A study of nanostructures formed in the hydrogen reduction of Fe(OH)<Subscript>3</Subscript>

A complex study of the hydrogen reduction of nanosized iron hydroxide Fe(OH)₃ at 400°C was performed. It was shown that, during the reduction of Fe(OH)₃ to iron metal α-Fe, intermediate compounds such as Fe(OH)₂, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH, and FeO are formed along with stable iron oxides α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄. A scheme of chemical and structural transformations that occur in the reduction of nanosized Fe(OH)₃ is presented. The scheme takes into account the possibility of the bifurcation mechanism of reaction development.     

Oxidation of iron studied by PAC and CEMS

A metallic Fe specimen, implanted with ¹¹¹In, was oxidized and subsequently annealed in a high vacuum for PAC spectroscopy. This treatment gave rise to a huge PAC signal. The magnitude of the hyperfine field was found to be one third of that in metallic Fe. CEMS on an enriched Fe foil given exactly the same treatment has revealed that a maghemite phase (γ-Fe₂O₃) is formed right after the oxidation treatment and a magnetite phase (Fe₃O₄) after the vacuum annealing. ¹¹¹In in the magnetite phase was found to give rise to a PAC signal with large amplitude. PAC spectroscopy in an external magnetic field has revealed that the site of ¹¹¹In is the tetrahedral site of the magnetite with the hyperfine field of +12 T, which is in excellent agreement with those in the ferrites. The present method of oxidation of metallic Fe with nuclear probes in it is quite useful for the study of oxidation processes. Also, it provides us with a simple means to prepare ferrite specimens incorporated with nuclear probes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Detection of an α-Muonium-substituted methyl radical

Mössbauer spectroscopic and magnetic measurements have been made on a novel magnetic protein produced by the controlled reconstitution of ferritin. The data indicate that the predominant mineral form in the iron-containing cores is maghemite (-Fe₂O₃) rather than magnetite (Fe₃O₄).     

Synthesis of iron oxide nanoparticles under oxidizing environment and their stabilization in aqueous and non-aqueous media

Synthesis of magnetite (Fe₃O₄) nanoparticles under oxidizing environment by precipitation from aqueous media is not straightforward because Fe²⁺ gets oxidized to Fe³⁺ and thus the ratio of Fe³⁺:Fe²⁺=2:1 is not maintained during the precipitation. A molar ratio of Fe³⁺:Fe²⁺ smaller than 2:1 has been used by many to compensate for the oxidation of Fe²⁺ during the preparation. In this work, we have prepared iron oxide nanoparticles in air environment by the precipitation technique using initial molar ratios Fe³⁺:Fe²⁺?2:1. The phases of the resulting powders have been determined by several techniques. It is found that the particles consist mainly of maghemite with little or no magnetite phase. The particles have been suspended in non-aqueous and aqueous media by coating the particles with a single layer and a bilayer of oleic acid, respectively. The particle sizes, morphology and the magnetic properties of the particles and the ferrofulids prepared from these particles are reported. The average particle sizes obtained from the TEM micrographs are 14, 10 and 9 nm for the water, kerosene and dodecane-based ferrofluids, respectively, indicating a better dispersion in the non-aqueous media. The specific saturation magnetization (σ_s) value of the oleic-acid-coated particles (∼53 emu/g) is found to be lower than that for the uncoated particles (∼63 emu/g). Magnetization σ_s of the dodecane-based ferrofluid is found to be 10.1 emu/g for a volume fraction of particles ?=0.019. Zero coercivity and zero remanance on the magnetization curves indicate that the particles are superparamagnetic (SPM) in nature.     

An optimal low-temperature tartrate precursor method for the synthesis of monophasic nanosized ZnFe2O4

In this study, the synthesis of monophasic nanocrystalline zinc ferrite (ZnFe₂O₄) was achieved by controlling the thermal decomposition conditions of a zinc–iron tartrate precursor method. Differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Fe²⁺ content analysis, transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) techniques were used to investigate the effect of heat treatment conditions on the calcined powders. The thermal decomposition of the precursor led to an intermediate phase formation of ZnO, Fe₃O₄, and γ-Fe₂O₃. It was found that the Fe₃O₄ → γ-Fe₂O₃ oxidation reaction is the key step in producing monophasic nanosized ZnFe₂O₄. The monophasic nanoparticles of ZnFe₂O₄ can be obtained when the precursor is heat treated under a low temperature (300–400 °C) and long residence time (4 h) process that can prompt the Fe₃O₄ oxidation and prevent the formation of α-Fe₂O₃.     

