Synthesis and formation mechanism of hematite hollow microspheres by a one-pot templateless surfactant-free hydrothermal process

Hollow microspheres of hematite (α-Fe₂O₃) were prepared by a simple hydrothermal treatment of ferrous ammonium sulphate hexahydrate ((NH₄)₂Fe(SO₄)₂·6H₂O) aqueous solution without any surfactants and additives at 180–200 °C. The products were characterized by XRD, FE-SEM and TEM. The results show that α-Fe₂O₃ hollow microspheres are about 1 μm in diameters and are composed of α-Fe₂O₃ nanoparticles with a diameter range from 50 nm to 150 nm. The effects of reaction parameters such as reaction time, temperature and [(NH₄)₂Fe(SO₄)₂·6H₂O] on the morphology of the final product were investigated. A potential formation mechanism of α-Fe₂O₃ hollow microspheres was also proposed, where the intermediate, urchin-like goethite, served as a sacrificial template for the formation of hollow structure. The reaction parameters-depended morphologies of the final products consisted well with the proposed formation mechanism.     

Prospects for the incorporation of cobalt into α-Fe2O3 nanorods during hydrothermal synthesis

A feasibility study on the incorporation of cobalt into α-Fe₂O₃ nanorods (NRs) during hydrothermal synthesis (HS) is presented as a function of FeCl₃ and CoCl₂ concentration, phosphate surfactant concentration and pH value, with samples assessed using X-ray diffractometry, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis. No evidence was found for the incorporation of cobalt into α-Fe₂O₃ NRs at low pH, whilst synthesis at intermediate and high pH values favoured the formation of CoFe₂O₄ NPs. The critical role of pH value over the precipitation, size and phase purity of the nanostructured reaction products is emphasised. At pH ~2, large, well crystalline α-Fe₂O₃ nanoparticles (NPs) and NRs were grown from FeCl₃ solution in the absence and presence of phosphate, respectively, whilst no evidence was found for Co precipitation or incorporation in α-Fe₂O₃ following HS in the presence of CoCl₂. At pH ~8, smaller α-Fe₂O₃ NPs, as well as Co₃O₄ and CoFe₂O₄ NPs were synthesised from FeCl₃, CoCl₂, or a mixture thereof. HS at pH ~12 produced a mixture of larger CoFe₂O₄ NPs and α-Fe₂O₃ NPs depending on the Fe:Co molar ratio. The formation of intermediate metastable (oxy)hydroxide phases is considered pH dependent, providing for a variety of different reaction pathways. Further, inclusion of preformed Co₃O₄ and CoFe₂O₄ NPs to the FeCl₃ solution at pH ~2 in the presence of phosphate surfactant resulted in the synthesis of α-Fe₂O₃ NRs with residual Co₃O₄ and CoFe₂O₄ NPs attached to their surfaces. The CoFe₂O₄ NPs encouraged local dissolution leading to the formation of α-Fe₂O₃ NR surface corrugations.     

Fabrication of CuO/Fe2O3 hollow hybrid microspheres

CuO/Fe₂O₃ hollow hybrid spheres with the size of 3–5 μm were successfully synthesized by a convenient hydrothermal method, using FeSO₄·7H₂O and CuSO₄·5H₂O as the starting materials and urea as the homogeneous precipitant. The samples were characterized by XRD, TEM, ED, SEM, EDX, IR and XPS measurements. XRD and XPS analyses indicated that the nanostructured materials consisted of CuO and α-Fe₂O₃. TEM and SEM measurements showed that the morphology of binary metal oxide was in the shape of hollow sphere. Careful observation from SEM measurements could find that CuO/Fe₂O₃ hollow microsphere shell was composed of uniform and dense metal oxide nanorods with about 20–40 nm in diameter and 100–200 nm in length. Moreover, the influence of calcination temperature on the thermal stability of the hollow structures was investigated. It showed that the hollow structure was stable after being calcined at 300 °C for 2 h. The formation mechanism of the CuO/Fe₂O₃ hollow spheres under hydrothermal condition was discussed.     

