Growth Of α-Fe2O3 Research Articles

Influence of ignition atmosphere on the structural properties of magnetic iron oxides synthesized via solution combustion and the NH3-SCR activity of W/Fe2O3 catalyst

Two kinds of magnetic iron oxides (Fe2O3-PO and Fe2O3-AO) are synthesized via the solution combustion coupled with the chemical oxidization of hydrogen peroxide. The magnetic carbon-containing Fe2O3-AO obtained at the absence of air presents the main γ-Fe2O3 crystal with smaller crystallite size and larger BET surface area, but not the aggregated and sintered α-Fe2O3 crystal of Fe2O3-PO. The doping of tungsten has no effect on the growth of α-Fe2O3 in Fe2O3-PO during the annealing process at 400 °C. However, the doping of tungsten effectively restrains the irreversible transformation of γ-Fe2O3 into α-Fe2O3 crystal and enhances the anti-collapse of the pore structure of Fe2O3-AO under the same annealing process at 400 °C by inducing a stronger interaction between tungsten species and γ-Fe2O3 crystal. In addition, the doping of tungsten improves the ratio of surface adsorbed oxygen, the Fe2+/(Fe2++Fe3+) ratio and the surface acidity of Fe2O3-PO-400 and Fe2O3-AO-400. Therefore, the doping of tungsten promotes the catalytic performance of Fe2O3-PO-400, especially its high-temperature activity, but exhibits a better promotional effect on the low-temperature activity of Fe2O3-AO-400. Magnetic 5W/Fe2O3-AO-400 shows the highest BET surface area, the largest Fe2+/(Fe2++Fe3+) ratio, the strongest surface acidity and the appropriate redox ability compared with the other tested catalysts, thus exhibits the highest low-temperature NH3-SCR activity and better resistance to SO2 and H2O than 5W/Fe2O3-PO-400. The formation of low crystallinity and high dispersive γ-Fe2O3 is an important reason on the good catalytic performance of magnetic 5W/Fe2O3-AO-400, which is also confirmed by the influence of annealing temperature. Furthermore, the results of steady-state kinetic experiments demonstrate that the 5W/Fe2O3-AO-400 catalyst mainly follows the Eley-Rideal mechanism.

In situ growth of α-Fe2O3 on Al2O3/YSZ hollow fiber membrane for oily wastewater

This work focused on in situ growth of α-Fe2O3 on an asymmetric Al2O3/YSZ hollow fiber via hydrothermal route. An asymmetric Al2O3/YSZ hollow fiber composite was prepared using the dry–wet phase inversion method and sintering process. The pristine hollow fiber and hollow fiber with α-Fe2O3 were systematically characterized using FESEM. The separation performance of the membranes was determined using the fluxes of both pure water and synthetic oily wastewater and membrane rejection. The FESEM images show more α-Fe2O3 particles grew and deposited on the hollow fiber with at increasing iron (Fe) concentration. The flux of the hollow fiber with α-Fe2O3 was higher compared to that of the pristine hollow fiber (1003 and 1148–1370 L∙m−2∙h−1, respectively). As for oily wastewater separation, the flux declined when the Fe concentration was increased from 0.05 to 0.5 M. However, an improvement in oil rejection up to 95% was obtained. Under visible light assisted filtration, fluxes and rejections of oil emulsion increased, which indicates that the α-Fe2O3 growth on top of the hollow fiber is prone to activate the photocatalytic activity for self-cleaning properties. Hence, the α-Fe2O3 supported Al2O3/YSZ hollow fiber can act as both separation membrane and photocatalytic membrane.

Fabrication of ultra-closely graphene-wrapped Ni foam substrate for supercapacitor electrode by flame induction and electrostatic interaction

Free-standing porous materials with good mechanical properties is quite important in practical applications. Here we develop a two-step process, firstly constructing an electrostatic adsorption surrounding to coat graphene oxide (GO) onto the surface of Ni foam, and secondly utilizing flame to turn it into reduced GO (RGO). RGO is found to closely wrap onto the surface of Ni foam with almost 100% RGO covering rate. As supercapacitor electrode, this RGO-modified Ni foam shows excellent performances in specific capacitance (335.7 F g−1), especially in coulombic efficiency (the highest value of 121.9%), energy efficiency (the highest value of 84.6%), and cycle stability (94.4% capacitance retention after 20000 cycles at 40 A g−1). The growth of α-Fe2O3 on this RGO wrapped Ni foam indicates apparently higher specific capacitance (1488.9 mF cm−2), energy efficiency, rate capacity, and cycle stability compared with direct growth on bare Ni foam. These outcomes provide new clues for preparing free-standing materials and designing new growth/loading substrates.

