Preparation and combustion properties of α-Fe2O3 coated Zr particles

Zirconium particles with irregular morphology and broad size distribution were uniformly coated by spherical α-Fe₂O₃ crystal grain via a facile route without polymer or surfactant as directing agents. The synthesized α-Fe₂O₃/Zr composite particles were characterized by X-ray diffraction, scanning electron microscopy, energy dispersion X-ray, UV-vis spectroscopy and Raman spectroscopy. The synthesis mechanism could be explained by cooperated heterogeneous nucleation and solid state transformation reaction. The combustion properties of α-Fe₂O₃/Zr composite particles were investigated. Compared with Zr particles, the combustion lasting time decreased from 16 s of Zr particles to 0.13 s of α-Fe₂O₃/Zr composite particles, and the top point of temperature reached in combustion increased from 2004 °C of Zr particles to 2378 °C of α-Fe₂O₃/Zr particles.     

The influence of different electrodeposition E/t programs on the photoelectrochemical properties of α-Fe2O3 thin films

In this work morphological, structural and photoelectrochemical properties of n-type α-Fe₂O₃ (hematite) thin films synthetized by means of two different electrochemical procedures: potential cycling electrodeposition (PC) and potential pulsed electrodeposition (PP) have been studied. The X-ray diffraction measurements showed that the films obtained after a thermal treatment at 520 °C present a nanocrystalline character. Scanning electron microscopy allowed finding that hematite films obtained by PP technique exhibit nanostructured morphology. The electrochemical and capacitance (Mott-Schottky and parallel capacitance) measurements showed that when in the PC and PP procedures the anodic limit E_λ,A is being made more anodic, a decrease of the majority carriers concentration (N_D) and the surface states number has been observed. The photovoltammetry measurements indicated that the hematite films formed with the PP technique present a photocurrent one order of magnitude higher than the ones exhibited by the iron oxide films formed by PC. For instance, PP hematite films exhibit photovoltaic conversion efficiencies of 0.96% which are 2.5 times higher than the corresponding to the PC ones (0.38%). The maximum incident photon-to-current efficiency measured at λ = 370 and 600 nm was observed for hematite films grown by the PP procedure. By means of the photocurrent transient technique a decrease in the recombination process for those samples synthesized by PP was observed. The results obtained are discussed considering the influence of the anodic limit of the potential employed during the preparation of the iron oxyhydroxide (β-FeOOH) precursor film, all of this related to a decrease of the oxygen defects in this material and to a decrease of Fe(II) amount that is formed during the electrodeposition process.     

Preparation and characterization of γ-Fe2O3/ZnO composite particles

γ-Fe₂O₃/ZnO composite particles were prepared via a simple solution method using surface-modified γ-Fe₂O₃ nanoparticles as seeds. The phases and purity of the as-prepared γ-Fe₂O₃/ZnO composite particles were characterized by XRD analysis, and the morphology was studied by SEM, which showed that the γ-Fe₂O₃/ZnO composites are of typical sphere-like morphology with diameters in the range of 300-400 nm. The γ-Fe₂O₃/ZnO composites exhibit magnetic response to an external magnet field and efficient characteristic emissions of ZnO under UV excitation, respectively, indicating that these nontoxic, emissive and magnetic nanoparticles may find use as chemical/biological sensors especially in areas that directly impact human health.     

Structural and magnetization studies on nanoparticles of Nd doped α-Fe2O3

It has been observed that, compared to bulk form, the nanocrystalline α-Fe₂O₃ is finding application in various areas. Magnetic properties of α-Fe₂O₃ are found to be influenced by the size of particles and are also sensitive to synthesis method employed for sample preparation. In the present work we have prepared a series of Nd doped α-Fe_2−xO₃ samples (x = 0.0–0.5) by combustion method, without using any fuel. The analysis of room temperature neutron diffraction patterns shows that all the compounds of the series form in the hematite (α-Fe₂O₃) structure, space group R−3c. Magnetization measurements show that there is a broad distribution of particle size in the samples. We find that the increase in the Nd content results in the dilution of magnetism of α-Fe₂O₃. From results we believe that inclusion of Nd in α-Fe₂O₃ drastically modifies the magnetic properties.     