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.     

Preparation of γ-Fe2O3 nanopowders by direct thermal decomposition of Fe-urea complex: reaction mechanism and magnetic properties

In this work, a novel method of producing maghemite (γ-Fe₂O₃) nanopowders has been developed, which can be performed by the direct thermal decomposition of an Fe–urea complex ([Fe(CON₂H₄)₆](NO₃)₃) in a single step. The reaction mechanism, particle morphology, and the magnetic properties of the γ-Fe₂O₃ nanopowders have been studied by using thermogravimetric (TG), differential scanning calorimetry (DSC), fourier transformed infrared (FTIR) spectroscopy, elemental analysis, X-ray powder diffraction (XRD), transmission electron micrograph (TEM) observations, and magnetic measurements. Thermal analyses together with the results of XRD show that the formation of γ-Fe₂O₃ occurs at ~200 °C through a two-stage thermal decomposition of the [Fe(CON₂H₄)₆](NO₃)₃ complex. The resulting iron oxide phases (i.e., γ-Fe₂O₃ and α-Fe₂O₃) are strongly dependent on the synthesis conditions of the [Fe(CON₂H₄)₆](NO₃)₃. When the molar ratio of Fe(NO₃)₃ · 9H₂O to CON₂H₄ that is used for the synthesis of [Fe(CON₂H₄)₆](NO₃)₃ is 1:6 (i.e., molar ratio in stoichiometry), a mixed phase of γ-Fe₂O₃ and α-Fe₂O₃ is formed. When the molar ratio is 1:6.2 (i.e., using an excess CON₂H₄), on the other hand, a pure γ-Fe₂O₃ is obtained. Magnetic measurements show that resulting nanopowders exhibit a ferromagnetic characteristic and their maximum saturation magnetization increases from 47.2 to 67.4 emu/g with an increase in the molar ratio of Fe(NO₃)₃ · 9H₂O to CON₂H₄ from 1:6 to 1:6.2.     

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.     

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.     

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.     

Mechanochemical synthesis of stoichiometric MgFe2O4 spinel

The influence of long-term milling of a mixture of (1) MgO and α-Fe₂O₃, (2) MgCO₃, and α-Fe₂O₃, and (3) Mg(OH)₂ and α-Fe₂O₃ powders in a planetary ball mill on the reaction synthesis of nanosized MgFe₂O₄ ferrites was studied. Mechanochemical reaction leading to formation of the MgFe₂O₄ spinel phase was followed by electron microscopy, (SEM and TEM), X-ray diffraction and magnetization measurements. The spinel phase was observed first in cases (1) and (2) after 5 h of milling, and its formation was observed in all cases after 10 h. The synthesized MgFe₂O₄ ferrite has a nanocrystalline structure with a crystallite size of about 11, 10, and 12 nm, respectively for cases (1)–(3). Magnetic measurements after 10 h of milling show magnetization values of 19.8 J/(Tkg), 23.5 J/(Tkg) and 13.8 J/(Tkg), respectively for the cases (1)–(3).     

Ferrite grade iron oxides from ore rejects

Iron oxyhydroxides and hydroxides were synthesized from chemically beneficiated high SiO₂/Al₂O₃ low-grade iron ore (57.49% Fe₂O₃) rejects and heated to get iron oxides of 96–99.73% purity. The infrared band positions, isothermal weight loss and thermogravimetric and chemical analysis established the chemical formulas of iron-oxyhydroxides as γ-FeOOH.0.3H₂O; α-FeOOH.0.2H₂O and amorphous FeOOH. The thermal products of all these were α-Fe₂O₃ excepting that of γ-FeOOH.0.3H₂O which gave mainly γ-Fe₂O₃ and some admixture of α-Fe₂O₃. The hydrazinated iron hydroxides and oxyhydroxides, on the other hand, decomposed autocatalytically to mainly γ-Fe₂O₃. Hydrazine method modifies the thermal decomposition path of the hydroxides. The saturation magnetization,J _s, values were found to be in the range 60–71 emu g⁻¹ which are close to the reported values for γ-Fe₂O₃. Mechanism of the γ-Fe₂O₃ formation by hydrazine method is discussed.     

