Green synthesis of mesoporous hematite (α-Fe2O3) nanoparticles and their photocatalytic activity

A green synthesis method for the preparation of mesoporous α-Fe₂O₃ nanoparticles has been developed using the extract of green tea (camellia sinensis) leaves. This simple and one-step method can suitably be scaled up for large-scale synthesis. The as-prepared mesoporous nanoparticles were characterized by SEM, TEM, XRD, XPS, Raman, UV–visible spectroscopy and N₂ adsorption analysis. The nanoparticles were highly pure and well crystallized with an average particle size of 60 nm. The photocatalytic activity of nanoparticles was evaluated by the amount of hydroxyl radical formation under visible light irradiation detected by fluorescence spectroscopy. The as-prepared α-Fe₂O₃ showed two times higher activity than commercial α-Fe₂O₃ in term of hydroxyl radical formation and enhanced performance in a photoelectrochemical cell. Also, a plausible mechanism for the formation of mesoporous α-Fe₂O₃ has been suggested.     

Ammonium mediated hydrothermal synthesis of nanostructured hematite (α-Fe2O3) particles

Uniform α-Fe₂O₃ particles of different shapes have been synthesized through hydrothermal process. The additives, the type of Fe(III) salts and reaction conditions in hydrothermal process were thoroughly investigated. The crystalline structure and morphology of the as-synthesized powder have been characterized by using X-ray powder diffraction, scanning electron microscopy and field emission scanning electron microscopy. Rod and ellipsoidal-shaped α-Fe₂O₃ were obtained with ferric chloride as a precursor, while only irregular-shaped particles were synthesized by using ferric nitrate as precursors in the absence of NH₄OH. Direct transformation of micro-rod hematite to ellipsoidal particles with FeCl₃ as precursor was also observed by adding NH₄OH. It is shown that the nanorod was formed through presumed directional aggregation of rapidly formed nucleus, while the formation of ellipsoidal hematite particles may undergo a nucleation–aggregation–dissolution–recrystallization process in the presence of ammonium.     

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.     

Synthesis and gas sensing properties of α-Fe(2)O(3)@ZnO core-shell nanospindles

α-Fe(2)O(3)@ZnO core-shell nanospindles were synthesized via a two-step hydrothermal approach, and characterized by means of SEM/TEM/XRD/XPS. The ZnO shell coated on the nanospindles has a thickness of 10-15 nm. Considering that both α-Fe(2)O(3) and ZnO are good sensing materials, we have investigated the gas sensing performances of the core-shell nanocomposite using ethanol as the main probe gas. It is interesting to find that the gas sensor properties of the core-shell nanospindles are significantly enhanced compared with pristine α-Fe(2)O(3). The enhanced sensor properties are attributed to the unique core-shell nanostructure. The detailed sensing mechanism is discussed with respect to the energy band structure and the electron depletion theory. The core-shell nanostructure reported in this work provides a new path to fabricate highly sensitive materials for gas sensing applications.     

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.     

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.     

Sonochemical fabrication and catalytic properties of α-Fe2O3 nanoparticles

In this study, α-Fe₂O₃ (hematite) nanoparticles were synthesised by a sonochemical method. The influence of different factors such as chemical composition of the precursors, atmosphere of the reactions and type of the sonicator on the chemical formula, crystallinity, morphology and size of the obtained products were investigated. Powder X-ray diffraction, scanning electron microscopy and IR spectroscopy, were used to characterise the nanostructures. The catalytic tests were performed in the reaction of methyl phenyl sulphide oxidation. The results exhibit the good catalytic performance of the as-prepared α-Fe₂O₃ nanoparticles.     

Fabrication, growth mechanism, and characterization of α-Fe(2)O(3) nanorods

This work reports the facile synthesis of α-Fe(2)O(3) nanorods and nano-hexagons and its application as sunlight-driven photocatalysis. The obtained products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), diffused reflectance spectroscopy (DRUV-vis), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The phase and crystallinity were confirmed from the XRD study. Electron microscopy study clearly indicates the formation of different morphologies of nanocrystals. These hematite nanostructures were used as a model system for studying the shape-dependent photocatalytic degradation of phenol, methylene blue, and congo red. Amongst all the nanostructured semiconductors, Pt-doped hematite nanorod showed 55% efficiency towards the decolonization of methylene blue and 63% toward congo red under sun light illumination. The difference in photocatalytic activity is discussed in terms of their crystallize size and morphological ordering.     

