Microwave Synthesis of γ-Fe2O3

Fine-particle Fe₂O₃ is prepared via microwave processing of Fe(NO₃)₃ · nH₂O, followed by low-temperature annealing. The particle size of the resulting -Fe₂O₃ is 5–6 nm after microwave processing and 80–110 nm after subsequent low-temperature heat treatment.     

Synthesis and Mossbauer studies of mesoporous γ-Fe2O3

A method of synthesis of mesoporous γ-Fe₂O₃ by thermal decomposition of iron citrate has been proposed. Investigations of the crystalline and magnetic structure of obtained materials were done. Nanodispersed maghemite (γ-Fe₂O₃) with the sizes of coherent scattering regions of about 4–7 nm consisted of one phase only after gel sintering at 200, 250 and 300 °C. The particles of synthesized materials were both in magnetically ordered, and superparamagnetic states, and they formed a grid-like mesoporous structure. The influence of magnetic dipole interparticle interaction on the parameters of Mossbauer spectra was observed. A phenomenological model of the differences between nanodispersed γ-Fe₂O₃ magnetic microstructures obtained after annealing at different temperatures was presented.     

Synthesis of ferrite grade γ-Fe2O3

Iron(II) carboxylato-hydrazinates: Ferrous fumarato-hydrazinate (FFH), FeC₄H₂O₄·2N₂H₄; ferrous succinato-hydrazinate (FSH), FeC₄H₄O₄·2N₂H₄; ferrous maleato-hydrazinate (FEH), FeC₄H₂O₄·2N₂H₄; ferrous malato-hydrazinate (FLH), Fein4H₄O₅·2N₂H₄; ferrous malonato-hydrazinate (FMH), FeC₃H₂O₄·1.5N₂H₄·H₂O; and ferrous tartrato-hydrazinate (FTH), FeC₄H₄O₆·N₂H₄·H₂O are being synthesized for the first time. These decompose (autocatalytically) in an ordinary atmosphere to mainly γ-Fe₂O₃, while the unhydrazinated iron(II) carboxylates in air yield α-Fe₂O₃, but the controlled atmosphere of moisture requires for the oxalates to stabilize the metastable γ-Fe₂O₃. The hydrazine released during heating reacts with atmospheric oxygen liberating enormous energy, N₂H₄ + O₂ → N₂ + H₂O; ΔH₂O = −621 kJ/mol, which enables to oxidatively decompose the dehydrazinated complex to γ-Fe₂O₃. The reaction products N₂ + H₂O provide the necessary atmosphere of moisture needed for the stabilization of the metastable oxide. The synthesis, characterization and thermal decomposition (DTA/TG) of the iron(II) carboxylato-hydrazinates are discussed to explain the suitability of γ-Fe₂O₃ in the ferrite synthesis.     

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.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part III – study of hydrogen iron oxide phase in γ-Fe2O3

Direct current electrical conductivity () measurements as a function of temperature have been carried out on -Fe₂O₃ prepared from precursors, iron (II) carboxylatohydrazinates, -FeOOH and hydrazinated -FeOOH. The conductivity variation obeys an Arrhenius equation, _I = \_oe^- E / kT and the plots of log versus 1/T of the as prepared -Fe₂O₃, which are in general linear, during the very first heating up to 350°C and cooling to room temperature (RT) do not overlap. This indicates a hysteresis behavior of conductivity, thereby suggesting involvement of two different conductivity mechanisms. When the heat treated sample was equilibrated in a known partial pressure of moisture at 200°C and then conductivity measured from RT, the log plots during heating and cooling did not overlap and a hysteresis behavior similar to the as prepared -Fe₂O₃ is observed again in the conductivity. Water is considered to be crucial during the synthesis of -Fe₂O₃ through magnetite, Fe₃O₄. Protons, H⁺, are thought to be introduced in the spinel Fe₃O₄ making it defective and the oxidation product of this is -Fe₂O₃ which retains few protons in its spinel structure. From the structural similarity of such proton incorporated -Fe₂O₃ and lithium ferrite, LiFe₅O₈, (Fe³⁺)₈ [Fe³⁺ ₁₂ Li¹⁺ ₄]O₃₂, a formula HFe₅O₈, (Fe³⁺)₈ [Fe³+₁₂H¹+₄]O₃₂ is suggested. A hydrogen iron oxide of formula H_1-xFe_5+x3O₈, where x 0.1 is probably formed as a maximum limit. Protons are removed during the very first heating of the as prepared sample in the present studies and hence the conductivity of proton free -Fe₂O₃ is different and therefore a hysteresis behavior is observed. Moisture equilibration reintroduces the protons. The lithiated samples in the present studies were found to substitute for protons in -Fe₂O₃ and no hysteresis behavior is observed in such samples even after moisture equilibration.     