Synthesis of magnetite (Fe3O4) nanoparticles without surfactants at room temperature

Iron oxide nanoparticles in the interval of 4–43 nm were synthesized by a colloidal method at room temperature, without use of surfactants and using precursors like FeCl₃·6H₂O and FeCl₂·4H₂O; deionizated water free of dissolved oxygen and ammonia solution (29% vol.) and using several aging times (2, 5 and 10 min). A detailed study by X- ray diffraction (XRD), Conventional Transmission Electron Microscopy (CTEM), High-Resolution Transmission Electron Microscopy (HRTEM) and electron diffraction patterns showed that with a reaction time less than 5 min nanoparticles of magnetite phase (Fe₃O₄) were synthesized, and with a bigger time of reaction the lepidocrocite phase (FeO(OH)) was identified. The minor particle average size measured was 6 nm in the sample, 0.0125 M with 2 min of aging time (0.0125M2 m). In addition it was possible to obtain a narrow nanoparticle size dispersion from 4 to 10 nm for small aging times.     

Hematite solid and hollow spindles: Selective synthesis and application in gas sensor and photocatalysis

Hematite solid spindles and hollow spindles have been selectively synthesized by a template-free, economical hydrothermal method, using FeCl₃·6H₂O as the starting materials and NaOH as the homogeneous precipitant. XRD analyses indicated that the products consisted of α-Fe₂O₃. SEM and TEM measurements showed that the morphologies of products were in the shape of solid spindles and hollow spindles, respectively. A possible formation process based on the nucleation-oriented aggregation-recrystallization mechanism is proposed. Moreover, the as-prepared hollow spindle-like α-Fe₂O₃ exhibits a good response and reversibility to some organic gas, such as 2-propanol and acetone. Compared with other hematite nanostructures, the porous hollow hematite spindles show outstanding performance in gas sensing due to their large surface area and porous hollow structure. Because of the unique porous hollow structures of the samples, the photocatalytic property of the spindle-like α-Fe₂O₃ was also investigated.     

Synthesis of hexagonal Al2−x Cr x O3 nanodisks by a thermal decomposition of mesoporous Cr4+:Al2O3 powders

Monodispersed hexagonal Al_2−x Cr _x O₃ nanodisks are synthesized through a reactive doping of Cr⁶⁺ cations in a hydrogenated mesoporous AlO(OH)·αH₂O powder followed by annealing at 1,200 °C in air. The reaction was carried out by a drop wise addition of an aqueous Cr⁶⁺ solution (0.5–1.0 M) to AlO(OH)·αH₂O, at room temperature. Al_2−x Cr _x O₃ nanostructure formation was controlled by the nucleation and growth from the intermediate amorphous mesoporous Cr⁴⁺:Al₂O₃ composites in the temperature range 400–1,000 °C. The nanodisks of ∼50 nm diameter and thickness of ∼16 nm is observed in the sample with x of 0.2 and similar nanodisks with a low dimension is observed at a higher value of x of 1.6 (after 2 h of heating at 1,200 °C). The Cr³⁺ ↔ Al³⁺ substitution, x ≤ 1.2, inhibits grain growth in small crystallites. The crystallites in x = 0.2 composition have 43 nm diameter while it is 15 nm in those with x = 1.2 composition.     

