Magnetic and resonance properties of ferrihydrite nanoparticles doped with cobalt

Powders of undoped ferrihydrite nanoparticles and ferrihydrite nanoparticles doped with cobalt in the ratio of 5: 1 have been prepared by hydrolysis of 3d-metal salts. It has been shown using Mössbauer spectroscopy that cobalt is uniformly distributed over characteristic crystal-chemical positions of iron ions. The blocking temperatures of ferrihydrite nanoparticles have been determined. The nanoparticle sizes, magnetizations, surface anisotropy constants, and bulk anisotropy constants have been estimated. The doping of ferrihydrite nanoparticles with cobalt leads to a significant increase in the anisotropy constant of a nanoparticle and to the formation of surface rotational anisotropy with the surface anisotropy constant K _u = 1.6 × 10^–3 erg/cm².     

α-Fe2O3-In2O3 mixed oxide nanoparticles synthesized under hydrothermal supercritical conditions

α-Fe₂O₃-In₂O₃ mixed oxide nanoparticles system has been synthesized by hydrothermal supercritical and postannealing route, starting with (1−x)Fe(NO₃)₃·9H₂O·xIn(NO₃)₃·5H₂O aqueous solution (x=0-1). X-ray diffraction and Mössbauer spectroscopy have been used to study the phase structure and substitutions in the nanosized samples. The concentration regions for the existence of the solid solutions in the α-Fe₂O₃-In₂O₃ nanoparticle system together with the solubility limits of In³⁺ ions in the hematite lattice and of Fe³⁺ ions in the cubic In₂O₃ structure have been evidenced. In general, the substitution level is considerably lower than the nominal concentration x. A justification of the processes leading to the formation of iron and indium phases in the investigated supercritical hydrothermal system has been given.     

Synthesis, structure and magnetic properties of non-crystalline ferrihydrite nanoflakes

Synthesis and characterization of the structural and magnetic properties of a 2-line (2L) ferrihydrite (FHYD) sample based on the composition Fe:Al:Cu=100:25:5 are reported. Typical of 2L-FHYDs, this sample also yields the two broad lines in X-ray diffraction and triplet in the 1400-1700 cm⁻¹ range in IR spectroscopy. However, in transmission electron microscopy, nanoflakes of about 5-20 nm size but without any hint of diffraction fringes characteristic of crystalline order were observed. Temperature dependence (2-380 K) of the magnetization M vs. applied field H (up to ±65 kOe) of this non-crystalline ferrihydrite is used to establish a blocking temperature T_B?20 K, Néel temperature T_N?365 K and a spin-glass ordering of the surface Fe³⁺ spins at T_S?6 K. These magnitudes of T_B and T_S are considerably smaller than those of a 5 nm undoped 2L crystalline ferrihydrite with T_B=70 K and T_S=30 K. The fit of the M vs. H data for several T>T_B to a modified Langevin function is shown to collapse onto a universal curve yielding a temperature-independent average magnetic moment μ_P=70(5)μ_B per nanoflake. Analysis of these parameters obtained from the fits of M vs. H data above T_B is used to show that the effective average volume of the nanoflakes is about 1/3 that of spherical 5 nm crystalline 2L-FHYD. It is argued that these lower magnitudes of μ_P, T_B, and T_S for the nanoflakes result from their smaller effective volume determined here.     

Synthesis of magnetite nanorods and porous hematite nanorods

Fe₃O₄ nanorods with average diameters of 40-50 nm and lengths of up to 1 μm were synthesized through hydrolysis of FeCl₃ and FeSO₄ solutions containing urea in the temperature range from 90 to 95 °C in reflux condition for 12 h, following an aging time of 12 h. The porous hematite nanorods were prepared by calcination of the precursor which was obtained from hydrolysis of FeCl₃ and FeSO₄ solutions containing urea at a temperature of 90 °C for 10 h in hydrothermal condition. The formation of the porosity of hematite was due to the decomposition of FeCO₃ and FeOOH. Urea played a key role in the formation of the iron oxide nanorods. Transmission electron microscopy (TEM) images showed that the morphology of magnetite particles is homogeneous in the shape of rods and hematite rods are full of porosity. The values of saturation magnetization (M) and coercivity (H) of magnetite nanorods are 67.55 emu/g and 114 Oe, respectively. The samples were also characterized by X-ray powder diffraction (XRD) and electron diffraction (ED). At last, the forming mechanism of both the magnetite and porous hematite nanorods was discussed.     