Mechanochemical preparation and thermal stability of γ-Fe2O3 derived from γ-FeOOH

The dehydration process of lepidocrocite, γ-FeOOH, induced by wet grinding procedures has been studied. Microcrystals of maghemite, γ-Fe₂O₃, are found in the product of ball-milling in hexane and cyclohexane media. In contrast, grinding in air leads to hematite α-Fe₂O₃. This change is accompanied by the development of a characteristic texture in which a slit-shaped porous system is present. On the other hand, mechanochemically prepared maghemite increases in its thermal stability. This fact has been attributed to the higher crystallinity of ground microcrystals as revealed by the values of crystallite size and microstrains.     

Synthesis of α-Fe2O3 via oxidative hydrolysis of iron(II) sulphate

The influence of the main reaction parameters (temperature, pH and concentration) in the oxidative hydrolysis of iron (II) sulphate in an acid medium on the properties of the obtained -Fe₂O₃ and its applicability in ferrite production has been studied. The addition of manganese(II) ions catalyses the process in the homogeneous phase, probably by activation of oxygen. The obtained results are discussed within the framework of the assumed reaction mechanism, which includes an homogeneous reaction and a heterogeneous one with the participation of the oxidative hydrolysis product -FeOOH.     

Synthesis, characterization and magnetic properties of polyaniline/γ-Fe2O3 composites

Conducting polyaniline/γ-Fe₂O₃ (PANI/FE) composites have been synthesized using an in situ deposition technique by placing fine-graded γ-Fe₂O₃ in a polymerization mixture of aniline. The composites are characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and infrared (IR) spectroscopy. The electrical properties such as d.c. and a.c. conductivities are studied by sandwitching the pellets of these composites between the silver electrodes. It is observed that the conductivity increases up to a composition of 20 wt.% of γ-Fe₂O₃ in polyaniline and decreases thereafter. The initial increase in conductivity is attributed to the extended chain length of polyaniline, where polarons possess sufficient energy to hop between favourable sites. Beyond 20 wt.% of γ-Fe₂O₃ in polyaniline, the blocking of charge carrier hop occurs, reducing conductivity values. The magnetic properties such as hysteresis characteristics and normalized a.c. susceptibility are also measured, which show a strong dependence on content of γ-Fe₂O₃ in polyaniline. Because of superparamagnetic behaviour of these composites, they may find extensive technological applications, especially for absorbing and shielding applications in microwave frequencies.     

Titanium-doped γ-Fe2O3: Reduction and oxidation properties

Titanium-doped -Fe₂O₃ has been prepared by the calcination of a solid formed by the addition of aqueous ammonia to an aqueous solution of titanium- and iron-containing salts and boiling the precipitate under reflux. As compared to pure -Fe₂O₃ made by similar methods, titanium-doped -Fe₂O₃ showed a higher surface area and a greater stability to reduction, thermal conversion to an -Fe₂O₃-related structure and the maintenance of a higher surface area during oxidation-reduction cycling.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part IV – preparation and characterization of magnesium ferrite, MgFe2O4 from γ-Fe2O3 obtained from hydrazinated iron oxyhydroxides and iron (II) carboxylato-hydrazinates

Electro-magnetic properties and microstructural characterization of MgFe₂O₄ synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe₂O₃ and traces of alpha-Fe₂O₃, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe₂O₃. The sources of -Fe₂O₃ are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe₂O₃ of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe₂O₃ gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe₂O₄ obtained from commercial alpha-Fe₂O₃ grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe₂O₄ from -Fe₂O₃ source while the alpha-Fe₂O₃ source gave the lowest value, 609. The Curie temperature, T_c, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest T_c of 737 K for MgFe₂O₄ from alpha-Fe₂O₃, while lower values are found for the ferrite prepared from -Fe₂O₃.     

