Rheological properties of a γ-Fe2O3 paraffin-based ferrofluid

A stable γ-Fe₂O₃ paraffin-based ferrofluid was prepared via high energy milling. The magnetic particles were characterized using X-ray diffraction, dynamic light scattering and vibrating sample magnetometer techniques. The rheological properties of the ferrofluid were studied using a standard rotating rheometer. The magnetoviscous effect and thixotropy in the ferrofluid were studied. The formation and destruction of magnetically induced structures and the interactions of nanoparticles and aggregates are discussed.     

Electrical property of nanocrystalline γ-Fe2O3 under high pressure

Using a microcircuit fabricated on a diamond anvil cell, in situ conductivity measurements on nanophase (NP) γ-Fe₂O₃ are obtained under high pressure. For NP γ-Fe₂O₃, the abrupt increase in electrical conductivity occurs at a pressure of 21.3 GPa, corresponding to a transition from maghemite to hematite. Above 26.4 GPa, conductivity increases smoothly with increasing pressure. No distinct abnormal change is observed during decompression, indicating that transformation is irreversible. The temperature-dependence of the conductivity of NP γ-Fe₂O₃ was investigated at several pressures, indicating the electrical conductivity of the sample increases with increasing pressure and temperature, and that a remarkable phenomenon of discontinuity occurs at 400 K. The abnormal change is attributed to the electronic phase transitions of NP γ-Fe₂O₃ due to the variation of inherent cation vacancies. Besides, the temperature-dependence of the electrical conductivity displays semiconductor-like behavior before 33.0 GPa.     

Adsorption of methyl orange on mesoporous γ-Fe2O3/SiO2 nanocomposites

Mesoporous γ-Fe₂O₃/SiO₂ nanocomposite containing 30 mol% of γ-Fe₂O₃ was prepared by a template-free sol-gel method, and its removal ability for methyl orange (MO) was investigated. The nanocomposite was characterized using X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), Fourier transform infrared (FTIR) absorption measurements, nitrogen adsorption-desorption measurements, and magnetic measurements. The synthesized γ-Fe₂O₃/SiO₂ nanocomposite has a mesoporous structure with an average pore size of 3.5 nm and a specific surface area of 245 m²/g, and it exhibits ferrimagnetic characteristics with the maximum saturation magnetization of 20.9 emu/g. The adsorption of MO on the nanocomposite reaches the maximum adsorbed percentage of ca. 80% within a few minutes, showing that most of MO can be removed in a short time. The MO adsorption data fit well with both Langmuir and Freundlich adsorption isotherms. The maximum adsorption capacity of MO is estimated to be 476 mg/g.     

Assembly of γ-Fe2O3/polyaniline nanofilms with tuned dipolar interaction

The internal morphology and magnetic properties of layer-by-layer assembled nanofilms of polyaniline (PANI) and maghemite (γ-Fe₂O₃—7.5-nm diameter) were probed with cross-sectional transmission electron microscopy (TEM) and magnetization measurements (magnetic hysteresis loops, magnetization using zero-field cooled/field-cooled protocols, and ac magnetic susceptibility). Additionally, simulations of the as-produced samples were performed to assess both the nanofilm’s morphology and the corresponding magnetic signatures using the cell dynamic system (CDS) approach and Monte Carlo (MC) through the standard Metropolis algorithm, respectively. Fine control of the film thickness and average maghemite particle–particle within this magnetic structure was accomplished by varying the number of bilayers (PANI/γ-Fe₂O₃) deposited onto silicon substrates or through changing the concentration of the maghemite particles suspended within the colloidal dispersion sample used for film fabrication. PANI/γ-Fe₂O₃ nanofilms comprising 5, 10, 25 and 50 deposited bilayers displayed, respectively, blocking temperatures (T _B) of 30, 35, 39 and 40 K and effective energy barriers (ΔE/k _B) of 1.0 × 10³, 2.3 × 10³, 2.8 × 10³ and 2.9 × 10³ K. Simulation of magnetic nanofilms using the CDS model provided the internal morphology to carry on MC simulation of the magnetic properties of the system taking into account the particle–particle dipolar interaction. The simulated (using CDS) surface–surface particle distance of 0.5, 2.5 and 4.5 nm was obtained for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. The simulated (using MC) T _B values were 33.0, 30.2 and 29.5 K for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. We found the experimental (TEM and magnetic measurements) and the simulated data (CDS and MC) in very good agreement, falling within the same range and displaying the same systematic trend. Our findings open up new perspectives for fabrication of magnetic nanofilms with pre-established (simulated) morphology and magnetic properties.     

