Use of ferromagnetic resonance to determine the size distribution of γ-Fe2O3 nanoparticles

Ferromagnetic resonance (FMR) experiments were performed as a function of temperature (10-300 K) on γ-Fe₂O₃ nanoparticles prepared by a sol-gel method. By measuring at several temperatures the relative intensity of the spectrum due to superparamagnetic particles and the anisotropy field of the spectrum due to ferrimagnetic particles, we determined the size distribution of γ-Fe₂O₃ nanoparticles. It was found to be a log-normal distribution with a most probable diameter D_m=8.1 nm and a standard deviation σ=0.25. Transmission electron microscopy measurements performed on the same samples yielded a log-normal distribution with D_m=11.2 nm and σ=0.23. The difference is attributed to the existence of a disordered surface layer in the particles.     

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.     

High concentration of hematite nanoparticles in a silica matrix: Structural and magnetic properties

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt% of hematite was prepared by the sol-gel method followed by heating in air at 200 °C. The so-obtained composite of iron(III) nanoparticles dissolved in glassy silica matrix was investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRPD confirms the formation of a single-phase hematite sample, whereas TEM reveals spherical particles in a silica matrix with an average diameter of 10 nm. DC magnetization shows bifurcation of the zero-field-cooled (ZFC) and field-cooled (FC) branches up to the room temperature with a blocking temperature T_B=65 K. Isothermal M(H) dependence displays significant hysteretic behaviour below T_B, whereas the room temperature data were successfully fitted to a weighted Langevin function. The average particle size obtained from this fit is in agreement with the TEM findings. The small shift of the T_B value with the magnetic field strength, narrowing of the hysteresis loop at low applied field, and the frequency dependence of the AC susceptibility data point to the presence of inter-particle interactions. The analysis of the results suggests that the system consists of single-domain nanoparticles with intermediate strength interactions.     

Magnetic field-induced ferromagnetism in iron oxide microspheres

A magnetic field of 12 T was applied during the annealing of amorphous α-Fe₂O₃ microspheres. The authors demonstrate that magnetic field processing leads to the formation of γ-Fe₂O₃ phases in the microspheres, and the field-treated microspheres exhibit typical room-temperature ferromagnetic behavior, whereas non-field-heated sample presents paramagnetism. The improvement in magnetic properties is considered to be a result of magnetic field-induced transformation from α-Fe₂O₃ to γ-Fe₂O₃.     

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.     

Preparation and medical application of magnetic beads conjugated with bioactive molecules

Ferrite nanobeads were synthesized from an aqueous solution utilizing Fe²⁺ to Fe³⁺ oxidation for use as magnetic carriers in bioscreening, bio-molecular recognition and anti-cancer diagnosis and therapy. The beads had a crystal structure that was intermediate between Fe₃O₄ and γ-Fe₂O₃. Functional biomolecules were strongly conjugated onto the surfaces of the ferrite beads via COOH and SH groups. The addition of ferrite seed crystals (3-8 nm in size) together with a disaccharide enabled the synthesis of monodisperse, spherical ferrite beads with average diameters (d¯) between 50 and 150 nm and relative deviation Δd/d¯=9-16%. Hollow ferrite nano-spheres (d¯=150-450 nm, Δd/d¯≈10%) were prepared using silica spheres as templates, which were dissolved in NaOH solution. Ferrite beads 40 nm in size were encapsulated in polymer spheres of styrene and polymerized glycidyl methacrylate (poly-GMA), 184±9 nm in diameter. They were used for high throughput bioscreening system for affinity purification of target proteins which make specific bindings to anti-cancer drugs, porphyrins, environment hormones, etc.     

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).     

Synthesis of Fe2O3 concentrations and sintering temperature on FTIR and magnetic susceptibility measured from 4 to 300 K of monolith silica gel prepared by sol–gel technique

Nano-crystalline ferric oxide was synthesized inside an amorphous silica matrix by the sol–gel method. The formation of ferric oxide can be detected, giving fine particles around 5–10 nm crystallite sizes calculated from win-fit program. This is associated with a specific role of the silica matrix, which facilitates the diffusion of the reacting cations, enhancing the ferric oxide formation. γ- to α- and/or ε-Fe₂O₃ transformations take place by increasing the Fe₂O₃ concentration for samples sintering at constant-heat treatment temperature. The dried monolith gel materials were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), as well as by scanning electron microscopy (SEM). The magnetic susceptibility at zero-field cooling (M_ZFC) of the prepared samples was evaluated using superconducting quantum interference device (SQUID) magnetometer in temperature range from 4 to 300 K at 1 T.     

