Preparation and characterization of α-Fe2O3–CeO2 composite

In our previous study we attempted to see the effect of cerium doping (Ce/Fe ratio 0.015 to 0.074) on goethite matrix and conversion of doped goethite to hematite. In the present communication, nano-structured α-Fe₂O₃–CeO₂ composite with Fe/Ce weight ratio as 1.1 has been synthesized by calcination of goethite-cerium hydroxide precursor prepared by co-precipitation method. It was observed that co-precipitation of cerium along with iron in hydroxide medium resulted in hindering the formation of crystalline order as the precursor formed showed poorly crystallized goethite and almost no crystallinity in Ce(OH)₄. Calcination of the precursor at 400 °C showed the formation of hematite together with a broad peak corresponding to cerium oxide whereas at 800 °C, two distinct phases of α-Fe₂O₃ and CeO₂ were observed. The Mössbauer spectra showed the presence of a paramagnetic component both for the precursor as well as for the sample calcined at 400 °C but on raising the calcination temperature to 800 °C, the paramagnetic component disappeared and the spectrum corresponding to pure α-Fe₂O₃ phase was observed. The microstructure of the product obtained by calcining at 800 °C showed rod like structure (30 to 50 nm width and 300 to 500 nm length) of α-Fe₂O₃ having equi-dimensional CeO₂ particles on and around the surface. Besides the rods, equi-dimensional particles and agglomerates corresponding to CeO₂ were also observed. The results show that co-precipitation followed by calcinations gives nanorods hematite with CeO₂ particles bonded to its surface.     

Thermal decomposition of β-FeOOH

β-FeOOH particles were prepared by a forced hydrolysis of the 0.1 M FeCl₃ + 5·10⁻³ M HCl solution, whereas sulfated β-FeOOH particles were prepared by forced hydrolysis of the 0.1 M FeCl₃ solution containing 5·10⁻³ M quinine hydrogen sulfate (QHS). β-FeOOH particles, as well as sulfated β-FeOOH particles, were thermally treated up to 600 °C. The samples were characterized using DTA, XRD, FT-IR and TEM. β-FeOOH particles showed a cigar-type morphology, whereas bundles of β-FeOOH needles were obtained in the presence of QHS. Heating of β-FeOOH particles at 300 °C and above yielded α-Fe₂O₃ particles. Specific adsorption of sulfate groups showed a strong effect on the thermal decomposition of β-FeOOH particles. Upon heating of sulfated particles between 300 and 500 °C the formation of an amorphous phase and a small fraction of α-Fe₂O₃ were observed. Needle-like morphology of amorphous particles in these samples was preserved. At 600 °C, α-Fe₂O₃ particles were obtained; however, they were much smaller than those obtained by heating a pure β-FeOOH.     

An investigation of thermal decomposition of β-FeOOH nanowire arrays assembled in AAO templates

β-FeOOH nanowire arrays were assembled into porous anodized aluminum oxide (AAO) templates by electrochemical deposition in the mixture solution of FeCl₃ and (NH₄)₂C₂O₄. In order to obtain well-crystallized α-Fe₂O₃ and other iron oxides nanowires, β-FeOOH nanowire arrays with amorphous crystal structure were heat-treated at different temperatures from 200 to 600 °C. The decomposition products were characterized by DTA, XRD, FTIR, and Mössbauer spectroscopy. When heat-treated at 200 °C, only 65% of β-FeOOH decomposed, whereas, when the temperature was up to 300 °C, it was completely decomposed and formed poorly crystallized β-Fe₂O₃. This transition temperature is higher than the 200 °C obtained on other β-FeOOH materials. However, when heated above 300 °C, the main products are characterized as poorly crystallized α-Fe₂O₃ nanowires, whereas, well-crystallized α-Fe₂O₃ nanowire arrays can be formed when the temperature was up to 600 °C, and this temperature is also higher compared with those temperatures observed on other β-FeOOH materials. From Mössbauer results, the α-Fe₂O₃ nanowires were composed of fine particles in which 66% of the particles are superparamagnetic.     

