Effect of pH, citrate treatment and silane-coupling agent concentration on the magnetic, structural and surface properties of functionalized silica-coated iron oxide nanocomposite particles

Superparamagnetic iron oxide nanoparticles were synthesized by coprecipitation of iron chloride salts at various pH values (9, 10, 11 and12) that were adjusted using an ammonia solution. Increasing the pH from 9 to 12 led to decreases in the size of iron oxide nanoparticles from 7.9±1.4 to 5±0.6 nm and the saturation magnetization (M_s) from 82.73 to 67.14 emu/g, respectively, when analyzed with transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). X-ray diffraction patterns as well as M_s values showed that magnetite is the dominantly synthesized phase in the examined pH values. Unmodified iron oxide nanoparticles were coated with silica via the hydrolysis and condensation of tetraethyl orthosilicate (TEOS), designated P1 particles. The size distribution diagram of P1 particles showed two regions with mean sizes of 143.3±15.4 and 216.9±13.7 nm corresponding to silica and iron oxide@silica particles, respectively. Stabilization of iron oxide nanoparticles using sodium citrate prior to coating with silica (P2 particles) resulted in nanocomposites with a mean size of 275±16.1 nm and an M_s value of 2.9 emu/g. Subsequently, the surface of P2 particles was functionalized by amine groups using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDS). Results obtained from the measurement of zeta potential revealed that the highest value of isoelectric point (PI) change, indicating a more efficient surface functionalization, occurs when the EDS concentration of 90 mM is used, as compared to that for particles aminated using 25 and 180 mM EDS.     

A simple thermal decomposition synthesis, magnetic properties, and cytotoxicity of La0.7Sr0.3MnO3 nanoparticles

This study reports the new and simple synthesis of magnetic La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles by thermal decomposition method using acetate salts of La, Sr and Mn as starting materials. To obtain the LSMO nanoparticles, thermal decomposition of the precursor is carried out at the temperatures of 600, 700, 800, 900, and 1000°C for 6 hours. The synthesized LSMO nanoparticles were characterized by XRD, FT-IR, TEM and SEM. Structural characterization shows that the prepared particles consisted of two phases of LaMnO₃ (LMO) and LSMO with crystallite sizes ranging from 18 to 55 nm. All the prepared samples have a perovskite structure which changes from cubic to rhombohedral with the increase in the thermal decomposition temperature. Basic magnetic characteristics such as saturation magnetization (M _S) and coercive field (H _C) are evaluated by sample vibrating magnetometry at room temperature (20°C). The samples show soft ferromagnetic behavior with M _S values of ∼9–55 emu/g and H _C values of ∼8–37 Oe, depending on the crystallite size and thermal decomposition temperature. The relationship between the crystallite size and the magnetic properties is presented and discussed. The cytotoxicity of synthesized LSMO nanoparticles was also evaluated with NIH 3T3 cells and the result showed that the synthesized nanoparticles were not toxic to the cells as determined from cell viability in response to the liquid extraction of LSMO nanoparticles.     

Magnetic properties of (Fe, Fe–B)/γ-Fe2O3 core shell nanostructure

Temperature-dependent magnetic properties of a core/shell nanostructure are reported employing magnetometry and electron magnetic resonance (EMR) spectroscopy. Structural characterization of the sample synthesized by NaBH₄ reduction of FeCl₃ was done by x-ray diffraction, TEM and Mössbauer spectroscopy and showed a core/shell nanostructure with a core of diameter D?20 nm consisting of α-Fe and amorphous Fe–B alloy and a shell of 7 nm thickness made up of principally γ-Fe₂O₃. Temperature-dependent EMR studies at 9.28 GHz show a narrow line with g?2.01 superimposed on a broad line with g?2.20. The narrow line assigned to the oxide shell disappears below about 60 K, in agreement with a blocking temperature T_B?30 K measured in SQUID magnetometry. The EMR parameters of the broad EMR line are similar to those reported for α-Fe nanoparticles imbedded in amorphous SiO₂ matrix. The magnitude of the saturation magnetization M_S=70 emu/g of the nanostructure is smaller than that of bulk α-Fe (M_S=220 emu/g) and bulk γ-Fe₂O₃ (M_S=88 emu/g). Size dependence is used to interpret the absence of exchange-bias in the field-cooled sample of the nanostructure.     

