Hydrothermal synthesis of Fe3O4 and α-Fe2O3 nanocrystals as anode electrode materials for rechargeable Li-ion batteries

This paper describes a facile, economical and environment-friendly hydrothermal method of fabricating Fe₃O₄ and α-Fe₂O₃ nanoparticles at 180 °C for 12 h, respectively. The as-obtained products were characterized in detail. X-ray powder diffraction and transmission electron microscopy were used to investigate the products’ properties of crystal form, size, and morphology. The results showed the Fe₃O₄ and α-Fe₂O₃ nanocrystals’ diameter were about 5 and 20 nm, respectively. Moreover, the electrochemical performances of the Fe₃O₄ and α-Fe₂O₃ nanoparticles as anode materials for Li-ion batteries were also evaluated. The first-discharge capacities of Fe₃O₄ and α-Fe₂O₃ nanocrystals were 1,380 and 1,280 mAh g^?1, and stabled about 96 and 75 mAh g^?1 after 20 cycles, respectively. These materials offer substantial promise for developing alternative, high capacity negative electrodes for safer lithium batteries as energy storage and conversion materials.     

Biomimetic fabrication of α-Fe2O3 with hierarchical structures as H2S Sensor

A facile sol–gel method is developed for the fabrication of α-Fe₂O₃ with quasi-honeycomb like structures inherited from Papilio paris butterfly wings. The exquisite hierarchical architecture is faithfully maintained in α-Fe₂O₃ from the skeleton of butterfly wings at the levels from macro to nano-scales. When used as a chemical sensor, the obtained α-Fe₂O₃ replica (P-α-Fe₂O₃) showed a much higher performance than that of the compared α-Fe₂O₃ nanoparticles synthesized under the same condition without biotemplate (S-α-Fe₂O₃). The P-α-Fe₂O₃-based sensor has a sensitivity of 19.2–50 ppm H₂S, which is four times more than that of S-α-Fe₂O₃, accompanied by a rapid response/recovery time within 1/10 s even at a relatively low working temperature of 180 °C. Compare to the S-α-Fe₂O₃, surface area of which cannot be detectable, the high sensing feature of P-α-Fe₂O₃ would be attributed to the relatively high-specific surface area 24.12 m²/g thus fabricated together with the unique 3D-network structures, which provide channel for the diffusion of H₂S. This strategy is expected to be used in fabrication of other kinds of metal oxide with unique structures for the potential application in gas sensor.     

Influence of synthesis variables on the properties of barium hexaferrite nanoparticles

Barium hexaferrite (BaFe₁₂O₁₉) nanoparticles were synthesized by sol–gel auto-combustion route. Prepared samples were sintered at 950 and 1100 °C with Fe³⁺/Ba²⁺ = 12 and 20 mol ratio. The formation mechanism of barium hexaferrite was investigated by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses. In addition, the effect of temperature and Fe³⁺/Ba²⁺ mole ratio on BaFe₁₂O₁₉ formation and magnetic properties, and the effect of increasing the Fe³⁺/Ba²⁺ upon gel ignition and subsequent phase development were investigated. Finally the magnetic behavior was monitored with VSM. DSC studies showed that pure barium hexaferrite phase was formed from maghemite (γ-Fe₂O₃), without the formation of hematite (α-Fe₂O₃). Also, XRD results confirmed the formation of barium hexaferrite phase in non stoichiometric Fe/Ba ratio. VSM results showed that the saturation magnetization was decreased and coercivity increased with decreasing the grain size.     

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.     

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.     

Synthesis and characterization of α-Fe2O3/polyaniline nanotube composite as electrochemical sensor for uric acid detection

We report the synthesis of α-Fe₂O₃/polyaniline nanotube (PAn NTs) composite as an electrochemical sensor for uric acid (UA) detection. Field emission scanning electron microscopy (FESEM) indicates a hexagonal shape of the α-Fe₂O₃ while a nanotube morphology of the PAn. Impedance spectroscopy results confirm a significant decrease in the charge transfer resistance of the glassy carbon electrode (GCE) modified with α-Fe₂O₃/PAn NTs due to the presence of PAn NTs. The results show that the increase in the conductivity of α-Fe₂O₃ in the presence of PAnNTs could improve the catalytic performance of α-Fe₂O₃/PAn NTs composite, compared to the pure α-Fe₂O₃ nanoparticles. From differential pulse voltammetry, a linear working range for the concentration of UA between 0.01?µM and 5?µM, with a LOD of 0.038?µM (S/N?=?3) was obtained. The sensitivity of the linear segment is 0.433?μA?µM^?1. The reliability of the modified electrode towards the detection of UA was investigated in the presence of interfering acids such as ascorbic acid, citric acid and succinic acid.     