Sodium niobate particles with controlled morphology synthesized by hydrothermal method and their use as templates in KNN fibers

Sodium niobate – NN (NaNbO₃) powders were synthesized by hydrothermal process to be used as template particles in the fabrication of textured lead free piezoelectric ceramics. Sodium hexaniobate–Na₈Nb₆O₁₉⋅13H₂O particles with rod-like morphology were synthesized at 120 °C. Particles with needle-like morphology and Na₂(Nb₂O₆)(H₂O) phase started to form at temperatures of 130 °C and above. Synthesis at 150 °C yields particles with totally needle-like morphology and consisting entirely of the Na₂(Nb₂O₆)(H₂O) phase. Sodium niobate–NaNbO₃ particles with cubic morphology were synthesized at temperatures of 160 °C and above. Rod-like and needle-like morphology was retained even after annealing at 400 °C for 1 h. A preliminary study was also done to integrate these anisometric template particles in the preparation of textured potassium sodium niobate (KNN) fibers.     

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.     

Hydrothermal synthesis,characterization and thermal stability studies of α-Fe2O3 hollow microspheres

A simple, cost-effective hydrothermal technique was used in this study to successfully fabricate hollow α-Fe₂O₃ microspheres, using only fructose and anhydrous ferric chloride without any organic solvent or additive. The synthesized α-Fe₂O₃ hollow microspheres were characterized by X-ray diffraction spectroscopy (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Based on the results, the shell was composed of aggregated α-Fe₂O₃ nanoparticles, while the fructose-derived carbon core was decomposed during calcination, leaving a hollow interior. XRD analysis confirmed the presence of the α-phase and the absence of γ-phase Fe₂O_3. A mean diameter of 595 nm was estimated for the microspheres by the Gaussian fit of the histogram constructed from the diameters measured over the SEM images. EDX spectrum of the sample showed signals attributed to Fe and O, and a homogenous distribution of these elements was confirmed by elemental mapping studies. ATR-FTIR analysis confirmed the bending and stretching vibration modes of the Fe-O bond. TGA-DTA data depicted that thermal stability of α-Fe₂O₃ hollow microsphere was achieved at 480 °C and no weight loss was observed up to 1000 °C. High-temperature calcination results showed that the material can maintain its hollow morphology up to 700 °C. This material has potential applications in drug delivery, gas sensing, and lithium storage.     

One-step synthesis of hematite (α-Fe2O3) nano-particles by direct thermal-decomposition of maghemite

In this research work, α-Fe₂O₃ nano-particles were prepared by direct thermal-decomposition of γ-Fe₂O₃. Precursor powders (γ-Fe₂O₃) were synthesized by wet chemical method at room temperature and then, the precursors were subsequently calcined in air for 1 h at 500 °C. Samples were characterized by thermal gravimetric analysis (TGA), X-ray diffraction (XRD), energy dispersive spectra (EDS), infrared spectrum (IR) and transmission electron microscopy (TEM), respectively. The XRD, EDS, and IR results indicated that the synthesized α-Fe₂O₃ particles were pure. The TEM image showed that the α-Fe₂O₃ nano-particles were spherical and 18 ± 2 nm in size. Magnetic properties have been detected by a vibrating sample magnetometer (VSM) at room temperature. The γ-Fe₂O₃ and α-Fe₂O₃ nano-particles exhibited a super-paramagnetic and weak ferromagnetic behavior at room temperature, respectively. Using the present method, hematite nano-particles can be produced without expensive organic solvent and complicated equipment.     

Processings of iron oxides at room temperature. I. Solid state conversion of pure α-FeOOH into distinctive α-Fe2O3

A distinct α-Fe₂O₃ variety is formed when pure synthetic α-FeOOH is dry ground at ambient temperature in a mechanically driven mortar at > 40 hr periods. XRD and IR spectroscopy indicate that its structural order is distinct from protohematite (obtained by heating α-FeOOH to ～450°C) and from well crystallized hematite. According to the changes observed in the WHH (width at half height) and So (surface area) values vs. grinding time, the broadening of α-Fe₂O₃ diffraction peaks is attributed mainly to decreasing particle size. Preferential disruption of some lattice planes occurs during the irreversible solid state conversion of α-FeOOH → defect α-Fe₂O₃. Variations in size, color and eventually in water content of the progressively formed α-Fe₂O₃ particles are observed.     

