Showing papers on "Magnetite published in 2008"
TL;DR: This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.
Abstract: We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.
423 citations
TL;DR: In this article, a new and highly active heterogeneous Fenton system based on iron metal and magnetite Fe 0 /Fe 3 O 4 composites has been prepared by controlled reduction of iron oxides.
Abstract: In this work, a new and highly active heterogeneous Fenton system based on iron metal and magnetite Fe 0 /Fe 3 O 4 composites has been prepared by controlled reduction of iron oxides. Temperature-programmed reduction experiments with H 2 showed that iron oxides, i.e. Fe 2 O 3 , FeOOH and Fe 3 O 4 , can be reduced to produce highly reactive Fe 0 /Fe 3 O 4 composites with different metal to oxide ratios as determined by Mossbauer spectroscopy and powder X-ray diffraction. Mossbauer measurements revealed that these composites are reactive towards gas phase molecules and can be oxidized rapidly by O 2 even at room temperature. The composites showed also very high activity for the Fenton chemistry, i.e. the oxidation of an organic model contaminant, the dye methylene blue, and the H 2 O 2 decomposition. The best results were obtained with the composites with 47% Fe 0 obtained by reduction of Fe 3 O 4 with H 2 at 400 °C for 2 h, which produced a very rapid discoloration with total organic carbon (TOC) removal of 75% after 2 h reaction. Conversion electron Mossbauer spectroscopy (CEMS) measurements before and after H 2 O 2 reaction showed that Fe 3 O 4 and especially Fe 0 are oxidized during the reaction. The reaction mechanism is discussed in terms of the formation of HO radicals by a Haber–Weiss initiated by an efficient electron transfer from the composite Fe 0 /Fe 3 O 4 to H 2 O 2 . The higher activity of the composites compared to the pure Fe 0 and iron oxides has been explained by two possible effects, i.e. (i) a thermodynamically favorable electron transfer from Fe 0 to Fe 3 O 4 producing Fe 2+ magnetite active for the reaction and (ii) by the formation of very reactive small particle size Fe 0 and Fe 3 O 4 .
310 citations
TL;DR: In this paper, the effect of surface inorganic−organic interactions on magnetic and structural properties of iron oxide magnetic nanoparticles functionalized by lipophilic stilbene molecules has been investigated.
Abstract: The effect of surface inorganic−organic interactions on magnetic and structural properties of iron oxide magnetic nanoparticles functionalized by lipophilic stilbene molecules has been investigated. The molecules have been grafted through either phosphonate or carboxylate coupling agents. Mossbauer spectra recorded at 300 and 77K suggest a global composition of Fe2.82O4 for the two types of functionalization. Complementary in-field Mossbauer and SQUID measurements have demonstrated that the nanoparticles consist in a magnetite core surrounded by an oxidized layer. The oxidized shell exhibits a spin canting in the carboxylate case leading to a decrease of the net magnetization of the oxide nanoparticle. No canting occurs in the phosphonate case, and the magnetic properties are therefore preserved. The magnetic properties thus depend on the coupling agent, e.g., surface interactions. This result is of primary importance to tune the magnetic properties of functionalized nanoparticles for biomedical and high ...
294 citations
TL;DR: The voluminous 2.5-Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling.
289 citations
TL;DR: In this article, the effect of different parameters on the nature and size of the particles as well as the mechanism of formation of the nanoparticles have been studied by different techniques and it was concluded that, under the electrochemical conditions used in this work, the NP mean size was found to be constant at around 20nm when the electrooxidation current density is increased from 10 to 200μm −2.
287 citations
TL;DR: In this paper, a method for the preparation of uniform magnetite nanoparticles with sizes around 30nm and stable in aqueous media at pH 7 has been developed, based on the precipitation of an iron (II) salt (FeSO4) in the presence of a base (NaOH) and a mild oxidant (KNO3).