Structure change analysis in γ-Fe2O3/carbon composite in the process of electrochemical lithium insertion

It was previously shown that γ-Fe₂O₃/carbon composite synthesized by the aqueous solution method exhibit good high-speed charge/discharge and cycle characteristics as the cathode material in lithium-ion rechargeable batteries. We examined the crystal structure of γ-Fe₂O₃ in γ-Fe₂O₃/carbon composite by X-ray diffraction and the Rietveld analysis before and during electrochemical insertion of lithium ions. Before insertion, the sample has a spinel structure belonging to the Fd3¯m space group with the following iron occupancies: 8a site, 0.92, 16c site, 0 and 16d site, 0.87. During insertion, iron occupancy at 16d site remains virtually constant, at 8a site decreases from 0.92 to 0, and at 16c site increases from 0 to 0.53. These results suggest that, during insertion, iron migrates from 8a to 16c site. In the most highly lithiated sample, iron occupancy at 8a site decreases to 0 and occupancies at 16c and 16d sites were not equalized. Thus, the crystal structure for this sample belongs not to the Fm3¯m space group that represents the rock salt structure, but rather to the Fd3¯m space group.     

Preparation, characterization and dye adsorption properties of γ-Fe2O3/SiO2/chitosan composite

A γ-Fe₂O₃/SiO₂/chitosan composite was prepared by water-in-oil emulsification, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). Effects of various factors, including adsorbent dosage, initial dye concentration, solution pH, and competing anions, on the adsorption of methyl orange from aqueous solutions by the resulting composite were studied by batch adsorption experiments. The adsorption kinetics was found to follow the pseudo-second-order kinetic model, and intraparticle diffusion was related to the adsorption, but not as a sole rate-controlling step. The equilibrium adsorption data were well described by the Freundlich isotherm model. Evaluation of the thermodynamic parameters ΔG°, ΔH°, and ΔS° revealed that the adsorption process was naturally feasible, spontaneous, and exothermic. The composite was proven to be efficient, suitable and promising for the removal of methyl orange from aqueous solutions since it has a relatively higher adsorption capacity than other low-cost adsorbents.     

γ-Fe2O3中的氢含量

利用热中子透射法测定γ-Fe₂O₃的氢含量。利用差热分析、磁分析以及穆斯堡尔效应研究γ-Fe₂O₃的相变,实验结果表明在γ-Fe₂O₃结构中确实含有一定量的氢,当γ-Fe₂O₃结构中的阳离子空位被H¹⁺,Co²⁺,Si⁴⁺,P⁵⁺等离子占据时,将 关键词：     

Vacancy ordering in γ-Fe2O3 nanocrystals observed by Fe NMR

The identification by ⁵⁷Fe internal field nuclear magnetic resonance (NMR) of hyperfine fields at four Fe sites in the (average) tetragonal unit cell of vacancy-ordered γ-Fe₂O₃ (maghemite) is reported. The effects of vacancy redistribution due to annealing the partially vacancy-ordered form has been observed in the ⁵⁷Fe lineshape. In addition, the reduction of the particle size of the vacancy-ordered form has been observed to gradually eliminate the vacancy ordering and then to cause a transition from ferrimagnetism to superparamagnetism.     