Magnetic properties of γ-iron oxide nanoparticles in a mesoporous silica matrix

Magnetic γ-Fe₂O₃/SiO₂ nanocomposites are synthesized by impregnating mesoporous silica with a hexane solution of γ-iron oxide nanoparticles. Materials with various structural and magnetic properties can be obtained using a subsequent thermal treatment of the synthesized samples (annealing for three hours in an air flow).     

Magneto-optic Faraday effect in maghemite nanoparticles/silica matrix nanocomposites prepared by the Sol–Gel method

Bulk monolithic samples of γ-Fe₂O₃/SiO₂ composites with different iron oxide/silica ratios have been prepared by the sol–gel technique. Iron oxide nanoparticles are obtained in-situ during heat treatment of samples and silica matrix consolidation. Preparation method was previously optimized to minimize the percentage of antiferromagnetic α-Fe₂O₃ and parallelepipeds of roughly 2×5×12 mm³, with good mechanical stability, are obtained. RT magnetization curves show a non-hysteretic behavior. Thus, magnetization measurements have been well fitted to an expression that combines the Langevin equation with an additional linear term, indicating that some of the nanoparticles are still superparamagnetic as confirmed by X-ray diffraction and electron microscopy measurements. Zero field cooled /field cooled experiments show curves with slightly different shapes, depending on the size and shape distribution of nanoparticles for a given composition. Magneto-optical Faraday effect measurements show that the Faraday rotation is proportional to magnetization of the samples, as expected. As a demonstration of their sensing possibilities, the relative intensity of polarized light, measured at 5° from the extinction angle, was plotted versus applied magnetic field.     

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.     

Synthesis and characterization of hybrid nanocomposites of polypyrrole filled with iron oxide nanoparticles

Hybrid polypyrrole (PPy)/α-Fe₂O₃ nanocomposite films were fabricated by spin coating on a glass substrate. X-Ray diffraction analysis revealed the crystalline structure of α-Fe₂O₃ nanostructures and the nanocomposites. The broad PPy peak weakened in intensity as the α-Fe₂O₃ content increased in PPy/α-Fe₂O₃ nanocomposites. Characteristic Fourier-transform IR peaks for pure PPy shifted to higher wavenumbers on addition of α-Fe₂O₃ to PPy/α-Fe₂O₃ nanocomposites. This can be attributed to better conjugation and interactions between PPy and α-Fe₂O₃ nanoparticles. Field-emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy images of the nanocomposites reveal a uniform distribution of α-Fe₂O₃ nanoparticles in the PPy matrix. UV-vis absorption spectroscopy revealed a blue shift from λ_max= 441 nm for PPy to λ_max= 392 nm for PPy/α-Fe₂O₃, reflecting strong interactions between PPy and α-Fe₂O₃ nanoparticles. The room-temperature dc electrical conductivity increased from 4.33×10⁻⁹ to 1.81×10⁻⁸ S/cm as the α-Fe₂O₃ nanoparticle content increased from 10 to 50 wt.% in PPy/α-Fe₂O₃ nanocomposites.     

Interparticle interaction effects on magnetic behaviors of hematite (α-Fe2O3) nanoparticles

The interparticle magnetic interactions of hematite (α-Fe₂O₃) nanoparticles were investigated by temperature and magnetic field dependent magnetization curves. The synthesis were done in two steps; milling metallic iron (Fe) powders in pure water (H₂O), known as mechanical milling technique, and annealing at 600 °C. The crystal and molecular structure of prepared samples were determined by X-ray powder diffraction (XRD) spectra and Fourier transform infrared (FTIR) spectra results. The average particle sizes and the size distributions were figured out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The magnetic behaviors of α-Fe₂O₃ nanoparticles were analyzed with a vibrating sample magnetometer (VSM). As a result of the analysis, it was observed that the prepared α-Fe₂O₃ nanoparticles did not perform a sharp Morin transition (the characteristic transition of α-Fe₂O₃) due to lack of unique particle size distribution. However, the transition can be observed in the wide temperature range as “a continuously transition”. Additionally, the effect of interparticle interaction on magnetic behavior was determined from the magnetization versus applied field (σ(M)) curves for 26±2 nm particles, dispersed in sodium oxalate matrix under ratios of 200:1, 300:1, 500:1 and 1000:1. The interparticle interaction fields, recorded at 5 K to avoid the thermal interactions, were found as ∼1082 Oe for 26±2 nm particles.     