Ordered mesoporous α-Fe2O3 (hematite) thin-film electrodes for application in high rate rechargeable lithium batteries

Herein is reported the synthesis of ordered mesoporous α-Fe(2)O(3) thin films produced through coassembly strategies using a poly(ethylene-co-butylene)-block-poly(ethylene oxide) diblock copolymer as the structure-directing agent and hydrated ferric nitrate as the molecular precursor. The sol-gel derived α-Fe(2)O(3) materials are highly crystalline after removal of the organic template and the nanoscale porosity can be retained up to annealing temperatures of 600 °C. While this paper focuses on the characterization of these materials using various state-of-the-art techniques, including grazing-incidence small-angle X-ray scattering, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and UV-vis and Raman spectroscopy, the electrochemical properties are also examined and it is demonstrated that mesoporous α-Fe(2)O(3) thin-film electrodes not only exhibit enhanced lithium-ion storage capabilities compared to bulk materials but also show excellent cycling stabilities by suppressing the irreversible phase transformations that are observed in microcrystalline α-Fe(2)O(3).     

The evolution of structure and properties in GdMn(1−x)TixO3 ceramics

In this work, the effects of Ti doping on the microstructure, dielectric, and magnetic properties of GdMn_(1?x)Ti_xO₃ (x?=?0.00–0.15) ceramic samples synthesized using a solid-state reaction were investigated. All the experimental samples formed a single-phase structure, and no structural transformation occurred within the experimental doping range; however, Ti doping caused lattice shrinkage. Ti doping reduced the grain size, and the microstructure of the synthesized samples appeared more compact in scanning electron microscopy images. The lattice distortion of GdMn_(1?x)Ti_xO₃ caused by Ti substitution at the Mn sites resulted in changes in the Raman vibration modes. X-ray photoelectron spectroscopy results showed that the valence state transition of the Ti and Mn ions occurred and the concentration of Ti⁴⁺, Mn³⁺ ions and oxygen vacancies changed due to the charge compensation induced by Ti doping. Ti doping had a significant influence on the size and concentration of cation vacancies in the GdMn_(1?x)Ti_xO₃ samples. Appropriate Ti doping was shown to reduce the dielectric loss, improve the frequency stability of the dielectric constant, and significantly affect the long-range ordering of Gd³⁺ magnetic moments and clearly reduce magnetization.

     

Behaviour of α-Fe2O3 (hematite), prepared by oxidation precipitation of ferrous sulphate,on heating

The effects of heating-Fe₂O₃ (hematite) prepared by oxidation precipitation of ferrous sulphate, at temperatures up to 700° C were studied. It was found that, in the course of heating, losses of structurally-bound water occured, accompanied by the formation and removal of pores, the lattice constants changed and the optical properties were modified, an effect which is important from the standpoint of the use of hematite as ferric pigment. With increasing annealing temperature, the complementary wavelength was shifted to higher values and the spectral purity of the pigment colour was decreased.     

Formation of three-dimensional urchin-like α-Fe₂O₃ structure and its field-emission application

A three-dimensional urchin-like α-Fe(2)O(3) microstructure is formed via a simple, template-free, and one-step thermal oxidation of Fe spheres in an air atmosphere at temperatures in the range of 300-450 °C. The urchin-like α-Fe(2)O(3) microstructure consists of crystalline α-Fe(2)O(3) nanoflakes grown perpendicularly on the surface of the sphere, a shell layer of α-Fe(2)O(3)/Fe(3)O(4), and an Fe core. During the oxidation process, the nanoflakes germinate and grow from cracks in the oxidation layer on the surface. The length of the nanoflakes increases with oxidation time. The tip diameters of the nanoflakes are in ranges of 10-20 nm at 300 °C, 20-30 nm at 350 °C, and 40-60 nm at 400 °C; the length can reach up to a few micrometers. The field-emission characteristics of the samples are experimentally studied and simulated. The results show that the urchin-like emitter has a low turn-on field of 2.8 V/μm, high field-enhancement factor of 4313, excellent emission uniformity of over 4 cm(2), and good emission stability during a 24 h test.     