Design of multifunctionalized γ-Fe2O3@SiO2 core–shell nanoparticles for enzymes immobilization

This article deals with the first covalent grafting of an enzyme on twice functionalized γ-Fe₂O₃@SiO₂ core–shell magnetic nanoparticles. First, amino-PEG functionalized nanoparticles were synthesized in order to comply with non-toxic platforms that would be stable in high concentration and would exhibit chemical groups to allow further coupling with biomolecules. This approach produces a colloidal suspension of covalently grafted enzymes that remains stable for months and mimics the enzyme–substrate interactions in solution. Secondly, nanoparticles synthesis and enzyme coupling process were reported and the catalytic properties of bound enzymes were measured and compared with that of the free one. These new materials appear to be useful tools for enzymatic catalysis research and may be extended to other biomolecules. Furthermore, magnetic properties of these materials open the way to separation, purification, and transport under magnetic field.     

Magnetically separable photocatalytic composite γ-Fe2O3@TiO2 synthesized by heterogeneous precipitation

Synthesis of magnetically separable photocatalytic active composite γ-Fe₂O₃@TiO₂ is the main objective of this work. In the first step, maghemite nanoparticles were prepared by a precipitation method and consequently covered by the citric acid in order to adjust the zeta-potential of the particle surface. The magnetic carrier was enfolded by TiO₂ via heterogeneous precipitation of TiOSO₄ using urea as a precipitation agent. The procedure was designed to minimize the production costs in order to be easily transferred into the industry scale conserving the high quality of the photoactive product. Nontoxic element oxides were used because of the ecological acceptance. Various methods were employed to characterize and study the intermediate (magnetic nanoparticles) and final materials (TiO₂-maghemite composite), respectively. Moreover, the influence of the subsequent annealing on the structure, phase composition and properties of the products is discussed.     

Single step thermal decomposition approach to prepare supported γ-Fe2O3 nanoparticles

γ-Fe₂O₃ nanoparticles supported on MgO (macro-crystalline and nanocrystalline) were prepared by an easy single step thermal decomposition method. Thermal decomposition of iron acetylacetonate in diphenyl ether, in the presence of the supports followed by calcination, leads to iron oxide nanoparticles supported on MgO. The X-ray diffraction results indicate the stability of γ-Fe₂O₃ phase on MgO (macro-crystalline and nanocrystalline) up to 1150 °C. The scanning electron microscopy images show that the supported iron oxide nanoparticles are agglomerated while the energy dispersive X-ray analysis indicates the presence of iron, magnesium and oxygen in the samples. Transmission electron microscopy images indicate the presence of smaller γ-Fe₂O₃ nanoparticles on nanocrystalline MgO. The magnetic properties of the supported magnetic nanoparticles at various calcination temperatures (350-1150 °C) were studied using a superconducting quantum interference device which indicates superparamagnetic behavior.     

Exchange bias behavior of monodisperse Fe3O4/γ-Fe2O3 core/shell nanoparticles

We have carried out systematic studies on well-characterized monodisperse Fe₃O₄/γ-Fe₂O₃ core/shell nanoparticles of 2-30 nm having a very narrow size distribution and possessing a uniquely mono-layer of surface γ-Fe₂O₃. This unique core-shell structure, probably having a disordered magnetic surface state, leads us to three key observations of unusual magnetic properties: i) a very large magnetic exchange anisotropy reaching over 7 × 10⁶ erg/cm³ for the smaller particles, ii) exchange bias behavior in the magnetization data of the core/shell Fe₃O₄/γ-Fe₂O₃ nanoparticles, and iii) the temperature dependence of the coercive field following an unusual exponential behavior.     

Pressure induced phase transition of nanocrystalline and bulk maghemite (γ-Fe2O3) to hematite (α-Fe2O3)

Phase transition and bulk moduli of bulk and nanocrystalline γ-Fe₂O₃ were studied using synchrotron X-ray diffraction under high pressure. Contrary to most other nanomaterials, nanocrystalline γ-Fe₂O₃ begins to transform into α-Fe₂O₃ at the same pressure as bulk γ-Fe₂O₃, which is caused by a special structure of γ-Fe₂O₃, in which there exist vacancies of crystal. It is believed that phase transition starts from a certain site of vacancy because of the stress concentration at vacancy sites. Compared to bulk material, nanocrystalline γ-Fe₂O₃ has a larger bulk modulus, which is ascribed to the large ratio of surface to volume.     