Structural and magnetic properties of polymer-stabilized tetragonal Ni nanoparticles

Ni nanoparticles (Ni-NPs), with diameter (D) ranging 5–30 nm, were synthesized by reducing nickel chloride with NaBH₄ in the presence of polymer molecules of poly-vinyl alcohol (PVA) in cold water. Nickel chloride was dispersed in the PVA molecules which stabilized the resulting Ni-NPs. Experiments were carried out with and without PVA to elucidate the effect of PVA molecules on the structural and magnetic properties of Ni-NPs. It was found that both uncoated (uc) and PVA-coated (pc) Ni-NPs exhibit a tetragonal (t) crystal structure, i.e. different from the cubic (fcc) structure of bulk nickel. pc Ni-NPs (paramagnetic in nature) converted to fcc Ni (spherical shape, D ～ 12 nm) on annealing at 573 K in air, exhibiting a saturation magnetization M _s = 20.5 emu/g, squareness ratio M _r /M _s = 0.48 and coercivity H _c = 248 Oe, which is higher than the bulk Ni (0.7 Oe). uc Ni-NPs showed little improvement in M _s and H _c on air annealing. The core–shell structure resulted in a high H _c value in stable pc Ni-NPs in air. Electron magnetic resonance revealed exchange interaction between the core and shell, which changes on annealing in air.     

Chemical synthesis and magnetic properties of HCP and FCC nickel nanoparticles

In this study, we successfully synthesized single-phase hexagonal closed packed (HCP) and face-centered cubic (FCC) nickel nanoparticles via reduction of nickel nitrate hexahydrate and nickel acetate tetrahydrate, respectively, in polyethylene glycol-200. Structural information of the as-synthesized nickel nanoparticles are studied by X-ray diffraction (XRD) as a function of the molar concentration of the nickel precursor. XRD results reveal that low concentrations of nickel precursor (0.005?M and below) favor the HCP, while high concentrations favor the mixture of HCP and FCC crystal structures. Particle size of HCP structure is found in the range of ～15?nm via transmission electron microscope analysis. Vibratory sample magnetometer is employed to study its magnetic behavior and the results reveal that FCC crystalline phase shows ferromagnetic nature with high saturation magnetization (M _s?～?39.6?emu?gm^?1) as compared to metastable HCP crystalline structure (M _s?～?2?emu?gm^?1). The surfactants bonding on the surface of nickel nanoparticles are studied.     

Effect of pH value on the structural and magnetic properties of nanocrystalline strontium hexaferrite thin films

Strontium hexaferrite SrFe₁₂O₁₉ thin films have been synthesized at different pH, adjusted by NH₄OH, on the Si (1 0 0) substrate using a spin coating sol-gel process. Fourier transform infrared spectroscopy analysis and theoretical calculations were conducted for determination and controlling metal citrates in solution precursors. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer were applied to evaluate the composition, microstructure, crystallite size and magnetic properties of the SrFe₁₂O₁₉ thin films. Using the solution with pH 7, the approximately single phase strontium hexaferrite thin films with optimum physical properties can be obtained at calcination temperature of 800 °C. The SrFe₁₂O₁₉ thin films derived from the solution with pH 7 after calcination at 800 °C exhibited crystallite size of 42 nm and magnetic properties of M_s=267 emu/cm³ (at 10 kOe), M_r=134 emu/cm³ and H_c=4290 Oe.     