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.     

The synthesis and selective gas sensing characteristics of SnO(2)/α-Fe(2)O(3) hierarchical nanostructures

SnO(2)/α-Fe(2)O(3) hierarchical nanostructures, in which the SnO(2) nanorods grow on the side surface of α-Fe(2)O(3) nanorods as multiple rows, were synthesized via a three-step process. The diameters and lengths of the SnO(2) nanorods are 6-15?nm and about 120?nm. The growth direction of SnO(2) nanorods is [001], significantly affected by that of α-Fe(2)O(3) nanorods. The hetero-nanostructures exhibit very good selectivity to ethanol. The sensing characteristics are related to the special heterojunction structures, confirmed by high-resolution transmission electron microscopy observation. Therefore, a heterojunction barrier controlled gas sensing mechanism is realized. Our results demonstrate that the hetero-nanostructures are promising materials for fabricating sensors and other complex devices.     

Anisotropy of electrical properties in α-Fe2O3 ceramics

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in -Fe₂O₃ single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of -Fe₂O₃ and limit the mobility of n type carriers.     

Studies on densification ofα-Fe2O3 ceramics

The densification of ceramics of -Fe₂O₃ depends on the processing parameters. The separate influences of milling, sieving, isostatic pressure and sintering atmosphere were investigated. The maximum density, with a value around 96%, was obtained in a sintering atmosphere of nitrogen.     

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.     

Green synthesis of α-Fe2O3 nanoparticles for photocatalytic application

In this study, the preparation of α-Fe₂O₃ nanoparticles using curcuma and tea leaves extract are reported. The curcuma and tea leaves are acted as a reductant and stabilizer. The crystal structure and particle size of the as-synthesized materials were measured through X-ray diffraction. X-ray diffraction patterns revealed that the as-prepared samples were α-Fe₂O₃ nanoparticles with well-crystallized rhombohedral structure and the crystallite sizes of the α-Fe₂O₃ nanoparticles are 4 and 5 nm. Scanning electron microscopy images showed that the prepared samples have spherical shape. The purity and properties of the as-synthesized α-Fe₂O₃ nanoparticles were measured by Raman spectroscopy. The chemical compositions of the as-prepared α-Fe₂O₃ nanoparticles have been analyzed by Fourier transform infrared spectroscopy. The absorption edge of the α-Fe₂O₃ nanoparticles are 561 and 551 nm. The photocatalytic activity of the α-Fe₂O₃ nanoparticles was measured by degradation of methylene orange and the α-Fe₂O₃ nanoparticles showed the excellent photocatalytic performance.     

Monodisperse α-Fe(2)O(3)@SiO(2)@Au core/shell nanocomposite spheres: synthesis, characterization and properties

A new hybrid spherical structure α-Fe(2)O(3)@SiO(2)@Au with a size of about 141?nm was designed, with a hematite cubic core surrounded by a thick silica shell and further decorated with gold nanoparticles. The monodisperse α-Fe(2)O(3)@SiO(2) spheres were first prepared by a sol-gel process based on the modified St?ber method. Subsequently, the surface of the α-Fe(2)O(3)@SiO(2) particles was functionalized by-NH(2) functional groups. The electrostatic attraction of -NH(2) groups will attach the negatively charged Au nanoparticles to the amino-functionalized α-Fe(2)O(3)@SiO(2) nanospheres in order to prepare α-Fe(2)O(3)@SiO(2) monodisperse hybrid spheres. The M(H) hysteresis loop for α-Fe(2)O(3)@SiO(2) and α-Fe(2)O(3)@SiO(2)@Au spheres indicates that the nanocomposite spheres exhibit superparamagnetic characteristics at room temperature. The optical properties and the application of these hybrid nanocomposites as catalysts for the conversion of CO to CO(2) have also been studied.