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 of orthorhombic and cubic PbF<Subscript>2</Subscript> by hydrothermal method

Orthorhombic (α-) and cubic (β-) PbF₂ have been successfully synthesized via a simple hydrothermal process at 200 °C for 8 h using Pb(C₂H₃O)₂ and NH₄F as the raw reaction materials. The crystal structure, morphology, and optical properties of the as-synthesized samples were characterized by X-ray powder diffraction, scanning electron microscopy, and photoluminescence spectroscopy. XRD and SEM results show that the uniform β-PbF₂ microspheres and porous-microspheres with the average diameter about 3 and 5 μm, respectively, were synthesized assisting with citric acid and α-PbF₂ microrods and microtubes with a diameter about 15 and 10 μm, respectively, were synthesized assisting with acetic acid. The reaction conditions influencing the synthesis of these PbF₂ microstructures, such as reaction time, the amount of CTAB, and the molar ratio of F/Pb were investigated. On the basis of a series of observations, phenomenological elucidation of a mechanism for the growth of the β-PbF₂ microspheres and multispheres has been presented. Room-temperature photoluminescence measurements indicated that the as-prepared α-PbF₂ microrods exhibit a strong green emission.     

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.     

Surfactant-assisted solvothermal preparation of submicrometer-sized hollow hematite particles and their photocatalytic activity

Submicrometer-sized hollow hematite particles were prepared through a surfactant-assisted solvothermal process. The amount of FeCl₃·H₂O and cetyltrimethylammonium bromide, and the acidity of the solution were systematically altered to study their effects on the final results. Hollow hematite particles with shapes from sphere, ellipsoid to peanut were obtained. Their sizes range from 500 nm to 2 μm with shell thickness from 100 to 500 nm. Powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and selected area electron diffraction were applied to investigate the products’ crystallinity, purity, morphology, size and structural features. Finally, the study on the photocatalysis of Fe₂O₃ for the destruction of diethyl phthalate in water was carried out. The result proved that Fe₂O₃ hollow particles were effective photocatalysts for the degradation of DEP, with 96.8% destruction ratio being obtained within 60 min.     

Hollow hydroxyapatite microspheres as a device for controlled delivery of proteins

Hollow hydroxyapatite (HA) microspheres were prepared by reacting solid microspheres of Li₂O–CaO–B₂O₃ glass (106–150 μm) in K₂HPO₄ solution, and evaluated as a controlled delivery device for a model protein, bovine serum albumin (BSA). Reaction of the glass microspheres for 2 days in 0.02 M K₂HPO₄ solution (pH = 9) at 37°C resulted in the formation of biocompatible HA microspheres with a hollow core diameter equal to 0.6 the external diameter, high surface area (~100 m²/g), and a mesoporous shell wall (pore size ≈13 nm). After loading with a solution of BSA in phosphate-buffered saline (PBS) (5 mg BSA/ml), the release kinetics of BSA from the HA microspheres into a PBS medium were measured using a micro bicinchoninic acid (BCA) protein assay. Release of BSA initially increased linearly with time, but almost ceased after 24–48 h. Modification of the BSA release kinetics was achieved by modifying the microstructure of the as-prepared HA microspheres using a controlled heat treatment (1–24 h at 600–900°C). Sustained release of BSA was achieved over 7–14 days from HA microspheres heated for 5 h at 600°C. The amount of BSA released at a given time was dependent on the concentration of BSA initially loaded into the HA microspheres. These hollow HA microspheres could provide a novel inorganic device for controlled local delivery of proteins and drugs.     

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.     

Synthesis of Eu2O3 hollow submicrometer spheres through a sol–gel template approach

Submicrometer-sized hollow Eu₂O₃ spheres with a shell thickness of about 75 nm and inner diameter about 690 nm have been synthesized through a sol–gel method using PS/PE microspheres as templates. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Field emission scanning electron microscopy (FESEM), and photoluminescence (PL) spectroscopy have been used for the characterization of the obtained hollow Eu₂O₃ spheres. The PL peak is obviously broadened compared with that of bulk Eu₂O₃. The mechanism of the formation of the hollow Eu₂O₃ spheres was discussed.