Synthesis of Fe x O y nanoparticles during iron oxidation by supercritical water

It is established that iron is oxidized by supercritical water (SCW) with the formation of H₂ and nanoparticles of iron oxides (Fe₃O₄, FeO, and γ-Fe₂O₃). The kinetics of H₂ production and iron oxidation has been studied by SCW injection at T = 673, 723, 773, 823, and 873 K into a reactor with iron particles. Data of X-ray diffraction and transmission electron microscopy show that the phase composition and morphology of synthesized oxide nanoparticles depend on the SCW temperature.     

Structure switch between α-Fe2O3, γ-Fe2O3 and Fe3O4 during the large scale and low temperature sol–gel synthesis of nearly monodispersed iron oxide nanoparticles

With same procedure and same starting materials, nearly monodispersed α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ nanoparticles were synthesized on an large scale of about 60 g in a single reaction through a low temperature sol–gel route. The simple preparation process includes the reactions between FeCl₂ and propylene oxide in ethanol solution at boiling point to form a sol and the following drying of the sol. The different iron oxide phases can be obtained just by changing of the drying conditions for the sol solution. The strategy developed in this study offers important advantages over the conventional routes for the synthesis of α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ nanoparticles, showing potential for its application in industrial production of iron oxides.     

Nanoneedles of maghemite iron oxide prepared from a wet chemical route

Nanometer-sized maghemite iron oxide (γ-Fe₂O₃) particles were produced first by synthesis of a precursor, γ-FeO(OH), in a surfactant-less microemulsion and subsequent heat treatment of the γ-FeO(OH). The precursors and γ-Fe₂O₃ powder were characterized using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Susceptometer from Quantum Design (SQUID) measurements. TGA and XRD analysis indicated the formation of single cubic phase when the samples were heat-treated at 240 °C. TEM reveals that the γ-Fe₂O₃ particles are needle-shaped with an aspect ratio of ∼20; typically 5-10 nm wide and over 150 nm long. It was found that these microemulsion-derived γ-Fe₂O₃ nanoneedles possess an intrinsic coercivity of 28 Oe at 300 K and 950 Oe at 2 K.     

Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on γ-Fe₂O₃ 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe₂O₃ phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe₂O₃ nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe₂O₃ is an intermediate phase between γ-Fe₂O₃ and α-Fe₂O₃ and a series of distinct Raman vibrational bands were identified with the ε-Fe₂O₃ phase. The structural transformation of γ-Fe₂O₃ into α-Fe₂O₃ occurs either directly or via ε-Fe₂O₃, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.     

Thin films of iron oxide by low pressure MOCVD using a novel precursor: tris(t-butyl-3-oxo-butanoato)iron(III)

Deposition of thin films of iron oxide on glass has been carried out using a novel precursor, tris(t-butyl-3-oxo-butanoato)iron(III), in a low-pressure metalorganic chemical vapor deposition (MOCVD) system. The new precursor was characterized for its thermal properties by thermogravimetry and differential thermal analysis. The films were characterized by X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy, and by optical measurements. XRD studies reveal that films grown at substrate temperatures below ∼550 °C and at low oxygen flow rates comprise only the phase Fe₃O₄ (magnetite). At higher temperatures and at higher oxygen flow rates, an increasing proportion of α-Fe₂O₃ is formed along with Fe₃O₄. Films of magnetite grown under different reactive ambients—oxygen and nitrous oxide—have very different morphologies, as revealed by scanning electron microscopic studies.     

Effect of chloride ion on crystalline phase transition of iron oxide produced by ultrasonic spray pyrolysis

Ultrafine spherical Fe₂O₃ powders with controllable morphology and crystal phase were synthesized by ultrasonic spray pyrolysis. In this experiment, we chose three common ferric salts (Fe(NO₃)₃·9H₂O, FeSO₄·7H₂O or FeCl₂·4H₂O) as precursor solution and regulated the concentration of chlorine ion (Cl^?) in precursor solution to produce Fe₂O₃ particles. The morphology, crystal structure and magnetic property of prepared Fe₂O₃ particles were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vibrating sample magnetometer (VSM). The diameter of the obtained Fe₂O₃ products ranged from 0.2 to 2?μm. And the product obtained from FeCl₂ precursor solution was magnetic, which was composed of hexagonal α-Fe₂O₃ and cubic γ-Fe₂O₃ from XRD results. We also calculated the weight percent of α-Fe₂O₃ and γ-Fe₂O₃ in the product through XRD quantitative analysis. However, with the addition of Cl^? in Fe(NO₃)₃ or FeSO₄ precursor solution, the products turned from non-magnetic to magnetic, whose pure α-Fe₂O₃ phase became to α-Fe₂O₃ and γ-Fe₂O₃ multi-phase. Besides, the weight percent of γ-Fe₂O₃ and the amount of M_s increased with the Cl^? concentration in precursor solution improving. According to the research, it can be inferred that the presence of Cl^? inhibits the phase transition of γ-Fe₂O₃ to α-Fe₂O₃ at high temperature.     