Regularities of Vacuum Oxidation of Iron in the Range of Low-Temperature Passivation According to the Data of Spectral Ellipsometry

The methods of spectral ellipsometry and nanotomography are used to study the kinetics of formation of oxide layer phase components (magnetite and hematite) on the iron surface under the conditions of vacuum treatment, in the region of low-temperature gas passivation manifestation. In the course of oxidation at the temperature of 300°C and vacuum treatment at 1 Torr, an island and, then, a solid layer of magnetite grows on the surface of iron. Further oxidation results in growth of α-Fe2O3 in the form of plates at the magnetite–gas boundary depthward into magnetite. It forms an island film consisting of hematite microcrystallites on the surface of magnetite when this magnetite surface is coated. Island coalescence occurs under longterm oxidation exposure, which leads to formation of a solid layer consisting of hematite microcrystallites with thin intergrain boundaries. Here, a “puzzle” surface structure is observed, in which crystallite boundaries approximately correspond to their neighbors and, therefore, result in complete coating of the surface. Such a layer efficiently hinders oxygen diffusion, which passivates the metal and prevents formation of a thick magnetite layer.

The influence of CTAB and gum arabic on the precipitation of α-FeOOH in a highly alkaline medium

The influence of cetyltrimethylammonium bromide (CTAB) and gum arabic (GA) on the precipitation of α-FeOOH in a highly alkaline medium was investigated using XRD, 57Fe Mössbauer, FT-IR and FE-SEM. In absence of CTAB and GA long α-FeOOH rods of nanosize width were obtained. For predetermined concentrations of CTAB added to the precipitation system the formation of a small fraction of α-Fe2O3 was noticed, whereas the length of the α-FeOOH rods significantly decreased and α-FeOOH dendrites were also found. The effect was specifically pronounced when GA was added to the precipitation system. The formation of ferrihydrite-like phase was detected, which suppressed the crystal growth of α-FeOOH on one side and forced the nucleation and growth of α-Fe2O3 on the other. Microstructural changes in the precipitates were also monitored. The phase transformation and changes in the nano/microstructure of the precipitates found can be explained taking into account the surface interactions between CTAB or GA with nuclei and crystallites (particles) during the kinetics investigated.

Fabrication, characterization, and photocatalytic property of α-Fe2O3/graphene oxide composite

Spindle-shaped microstructure of α-Fe2O3 was successfully synthesized by a simple hydrothermal method. The α-Fe2O3/graphene oxide (GO) composites was prepared using a modified Hummers’ strategy. The properties of the samples were systematically investigated by X-ray powder diffraction (XRD), UV–Vis diffuse reflectance spectrophotometer, transmission electron microscope, atomic force microscope, X-ray photoelectron spectroscopy, and Raman spectroscopy (Raman) techniques. GO nanosheets act as supporting materials for anchoring the α-Fe2O3 particles. The average crystallite sizes of the α-Fe2O3 and α-Fe2O3/GO samples are ca. 27 and 24 nm, respectively. The possible growth of α-Fe2O3 onto GO layers led to a higher absorbance capacity for visible light by α-Fe2O3/GO than α-Fe2O3 composite. The photocatalytic degradation of toluene over the α-Fe2O3 and α-Fe2O3/GO samples under xenon-lamp irradiation was comparatively studied by in situ FTIR technique. The results indicate that the α-Fe2O3/GO sample synthesized exhibited a higher capacity for the degradation of toluene. The composite of α-Fe2O3/GO could be promisingly applied in photo-driven air purification.