Abstract: A direct method for the preparation of uniform magnetite nanoparticles with sizes around 30?nm and stable in aqueous media at pH 7 has been developed. This method is based on the precipitation of an iron (II) salt (FeSO4) in the presence of a base (NaOH) and a mild oxidant (KNO3). Reaction rate seems to be controlled by the iron salt concentration and the presence of ethanol in the media. Thus lower iron concentration and a water/ethanol ratio equal to one lead to the formation of the smallest particles, 30?nm in diameter. Colloidal suspensions of these particles were directly obtained by simple ultrasonic treatment of the powders leading to very stable ferrofluids at pH 7. Sulphate anions present at the particle surface seem to be responsible for the colloidal stability, providing a biocompatible character to the suspensions. The structural, morphological and magnetic characterization of the nanoparticles is also described and suggests that the smallest particles have a diameter close to the limit between monodomain?multidomain magnetic structure, which could account for the high powder absorption of magnetic fields. According to this calorimetric experiments resulted in specific power absorption rates of ca 80?95?W?g?1, which are among the highest values reported in the literature and make these nanoparticles very interesting for hyperthermia.
254 citations
TL;DR: In this paper, a template-free solvothermal route was used to synthesize uniform-sized, monodisperse, and single-crystal magnetite hollow spheres with a diameter of 200−300 nm and a shell thickness of ∼50 nm.
Abstract: Uniform-sized, monodisperse, and single-crystal magnetite hollow spheres with a diameter of 200−300 nm and a shell thickness of ∼50 nm have been successfully synthesized in high yield using a template-free solvothermal route. The reaction duration and the ethylenediamine amount are shown to play important roles in the formation of the magnetite hollow spheres. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier tranform IR, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM were used to characterize the products. The results show that the prepared single-crystalline hollow spheres are composed of well-aligned magnetite and have a cubic structure. The magnetite hollow spheres show a high saturation magnetization of ca. 68 emu/g, a remnant magnetization of ca. 13 emu/g, and a coercivity of ca. 94 Oe at room temperature. A possible mechanism for the formation of magnetite hollow spherical structures is proposed based on the experimental observations. The pr...
233 citations
TL;DR: In this paper, the adsorption of arsenite and arsenate to siderite, fougerite and magnetite using batch sorption experiments and EXAFS spectroscopy was studied.
226 citations
TL;DR: It is shown here that selenite is rapidly reduced within one day by nanoparticulate mackinawite and magnetite, while only one third of selenites is reduced by micrometer-sized siderite.
Abstract: Suboxic soils and sediments often contain the Fe(II)-bearing minerals mackinawite (FeS), siderite (FeCO3) or magnetite (Fe3O4), which should be able to reduce aqueous selenite, thereby forming solids of low solubility. While the reduction of selenate or selenite to Se(0) by green rust, pyrite and by Fe2+ sorbed to montmorillonite is a slow (weeks), kinetically limited redox reaction as demonstrated earlier, we show here that selenite is rapidly reduced within one day by nanoparticulate mackinawite and magnetite, while only one third of selenite is reduced by micrometer-sized siderite. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, red and gray elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2), was not observed. The local structures of the reaction products suggest formation of nanoscale clusters, which may be prone to colloid-facilitated transport, and may have a hig...
220 citations
TL;DR: In this paper, the magnetite particle size is controlled by variation of pH and temperature and the increase of oxidation increases with decreasing particle size, and the variation of magnetite saturation magnetizations with particle size was interpreted using a magnetic core-shell particle model.
197 citations
TL;DR: The bacterium Actinobacter sp.
Abstract: The bacterium Actinobacter sp. has been shown to be capable of extracellularly synthesizing iron based magnetic nanoparticles, namely maghemite (γ-Fe2O3) and greigite (Fe3S4) under ambient conditions depending on the nature of precursors used. More precisely, the bacterium synthesized maghemite when reacted with ferric chloride and iron sulfide when exposed to the aqueous solution of ferric chloride-ferrous sulfate. Challenging the bacterium with different metal ions resulted in induction of different proteins, which bring about the specific biochemical transformations in each case leading to the observed products. Maghemite and iron sulfide nanoparticles show superparamagnetic characteristics as expected. Compared to the earlier reports of magnetite and greigite synthesis by magnetotactic bacteria and iron reducing bacteria, which take place strictly under anaerobic conditions, the present procedure offers significant advancement since the reaction occurs under aerobic condition. Moreover, reaction end p...
TL;DR: In this article, the first part of a global study on the oxidation process of a Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi.