Synthesis and vibrational properties of hematite (α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanoparticles

α- Fe₂O₃ nanoparticles have been synthesized by gel evaporation method in air at 300°C. The average size of as synthesized α-Fe₂O₃ nanoparticle was estimated to be 30 nm and the particles were of good crystalline nature. Shape of the nanoparticles were slightly deviated from spherical which is attributed to the asymmetric growth of primary nuclei. MicroRaman and X-ray diffraction results have shown mixed phases of α-Fe₂O₃ and γ-Fe₂O₃. However, the α-Fe₂O₃ phase is more predominant than γ-Fe₂O₃ due to the incomplete nucleation of α-Fe₂O₃ particles at the size of 30 nm. The vibrating sample magnetometer measurement shows that the nanoparticles possess ferromagnetic property.     

Templating synthesis of waxberried hematite hollow spheres

Novel hematite (α-Fe₂O₃) hollow spheres were prepared through a surfactant-assisted solvothermal process. The XRD, SEM and TEM characterization data confirm that the formation of α-Fe₂O₃ hollow spheres exhibits waxberry-like architectures with spindle nanoparticles, the length in the range of 150-400 nm, as building block. Their tips of these nanoparticles were concentrated on a center. The sizes of α-Fe₂O₃ waxberry are less than 3 µm. They possess good photocatalytic properties when used for the degradation of salicylic acid in water. The formation mechanism of α-Fe₂O₃ waxberry is also discussed.     

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.     

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.     

Structural characterization and catalytic activities of titania supported iron (III) oxide catalysts: Effect of Li+ impregnation

Pure Fe₂O₃/TiO₂ catalysts (1–35 wt.% Fe₂O₃) and Li-impregnated Fe₂O₃/TiO₂ catalysts (1–10 wt.% Li₂O) were prepared. The structural characteristics of pre-calcined samples were examined by using X-ray diffraction (XRD), thermogravimetric analysis (TGA) and ESR techniques. The surface acidity was measured by the adsorption of n-butylamine in non-aqueous media. The catalytic conversion of 2-propanol at 320–360 °C using the microcatalytic pulse technique was studied.XRD of 650 °C-calcination products showed the existence of pure oxide phases as well as other spinels depending on the chemical composition. ESR spectra indicated the existence of Fe(III) in α-Fe₂O₃ phase and in spinel phase. The chemical composition, surface acidity of the catalyst and reaction temperature affect the catalytic conversion of 2-propanol.     

Thermal decomposition synthesis of 3D urchin-like α-Fe2O3 superstructures

Urchin-like α-Fe₂O₃ superstructures have been deposited on Si substrate using thermal decomposition FeCl₃ solution at 200–600 °C in the oven. The morphologies and structures of the synthesized urchin-like superstructures have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that urchin-like α-Fe₂O₃ superstructures were a polycrystal with the rhombohedral structure and typical diameters of 16–20 nm and lengths up to 1.0 μm. The as-prepared α-Fe₂O₃ superstructures have a high Brunauer–Emmett–Teller (BET) surface area of about 60.24 m²/g. The photoluminescence spectrum of the urchin-like α-Fe₂O₃ superstructures consists of one weak emission peak at 548 nm (2.26 eV). A possible new mechanism for the formation of the urchin-like superstructures was also preliminarily discussed.     

In situ characterization of Cu–Fe–O<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> catalyst for water–gas shift reaction

Cu–Fe–O catalyst consisting of α-Fe₂O₃ with supported CuO nanoparticles was prepared through co-precipitation. Integrated analyses of surface and bulk of the catalyst particles showed that it consists of α-Fe₂O₃ with supported CuO nanoparticles. The supported CuO can be reduced to metallic Cu at a temperature of 100–150 °C with a following transformation of α-Fe₂O₃ to Fe₃O₄ at a temperature of 150–200 °C. The composition of Cu/Fe₃O₄ was identified by in situ X-ray diffraction, high-angle annular dark field scanning transmission electron microscope and ambient pressure X-ray photoelectron spectrometer. The formed Cu/Fe₃O₄ exhibits high activity for water–gas shift reaction in the temperature range of 180–250 °C. Activation barrier of WGS on Cu/Fe₃O₄ is lower than Cu/Al₂O₃ by 10–15 kJ/mol, suggesting that Fe₃O₄ participates into the WGS in the low-temperature range. Ambient pressure X-ray photoelectron spectroscopy studies show that the active surface phase during WGS consists of metallic Cu nanoparticles and Fe₃O₄ nanoparticles.     