A new method for preparation of magnetite from iron oxyhydroxide or iron oxide and ferrous salt in aqueous solution

In this study, a new method is proposed for the preparation of Fe₃O₄ from iron oxyhydroxides (goethite, akaganeite, lepidocrocite, feroxyhyte and ferrihydrite) or iron oxide (hematite) and ferrous salt in aqueous solution. The product is magnetite with various particle sizes. Products are characterized by X-ray powder diffraction, IR spectra and vibrating sample magnetometery.     

Mössbauer study of biofilms formed at spring caves of Buda Karst,Hungary

Biofilms formed at spring caves of Buda Karst, Hungary, were investigated by ⁵⁷Fe Mössbauer spectroscopy. 78 K Mössbauer spectra were decomposed into a sextet and two doublets. The subspectra were assigned to goethite, hematite, ferrihydrite and siderite, according to their known Mössbauer parameters. The room temperature spectra indicated that goethite and/or hematite are in the superparamagnetic state at room temperature. The results can be interpreted in terms of karstification of hypogenic caves by the role of biofilms via discharge features.     

Al‐for‐Fe substitution in hematite: the effect of low Al concentrations in the Raman spectrum of Fe2O3

Five different mixed Al‐hematites with an aluminium molar content varying from 0 to 10% were investigated by micro‐Raman spectroscopy to study the effect of Al‐for‐Fe substitution on the hematite lattice. A red shift of hematite vibrational wavenumbers and a line broadening were observed; also a shoulder located near 430 cm⁻¹ and a broad band at 670 cm⁻¹ developed. The variation of the spectral features is discussed in terms of a local disorder correlated to the insertion of Al³⁺ ions into the Fe(O)₆ octahedra constituting hematite structure. A multivariate analysis was also carried out on the spectral data to distinguish between the doped samples analysed. Copyright © 2007 John Wiley & Sons, Ltd.     

EPR and optical spectroscopy of Cr3+ ions in Y3Ga5O12 crystalline thin films

The emission spectrum of Cr³⁺ in Y₃Ga₅O₁₂ crystalline thin films at 10K is composed of the sharp R ₁-line, its phonon sideband, and the very weak broadband. The lineshape of R ₁-line at the peak wavelength of 689.8 nm cannot be fitted to a single Gaussian. This suggests that there exist several different Cr³⁺ sites in the thin film. This fact is confirmed by lifetime measurements; the radiative decay of the fluorescence is decomposed into multiple-exponential curves. The dominant component (2.8 ms) of the decay curve for the broadband emission is equal to those (2.4 ms) for R ₁-line and its phonon sidebands. This result is discussed in terms of a tunneling model.     

Mössbauer and X-Ray Diffraction Investigations of a Series of B-Doped Ferrihydrites

X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy are used to characterize the influence of borate on two-line ferrihydrite's structure and develop likely models for its attachment. Particle sizes were in the 2–4 nm range, and as borate sorption increased, the ferrihydrite particle size decreased. The d-spacings of two-line ferrihydrite increased with increased borate adsorption. Isomer shift and quadrupole splitting exhibit slight increasing trends as well. Also, the phase transformation temperature of ferrihydrite to hematite is significantly raised due to borate coating of the surface. We suggest borate is sorbed onto the surface by attachment to the oxygen corners of the iron octahedra that are on the surface of the nanoparticles, placing boron in a tetrahedral molecular geometry.     