Synthesis by the solution combustion process and magnetic properties of iron oxide (Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>) particles

This article describes the solution combustion synthesis technique as applicable to iron oxide powder production using urea as fuel and ferric nitrate as an oxidizer. It focuses on the thermodynamic modeling of the combustion reaction under different fuel-to-oxidant ratios. X-ray diffraction showed magnetite (Fe₃O₄) and hematite (α-Fe₂O₃) phase formations for the as-synthesized powders. The smallest crystallite size was obtained by stoichiometric chemical reaction. The magnetic properties of the samples are also carefully discussed as superparamagnetic behavior.     

Effect of precipitation conditions on the phase composition, particle morphology, and properties of iron(III,II) hydroxide precipitates

The phase composition and particle shape of amorphous iron hydroxide-iron oxide precipitates were correlated with the oxidation state of iron in the starting solution, Fe₂O₃ : FeO ratio, and precipitation procedure (coprecipitation or successive precipitation) using Mössbauer spectroscopy, atomic absorption, lowtemperature nitrogen BET, and electron microscopy data. A correlation was established between the amount of iron(II) in the starting solution, the amount of the metastable phase γ-Fe₂O₃ in the precipitate, particle shape, and the activity of the final heat-treatment products—α-Fe₂O₃ and mixed oxides of iron.     

Direct preparation of γ-Fe2O3 thin film recording media by reactive r.f. sputtering technique

A method for the direct preparation of γ-Fe₂O₃ thin films from iron metal was studied. The technique employs an intermittent iron sputter deposition process using a partially masked revolving substrate and a subsequent oxidation process of each deposited layer. Electron diffraction analysis of the films obtained shows superlatice reflections due to iron vacancy ordering, indexed as (110), (210) and (211), together with the reflections of the spinel structure. No reflections from an α-Fe₂O₃ structure can be detected in the films. From their electrical resistivity and lattice parameter values the films are identified as being in an intermediate state between stoichiometric Fe₃O₄ and γ-Fe₂O₃. They exhibit a typical coercive force of 600 Oe, a squareness ratio of 0.75, a saturation magnetization of 3600 G and a coercive squareness ratio of 0.8 for an Fe_0.945Co_0.025Cu_0.03 target. These characteristics confirm that the films are applicable to magnetic recording media.     

Synthesis of maghemite (γ-Fe2O3) nanoparticles by thermal-decomposition of magnetite (Fe3O4) nanoparticles

In this research work, we prepared γ-Fe₂O₃ nanoparticles by thermal-decomposition of Fe₃O₄. The Fe₃O₄ nanoparticles were synthesized via co-precipitation method at room temperature. This simple, soft and cheap method is suitable for preparation of iron oxide nanoparticles (γ-Fe₂O₃; Fe₃O₄). The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), vibrating sample magnetometer and differential scanning calorimeter (DSC). The XRD and FT-IR results indicated the formation of γ-Fe₂O₃ and Fe₃O₄ nanoparticles. The TEM images showed that the γ-Fe₂O₃ and Fe₃O₄ were spherical, and their size was 18 and 22 nm respectively. Magnetic properties have been measured by VSM at room temperature. Hysteresis loops showed that the γ-Fe₂O₃ and Fe₃O₄ nanoparticles were super-paramagnetic.     

Fabrication and characterization of Fe3O4 thin films deposited by reactive magnetron sputtering

Fe-O thin films with different atomic ratio of iron to oxygen were deposited on glass and thermally oxidized silicon substrates at temperatures of 300, 473 and 593 K, by reactive magnetron sputtering in Ar+O₂ atmosphere. The composition and structure of the thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrical resistivity. It was found from XRD that with increasing the oxygen partial pressure in the working gas, the crystalline structure of the Fe-O films deposited at the substrate temperature of 473 K gradually changed from α-Fe, amorphous Fe-O, Fe₃O₄, γ-Fe₂O₃ to Fe_21.34O₃₂. The structure and chemical valence of the Fe₃O₄ films were analyzed by electron microscopy and XPS, respectively.     