Optical and magnetic properties of elliptical hematite (α-Fe2O3) nanoparticles coated with uniform continuous layers of silica of different thickness

Elliptical-type α-Fe₂O₃ nanoparticles with/without silica shell have been prepared. The core particles were coated with uniform continuous layers of silica of two different thicknesses by hydrolysis of TEOS. The obtained HCP structure elliptical α-Fe₂O₃ nanoparticles with ∼ 240 nm length and 100 nm width is polycrystalline in nature. The thicknesses of SiO₂ shell coated on α-Fe₂O₃ are about 55 and 30 nm, respectively. The optical and magnetic properties of these nanoparticles have been investigated.     

The effect of temperature on the crystallization of α-Fe2O3 particles from dense β-FeOOH suspensions

The effect of temperature on the crystallization of α-Fe₂O₃ particles from dense β-FeOOH suspensions was monitored by ⁵⁷Fe Mössbauer spectroscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and energy dispersive spectroscopy. Dense suspensions of very long laterally arranged β-FeOOH fibrils were obtained at 90 °C. Crystallization at 120 °C between 18 and 72 h yielded monodisperse α-Fe₂O₃ particles of a shape close to that of double spheres with ring. The double spheres with ring showed two narrow particle size distributions. In these particles a substructure was detected, i.e., the spheres consisted of the linear chains of interconnected α-Fe₂O₃ subparticles. With further rise in the crystallization temperature the increase in α-Fe₂O₃ particles and porosity became pronounced. Obviously, the aggregation mechanism played an important role in the formation of α-Fe₂O₃ particles.     

Influence of nano-particle size on the oxidation behaviour of Al,Fe and Cu

Abstract

The addition of ultra-fine and nano-sized particles in composite materials influences their properties. In order to understand the influence of the nano-size of metal particles on the oxidation behaviour, a series of in situ studies by thermogravimetry (TG) and high temperature X-ray diffraction (XRD) was performed on Al, Fe and Cu during heating with post oxidation analysis by field emission – scanning electron microscope (FE-SEM). The oxidation reaction depends, for all three studied metals, on the particle size both in regard to transient oxide formation and formation temperatures. On a nano-scale Al forms θ-Al₂O₃ and γ-Al₂O₃ simultaneously at 500°C, but both of these oxides transform to α-Al₂O₃ at 975°C producing nano-scaled oxide particles. Aluminium particles with sizes from 2 to 25 μm form α-Al₂O₃ only, starting at temperatures close to the melting point. Nano-sized Fe forms on heating from 340°C α-Fe₂O₃ only and no other oxides. On nano-sized Cu particles the formation of Cu₂O starts at 140°C, transforming completely to CuO at 300°C.     

A mild solvothermal route to α-Fe2O3 nanoparticles

A mild solvothermal route has been developed to synthesize α-Fe₂O₃ nanoparticles using Fe(NO₃)₃ as a starting material. The results from XRD and TEM indicate the α-Fe₂O₃ powders possess a rhombohedrally centered hexagonal structure, and the size of particles from alcohothermal method at 160 °C is about 50-100 nm.     

Preparation of polycrystalline YAG/alumina composite fibers and YAG fiber by sol–gel method

Polycrystalline yttrium–aluminum garnet, Y₃Al₅O₁₂ (YAG) fiber and α-alumina and YAG matrix composite fiber were prepared by the sol–gel method. α-Alumina and YAG matrix composite fiber with fine and homogeneous microstructure could be successfully fabricated by interpenetrating YAG in alumina matrix and adding α-alumina of seed particles to fibers. Effect of α-alumina seed particles and YAG on crystallization and microstructure of composite fiber were discussed. The size of alumina matrix of the composite fibers heated at 1600°C for 4 h was below 2 μm. The tensile of strength alumina fiber heat-treated at 1500°C was 0.2 GPa, while that of the composite fiber was 1.1 GPa.