TL;DR: The sorption mechanism proposed is in accordance with a ligand exchange reaction where both monodentate and bidentate complexes could exist at low surface loading level, the relative proportion of the complexes being due to both pH and concentration in solution.
TL;DR: In this article, the temperature-dependent Fe isotope fractionation between fayalite and magnetite is described by 10 3 ln α 57 Mag-Fa = 0.30 (+/− ǫ 0.024) × 10 6 / T 2.
TL;DR: A series of modified ferrites were prepared by doping iron oxide with various transition/non-transition/innertransition metal ions [M = Cr, Mn, Co, Ni, Cu, Zn and Ce] in situ during synthesis as discussed by the authors.
Abstract: A series of modified ferrites were prepared by doping iron oxide with various transition/non-transition/inner-transition metal ions [M = Cr, Mn, Co, Ni, Cu, Zn and Ce] in situ during synthesis. All the modified ferrites thus obtained exhibit remarkably high surface areas, greater than that of pure iron oxide (Fe2O3) sample. The efficacy of the dopant ions in modifying the resultant specific surface area, could be directly related to variations in the rate of crystal growth. The nature and concentration of the foreign cations present in the system govern this variation. Interestingly all the modified ferrites, exhibit a narrow pore size distribution in the range of 4.9–25 nm. XRD analysis revealed the existence of hematite (Fe2O3) phase in all the as-prepared samples. The X-ray diffraction experiments performed on activated catalysts, confirmed the existence of magnetite (Fe3O4) phase with a nominal composition of Fe2.73M0.27O4. These inverse or mixed spinels with general formula A(1−δ)Bδ[AδB(2−δ)]O4, possess highly facile Fe3+ ⇔ Fe2+ redox couple, the degree of facileness depends on the extent of synergistic interaction between iron and the other substitutent metal ion. The rapid electron hopping between Fe3+ ⇔ Fe2+ in the Fe3O4 lattice system is essential to catalyze WGS reaction. From TPR it was observed that, incorporation of metal cations into the hematite (α-Fe2O3) crystal structure alters the reducibility of the hematite particles, which in turn depends on the nature of the incorporated metal cation. A plausible explanation for the WGS activity over various modified ferrites has been attempted with the help of TPR analysis.
TL;DR: In this paper, the authors measured the d 56 Fe values of bulk granitic and volcanic rocks, as well as magnetite and Fe silicates, and showed that loss of a low-d 56 Fe ferrous chloride fluid is the most likely explanation for the high d 56Fe values in the bulk rocks.
TL;DR: In this paper, the second part of a global study on the oxidation process of an Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi.
TL;DR: In this article, a dry-jet wet spinning process for making magnetically active cellulose fibers has been developed using the ionic liquid (IL) 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl).
Abstract: A dry-jet wet spinning process for making magnetically active cellulose fibers has been developed using the ionic liquid (IL) 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl). Cellulose from different sources with various degrees of polymerization (DP) was used for making fibers by first dissolving the cellulose in the IL, dispersing particles of magnetite in the solution, and then coagulating the fibers in a water bath under appropriate spinning conditions. The variation of fiber properties with cellulose source and concentration of magnetite is discussed. Fiber texture was found to be related to overall magnetite concentration, cellulose concentration, and molecular weight in the spinning solution. In general, it was found that increasing DP and/or cellulose concentration resulted in more robust fibers, and conversely the addition of magnetite particles weakened the overall mechanical properties.
TL;DR: Results indicate a ferrihydrite-magnetite core-shell structure and the presence of magnetite in the ferritin cores of patients with Alzheimer's disease is not a cause of their increased brain iron(II) concentration.
Abstract: Transmission Electron Microscopy (TEM), X-ray Absorption Near Edge Spectroscopy (XANES), Electron Energy-Loss Spectroscopy (EELS), Small-Angle X-ray Scattering (SAXS), and SQUID magnetic studies were performed in a batch of horse spleen ferritins from which iron had been gradually removed, yielding samples containing 2200, 1200, 500, and 200 iron atoms. Taken together, findings obtained demonstrate that the ferritin iron core consists of a polyphasic structure (ferrihydrite, magnetite, hematite) and that the proportion of phases is modified by iron removal. Thus, the relative amount of magnetite in ferritin containing 2200 to 200 iron atoms rose steadily from approximately 20% to approximately 70% whereas the percentage of ferrihydrite fell from approximately 60% to approximately 20%. These results indicate a ferrihydrite-magnetite core-shell structure. It was also found that the magnetite in the ferritin iron core is not a source of free toxic ferrous iron, as previously believed. Therefore, the presence of magnetite in the ferritin cores of patients with Alzheimer's disease is not a cause of their increased brain iron(II) concentration.