Magnetic properties of nanosized γ-Fe2O3 and α-(Fe2/3Cr1/3)2O3, prepared by thermal decomposition of heterometallic single-molecular precursor

Thermal decomposition of the trinuclear complex [Fe₂CrO(CH₃COO)₆(H₂O)₃]NO₃ at 300, 400 and 500 °C gave γ-Fe₂O₃ nanoparticles along with amorphous chromium oxide, while decomposition of the same starting compound at 600 and 700 °C led to the formation of α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles. Size of γ-Fe₂O₃ nanoparticles, determined by X-ray diffraction, was in the range from 9 to 11 nm and increased with formation temperature growth. Average size of α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles was about 40 nm and almost did not depend on the temperature of its formation. γ-Fe₂O₃ nanoparticles possessed superparamagnetic behavior with blocking temperature 180-250 K, saturation magnetization 29-35 emu/g at 5 K, 44-49 emu/g at 300 K and coercivity 400-600 Oe at 5 K. α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles were characterized by low magnetization values (2.7 emu/g at 70 kOe). Such magnetic properties can be caused by non-compensated spins and defects present on the surface of these nanoparticles. The increase of α-(Fe_2/3Cr_1/3)₂O₃ formation temperature led to decrease of magnetization (being compared for the same fields), which may be caused by decrease of the quantity of defects or non-compensated spins (due to decrease of particles' surface).     

Thickness dependent activity of nanostructured TiO2/α-Fe2O3 photocatalyst thin films

The effect of thickness of TiO₂ coating on synergistic photocatalytic activity of TiO₂ (anatase)/α-Fe₂O₃/glass thin films as photocatalysts for degradation of Escherichia coli bacteria in a low-concentration H₂O₂ solution and under visible light irradiation was investigated. Nanograined α-Fe₂O₃ films with optical band-gap of 2.06 eV were fabricated by post-annealing of thermal evaporated iron oxide thin films at 400 °C in air. Increase in thickness of the Fe₂O₃ thin film (here, up to 200 nm) resulted in a slight reduction of the optical band-gap energy and an increase in the photoinactivation of the bacteria. Sol-gel TiO₂ coatings were deposited on the α-Fe₂O₃ (200 nm)/glass films, and then, they were annealed at 400 °C in air for crystallization of the TiO₂ and formation of TiO₂/Fe₂O₃ heterojunction. For the TiO₂ coatings with thicknesses ≤50 nm, the antibacterial activity of the TiO₂/α-Fe₂O₃ (200 nm) was found to be better than the activity of the bare α-Fe₂O₃ film. The optimum thickness of the TiO₂ coating was found to be 10 nm, resulting in about 70 and 250% improvement in visible light photo-induced antibacterial activity of the TiO₂/α-Fe₂O₃ thin film as compared to the corresponding activity of the bare α-Fe₂O₃ and TiO₂ thin films, respectively. The improvement in the photoinactivation of bacteria on surface of TiO₂/α-Fe₂O₃ was assigned to formation of Ti-O-Fe bond at the interface.     

Influence of hydrothermally modified γ-Al2O3 on the properties of NiMo/γ-Al2O3 catalyst

The influence of hydrothermal modification of γ-Al₂O₃ on the properties of NiMo/γ-Al₂O₃ catalyst was investigated in this paper. The experimental results showed that the use of the modified γ-Al₂O₃ in the preparation of the NiMo/γ-Al₂O₃ catalyst led to the increase of the dispersion of the surface Mo and Ni oxides, favored the formation of the poly-molybdates and promoted the reduction of the active Mo oxides owing to the increase of the surface acidity of the modified γ-Al₂O₃. Therefore, the NiMo/γ-Al₂O₃ catalyst supported on the modified γ-Al₂O₃ exhibited a higher hydrodenitrogenation (HDN) activity than that supported on the untreated γ-Al₂O₃ in the temperature range of 300-340 °C.     