Magnetic response of microbially synthesized transition metal- and lanthanide-substituted nano-sized magnetites

The magnetic susceptibility (κ_RT) and saturation magnetization (M_S) of microbially synthesized magnetites were systematically examined. Transition metal (Cr, Mn, Co, Ni and Zn)- and lanthanide (Nd, Gd, Tb, Ho and Er)-substituted magnetites were microbially synthesized by the incubation of transition metal (TM)- and lanthanide (L)-mixed magnetite precursors with either thermophilic (TOR-39) or psychrotolerant (PV-4) metal-reducing bacteria (MRB). Zinc incorporated congruently into both the precursor and substituted magnetite, while Ni and Er predominantly did not. Microbially synthesized Mn- and Zn-substituted magnetites had higher κ_RT than pure biomagnetite depending on bacterial species and they exhibited a maximum κ_RT at 0.2 cationic mole fraction (CMF). Other TMs’ substitution linearly decreased the κ_RT with increasing substitution amount. Based on the M_S values of TM- and L-substituted magnetite at 0.1 and 0.02 CMF, respectively, Zn (90.7 emu/g for TOR-39 and 93.2 emu/g for PV-4)- and Mn (88.3 emu/g by PV-4)-substituted magnetite exhibited higher M_S than standard chemical magnetite (84.7 emu/g) or pure biomagnetite without metal substitution (76.6 emu/g for TOR-39 and 80.3 emu/g for PV-4). Lanthanides tended to decrease M_S, with Gd- and Ho-substituted magnetites having the highest magnetization. The higher magnetization of microbially synthesized TM-substituted magnetites by the psychrotroph, PV-4 may be explained by the magnetite formation taking place at low temperatures slowing mechanics, which may alter the magnetic properties compared to the thermophile, through suppression of the random distribution of substituted cations.     

Mn–ferrite nanoparticles via reverse microemulsions: synthesis and characterization

Mn–ferrite nanoparticles were synthesized by thermal treatment at 800 °C of manganese and iron oxo-hydroxides obtained via water-in-oil microemulsions consisting of n-hexanol as continuous phase, cetyl trimethyl ammonium bromide (CTAB) as the cationic surfactant and aqueous solutions of metal salts and precipitant agent (tetramethyl ammonium hydroxide) as reagents. Nanoparticles were synthesized using a multi-microemulsion approach. Two different co-precipitation routes are described depending on the Fe(II) or Fe(III) precursor salts. The influence of salt concentration and digestion process on the final products was examined. The nanoparticles were characterized by X-ray diffraction accompanied by Rietveld analysis, transmission electron microscopy, thermal analysis, infrared spectroscopy, and SQUID magnetometry. In all the synthesis reported in this study MnFe₂O₄ was observed only after thermal treatment at 800 °C of the as-prepared precursors. Almost spherical nanocrystalline MnFe₂O₄ ranging from 12 to 39 nm was obtained starting from chlorides or mixed chloride–sulfate salts as precursors. Low values of reduced remanent magnetization (M _r/M _s) and coercive field (H _c) induce to believe that a fraction of superparamagnetic particle is present at room temperature.     

Synthesis and characterization of manganese ferrite nanoparticles by thermal treatment method

Cubic structured manganese ferrite nanoparticles were synthesized by a thermal treatment method followed by calcination at various temperatures from 723 to 873 K. In this investigation, we used polyvinyl pyrrolidon (PVP) as a capping agent to control the agglomeration of the nanoparticles. The characterization studies were conducted by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average particle sizes of manganese ferrite nanoparticles were determined by TEM, which increased with the calcination temperature from 12 to 22 nm and they had good agreement with XRD results. Fourier transform infrared spectroscopy confirmed the presence of metal oxide bands at all temperatures and the absence of organic bands at 873 K. Magnetic properties were demonstrated by a vibrating sample magnetometer, which showed a super-paramagnetic behavior for all samples and also saturation magnetization (M_s) increases from 3.06 to 15.78 emu/g by increasing the calcination temperature. The magnetic properties were also confirmed by the use of electron paramagnetic resonance spectroscopy, which revealed the existence of unpaired electrons and also measured peak-to-peak line width, resonant magnetic field and the g-factor.     