Formation of α-Fe<sub>2</sub>O<sub>3</sub> Nanoflakes by Heating Fe in Air

Flake-shaped hematite (α-Fe2O3) nanostructure has been successfully fabricated by using a hot-plate to directly heat Fe foil or Fe-coated substrates in air at 300oC. After heating, the surface of the samples was found to be populated with α-Fe2O3 nanoflakes. Such growth of α-Fe2O3 nanoflakes was very substrate-friendly. They can be formed on blank Si wafer, patterened Si, AFM tips, silica sphere, quartz, glass slide, Al foil and electrochemically etched W tip. The formation process and the final products were investigated by glancing angle x-ray diffraction (GAXRD), micro-Raman, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate the final products are single crystalline α-Fe2O3 nanoflakes vertically standing on the Fe3O4 film that acts as the precursor for growth of α-Fe2O3. The α-Fe2O3 nanoflakes formed by this method show very sharp tip with the tip radii as small as several nanometers and large surface to volume ratio. Such nanoflakes may be potentially useful as novel candidates for future electron field emission and gas senor devices. Furthermore, it is believed that this simple and substrates-friendly method is useful in extending the applications of α-Fe2O3 nanostructures.

Morphological and structural characterization of epitaxial α-Fe2O3 (0001) deposited on Al2O3 (0001) by dc sputter deposition

We investigated α-Fe2O3 (0001) film growth on α-Al2O3 (0001) using a laboratory-built dc faced magnetron sputtering system with x-ray diffraction, x-ray reflectivity, and atomic force microscopy (AFM). The films were deposited from iron targets at a growth rate of ∼6.3nm∕min in an ambient argon and oxygen mixture. The structural properties and quality of the α-Fe2O3 thin films were characterized using high-resolution synchrotron x-ray scattering. Films thinner than 16nm were to be found highly strained due to the large lattice mismatch between film and substrate of 5.8%. This strain was found to decrease as the film thickness increased, and most of the strain was released through surface modulation once the film thickness reached 20nm. The epitaxial relationships were found to be α-Fe2O3 [0001]‖α-Al2O3 [0001] in the out-of-plane direction. That the film and substrate display the in-plane alignment anticipated for the epitaxy of isomorphous materials was established by verifying that the [112¯0] directions of film and substrate were coincident. X-ray reflectivity and AFM were used to show that layer-by-layer growth of α-Fe2O3 (0001) occurs up to a thickness of 11nm despite the large lattice mismatch.

Growth of nano- -Fe2O3 in a titania matrix by the sol–gel route

Nano-α-Fe2O3 in a titania matrix was prepared by the sol–gel route. The nanocomposites containing various sizes of α-Fe2O3 were precipitated by the heat treatment of the dried gel in a temperature range 100–1200 °C. Differential thermal analysis of the TiO2–Fe2O3 system showed peaks at 325, 390 and 730 °C. The formation of anatase as well as the rutile phase of TiO2 and the growth of α-Fe2O3 were predicted from the above peak analysis. These results were in agreement with the X-ray diffraction studies. The sizes of the nanoparticles were analysed by transmission electron microscopic studies. The Mossbauer spectra of TiO2–Fe2O3 nanocomposites showed paramagnetic and superparamagnetic doublets for all the specimens containing nanoparticles of α-Fe2O3. The electron paramagnetic resonance study indicated the presence of a paramagnetic phase in the nanocomposite samples. © 1998 Chapman & Hall

Synthesis of epitaxial films of Fe3O4 and α-Fe2O3 with various low-index orientations by oxygen-plasma-assisted molecular beam epitaxy

Epitaxial thin films of pure-phase Fe3O4(110), Fe3O4(111), α-Fe2O3(112̄0), and α-Fe2O3(11̄02) have been grown on MgO(110), α-Al2O3(0001), α-Al2O3(112̄0), and α-Al2O3(11̄02) substrates, respectively, by molecular beam epitaxy using an elemental Fe source and an electron cyclotron resonance oxygen plasma source. Characterization of the crystal structures, chemical states, and epitaxial relationships was carried out using a variety of techniques, including in situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction, x-ray photoelectron spectroscopy/diffraction, and ex situ x-ray reflectivity and diffraction. Real-time RHEED reveals that Fe3O4 growth on MgO appears in a step-flow fashion, whereas the growth of Fe3O4(111) on α-Al2O3(0001) occurs initially by island formation, and then island coalescence. However, the growth of α-Fe2O3 on α-Al2O3 appears to follow an intermediate growth mode. The formation of pure-phase films is controlled largely by oxygen partial pressure, plasma power, and growth rate, but appears to be independent of growth temperature, at least from 250 to 550 °C. The present study demonstrates that selective growth of pure-phase iron oxides with various low-index orientations can be achieved by controlling the growth conditions and selecting suitable substrates.