TL;DR: In this paper, thermal volume changes and oxidation mechanisms in magnetite iron ore green pellets balled with 0.5% bentonite binder, as a function of raw material fineness and pellet porosity, are shown.
TL;DR: In this paper, four mixed oxides of iron and silica including quartz/amorphous iron(III) oxide (Q 1), quartz/maghemite (Q 2 ), quartz/Maghemite(Q 3 ) and quartz/goethite (q 4 ) were prepared and characterized by XRD, BET surface area and chemical analyses.
TL;DR: The results indicate the possibility that sensory cells in bats contain freely rotating magnetite particles, which appears not to be the case in birds, and suggest that big brown bats use magnetite to detect the magnetic field as a compass.
Abstract: While the role of magnetic cues for compass orientation has been confirmed in numerous animals, the mechanism of detection is still debated. Two hypotheses have been proposed, one based on a light dependent mechanism, apparently used by birds and another based on a “compass organelle” containing the iron oxide particles magnetite (Fe3O4). Bats have recently been shown to use magnetic cues for compass orientation but the method by which they detect the Earth's magnetic field remains unknown. Here we use the classic “Kalmijn-Blakemore” pulse re-magnetization experiment, whereby the polarity of cellular magnetite is reversed. The results demonstrate that the big brown bat Eptesicus fuscus uses single domain magnetite to detect the Earths magnetic field and the response indicates a polarity based receptor. Polarity detection is a prerequisite for the use of magnetite as a compass and suggests that big brown bats use magnetite to detect the magnetic field as a compass. Our results indicate the possibility that sensory cells in bats contain freely rotating magnetite particles, which appears not to be the case in birds. It is crucial that the ultrastructure of the magnetite containing magnetoreceptors is described for our understanding of magnetoreception in animals.
TL;DR: In this paper, the tridentate hexanuclear As(III)O3 pyramids occupied vacant tetrahedral sites on {1 1 1 1} surfaces of magnetite particles.
TL;DR: The present results yield useful constraints on As(V) and As(III) adsorption on high surface-area powdered maghemite, which may help in modeling the behavior of arsenic at the Maghemite-water interface.
Abstract: Arsenic sorption onto maghemite potentially contributes to arsenic retention in magnetite-based arsenic removal processes because maghemite is the most common oxidation product of magnetite and may...
TL;DR: In this paper, two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of the ferrous ions with oxalic acid: α-Fe(C2O4) · 2H2O with a monoclinic unit cell, obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature.
Abstract: Two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of ferrous ions with oxalic acid: α-Fe(C2O4) · 2H2O with a monoclinic unit cell is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be tailored from prismatic crystals of the α-polymorph over star-like aggregates of α/β-mixtures to non-agglomerated crystallites of β-oxalate. Thermal decomposition in air gives hematite at T ≥ 250 °C; if the thermolysis reaction is performed at low oxygen partial pressures (e.g., T = 500 °C and pO2 = 10−25 atm) magnetite is obtained. The synthesized magnetite is stoichiometric as signaled by lattice parameters of a0 = 8.39 A. The thermal decomposition of ferrous oxalate is monitored by thermal analysis, XRD, and IR-spectroscopy. The morphology of the oxalate crystals is preserved during thermal decomposition; the oxalates are transformed into spinel particle aggregates of similar size and shape. The crystallite size of the magnetite particles increases with temperature and is 40 or 55 nm, if synthesized from β-oxalate at 500 °C or 700 °C, respectively. The saturation magnetization of the magnetite particles decreases with decreasing particle size. Since the particles are larger than the critical diameter for superparamagnetic behavior they display hysteresis behavior at room temperature.