The influence of carboxylate ions on the growth of β-FeOOH particles

Colloidal -FeOOH particles were produced by hydrolysis of FeCl₃ solutions doped with varied amounts of formate, lactate, oxalate, tartarate, pyromellitate and EDTA ions at 100 °C. The resulting particles were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric-differential thermal analysis, Fourier transform-infrared spectroscopy, elemental analysis, and the adsorption of nitrogen and water. With increasing concentration of added carboxylate ions, the mean modal size of the particles formed increased at low concentrations and decreased after passing the maximum. The crystallite sizes also revealed a maximum on adding EDTA, oxalate and lactate ions, while they monotonically decreased with the addition of other ions. Doping with tartarate and pyromellitate ions by more than 7 and 10 mol%, respectively, produced amorphous particles containing a large amount of these ions. The particles formed with 20 mol% tartarate ions adsorbed water selectively.     

Anomalous behaviour of the magnetic hysteresis loop in the α-Fe2O3/SiO2 nanocomposite

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt.% of hematite was prepared by the sol-gel method and characterized by using X-ray diffractometer (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometer. TEM microscopy showed spherical particles with average size about 10 nm, whereas XRD diffraction confirmed the formation of the hematite phase. The magnetic measurements showed anomalous behavior of the hysteresis loops including decrease of high field isothermal magnetization and overlap of initial, remagnetization and magnetization curves. This anomalous behavior represents a novel effect for α-Fe₂O₃/SiO₂ nanocomposites. We conjecture that a field-induced antiferromagnetic coupling between nanoparticles may produce this effect.     

Morphological and magnetic properties of the hydrothermally prepared α-Fe2O3 nanorods

α-Fe₂O₃ nanorods were synthesized via hydrothermal method. X-ray powder diffraction revealed the formation of rhombohedral α-Fe₂O₃ single crystal phase with fiber texture. Scanning and transmission electron micrographs analyses showed that the rhombohedral α-Fe₂O₃ has nanorods in shape with diameters of 40–85 nm and lengths of 150–45,000 nm. Isothermal magnetization vs. applied magnetic field curves measured at room and liquid nitrogen temperatures displayed a variation on magnetic ordering: from weak ferromagnetism at room temperature to not hysteretic behavior at liquid nitrogen temperature that is well described by a Langevin function. Moreover, the zero field cooling-field cooling curves under applied magnetic field of 100 Oe confirms the decreasing of Morin temperature transition due to nanometric size of the samples.     

Synthesis of monodisperse shuttle-like α-Fe2O3 nanorods via the EDA-assisted method

Monodisperse hematite shuttle-like nanorods were synthesized successfully by the ethylenediamine (EDA)-assisted method. The structure and morphology were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectrometer (EDS). XRD studies indicated that the as-prepared product was well-crystallized orthorhombic phase of α-Fe₂O₃. TEM and SEM images showed that the α-Fe₂O₃ nano-particles were of rod shape with an average length of 400 nm and diameter of about 80 nm in the middle part.     

Green synthesis of hematite (α-Fe2O3) submicron particles

Iron oxide microparticles have been synthesized through a green technique using hydrogen peroxide under sunlight irradiation. The X-ray powder diffraction measurement shows that these particles are hematite (α-Fe₂O₃). The microstructure and particle size were investigated using scanning electron microscopy. The magnetic characterization shows the presence of Morin transition at about 258 K, which is very close to the normal value (263 K) of bulk hematite. These hematite particles show typical antiferromagnetic behavior at low temperature and weak ferromagnetic behavior at room temperature.     

Preparation and characterization of hollow α-Fe2O3 irregular microspheres

Hollow α-Fe₂O₃ irregular microspheres were prepared at 160 °C from a hydrolyzing Fe(ClO₄)₃ solution by adding sodium polyanethol sulphonate. The particles were characterized by ⁵⁷Fe Mössbauer, X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. The walls of these hollow particles consisted of elongated subunits composed of elongated and thin α-Fe₂O₃ rods. The precipitation of hollow α-Fe₂O₃ irregular microspheres was governed by the preferential adsorption of sulphonate/sulphate groups. The lateral aggregation of elongated thin rods and subunits also played an important role in the formation of hollow α-Fe₂O₃ irregular microspheres.     