包钴型γ-Fe2O3磁粉矫顽力的研究

包钴型γ-Fe₂O₃磁粉分为包钴γ-Fe₂O₃(简记为Co-γ-Fe₂O₃)和包钴包亚铁γ-Fe₂O₃(简记为CoFe-γ-Fe₂O₃)两种,它们的矫顽力可比γ-Fe₂O₃磁粉的提高100至400Oe左右,本工作对这两种磁粉矫顽力增大的原因作了探讨,认为它们矫顽力增大的机制不同:CO-γ-Fe₂O₃矫顽力增大是由于表面包覆一层Co(OH)₂使表面各向异性增大,而CoFe-γ-Fe₂O₃则是由于表面包覆的是钴铁氧体,γ-Fe₂O₃与钴铁氧体之间发生耦合作用,使矫顽力增大。     

Synthesis and magnetic properties of platelet γ-Fe2O3 particles for medical applications using hysteresis-loss heating

Platelet γ-Fe₂O₃ particles of particle size less than 100 nm were prepared for medical applications that use the hysteresis-loss heating of ferromagnetic particles. The γ-Fe₂O₃ particles were obtained through the dehydration, reduction, and oxidation of platelet α-FeOOH particles, which were synthesized by the precipitation of ferric ions in an alkaline solution containing ethanolamine, and the crystals grown using a hydrothermal treatment. The γ-Fe₂O₃ particles contained dimples formed by the dehydration of α-FeOOH particles. The coercive force and the saturation magnetization of the γ-Fe₂O₃ particles were in the ranges 11.9 to 12.7 kA/m (150 to 160 Oe), and 70 to 72 Am²/kg (70 to 72 emu/g), respectively. The specific loss power of the γ-Fe₂O₃ particles, estimated from their temperature-raising property measured under a peak magnetic field of 50.9 kA/m (640 Oe) and at a frequency of 117 kHz, was 590 W/g. This value is higher than that of spherical cobalt-containing iron oxide particles having equivalent coercive force and saturation magnetization, reflecting the larger area of the minor hysteresis loop measured under a peak magnetic field of 50.9 kA/m (640 Oe).     

Magnetic and microwave absorbing properties of polyaniline/γ-Fe2O3 nanocomposite

The conducting protonated polyaniline (ES)/γ-Fe₂O₃ nanocomposite with the different γ-Fe₂O₃ content were synthesized by in-situ polymerization. Its morphology, microstructure, DC conductivity and magnetic properties of samples were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), four-wire-technique, and vibrating sample magnetometer (VSM), respectively. The microwave absorbing properties of the nanocomposite powders dispersing in wax coating with the coating thickness of 2 mm were investigated using a vector network analyzers in the frequency range of 7–18 GHz. The pure ES has shown the absorption band with a maximum absorption at approximately 16 GHz and a width (defined as frequency difference between points where the absorption is more than 8 dB) of 3.24 GHz, when 10% γ-Fe₂O₃ by weight is incorporated , the width is broadened to 4.13 GHz and some other absorption bands appear in the range of 7–13 GHz. The parameter dielectric loss tan δ_e (=ε″/ε′) in the 7–18 GHz is found to decrease with increasing γ-Fe₂O₃ contents with 10%, 20%, 30%, respectively, but magnetic loss tan δ_m (=μ″/μ′) increases with increasing γ-Fe₂O₃ contents. The results show that moderate content of γ-Fe₂O₃ nanoparticles embedded in protonated polyaniline matrix may create advanced microwave absorption properties due to simultaneous adjusting of dielectric loss and magnetic loss.     

Temperature-dependent linear or nonlinear gas sensing characteristics of In2O3 mixed α-Fe2O3 nanorods with high sensitivity

Highly sensitive gas sensors are realized from In₂O₃ mixed α-Fe₂O₃ nanorods. At 200 °C, the sensitivity of the sensors upon exposure to 200 ppm ethanol is 31.3, and the sensors exhibit linear dependence of the sensitivity on the ethanol concentration at 100 °C and 200 °C. In contrast, nonlinear gas sensing characteristics are observed at 300 °C and 400 °C. The relationship between sensitivity and ethanol concentration is discussed by using the conduction model, and the experimental data are in good agreement with the obtained equations. Our results imply that In₂O₃ mixed α-Fe₂O₃ nanorods are good candidates for nano-scale gas sensors and the relationship between sensitivity and ethanol concentration is significantly influenced by temperatures.