NMR analysis of ferromagnets: Fe oxides

A short historical review is given on internal field NMR of ferromagnets, illustrated with recent pulsed NMR spectra of the elemental ferromagnets Fe, Co and Ni and the Fe-oxides magnetite, maghemite and hematite, which, with the exception of maghemite, have resonance frequencies first reported over 45 years ago. Since the magnetic hyperfine field at the nucleus is not known a priori, the original search frequency motivations are discussed along with the mechanisms for the initially much larger than expected (～10³) NMR signals that were observed. The ⁵⁷Fe spectra of the three principal Fe-oxide ferromagnets, magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), obtained here under uniform spectroscopic conditions, are then discussed in more detail, with a focus on the influence of particle size and vacancy content on the hyperfine fields     

Magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> surface

The magnetic properties of the magnetite Fe₃O₄(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe²⁺) and trivalent (Fe³⁺) ions on the Fe₃O₄(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.     

Assembly of γ-Fe2O3/polyaniline nanofilms with tuned dipolar interaction

The internal morphology and magnetic properties of layer-by-layer assembled nanofilms of polyaniline (PANI) and maghemite (γ-Fe₂O₃—7.5-nm diameter) were probed with cross-sectional transmission electron microscopy (TEM) and magnetization measurements (magnetic hysteresis loops, magnetization using zero-field cooled/field-cooled protocols, and ac magnetic susceptibility). Additionally, simulations of the as-produced samples were performed to assess both the nanofilm’s morphology and the corresponding magnetic signatures using the cell dynamic system (CDS) approach and Monte Carlo (MC) through the standard Metropolis algorithm, respectively. Fine control of the film thickness and average maghemite particle–particle within this magnetic structure was accomplished by varying the number of bilayers (PANI/γ-Fe₂O₃) deposited onto silicon substrates or through changing the concentration of the maghemite particles suspended within the colloidal dispersion sample used for film fabrication. PANI/γ-Fe₂O₃ nanofilms comprising 5, 10, 25 and 50 deposited bilayers displayed, respectively, blocking temperatures (T _B) of 30, 35, 39 and 40 K and effective energy barriers (ΔE/k _B) of 1.0 × 10³, 2.3 × 10³, 2.8 × 10³ and 2.9 × 10³ K. Simulation of magnetic nanofilms using the CDS model provided the internal morphology to carry on MC simulation of the magnetic properties of the system taking into account the particle–particle dipolar interaction. The simulated (using CDS) surface–surface particle distance of 0.5, 2.5 and 4.5 nm was obtained for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. The simulated (using MC) T _B values were 33.0, 30.2 and 29.5 K for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. We found the experimental (TEM and magnetic measurements) and the simulated data (CDS and MC) in very good agreement, falling within the same range and displaying the same systematic trend. Our findings open up new perspectives for fabrication of magnetic nanofilms with pre-established (simulated) morphology and magnetic properties.     

Growth and properties of epitaxial iron oxide layers

Epitaxial layers of iron oxides have been grown on a MgO(001) substrate by evaporating natural Fe or⁵⁷Fe from Knudsen cells in the presence of a NO₂ flow directed to the substrate. The resulting layers have been investigated in situ with LEED, RHEED, AES and XPS and ex situ with CEMS and ion beam analysis. For substrate temperatures between 200 and 400°C we observe RHEED oscillations during deposition, indicative of layer-by-layer growth. By adjusting the flux of NO₂ at the surface, all stable and metastable cubic phases in the Fe-O system could be grown: FeO (wustite), Fe₃O₄ (magnetite), -Fe₂O₃ (maghemite) and solid solutions between the latter two phases. Rutherford backscattering spectra show a relatively high minimum yield in the channel directions.     