Template-free synthesis and characterization of snowflake-like α-Fe2O3 microstructures

Single-crystalline α-Fe₂O₃ with a micro-snowflake-like morphology has been synthesized though a hydrothermal reaction in a K₃[Fe(CN)₆] solution without the assistance of any template or surfactant. The morphology and structure of the synthesized hematite were characterized in detail by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A possible growth process of α-Fe₂O₃ crystals has been proposed, and NaOH plays a crucial role in the formation of the snowflake-like structure. Additionally, magnetic investigations show that the α-Fe₂O₃ crystals exhibit a weakly ferromagnetic property at room temperature with a coercive force of 134 Oe and remnant magnetization of 0.67 emu g^− 1.     

Microwave-assisted synthesis and humidity sensing of nanostructured α-Fe2O3

Nanocrystalline α-Fe₂O₃ has been prepared on a large-scale by a facile microwave-assisted hydrothermal route from a solution of Fe(NO₃)₃·9H₂O and pentaerythritol. A systematic study of the morphology, crystallinity and oxidation state of Fe using different characterization techniques, such as transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was performed. It reveals that nanostructured α-Fe₂O₃ comprises bundles of nanorods with a rhombohedral crystalline structure. The individual nanorod has 8-10 nm diameter and ∼50 nm length. The as-prepared nanostructured α-Fe₂O₃ (sensor) gives selective response towards humidity. The sensor shows high sensitivity, fast linear response to change in the humidity with almost 100% reproducibility. The sensor works at room temperature and rejuvenates without heat treatment. The as-prepared nanostructured α-Fe₂O₃ appears to be a promising humidity sensing material with the potential for commercialization.     

Effect of α-Fe2O3 addition on the morphological,optical and decolorization properties of ZnO nanostructures

Visible light sensitive photocatalysts of Fe₂O₃/ZnO nanocomposites were prepared by a simple solid-state reaction method, using zinc acetate, α-Fe₂O₃ and sodium hydroxide at room temperature. The products were characterized by scanning electron microscopy, powder X-ray diffraction, N₂ adsorption–desorption measurement, UV–vis absorption, and photoluminescence spectroscopy and used for photodecolorization of Congo red. The characterization results showed that the morphology, crystallite size, BET surface area and optical absorption of the samples varied significantly with the Fe³⁺ to Zn²⁺ ratios. The nanocomposites show two absorption edges at ultraviolet and visible region. The optical band gap values of these nanocomposites were calculated to be about 3.98–3.81 eV and 2.88–2.98 eV, which show a red shift from that of pure ZnO. These red shifts are related to the formation of Fe s-levels below the conductive band edge of ZnO and effectively extend the absorption edge into the visible region. The growth mechanisms of the samples are proposed. These nanocomposites showed high decolorization ability in visible light with wavelength up to about 400 nm. Among the samples, Fe₂O₃/ZnO nanoflower (molar ratio of Fe³⁺ to Zn²⁺ is 1:100) exhibited higher decolorization efficiency than the other nanocomposites. It could be considered as a promising photocatalyst for dyes treatment.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Effect of particle size distribution on the magnetic properties γ-Fe2O3 nanoparticles

The magnetic response of nanocomposites formed by non-interacting well dispersed γ-Fe₂O₃ nanoparticles in a polymer matrix is presented. Various low loading fraction of particles in polymer leads to an observation of similar values of blocking temperatures and coercive fields. ac response confirms that particles are non-interacting and follow Neel–Brown model. Effect of particle size distribution on hysteresis behavior and saturation magnetization as a function of temperature is discussed. Since particles have a size distribution, the experimental results of magnetic response are compared with simulations based on Stoner–Wohlfarth model of single size particles. We have devised a measurement method in which a constant magnetic field was applied while the thermal energy is varied by sequentially heating and cooling the sample below the blocking temperature. Nanoparticle–polymer composites show reversible magnetization behavior for sequential heating/cooling cycles. However, simulation based on single size particle system shows irreversible magnetization behavior during the heating and cooling cycles. These observations are qualitatively explained in terms of different behavior of magnetization as a function of temperature for smaller superparamagnetic particles and larger blocked particles below overall blocking temperature of the composite.     