Formation Mechanism of Monodispersed α-Fe2O3Particles in Dilute FeCl3Solutions

Some ambiguity is still involved in the interpretation of the growth mechanism of monodispersed hematite (α-Fe2O3) particles in dilute FeCl3solutions. Namely, there are two entirely different proposals on this issue, viz. aggregation of preformed primary particles of α-Fe2O3itself and reprecipitation of the ionic species through dissolution of the preformed β-FeOOH particles. In order to resolve this problem, the formation process was followed in detail through TEM, Electron Diffraction, XRD, FT-IR, and ICP spectrometry along with quantitative analyses on seed effects. As a result, it has been concluded that the nuclei of the hematite particles are initially generated with the formation of β-FeOOH particles and that they are grown by deposition of the solute originally present in the solution phase and indirectly furnished from the β-FeOOH by dissolution. As the concentration of the solute is lowered by the growth of the hematite particles, they continue to grow with the solute provided mainly from the β-FeOOH in a steady-state of the dissolution of β-FeOOH and growth of α-Fe2O3. The basic formation mechanism is common to the ellipsoidal particles grown in the presence of phosphate ions and spherical particles in their absence.

In situ RHEED and XPS studies of epitaxial thin α-Fe2O3(0001) films on sapphire

In situ RHEED and XPS measurements of epitaxial α-Fe2O3(0001) films are reported as a function of the number of deposited monolayers. The films were prepared on α-Al2O3(0001) substrates by MBE. The RHEED patterns suggest that layer-by-layer growth of α-Fe2O3(0001) occurs for the first few monolayers. Subsequently, the growth mode changes to three-dimensional growth. The in-plane lattice constant of the first monolayer of α-Fe2O3(0001) is expanded relative to that of the bulk, although in the case of lattice matching between α-Al2O3 and α-Fe2O3 a contraction would be expected. This can be explained by assuming a basic hexagonal structure for the first monolayer with a random distribution of ferric ions over the octahedral sites between the close-packed oxygen layers. Beyond the first monolayer, the ordered corundum structure is formed. The lineshapes of the XPS Fe 2p core level spectra are also found to be thickness-dependent. The deviation of the Madelung potential at the surface shifts the positions of the Fe 2p peaks to lower binding energies. For the first few monolayers, the satellite intensity is reduced because the interplanar contraction leads to a shorter Fe-O distance.

Magnetic changes accompanying the thermal decomposition of nontronite (in air) and its relevance to Martian mineralogy

Thermal treatment of nontronite in air, for long periods at 700°C or short periods at 900°C, results in destruction of the nontronite structure, a distinct reddening in color, and a spectacular increase in magnetic susceptibility and saturation magnetization (up to 4.4 Am2 /Kg). Magnetic property measurements and calculations suggest that the magnetism is due to the presence of ultrafine particles of α or γFe2O3; the precise identity has not yet been resolved. The highly magnetic thermally treated nontronite is amorphous to X rays consistent with an ultrafine grain size. Prolonged heating results in the growth of αFe2O3, as suggested by X ray, IR, and magnetic measurements. Reflectivity spectra of a sample heated for 1 hour at 900°C indicates, in addition to αFe2O3, the presence of an opaque, magnetite‐like phase. Given the composition of the starting material, and the experimental conditions, the presence of Fe3O4, is unlikely; the cause of this opacity is as yet unknown. Thermally treated nontronite has chemical, color and magnetic properties akin to those found by Viking on Mars. These results favor an origin for the fine grained Martian surface material by repeated impacts into an Fe‐rich smectite‐charged regolith (Weldon et al., 1980).