Synthesis of phase pure iron oxide polymorphs thin films and their enhanced magnetic properties

The α-Fe₂O₃ thin film was prepared on liquid–vapor interface at room temperature by a facile and cost effective method, which was converted to Fe₃O₄ and γ-Fe₂O₃ films by reduction and oxidation process. The morphological and structural characterizations reveal the average crystallites size in α-Fe₂O₃, Fe₃O₄ and γ-Fe₂O₃ films 12.8, 9.2 and 19 nm with rms roughness 4.35, 4.60 and 8.21 nm, respectively. From magnetic measurements, the α-Fe₂O₃ thin film shows a room temperature super-paramagnetic behavior with saturation magnetization 18 emu/cm³, while Fe₃O₄ and γ-Fe₂O₃ thin films exhibit ferrimagnetic behavior with saturation magnetization values 414.5 and 148 emu/cm³, respectively. A significantly higher value of saturation magnetization is observed in α-Fe₂O₃ film, which is trusted due to the uncompensated surface spins in the film. The converted Fe₃O₄ film also shows enhanced saturation magnetization due to the reduction in antiphase boundaries, whereas the magnetization in γ-Fe₂O₃ film decreases comparatively. The magnetic property of the γ-Fe₂O₃ is explained on the basis of the Fe³⁺ ions vacancy at the octahedral position in its structure.     

Electrochemical characterization of core-shell carbon-encapsulated magnetic nanoparticles

In this paper we studied the electrochemical behaviour of core-shell carbon-encapsulated magnetic nanoparticles (CEMNPs). CEMNPs have core diameters between 15 and 35 nm and are comprised of Fe, Fe₃C and NdC₂ nanoparticles encapsulated in crystalline carbon cages. Direct current cyclic voltammetry (CV) studies showed that carbon-encapsulated magnetic nanoparticles are stable in electrolyte environments. The graphitic coating perfectly isolates the encapsulated particles from the electrolyte in a wide range of potentials. CEMNP-based electrodes have low resistance (0.43-1.44 Ω cm²) and posses a specific capacity of 10-40 F g^− 1, which depends on the surface area and the crystallinity. It was shown, that CEMNPs are interesting multi-functional materials with a high potential to be used in various electrochemical devices.     

Hierarchical pseudo-cubic hematite nanoparticle as formaldehyde sensor

To develop a low cost and scalable gas, sensor for the detection of toxic and flammable gases with fast response and high sensitivity is extremely important for monitoring environmental pollution. This work reports a facile method for preparing pseudo-cubic hierarchical α-Fe₂O₃ nanostructured materials as well as their implementation in gas sensor application. The α-Fe₂O₃ is developed using Fe(NO₃)₃ and ethylene glycol followed by a facile and one-step solvo-thermal reaction without subsequent heat treatment. The pseudo-cubic nanostructures were having an average edge length of 5–10 nm. The solvent played the role of ligand and synergistically affected olation and oxolation process along with dehydration to form final product. The sensor performance of α-Fe₂O₃ in the detection of toxic and flammable gases such as formaldehyde (HCHO), ethanol (C₂H₅OH), and carbon monoxide (CO) was evaluated. As-synthesized nanostructured hematite showed better sensing performance towards formaldehyde. The fabricated gas sensor showed temperature sensitivity sensing performance for the same gas. In addition, ethanol, formaldehyde vapours, and carbon monoxide gas-sensing properties were tested and the sensing performance of the synthesized material was found to be in the order of HCHO > C₂H₅OH > CO. This sensing performance is attributed to the large specific surface area of the pseudo-cubic nanoparticles.     

Novel process for synthesis of α-Fe2O3: microstructural and optoelectronic investigations

Novel chemical synthesis method has been successfully employed for the preparation of n type α-Fe₂O₃ nanoparticles. Thin films of annealed Fe₂O₃ powders processed on glass substrates using spin coating technique. The effects of process temperature on the structural, morphological, electrical transport and optical properties were studied. X-ray diffraction study revealed formation of single phase nanocrystalline hexagonal α-Fe₂O₃. Microstructural analysis confirms nanostructured morphology. Dc electrical conductivity measurement reveled the semiconducting nature with room temperature electrical conductivity increased from 10^?4 to 10^?3 (Ω cm)^?1 as process temperature of Fe₂O₃ increased from 400 to 700 °C respectively. The n-type electrical conductivity is confirmed from thermo-emf measurement with no appreciable change in thermoelectric power after increasing processing temperature. The decrease in the band gap energy from 3.88 to 2.62 eV was observed after increasing process temperature.     