Phase-transitions of α- and β-Bi2O3 nanowires

The transition temperatures for the α or β to δ and the δ to liquid phase for α- and β-Bi₂O₃ nanowires were investigated. We found that there is a size effect for the δ to liquid phase transition but not the α or β to δ phase transition. This is because the δ to liquid phase transition involves bond rupture as well as surface area reduction, whereas the α or β to δ is only solid-solid phase transition, which requires only the reorganization of the bonds. This is the first time the phase transitions of the Bi₂O₃ nanowires have been investigated and their size effect revealed.     

Novel rose-type magnetic (Fe3O4, γ-Fe2O3 and α-Fe2O3) nanoplates synthesized by simple hydrothermal decomposition

Rose-type magnetic nanoplates (RTMNPs) were synthesized by a simple hydrothermal decomposition method where FeCl₂·4H₂O was solely used as a precursor. The synthesized nanoplates were characterized using XRD, FE-SEM, UV-vis absorption (reflectance) spectra and magnetic hysteresis loops. The resulting nanoplates were in the ranges of size 350-500 nm and width 60-70 nm with high crystallinity, purity (shown by XRD) and reproducibility. These iron oxide nanoplates have a great potential in magnetic nanodevices and biomagnetic applications.     

Preparation and characterization of α-Fe2O3 polyhedral nanocrystals via annealing technique

Polyhedral nanocrystals of α-Fe₂O₃ are successfully synthesized by annealing FeCl₃ on silicon substrate at 1000 °C in the presence of H₂ gas diluted with argon (Ar). Uniformly shaped polyhedral nanoparticles (diameter ~ 50-100 nm) are observed at 1000 °C and gases flow rate such as; Ar = 200 ml/min and H₂ = 150 ml/min. Non-uniform shaped nanoparticles (diameter ~ 20-70 nm) are also observed at an annealing temperature of 950 °C with lower gases flow rate (Ar = 100 ml/min and H₂ = 75 ml/min). Nanoparticles are characterized in detail by field-emission electron microscopy (FE-SEM), energy dispersive X-ray (EDX) and high resolution transmission electron microscopy (HRTEM) techniques. HRTEM study shows well resolved (110) fringes corresponding to α-Fe₂O₃, and selected area diffraction pattern (SADP) confirms the crystalline nature of α-Fe₂O₃ polyhedral nanoparticles. It is observed that polyhedral formation of α-Fe₂O₃ nanocrystals depends upon annealing temperature and the surface morphology highly rely on the gas flow rate inside the reaction chamber.     

Fabrication and magnetic property of α-Fe2O3 nanoparticles/TiO2 nanowires hybrid structure

α-Fe₂O₃ nanoparticles/TiO₂ nanowires hybrid structure is fabricated by two-step hydrothermal treatment. TiO₂ nanowires are prepared by heating of titanate nanowires, which are obtained by hydrothermal treatment of TiO₂ powder and further repeated HCl treatment. α-Fe₂O₃ nanoparticles are deposited on the surface of TiO₂ nanowires by hydrothermal treatment in Fe(NO₃)₃ solution. The HRTEM images confirm the junctions between α-Fe₂O₃ nanoparticles and TiO₂ nanowires. The formation of hybrid structures has significant influence on the magnetic properties of α-Fe₂O₃. The Morin transition temperature of α-Fe₂O₃ nanoparticles/TiO₂ nanowires hybrid structure is 190 K, which is determined by the sharp peak in the differential ZFC curve. Whereas there is no observable Morin transition for the corresponding isolated α-Fe₂O₃ nanoparticles with similar average particles size of ca. 20 nm.     

Synthesis,characterization and electrochemical properties of three-dimensionally ordered macroporous α-Fe2O3

Three-dimensionally ordered macroporous (3DOM) α-Fe₂O₃ electrode materials with large pore sizes and interconnected macroporous frameworks were successfully synthesized by a simply modified colloidal crystal templating strategy. The obtained samples were characterized by means of thermogravimetry, powder X-ray diffraction, nitrogen physisorption, scanning and transmission electron microscopy. The electrochemical properties of the 3DOM α-Fe₂O₃ were evaluated with cyclic voltammetry and discharge–charge experiments in an organic electrolyte containing a lithium salt. The results showed that the 3DOM α-Fe₂O₃ possessed a potential to be used as an anode material for lithium ion batteries with high initial discharge and charge capacities of 1883 and 1139 mAh g⁻¹, respectively. After 60th cycle, the reversible capacity could still be as high as 681 mAh g⁻¹ with a stable Coulombic efficiency of around 95%.