TL;DR: In this article, a surface-initiated atom transfer radical polymerization (ATRP) was carried out on magnetite nanoparticles by using diblock copolymer brushes consisting of poly(ethylene glycol) methacrylate blocks.
TL;DR: In this paper, a set of samples with defined ratios of haematite and magnetite were used for detailed rock magnetic investigations and the measured parameters provided a reference for interpreting common rock magnetic parameters in investigations of sediments.
Abstract: SUMMARY
Detailed rock magnetic investigations were carried out on a set of samples with defined ratios of haematite and magnetite. The measured parameters provide a reference for interpreting common rock magnetic parameters in investigations of sediments. The contribution of haematite to the magnetic fraction must exceed 95 wt-% of the magnetic fraction when mixed with magnetite in order to visibly influence grain size and coercitivity indicative magnetic parameters. Coercivity of remanence (BCR) and coercive force (BC) do not change in the same way with increasing haematite content, which results in a peak BCR/BC value at around 99.5 wt-% haematite. Variations in haematite content can be ignored when interpreting most rock magnetic parameters, especially grain size indicative parameters for samples where haematite contents range from 0 to 98 wt-%. The S-ratio is still the most sensitive parameter for estimating the relative amount of haematite in magnetite/haematite mixtures. A combination of S-ratio, the saturation isothermal remanent magnetization divided by the low field magnetic susceptibility (SIRM/κLF) and BCR is the most effective way to identify haematite in natural samples. Our results agree with literature data and fill the gap between results obtained either from pure magnetite or haematite with comparable grain sizes.
Journal Article•
TL;DR: It is shown that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature, and the signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage.
TL;DR: In this paper, groundmass spinel grains in 46 kimberlite and related rocks have been analyzed and compared, and the majority of the spinel analyses are classified as high-chromium chromite (Chr) and magnesioulvo« spinel^magnetite (Mum) and represent two significant stages of spinel growth.
Abstract: Groundmass spinel grains in 46 kimberlite and related rocks have been analyzed and compared. The majority of the spinel analyses are classified as high-chromium chromite (Chr) and magnesioulvo« spinel^magnetite (Mum) and represent two significant stages of spinel growth. There are also a significant number of spinel grains that are classified as xenocryst spinel (Xen), pleonaste spinel (Ple) and magnetite (Mag). Eight different spinel zoning trends are identified.The majority of the Chr spinel grains are interpreted as a primary phase that crystallized as small octahedra from kimberlite magma on the journey from the upper mantle to the final resting place in the upper crust. Three zoning trends lead directly away from primary chromite. The major zoning trend, Trend 1, is from chromite to magnesio-ulvo« spinel^magnetite. This zoning trend is unique to spinel in kimberlite, carbonatites and lamprophyres. We suggest that this somewhat oxidizing, and more magnesian, trend was influenced by the high carbonate content of Group I kimberlites and the rapid crystallization of the minerals during the evolution of volatiles.The zoningTrend 2 involves increasing titanium and ferric iron as a function of increasing Fe 2þ /(Fe 2þ þ Mg). This trend is similar to the zoning of spinel in basalt and is thought to be due to co-crystallization of magnesium- and aluminum-rich silicate minerals such as olivine and phlogopite in kimberlites, or pyroxene and plagioclase in basalt. Zoning Trend 3 in kimberlite leads away from primary chromite and towards an aluminous pleonaste (Ple) spinel. This trend is characterized by a large decease of Cr/(Cr þAl) parallel to so-called olivine^spinel iso-potential lines. Similar trends of lesser magnitude and cyclic Al^Cr zoning have been identified in basaltic spinel. This trend is thought to be due to very rapid crystallization under conditions of supersaturation where the crystallization of spinel affects the local environment ahead of the growing spinel crystal (i.e. diffusion-controlled crystallization). The tendency for immiscibility between ferrite- or titanate-rich spinel, and aluminate-rich spinel (pleonaste) has a great influence on Trends 1 and 3 zoning and also on atoll-spinel formation. Very local conditions such as nucleation, or lack of nucleation, of other minerals can influence both the textural environment and composition of kimberlitic spinel.
TL;DR: In this paper, two fly ash samples from a black coal-fired power plant (Bexbach, Germany) were investigated for their magnetic properties, particle structure, grain-size distribution and chemical composition.