Magnetic and optical properties of Gd-Tl substituted M-type barium hexaferrites synthesized by co-precipitation technique

The substitution of numerous cations into hexagonal ferrite has been extensively used to endow novel properties and functionalities for various applications. In the present work Gd-Tl substituted barium hexaferrites prepared by co-precipitation process, having the composition Ba_0.75Cu_0.25(Gd_xTl_0.5-x)Fe_11.5O₁₉ (x = 0.0, 0.25 and 0.50). The hexaferrite formation during calcination of sample x = 0.25 was confirmed by TGA/DSC which was processed at 1000 °C for 3 h. The analysis of X-ray diffraction depicts the existence of magneto-plumbite structure with the formation of a minor secondary α–Fe₂O₃ phase x ≤ 0.0 and BaFe₂O₄ phase x ≤ 0.50. UV–Vis spectra reveal the dropping down behavior in the optical energy band gap from 2.47 eV to 1.74 eV. The grains with hexagonal platelet-like shape having size of 0.415–0.446 μm of magnetic powder nanoparticles (MPs) are observed by SEM images. The energy dispersive spectrometer (EDS) analysis was employed for presence of ferrite elements within a single particle. Hysteresis loops signifies the magnetization (M_s) and remnant magnetization (M_r) first increases up to x = 0.25 then reduces with the substitution (x) increment; contrarily, the coercivity (H_c) exhibited initially decreased with maximum content of Tl at x = 0.0 then increases at x = 0.25 after that it decreases at x = 0.50. Maximum values such as M_s (51.727 emu/g), M_r (28.061 emu/g), and H_c (4.057 kOe) are attained for x = 0.25 at room temperature. The synthesized magnetic nanoparticles are found to be suitable for microwave absorbing materials, permanent magnets, catalyst, high density recording media and optoelectronic devices.     

Enhanced magnetization of nanoparticles of Mg x Fe(3? x )O4 (0.5≤ x ≤1.5) synthesized by combustion reaction

For the first time nanocrystalline magnetic particles of Mg _x Fe_(3−x)O₄ with x ranging from 0.5 to 1.5 have been synthesized by a combustion reaction method using iron nitrate Fe(NO₃)₃.9H₂O, magnesium nitrate Mg(NO₃)₂.6H₂O, and urea CO(NH₂)₂ as fuel without intermediate decomposition and/or calcining steps. X-ray diffraction patterns of all systems showed broad peaks consistent with cubic inverse spinel structure of MgFe₂O₄. The absence of extra reflections in the diffraction patterns of as-prepared materials ensures the phase purity. The mean crystallite sizes determined from the prominent (311) peak of the diffraction using Scherrer’s equation and transmission electron microscopy micrographs were c.a. 40 nm with spherical morphology. Fourier transform infrared spectra of the as-prepared material showed traces of organic and metallic salt by-products; however, these could be removed by washing with deionized water. Typical hysteresis curves were obtained for all specimens in magnetic field up to 14 T between 4 and 340 K. The saturation magnetization was 48.3 emu/g and 31.3 emu/g, 44.8 emu/g, and 28.4 emu/g for x=1.0 and 0.8 at 4 K and 340 K, respectively. The saturation magnetization, M _s , of nanoparticles of the MgFe₂O₄ specimen is about 50% higher when compared to the bulk. The enhanced magnetization measured in our nanoparticles MgFe₂O₄ specimens may be attributed to the uncompensated magnetic moment of iron ions between the A- and B-sites, i.e., changes in the inversion factor. Our magnetization results of MgFe₂O₄ specimens are comparable to the existing data for the same compound but with different particle size and prepared by different synthesis methods.     

Effect of reaction condition on microstructure and properties of (NiCuZn)Fe2O4 nanoparticles synthesized via co-precipitation with ultrasonic irradiation

Nano-spinel ferrites synthesized via chemical co-precipitation method are small in size and have serious agglomeration phenomenon, which makes separation difficult in the subsequent process. Ni_0.4Cu_0.2Zn_0.4Fe₂O₄ ferrites nanoparticles were synthesized via co-precipitation assisted with ultrasonic irradiation produced by ultrasonic cleaner with 20 kHz frequency using chlorinated salts and KOH as initial materials. The effects of ultrasonic power (0, 40 W, 60 W, 80 W) and reaction temperature on the microstructure and magnetic properties of ferrite nanoparticles were investigated. The structure analyses via XRD revealed the successful formation of pure (NiCuZn)Fe₂O₄ ferrites nanospinel without any impurity. The crystallites sizes were less than 40 nm and the lattice constant was near 8.39 Å. The TEM showed ferrite particle polygonal. M−H analyses performed the saturation magnetization and coercivity of ferrite nanoparticles obtained at the reaction temperature of 25℃ were higher than at 50℃ with same power. The samples exhibited the highest values of Ms 55.67 emu/g at 25℃ and 47.77 emu/g at 50℃ for 60 W and the lowest values of Hc 71.23 Oe at 25℃ for 40 W and 52.85 Oe at 50℃ for 60 W. The squareness ratio (SQR) were found to be lower than 0.5, which revealed the single magnetic domain nature (NiCuZn)Fe₂O₄ nanoparticles. All the outcomes show the ultrasonic irradiation has positive effects on improving the microstructure and increasing magnetic properties.     