Superparamagnetic silica nanoparticles with immobilized metal affinity ligands for protein adsorption

Superparamagnetic silica-coated magnetite (Fe₃O₄) nanoparticles with immobilized metal affinity ligands were prepared for protein adsorption. First, magnetite nanoparticles were synthesized by co-precipitating Fe²⁺ and Fe³⁺ in an ammonia solution. Then silica was coated on the Fe₃O₄ nanoparticles using a sol–gel method to obtain magnetic silica nanoparticles. The condensation product of 3-Glycidoxypropyltrimethoxysilane (GLYMO) and iminodiacetic acid (IDA) was immobilized on them and after charged with Cu²⁺, the magnetic silica nanoparticles with immobilized Cu²⁺ were applied for the adsorption of bovine serum albumin (BSA). Scanning electron micrograph showed that the magnetic silica nanoparticles with an average size of 190 nm were well dispersed without aggregation. X-ray diffraction showed the spinel structure for the magnetite particles coated with silica. Magnetic measurement revealed the magnetic silica nanoparticles were superparamagnetic and the saturation magnetization was about 15.0 emu/g. Protein adsorption results showed that the nanoparticles had high adsorption capacity for BSA (73 mg/g) and low nonspecific adsorption. The regeneration of these nanoparticles was also studied.     

Extraterrestrial magnetite studied by Mössbauer spectroscopy

The meteorite Orgueil is a carbonaceous chondrite of type CI. Carbonaceous chondrites contain Fe(III), Fe(II) and in some cases metallic iron, indicating that they are in a state far from thermodynamic equilibrium. In Orgueil about 40% of the iron is present in magnetite (Fe₃O₄). In this work a sample of magnetite grains extracted from Orgueil has been studied by Mössbauer spectroscopy. It has been found that the magnetic phase contains about 11% of maghemite and that the remaining magnetite has a vacancy concentration smaller than 0.006, corresponding to the formula Fe_2.994O₄.     

Enhanced magneto-optic activity of magnetite-based ferrofluids subjected to gamma irradiation

We report here the effect of γ-irradiation on the particle size and size distribution dependent spectroscopic and magneto-optic properties of ferrofluids, synthesized by a co-precipitation method. The X-ray diffraction (XRD) study exhibits magnetite (Fe₃O₄) phase of the particles while electron microscopic and dynamic light scattering (DLS) studies have predicted particle growth upon γ-irradiation. Further, Fourier transform infrared (FT-IR) spectroscopy studies ensured that no dissociation has occurred due to irradiation effect. As a consequence of magneto-optic behavior reflected in the Faraday rotation (FR) measurement, the Verdet constant increased from a value of 0.64×10⁻² for the pristine sample to 5.6×10⁻² deg/Gauss-cm for the sample irradiated with the highest dose (2.635 kGy). The substantial enhancement in the FR is assigned to the improvement in associated chaining effect owing to adequate particle growth where an increased stoichiometry variation of Fe²⁺/Fe³⁺ is assured.     

Characterization of iron oxide layers using Auger electron spectroscopy

Metals can form several kinds of oxides. Iron forms wustite (FeO), magnetite (FeO + Fe₂O₃ or Fe₃O₄) and haematite (Fe₂O₃). Iron oxides, especially magnetite, are used for insulation between the lamellas of an electromotor made of electromagnetic sheet. In this work, iron oxide layers were characterized on industrial samples of electromagnetic sheet by AES depth profile analysis, and iron oxides with known chemical composition were used as reference samples, i.e. a magnetite mineral and a standard haematite reference sample. The magnetite mineral was chosen because it can be found in nature in a very pure form. The selection of reference samples was also verified on samples with an oxide layer of known composition, which were prepared by sputter deposition. The composition of the sputtered oxide layers was analysed by the weight-gain method and Rutherford backscattering without the use of standard reference materials (SRM), and the results were then compared with those obtained by AES depth profile analysis.     

The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman micro‐spectroscopy and electrochemical polarisation. Part I: near‐resonance enhancement of the Raman spectra of iron oxide and oxyhydroxide compounds

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.     

Mössbauer studies in the spinel system coxfe3−xo4

Mössbauer spectra have been obtained for the cobalt-iron spinels having the general formula CO_xFe_3?xO₄ for the whole range of composition, materials from x = 0 to x = 2.76 having been studied. The spectra fall into three groups: (A) Those for spinels for which 0 < χ < 1 which show two six-peak sets of lines corresponding to exchanging Fe²⁺/Fe³⁺ and to Fe³⁺ iron; (B) those for spinels containing Fe³⁺ iron only which show magnetic splitting 1 < χ < 2; (C) those for spinels containing Fe³⁺ iron only which show quadrupole splitting but no magnetic splitting (x = 2.3, 2.5 and 2.8). The significance of the various spectra in relation to the structure of these mixed oxides is discussed.     