Optical properties of γ-Fe2O3 nanoparticles dispersed on sol–gel silica spheres

Two types of γ-Fe₂O₃ nanoparticles, pure γ-Fe₂O₃ and γ-Fe₂O₃ dispersed on sol–gel silica spheres (γ-Fe₂O₃/SiO₂) in thin film form were prepared by the sol–gel technique. Transmission electron microscopy, X-ray diffraction, optical transmittance and FTIR studies along with photoluminescence measurements were carried out for characterizing the samples. The X-ray diffraction patterns of both γ-Fe₂O₃ nanoparticles and γ-Fe₂O₃/SiO₂ indicated their phase-pure forms which were supported by the FTIR spectra. The average sizes of the nanoparticles obtained from transmission electron microscopy studies were 4 nm for both types of samples. Optical transmittance studies indicated direct allowed transitions with two band gaps at 2.43 and 3.07 eV. Although both types of samples showed excitonic luminescence at 2.38 eV (at room temperature), the luminescence intensity of the γ-Fe₂O₃/SiO₂ was higher than that of pure γ-Fe₂O₃.     

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).     

Roles of γ-Fe2O3 in fly ash for mercury removal: Results of density functional theory study

First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe₂O₃ in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe₂O₃ is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe₂O₃ (0 0 1) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe₂O₃ (0 0 1) perfect surface, with binding energy of −54.3 kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6 kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe₂O₃ (0 0 1) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.     

Preparation and characterization of hexadecyl functionalized magnetic silica nanoparticles and its application in Rhodamine 6G removal

In this paper, a new adsorbent, hexadecyl functionalized magnetic silica nanoparticles (C₁₆/SiO₂-Fe₃O₄ NPs), was prepared by a facile method. The final product was characterized by X-ray diffractometer, transmission electron microscope, Fourier transform infrared spectrometer and vibration sample magnetometer. The preparation and adsorption conditions of the adsorbent were optimized. The adsorbent prepared maintaining volume ratio of tetraethylorthosilicate to hexadecyltrimethoxysilane at 1:0.5 and their total volume at 1100 μL exhibited high adsorption capacity. The optimum pH value for the adsorption experiments was 11.00. The adsorption behavior of Rhodamine 6G onto C₁₆/SiO₂-Fe₃O₄ NPs obeyed pseudo-second-order kinetic model and Langmuir isotherm. Thermodynamic data indicated that the adsorption process was spontaneous and exothermic. The adsorption capacity of the adsorbent could reach to 35.6 mg g⁻¹, owing to the hydrophobic attraction and the enhanced electrostatic attraction. The saturation magnetization of the magnetic adsorbent was 35 emu g⁻¹, which ensured the magnetic separation after adsorption.     

Novel carbon nanotube iron oxide magnetic nanocomposites

A novel magnetic nanocomposite of γ-Fe₂O₃ nanoparticles decorated multiwalls carbon nanotubes (MWNTs) was synthesized for the first time by a simple chemistry precipitation method. The structure and morphology of the composite was characterized by X-ray powder diffractometer (XRD), TEM and EDS. The results of XRD and TEM show that γ-Fe₂O₃ nanoparticles is immobilized on the side wall of the MWNTs, the size of most of the particle is <5 nm.The EDS analysis shows that the atomic ratio of Fe to O is 2:3. The magnetization curves of the MWNTs and γ-Fe₂O₃ decorated MWNTs were measured by VSM at room temperature, which indicate that the saturated magnetization (Ms), remanence (Mr) and coercivity (Hc) of the decorated MWNTs are much larger than those of MWNTs, and the decorated MWNTs exhibit well magnetic properties.     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

Textural and electronic characteristics of mechanochemically activated composites with nanosilica and activated carbon

Nanosilicas (A-50, A-300, A-500)/activated carbon (AC, S_BET = 1520 m²/g) composites were prepared using short-term (5 min) mechanochemical activation (MCA) of powder mixtures in a microbreaker. Smaller silica nanoparticles of A-500 (average diameter d_av = 5.5 nm) can more easily penetrate into broad mesopores and macropores of AC microparticles than larger nanoparticles of A-50 (d_av = 52.4 nm) or A-300 (d_av = 8.1 nm). After MCA of silica/AC, nanopores of non-broken AC nanoparticles remained accessible for adsorbed N₂ molecules. According to ultra-soft X-ray emission spectra (USXES), MCA of silica/AC caused formation of chemical bonds Si-O-C; however, Si-C and Si-Si bonds were practically not formed. A decrease in intensity of OK_α band in respect to CK_α band of silica/AC composites with diminishing sizes of silica nanoparticles is due to both changes in the surface structure of particles and penetration of a greater number of silica nanoparticles into broad pores of AC microparticles and restriction of penetration depth of exciting electron beam into the AC particles.