Photoelectrochemical activity of as-grown, α-Fe2O3 nanowire array electrodes for water splitting

Undoped hematite nanowire arrays grown using plasma oxidation of iron foils show significant photoactivity (~0.38 mA cm(-2) at 1.5 V versus reversible hydrogen electrode in 1 M KOH). In contrast, thermally oxidized nanowire arrays grown on iron exhibit no photoactivity due to the formation of a thick (>7 μm Fe(1-x)O) interfacial layer. An atmospheric plasma oxidation process required only a few minutes to synthesize hematite nanowire arrays with a 1–5 μm interfacial layer of magnetite between the nanowire arrays and the iron substrate. An amorphous oxide surface layer on hematite nanowires, if present, is shown to decrease the resulting photoactivity of as-synthesized, plasma grown nanowire arrays. The photocurrent onset potential is improved after removing the amorphous surface on the nanowires using an acid etch. A two-step method involving high temperature nucleation followed by growth at low temperature is shown to produce a highly dense and uniform coverage of nanowire arrays.     

Solar light-driven reduction of crystal violet by a composite of g-C3N4, β-Ag2Se, γ-Fe2O3 and graphite

Abstract

In this work, a new g-C₃N₄-based Z-scheme with γ-Fe₂O₃ and β-Ag₂Se both n-type semiconductors, and graphite to favor electron exchange is presented. The composite material was studied by XRD, FTIR, UV-Vis, TEM, XPS, TGA, DSC and TOF-SIMS, and the ability of this photocatalytic system to act as a photo-reductant was assessed using crystal violet (CV⁺) dye. Solar light driven photo-reduction of CV⁺ in the presence of tri-sodium citrate evidenced a synergistic enhancement of the activity of the composite toward reduction, with ～20 times higher conversion rates per unit of surface area than those of g-C₃N₄. Photo-oxidation experiments under Xe lamp irradiation in the presence of H₂O₂ also showed that the AgFeCN composite featured a higher activity (～8×) than g-C₃N₄. This Z-scheme may deserve further study as a photo-reductant to obtain hydrogen or hydrogenated compounds. Moreover, the use of CV⁺ may represent a facile procedure that can aid in the selection of new photocatalysts to be used in hydrogen production.     

Fabrication,superhydrophobicity, and microwave absorbing properties of the magnetic γ-Fe2O3/bamboo composites

Magnetic γ-Fe₂O₃ nanoparticles were successfully deposited on the surface of the bamboo via a coprecipitation process at room temperature. Spherical-like magnetic γ-Fe₂O₃ nanoparticles with a diameter of about 17 nm displayed well superparamagnetic behavior and were chemically bonded to the bamboo surface through the combination of hydrogen groups. With further modification by 1H,1H,2H,2H-perfluorodecyltri ethoxysilane (FAS-17), magnetic γ-Fe₂O₃/bamboo composites (MBCs) expressed superhydrophobic performances to not only water but also common liquids like coffee, milk, ink, tea, and coke. When immersed into the corrosive solutions including strong acid (pH = 1), heavy alkaline (pH = 14), and salt with high concentration (5 M) for 24 h, superhydrophobic magnetic γ-Fe₂O₃/bamboo composites (SMBC) still remained magnetism as well as superhydrophobicity. Also, under harsh conditions like boiled at 100 °C for 4 h, frozen at − 40 °C for 24 h, SMBCs were kept a robust magnetism and superhydrophobicity. Additionally, SMBC was a typical ferromagnet and exhibited some microwave absorbabilities.     

Characterization of oxygen passivated iron nanoparticles and thermal evolution to γ-Fe2O3

Nanocrystalline iron powders have been prepared by the inert gas evaporation method. After preparation the material has been passivated by pure oxygen and air exposure. In the present paper we describe new characterization studies of this sample by Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS), Electron Energy Loss Spectroscopy (EELS) and Mössbauer Spectroscopy (MS), giving a complete chemical and structural characterization of the nanocomposite material in order to correlate its microstructure with its singular magnetic behavior.This nanocomposite was later heated following different thermal treatments. It was found that the sample heated successively in high vacuum (10^–7 torr) at 383 K for 1 h and under a residual oxygen pressure of 4 × 10^–4 torr at 573 K for 3 h, results in a powder formed by nanoparticles of -Fe₂O₃ as stated from XRD, XAS and MS. This material is stable during several years and behaves almost totally like superparamagnetic at room temperature.     