Facile synthesis of capped γ-Fe2O3 and Fe3O4 nanoparticles

Facile methods for the selective preparation of capped iron oxide nanoparticles (γ-Fe₂O₃, Fe₃O₄) are described. The magnetic oxides are obtained via oxidative transformation of an iron hydroxide gel using H₂O₂ or (NH₄)₂S₂O₈ solutions as oxidants. Capping with oleic or other aliphatic acids is established simultaneously in one step by adding a toluene solution of the capping agent and refluxing the resulting biphase system. The method is simple, soft and affords nanoparticles of γ-Fe₂O₃ or Fe₃O₄ of controlled size depending on the reaction conditions. The capped nanoparticles are readily soluble in organic or aqueous media according to the nature of the sheath surrounding the surface of the particles, providing stable and high concentration ferrofluids.     

Synthesis of multicomponent nanocomposites containing filamentary carbon nanostructures

Abstract

In this work, the multicomponent nanocomposites containing filamentary carbon nanostructures were synthesized using materials based on iron oxides with a predominant content of the epsilon phase (ε-Fe₂O₃). These iron oxide-based materials were obtained by a direct plasma-dynamic synthesis with supersonic outflow of an iron-containing electric discharge plasma into an oxygen atmosphere. Subsequently, they were used as an initial precursor and placed in the plasma-chemical reactor, where the multicomponent C/Si_xO_y/Fe₂O₃ nanostructures were synthesized under the influence of the pulsed electron beam. This method was based on the volume excitation of the reaction gas by a pulsed electron beam in such a way as to control the uniform process implementation in the entire excitation region. The morphology and phase composition of the synthesized C/Si_xO_y/Fe₂O₃ nanocomposites were studied. A typical morphological feature of the C/Si_xO_y/Fe₂O₃ samples was found to be the formation of filamentary nanostructures. Their diameter does not exceed 10–20?nm, while their length varies up to 1?μm.     

Preparation and characterization of α-Fe2O3 nanorod-thin film by metal-organic chemical vapor deposition

Alpha iron oxide (α-Fe₂O₃) films were grown on catalyst-free silicon substrate using a vertical type metal-organic chemical vapor deposition process. X-ray powder diffraction and field-emission transmission electron microscopy measurements showed that these α-Fe₂O₃ films consisted of bundles of one dimensional (1D) nanorods and the nanorods in these α-Fe₂O₃ films were single crystalline with a well-ordered rhombohedral structure. The nanorods showed a preferred growth orientation in the [104] direction. Magnetic force microscopy image suggests that spin domains were formed in the α-Fe₂O₃ nanorods. Photo-catalytic property of these nanorod films was confirmed through the photo-degradation of Rhodamine B by UV irradiation. These α-Fe₂O₃ film/nanorod materials could be used as building blocks for nanodevice applications.     

One-step solution auto-combustion process for the rapid synthesis of crystalline phase iron oxide nanoparticles with improved magnetic and photocatalytic properties

We report the solution auto-combustion (AC) process for the rapid synthesis of Fe₃O₄ nanoparticles derived from the sol–gel (SG) process. The citric acid (CA) and tartaric acid (TA) is used as gelling agents in the SG process, where the citric acid turns into a fuel that combusts the gel and yields a highly magnetic crystalline phase Fe₃O₄ nanoparticles in one step with an average particle size of 50 nm. In contrast, the citric acid at different concentrations and tartaric acid at any concentrations do not lead to any combustion process and yield amorphous iron oxides. Upon annealing, these CA and TA derived iron oxide samples are turned to crystalline phase α-Fe₂O₃ particles. In contrast, the as-synthesized AC sample (i.e. Fe₃O₄) is oxidized to γ-Fe₂O₃ phase, which is confirmed from their respective XRD, Rietveld refinement and XPS studies. All the synthesized iron oxide phases showed broad visible light absorption. The room temperature M?H hysteresis curves obtained from VSM revealed that the Fe₃O₄ and α-Fe₂O₃/γ-Fe₂O₃ phases exhibit super-paramagnetic and ferromagnetic properties, respectively. The photocatalytic efficiencies of the samples are found to be in the order of Fe₃O₄ > γ-Fe₂O₃ > α-Fe₂O₃ with 98, 87, 79/73% degradation of rhodamine B dye at the end of 3 h and H₂ evolution rate over these systems is found to be 2.1, 1.3 and 0.92/0.89 mmol/h/g, respectively under simulated solar light irradiation. The photocatalytic recycle studies demonstrated that all the synthesized photocatalysts possess excellent chemical and photo-stabilities.     