Fe-doped TiO2 prepared by mechanical alloying

The effect of different milling conditions on the formation of Fe-doped TiO₂ powders by mechanical alloying was investigated by Mössbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO₂ induce the anatase to rutile phase transformation via high pressure srilankite. Mössbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe^3?+? incorporated in the TiO₂ structure, and to the Fe^2?+? located either at the surface or the interstitial sites of TiO₂. A greater incorporation of Fe in the TiO₂ structure was observed when samples were prepared from hematite instead of α-Fe.     

Photocatalytic efficiency of iron oxides: Degradation of 4-chlorophenol

The photocatalytic activity of ferrihydrite Fe₅O₇(OH)×4H₂O synthesized by homogeneous precipitation with urea and products obtained by calcinations of as-precipitated ferrihydrite at different temperatures (200–1000 °C) was studied. The microstructure and surface properties of raw precipitate and all heated samples were characterized by means of HRTEM, SEM, BET/BJH and RTG analyses. Kinetics of disappearance of 4-chlorophenol (4-CP) in aqueous solution was used as a test reaction. We have found that hematite Fe₂O₃ obtained at 1000 °C exhibited satisfied photocatalytic efficiency on the degradation of 4-CP.     

Weathering of Martian and Earth surface studied by Mössbauer spectroscopy

Martian regolith and Earth’s basaltoid samples have been investigated by means of Mössbauer spectroscopy. The identification of the same minerals: olivine, pyroxene, magnetite, hematite and confrontation of the Fe^3?+?/Fe^2?+?, Fe^3?+?/Fe_tot, Fe^2?+?/Fe_tot ratios are presented. Co-existence of olivine and hematite in Martian regolith, absent in presented by authors terrestrial samples has been tentatively explained.     

Transformation from δ-FeOOH to hematite in the presence of trace Fe(II)

δ-FeOOH, a poorly crystalline iron oxyhydroxide, can transform to the more stable iron oxyhydroxide or oxide. In the present work, the transformation from δ-FeOOH to goethite and hematite in the presence of trace Fe(II) has been investigated. The results show that Fe(II) can catalyze this transformation of δ-FeOOH. Based on experimental results and literature data, it is confirmed that two transformation mechanisms exist in the current system. One is the catalytic dissolution of δ-FeOOH, which leads to the formation of both hematite and goethite. The other is the catalytic solid-state transformation from δ-FeOOH to hematite. Which mechanism predominates depends on the temperature, pH, heating rate, etc. The results reveal that high temperatures (in the range from room temperature (RT) to 100 °C) favor the solid-state transformation as well as the formation of hematite. Given the structural relationships observed between δ-FeOOH and hematite, it is possible that the solid-state transformation from δ-FeOOH to hematite can exist.     

A rubidium-stabilized ring-cavity resonator for optical frequency metrology: precise measurement of the D1 line in 133Cs

We have developed a ring-cavity resonator that can be used to measure the absolute frequencies of optical transitions with an uncertainty below 40 kHz. The length of the resonator is calibrated against a reference laser locked to the D₂ line of ⁸⁷Rb, the frequency of which is known with 6 kHz accuracy. We demonstrate the power of this technique by measuring the absolute frequencies of various hyperfine transitions in the D₁ line of ¹³³Cs. Our results agree with earlier measurements using the frequency-comb technique, and have similar accuracy. Measurement of the D₁-line frequency could lead to a more precise determination of the fine-structure constant. We also report a precise value of A=291.918(8) MHz for the hyperfine constant in the 6P_1/2 state.     