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.     

Magnetic nanocomposites of mixed oxides of iron and barium synthesized under different oxidative environments

Synthesis of nanocomposites of mixed oxides of iron and barium in a copolymer matrix of aniline and formaldehyde using a chemical route at room temperature is reported. X-ray diffraction, infrared, ⁵⁷Fe Mossbauer studies, and scanning electron microscopy on as-synthesized samples, as well as samples obtained on heating at different temperatures, are described. X-ray diffraction, ⁵⁷Fe Mossbauer, and scanning electron microscopy show the formation of nanoparticles of barium ferrites in the polymer matrix. These studies further show the formation of solid solution of iron and barium oxide on heating the samples at temperatures from 400 to 700°C. From the Mossbauer and x-ray diffraction studies, it has been found that γ-Fe₂O₃, which normally transforms into α-Fe₂O₃ on heating at 500°C, persists up to 700°C in the present samples containing barium ions. Infrared studies indicate that the polymeric backbone is strongly influenced by different reaction conditions and lead to variable magnetic character in the heated samples. The text was submitted by the authors in English.     

Pinning enhancement in MgB2 superconducting thin films by magnetic nanoparticles of Fe2O3

MgB₂ thin films were fabricated on r-plane Al₂O₃ ( ${1} \overline{{1}} {0} {2})$ substrates. First, deposition of boron was performed by rf magnetron sputtering on Al₂O₃ substrates and followed by a post-deposition annealing at 850 °C in magnesium vapour. In order to investigate the effect of Fe₂O₃ nanoparticles on the structural and magnetic properties of films, MgB₂ films were coated with different concentrations of Fe₂O₃ nanoparticles by spin coating process. The magnetic field dependence of the critical current density J _c was calculated from the M–H loops and magnetic field dependence of the pinning force density, f _p(b), was investigated for the films containing different concentrations of Fe₂O₃ nanoparticles. The critical current densities, J _c, in 3T magnetic field at 5 K were found to be around 2·7 × 10⁴ A/cm², 4·3 × 10⁴ A/cm², 1·3 × 10⁵ A/cm² and 5·2 × 10⁴ A/cm² for films with concentrations of 0, 25, 50 and 100% Fe₂O₃, respectively. It was found that the films coated with Fe₂O₃ nanoparticles have significantly enhanced the critical current density. It can be noted that especially the films coated by Fe₂O₃ became stronger in the magnetic field and at higher temperatures. It was believed that coated films indicated the presence of artificial pinning centres created by Fe₂O₃ nanoparticles. The results of AFM indicate that surface roughness of the films significantly decreased with increase in concentration of coating material.     

Phase evolution and magnetic properties study of Fe/Cobalt ferrite nanocomposites

In the work presented here attempt is made to investigate phase evolution and magnetic properties of Co/α-Fe₂O₃:1:6 (molar ratio) powder mixtures subjected to high energy milling (30 h) followed by annealing in air and subsequent heat-treatment in a reducing atmosphere at 400 °C for 20 min. The latter process gave rise to the formation of a nanocomposoite compound composed of CoFe₂O₄, Fe₃O₄ and α-Fe phases, as evidenced by Mössbauer and XRD results. Rather high maximum magnetization M_max (88 emu/g) and reasonable _iH_C (1.03 kOe) values were obtained for the nanocomposite sample prepared. This is possibly attributed to the existence of well exchange coupled soft and hard magnetic phases. Further, the structural-magnetic properties relationship of the various powders prepared is discussed in detail.     

Samarium-doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles with improved magnetic and supercapacitive performance: a novel preparation strategy and characterization

Sm³⁺-doped magnetite (Fe₃O₄) nanoparticles were synthesized through a one-pot facile electrochemical method. In this method, products were electrodeposited on a stainless steel (316L) cathode from an additive-free 0.005 M Fe(NO₃)₃/FeCl₂/SmCl₃ aqueous electrolyte. The structural characterizations through X-ray diffraction, field-emission electron microscopy, and energy-dispersive X-ray indicated that the deposited material has Sm³⁺-doped magnetite particles with average size of 20 nm. Magnetic analysis by VSM revealed the superparamagnetic nature of the prepared nanoparticles (Ms = 41.89 emu g^?1, Mr = 0.12 emu g^?1, and H _Ci = 2.24 G). The supercapacitive capability evaluation of the prepared magnetite nanoparticles through cyclic voltammetry and galvanostat charge–discharge showed that these materials are capable to deliver specific capacitances as high as 207 F g^?1 (at 0.5 A g^?1) and 145 F g^?1 (at 2 A g^?1), and capacity retentions of 94.5 and 84.6% after 2000 cycling at 0.5 and 1 A g^?1, respectively. The results proved the suitability of the electrosynthesized nanoparticles for use in supercapacitors. Furthermore, this work provides a facile electrochemical route for the synthesis of lanthanide-doped magnetite nanoparticles.     