Hydrothermal synthesis of surface-modified iron oxide nanoparticles

We synthesized surface-modified iron oxide nanoparticles in aqueous phase by heating an aqueous solution of iron sulfate (FeSO₄) at 473 K with a small amount of either n-decanoic acid (C₉H₁₉COOH) or n-decylamine (C₁₀H₂₁NH₂), which is not miscible with water at room temperature. Transmission electron microscopy showed that the addition of n-decanoic acid or decylamine changed the shape of the obtained nanoparticles. X-ray diffraction spectra revealed that the synthesized nanoparticles were in α-Fe₂O₃ or Fe₃O₄ phase while Fourier transform infrared spectroscopy and thermogravimetry indicated the existence of an organic layer on the surface of the nanoparticles. In the synthetic condition, decreased dielectric constant of water at higher temperature increased the solubility of n-decanoic acid or n-decylamine in water to promote the reaction between the surface of iron oxide nanoparticles and the organic reagents. After the synthesis, the used organic modifiers separated from the aqueous phase at room temperature, which may help the environmentally benign synthesis of surface-modified metal oxide nanoparticles.     

Synthesis, morphology and optical properties of multi-pods Au/FeO(OH) and Au/Fe2O3 nanostructures

Multi-pods Au/FeOOH nanostructures were synthesized by a hydrothermal treatment of an aqueous solution of mixed micellar formed by gold nanoparticles, hexadecyltrimethyl ammonium bromide (CTAB), and (NH₄)₃[FeF₆] at 160 °C for 48 h and sequential calcined at 290 °C for 1.5 h, resulting in the formation of multi-pods Au/Fe₂O₃ nanostructures. The as-obtained products were characterized by powder X-ray diffraction, transmission electron microscopy, selected area electron diffraction, field emission scanning electron microscopy, and UV-vis spectroscopy. Surface plasmon resonance band of gold nanoparticles was observed in the multi-pods Au/FeOOH nanostructures. However, a similar behavior was not seen with multi-pods Au/Fe₂O₃ nanostructures. The critical role of F⁻ ions and CTAB molecules in the formation of FeO(OH) multipods and the probable mechanism of the formation of multi-pods Au/FeOOH and Au/Fe₂O₃ nanostructures were discussed.     

Facile hydrothermal route to the controlled synthesis of α-Fe2O3 1-D nanostructures

Single-crystalline α-Fe₂O₃ 1-D nanostructures can be obtained via a facile one-step hydrothermal synthetic route. It was found that the introduction of SnCl₄ played a key role in determining the composition and morphology of α-Fe₂O₃. The addition of SnCl₄ favours the formation of Fe₂O₃ rather than FeOOH, and the morphology can be tuned from nanorod to double-shuttle as the increase of SnCl₄ concentration. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). This simple method does not need any seed, catalyst, or template, thus is promising for large-scale and low-cost production.     

Preparation and characterisation of γ-Fe2O3 as tape recording material

Optimum conditions for the preparation of tape recording quality γ-Fe₂O₃ by the thermal decomposition of ferrous oxalate dihydrate have been established. Formation of the intermediate Fe₃O₄ which is most important in forming γ-Fe₂O₃ takes place only in the presence of water vapour. Various stages of decomposition have been characterised by DTA, TG, DTG, and x-ray powder diffraction. The method for the preparation of acicular γ-Fe₂O₃ that matches very well with the commercial tape recording material has been developed.     

Joint mechanical activation of MnO2, Fe2O3 and graphite: Mutual influence on the structure

The structural changes of MnO₂, Fe₂O₃ and graphite under separate and joint mechanical activation in high-energy planetary ball mill were studied by X-ray diffraction analysis, Raman spectroscopy and chemical analysis. Separate mechanical processing resulted in nanostructured states of MnO₂, Fe₂O₃ and graphite with the size of coherent scattering regions of 25, 12 and 6 nm, respectively, and the average particle size of 15–20 nm. Along with nanoparticles of globular shape, Fe₂O₃ nanorods were found to be formed during separate milling. No mechanochemical effect was found after separate milling. Under joint mechanical activation of nanostructured manganese and iron oxides with graphite, phase transformations toward less stable forms of oxides (Mn₂O₃, Mn₃O₄, Fe₃O₄) were found. When co-milled with α-Fe₂O₃, graphite was found to exfoliate to graphene layers. The graphite phase remained under the combined mechanical activation with MnO₂. Dynamic recrystallization of α-Fe₂O₃ phase also proceeded during joint mechanical activation of nanostructured Fe₂O₃ and graphite.