Iron oxide nanoparticles for plant nutrition? A preliminary Mössbauer study

One of the most important micronutrients for plants is iron. We have prepared iron(III) oxyhydroxide and magnetite nanoparticles with the aim to use them as possible nutrition source for plants. The iron(III)-oxide/oxyhydroxide nanoparticles prepared under our experimental conditions as colloidal suspensions proved to be 6-line ferrihydrite nanoparticles as verified by XRD, TEM/SAED and Mössbauer spectroscopy measurements. ⁵⁷Fe Mössbauer spectra of magnetite nanoparticles prepared under different preparation conditions could be analyzed on the basis of a common model based on the superposition of four sextet components displaying Gaussian-shaped hyperfine magnetic field distributions.     

Reduction of hematite with ethanol to produce magnetic nanoparticles of Fe3O4, Fe1 − x O or Fe0 coated with carbon

The production of magnetic nanoparticles of Fe₃O₄ or Fe⁰ coated with carbon and carbon nanotubes was investigated by the reduction of hematite with ethanol in a Temperature Programmed Reaction up to 950°C. XRD and Mössbauer measurements showed after reaction at 350°C the partial reduction of hematite to magnetite. At 600°C the hematite is completely reduced to magnetite (59%), wüstite (39%) and metallic iron (7%). At higher temperatures, carbide and metallic iron are the only phases present. TG weight losses suggested the formation of 3–56 wt.% carbon deposits after reaction with ethanol. It was observed by SEM images a high concentration of nanometric carbon filaments on the material surface. BET analyses showed a slight increase in the surface area after reaction. These materials have potential application as catalyst support and removal of spilled oil contaminants.     

Further investigation on the formation mechanisms of (NH4)2SiF6 synthesized by dry etching technique

The validity of two formation mechanisms of ammonium silicofluoride (ASF), which are proposed to take place when a silicon surface is exposed to the vapor of HNO₃/HF acid mixture is investigated. Of the two proposed mechanisms regarding the synthesis of ASF on silicon surface, validity of the first predicting the release of hydrofluosilicic acid (H₂SiF₆) at the intermediate stage is examined by FTIR spectroscopy and the second mechanism suggesting O₂ release is investigated using the Winkler technique. IR absorbance bands of SiF₆²⁺ are observed on the fresh samples prepared at low (1/100) HNO₃/HF volume fractions. No significant amount of oxygen is detected during the synthesis of ASF films on silicon surface by dry etching technique. These two observations together provide firmer support for the validity of the second mechanism.     

Mössbauer study of iron oxide modified montmorillonite

Montmorillonite particles were modified by iron oxides using the precipitation process with the aim to monitor the differences in the structural and magnetic properties of intercalated and adsorbed Fe³⁺. The Mössbauer spectra recorded at 5 K in zero and 6 T external fields, IR spectra and TG curves measured in zero and 32 mT fields identified the ferrihydrite pillars in an interlayer space of the montmorillonite structure and γ-Fe₂O₃ nanoparticles adsorbed on the mineral surface. The temperature dependent Mössbauer spectra (25–300 K) reflect the superparamagnetic behaviour of maghemite nanoparticles and ferrihydrite pillars with the blocking temperatures of about 80 and 25 K, respectively.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

High concentration of hematite nanoparticles in a silica matrix: Structural and magnetic properties

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt% of hematite was prepared by the sol-gel method followed by heating in air at 200 °C. The so-obtained composite of iron(III) nanoparticles dissolved in glassy silica matrix was investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRPD confirms the formation of a single-phase hematite sample, whereas TEM reveals spherical particles in a silica matrix with an average diameter of 10 nm. DC magnetization shows bifurcation of the zero-field-cooled (ZFC) and field-cooled (FC) branches up to the room temperature with a blocking temperature T_B=65 K. Isothermal M(H) dependence displays significant hysteretic behaviour below T_B, whereas the room temperature data were successfully fitted to a weighted Langevin function. The average particle size obtained from this fit is in agreement with the TEM findings. The small shift of the T_B value with the magnetic field strength, narrowing of the hysteresis loop at low applied field, and the frequency dependence of the AC susceptibility data point to the presence of inter-particle interactions. The analysis of the results suggests that the system consists of single-domain nanoparticles with intermediate strength interactions.