Study on the effects of Cr2O3 on the reduction behavior of γ-Fe2O3

XRD and TG reduction analysis show that -Fe₂O₃ Fe-Cr catalysts, which contains 0.0 to 14.0 wt %. Cr₂O₃ and prepared by coprecipitating method, consist of crystalline -Fe₂O₃ and non-crystalline Fe₂O₃. Between 150–450 °C, three reduction stages are observed in the catalyst. The first stage is non-crystalline Fe₂O₃ reduced to non-crystalline Fe₃O₄, the second is crystalline -Fe₂O₃ to crystalline Fe₃O₄ and the third is non-crystalline Fe₂O₃ reduced to non-crystalline FeO. About 5 wt %. Cr₂O₃ can enter the lattices of -Fe₂O₃ to form solid solution. With the increasing of Cr₂O₃ content, the relative abundance of non-crystalline Fe₂O₃ and the amount of soluble Cr₂O₃ in non-crystalline increases, while the crystalline size of -Fe₂O₃ decreases.     

Efficient growth of aligned SnO2 nanorod arrays on hematite (α-Fe2O3) nanotube arrays for photoelectrochemical and photocatalytic applications

SnO₂ nanorod arrays were fabricated on hematiete nanotube arrays by an efficient hydrothermal method. The hematiete nanotube arrays were prepared by anodization of pure iron foil in an ethylene glycol solution. SnO₂ nanorod arrays grew from the bottom of hematite nanotubes and were firmly combined with the iron foil substrate. The morphology and microstructure of SnO₂ nanorod arrays are investigated by field-emission scanning electron microscopy, grazing incidence X-ray diffraction and UV–Vis absorbance spectra. The sample presented typical SnO₂ nanorod arrays (reacted for 2 h) generally of 400 nm in length and 50 nm in side width showed the best photocatalytic activity and photoelectrochemical response under the UV illumination. It should be attributed to the effective electron–hole separation and the excellent electron transfer pathway along the 1D SnO₂ nanorod arrays and hematiete nanotube arrays.     

Sintering of pseudo-boehmite and γ-Al2O3

Sintering of pseudo-boehmite, acicular-Al₂O₃ produced by dehydration of pseudo-boehmite, and-Al₂O₃ ex alum was investigated. The sintering process was studied by X-ray diffraction, transmission electron microscopy with selected area electron diffraction and BET surface area measurements. The solid state reaction to-Al₂O₃ causes a steep drop of the surface area to less than 10 m²g^–1. The acicular pseudo-boehmite and-Al₂O₃ supports exhibit an intermediate state where the acicular particles assume a rod-like shape and the surface area falls from about 300 to 100 m²g^–1. It was established that reaction to -Al₂O₃ and, hence, sintering proceeds via a nucleation and growth mechanism. The rate-limiting step is nucleation of -Al₂O₃. Consequently, the contacts between the elementary alumina particles dominate the sinter process. The contact between the acicular elementary particles of pseudoboehmite and-Al₂O₃ studied leads to the reaction to -Al₂O₃ to be almost complete after keeping samples for 145 h at 1050 °C. Decomposition of alum produces very small particles showing negligible mutual contacts. Consequently an elevated thermal stability is exhibited. Treatment of the alumina ex alum with water and drying results in a xerogel in which contact between elementary particles is much more intimate. Accordingly, treatment at 1050 °C causes a sharp drop in surface area.     

Synthesis and characterization of γ-Bi2O3 based solid electrolyte doped with Nb2O5

γ-phase bismuth oxide is a well known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). This study aims to determine new phases of Bi₂O₃-Nb₂O₅ binary system and the temperature dependence of the electrical transport properties. The reaction products obtained in open air atmosphere were characterized by X-ray powder diffractions (XRD). The unit cell parameters were defined from the indexes of the powder diffraction patterns. The γ-Bi₂O₃ crystal system were obtained by doping 0.01 < mole% Nb₂O₅ < 0.04 at 750 °C for 48 and 96 h. Thermal behaviour and thermal stability of the phases were investigated by thermal analysis techniques. Surface and grain properties of the related phases were determined by SEM analysis. The temperature dependence of the electrical properties of γ-Bi₂O₃ solid solution was measured by four-point probe d.c. conductivity method. In the investigated system, the highest value of conductivity was observed for σ _T = 0.016 ohm^?1 cm^?1 at 650 °C on 4 mole% Nb₂O₅ addition. The electrical conductivity curves of studied materials revealed regular increase with temperature in the form of the Arrhenius type conductivity behaviour.     

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.