Investigation of metabolic changes in blood and tissue of mice γ-irradiated with sublethal doses by direct observation of EPR signals from Hb-NO complexes

The metabolic changes in probes of blood and tissue (spleen, liver and kidney) of mice under total γ-irradiation with the doses varied in the interval of 1–10 Gy at the dose rate of 0.073 Gy/min were studied in the early postirradiation period by ex vivo electron paramagnetic resonance (EPR). It was established that the impact with the lower dose rate leads to more intensive nitric monoxide biosynthesis in comparison with higher dose rates. In the early postirradiation period (from 2 up to 6 h), irradiation with doses higher than 2 Gy brings about an increase of the NO concentration and, hence, the appearance of nitrosyl complexes which were registered directly by EPR in blood and spleen. The observed line is identified as the signal from α-(Fe²⁺-NO)₂β(Fe³⁺)₂ or α-(Fe²⁺-NO) α(Fe²⁺)β(Fe³⁺)₂ complexes since the methemoglobin concentration also increases in comparison with the control level. The concentration of Hb-NO complexes in blood and spleen depends on the dose and individual radiosensitivity of the organism. Therefore, the intensity of the Hb-NO signal may serve as a criterion of the radiation injury level during the first hours after the irradiation. 30 h after the impact, the Hb-NO complexes were no longer detected. For the first day, the concentration of Fe³⁺-transferrin in blood increases with the dose and time passed after the irradiation. The intensity of the EPR signal from Fe³⁺-transferrin in blood may also serve as a measure of the radiation injury level.     

Optical emissions of products sputtered from Fe, Fe2O3 and Fe3O4 powders

Powder iron has been bombarded by a 5 keV Kr⁺ ions in a vacuum better than 10^-7 torr and under few 10^-6 torr ultra pure oxygen partial pressure. The optical spectra of the sputtered particles were recorded between 340.0 nm and 410.0 nm. These spectra exhibit discrete lines, which are attributed to neutral excited atoms of iron. Two iron oxides, namely hematite (Fe₂O₃)_{3}) and magnetite (Fe₃O₄)_{4}), in powder form, were studied under the same experimental conditions and identical lines were observed in the obtained spectra. The absolute intensities of the spectral lines in all spectra were measured and the differences in the recorded yield photons were discussed in term of electron-transfer processes between the excited sputtered atom and the bombarded surface. In accordance with the proposed interpretation, we suggest values for the energy gaps and electronic affinities for the studied oxides and for the oxide layer that might be formed by the adsorption of oxygen atoms.     

Microwave-assisted synthesis and magnetic property of magnetite and hematite nanoparticles

Nanoparticles of magnetite (Fe₃O₄) and hematite (α-Fe₂O₃) have been prepared by a simple microwave heating method using FeCl₃, polyethylene glycol and N₂H₄·H₂O. The amount of N₂H₄·H₂O has an effect on the final phase of Fe₃O₄. The morphology of α-Fe₂O₃ was affected by the heating method. Crystalline α-Fe₂O₃ agglomerates were formed immediately at room temperature and most of these nanoparticles within agglomerates show the same orientation along [110] direction. After microwave heating, ellipsoidal α-Fe₂O₃ nanoparticles were formed following an oriented attachment mechanism. Both Fe₃O₄ and α-Fe₂O₃ nanoparticles exhibit a small hysteresis loop at room temperature.     

Controlled manufacturing of nanoparticles by the laser pyrolysis: Application to cementite iron carbide

The laser pyrolysis is an attractive technique for the synthesis of different nanostructures from gas-phase precursors. The characteristics of this synthesized method are here exemplified by the production of almost pure cementite Fe₃C nanomaterials, obtained by the pyrolysis of methyl methacrylate and iron pentacarbonyl (vapors). Those nanopowders exhibited core (Fe₃C)-shell (MMA polymer-based) morphologies and mean particle diameters of about 8-9 nm. Preliminary magnetic measurements indicate rather high values for the saturation magnetization. By irradiating the same reactive mixture with a lower intensity radiation, the chemical content of nanopowders shifts towards mixtures of iron and maghemite/magnetite iron oxides.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.