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.     

Multi-walled carbon nanotubes-supported Fe(naph)3 nanoparticles to prepare polyacetylene/multi-walled carbon nanotubes nanocomposites

Carbon nanotubes (CNTs) are a promising candidate for preparing conductive polymer/CNT nanocomposites. CNTs are also an alternative to conventional catalyst support. This report studies multi-walled carbon nanotubes (MWNTs) supported-Fe(naph)₃ nanoparticles to prepare polyacetylene (PA)/MWNT nanocomposites with core–shell structure. The XPS spectra and HRTEM images demonstrate the Fe(naph)₃ nanoparticles successfully deposited on the walls of MWNTs and partially transformed to γ-Fe₂O₃ nanoparticles after heated at 100 °C for 2 h. XRD analysis indicates the formation of PA on the walls of MWNTs. Structural analysis using HRTEM shows that PA/MWNT nanocomposites exhibit core–shell structure. TGA data reveals the stability of PA grown on the exterior walls of MWNTs has been improved. The growth mechanism of PA/MWNT nanocomposites can be explained by a heterogeneous process. The conductivity of the nanocomposites was studied by a four-probe approach and a relatively high conductivity was observed.     

Influence of different parameters and their coupled effects on the stability of alumina nanofluids by a fractional factorial design approach

Nanofluids have been introduced as new-generation fluids able to improve energy efficiency in heat exchangers. However, stability problems related to both agglomeration and sedimentation of nanoparticles have limited industrial-level scaling. A fractional factorial experimental 2^k?1 design was applied in order to evaluate the effects of nanoparticle concentration, surfactant type and concentration, ultrasonic amplitude as well as ultrasonic time on the stability of alumina (Al₂O₃) nanofluids. Commercial alumina nanoparticles (particle diameter <50 nm) were dispersed in deionized water using ultrasonic probe dispersion equipment. Sodium dodecylbenzenesulfonate (SDBS) and cetyltrimethylammonium bromide (CTAB) were used as surfactants. The stability of the nanofluids in static mode was monitored by visual inspection and UV visible spectroscopy. The results of the experimental design showed that the coupled effects between surfactant type and surfactant concentration and between ultrasonication tip amplitude and ultrasonication time had the most pronounced effects on nanofluid stability. The experimental conditions providing the best stability were 0.5 wt% of Al₂O₃, CTAB, critical micelle surfactant concentration, 30% ultrasonic amplitude and 30 min of ultrasonication.     

Joint mechanical activation of MnO2, Fe2O3 and graphite: Mutual influence on the structure

The structural changes of MnO₂, Fe₂O₃ and graphite under separate and joint mechanical activation in high-energy planetary ball mill were studied by X-ray diffraction analysis, Raman spectroscopy and chemical analysis. Separate mechanical processing resulted in nanostructured states of MnO₂, Fe₂O₃ and graphite with the size of coherent scattering regions of 25, 12 and 6 nm, respectively, and the average particle size of 15–20 nm. Along with nanoparticles of globular shape, Fe₂O₃ nanorods were found to be formed during separate milling. No mechanochemical effect was found after separate milling. Under joint mechanical activation of nanostructured manganese and iron oxides with graphite, phase transformations toward less stable forms of oxides (Mn₂O₃, Mn₃O₄, Fe₃O₄) were found. When co-milled with α-Fe₂O₃, graphite was found to exfoliate to graphene layers. The graphite phase remained under the combined mechanical activation with MnO₂. Dynamic recrystallization of α-Fe₂O₃ phase also proceeded during joint mechanical activation of nanostructured Fe₂O₃ and graphite.     

Phase composition and magnetic properties of iron oxide nanoparticles obtained by impulse electric discharge in water

The chemical composition and the magnetic properties of iron oxide nanoparticles obtained by impulse electric discharge in water are investigated. The phase composition of the Fe₃O₄, Fe₂O₃, and Fe nanoparticles is determined. By means of the nuclear magnatiec resonance (NMR) technique, the magnetic moments of the nanoparticles are determined. The magnetic moment of the spherical nanoparticles equals to 2.39 × 10^–19 A m², and that of the cubical ones is 4.56 × 10^–19 A m².