is proved, which points to the leading role of variable redox conditions in the synthesis of hydroxides. The structure of iron hydroxides is stabilized by inorganic elements, i.e., ferrihydrite, by silicon; feroxyhyte, by manganese; lepidocrocite, by phosphorus; and goethite, by aluminum. Ferrihydrite and feroxyhyte are formed with the participation of biota, whereas the abiotic formation of lepidocrocite and goethite is possible. The iron hydroxidogenesis is more pronounced in podzolic soils than in chernozems, and it is more pronounced in iron-manganic nodules than in the fine earth. Upon the dissolution of iron hydroxides, iron isotopes are fractioned with light-weight 54Fe atoms being dissolved more readily. Unstable hydroxides are transformed into stable (hydr)oxides, i.e., feroxyhyte is spontaneously converted to goethite, and ferrihydrite, to hematite or goethite.

Bioreduction of Nitrobenzene, Natural Organic Matter, and Hematite by Shewanella putrefaciens CN32

Abstract: We examined the reduction of nitrobenzene by Shewanella putrefaciens CN32 in the presence of natural organic matter (NOM) and hematite. Bioreduction experiments were conducted with combinations and varied concentrations of nitrobenzene, soil humic acid, Georgetown NOM, hematite, and CN32. Abiotic experiments were conducted to quantify nitrobenzene reduction by biogenic Fe(II) and by bioreduced NOMs. We show that S. putrefaciens CN32 can directly reduce nitrobenzene. Both NOMs enhanced nitrobenzene reduction and the degree of enhancement depended on properties of the NOMs (aromaticity, organic radical content). Hematite enhanced nitrobenzene reduction by indirect reaction with biogenic-Fe(II), however, enhancement was dependent on the availability of excess electron donor. Under electron donor-limiting conditions, reducing equivalents diverted to hematite were not all transferred to nitrobenzene. In systems that contained both NOM and hematite we conclude that NOM-mediated reduction of nitrobenzene was more important than Fe(II)-mediated reduction.

Vertically aligned and ordered hematite hierarchical columnar arrays for applications in field-emission, superhydrophilicity, and photocatalysis

Abstract: We achieved the vertically aligned and ordered hematite hierarchical columnar arrays using pulsed laser deposition (PLD) with polystyrene (PS) colloid spheres as templates Crystallized hematite hexagonal-close-packed columnar arrays are fabricated at room temperature and can be transformed to hexagonal-non-close-packed columnar arrays at an annealing temperature of 450 °C The hierarchical columns consist of assembled nanoplates or nanoparticles, depending on the oxygen pressures during the PLD process The improved performance in field emission (FE), surface wetting, and photocatalytic properties of the hematite hierarchical columnar arrays is due to the unique morphology of the superstructures and controlled compositions

Determining the modal mineralogy of Martian soils

Abstract: [1] A method for identifying the major and minor minerals and quantifying their proportions (by weight) in soils at the Mars Exploration Rovers landing sites is presented. The procedure utilizes modeled Mossbauer data for iron-bearing minerals, chemical calculations of Alpha Particle X-ray Spectrometer data for iron-absent minerals, and modeling of MiniTES thermal infrared spectra for silica and clays. Two models are formulated, using different assumptions about the mineralogy of sulfur and chlorine: sulfate + chloride, or incorporated in or adsorbed onto schwertmannite + akaganeite. The actual soil mineralogy may be bracketed by these results. Using either model, average dark soils on the Gusev crater plains and on Meridiani Planum are shown to be composed of a mixture of igneous (olivine, pyroxene, plagioclase, Fe-Ti-Cr spinels, phosphate) and alteration (amorphous silica, hematite, nanophase oxides, clays, plus sulfate + chloride or oxysulfate + oxychloride) minerals. This assemblage suggests that soil alteration did not occur in situ and that the igneous and alteration components were likely derived from different sources. The (model dependent) mixing ratio of igneous and alteration components is identical at the two sites on opposite sides of the planet, implying that Martian soils may provide a representative mineralogical sampling of the exposed crust.