Synthesis of aqueous ferrofluids of ZnxFe3−xO4 nanoparticles by citric acid assisted hydrothermal-reduction route for magnetic hyperthermia applications

Superparamagnetic and monodispersed aqueous ferrofluids of Zn substituted magnetite nanoparticles (Zn_xFe_3−xO₄, x=0, 0.25, 0.3, 0.37 and 0.4) were synthesized via hydrothermal-reduction route in the presence of citric acid, which is a facile, low energy and environmental friendly method. The synthesized nanoparticles were characterized by X ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, scanning and transmission electron microscopy (SEM and TEM) and the dynamic light scattering (DLS) method. The results showed that a certain amount of citric acid was required to obtain single phase Zn substituted magnetite nanoparticles. Citric acid acted as a modulator and reducing agent in the formation of spinel structure and controlled nanoparticle size and crystallinity. Mean particle sizes of the prepared nanoparticles were around 10 nm. The results that are obtained from XRD, magnetic and power loss measurements showed that the crystallinity, saturation magnetization (M_S) and loss power of the synthesized ferrofluids were all influenced by the substitution of Zn in the structure of magnetite. The Zn substituted magnetite nanoparticles obtained by this route showed a good stability in aqueous medium (pH 7) and hydrodynamic sizes below 100 nm and polydispersity indexes below 0.2. The calculated intrinsic loss power (ILP) for the sample x=0.3 (e.g. 2.36 nH m²/kg) was comparable to ILP of commercial ferrofluids with similar hydrodynamic sizes.     

Inverse magnetocaloric effect in sol–gel derived nanosized cobalt ferrite

The magnetocaloric properties of cobalt ferrite nanoparticles were investigated to evaluate the potential of these materials as magnetic refrigerants. Nanosized cobalt ferrites were synthesized by the method of sol–gel combustion. The nanoparticles were found to be spherical with an average crystallite size of 14 nm. The magnetic entropy change (ΔS _m) calculated indirectly from magnetization isotherms in the temperature region 170–320 K was found to be negative, signifying an inverse magnetocaloric effect in the nanoparticles. The magnitudes of the ΔS _m values were found to be larger when compared to the reported values in the literature for the corresponding ferrite materials in the nanoregime.     

Surface-dependent cytotoxicity on bacteria as a model for environmental stress of halloysite nanotubes

Nanoparticles of CoDy_0.1Fe_1.9O₄ were synthesized by sol–gel auto-combustion method. Four samples were synthesized from precursor solutions having different pH values in the range of 2.5–10. X-ray diffraction patterns were analyzed to determine the crystal phase of CoDy_0.1Fe_1.9O₄ nanoparticles synthesized at different pH. The XRD patterns confirm the formation of cubic spinel structure for CoDy_0.1Fe_1.9O₄ nanoparticles synthesized at different pH values. However the single phase is obtained only at pH 7.5, while γ and α-Fe₂O₃ phases appeared at pH 2.5, 5.0, and 10.0. Transmission electron microscopy shows that the structural morphology of the nanoparticles is highly dependent on the pH concentration. Particle size varies in between 24 and 36 nm with respect to change in pH concentration of the sample. Particle size increases to 36 nm with increase in pH up to 7.5 followed by a decrease in particle size for further increase in pH value. Magnetic properties were explored by vibrating sample magnetometry. The variation in pH during the synthesis process changes the size and structural morphology of the nanoparticles, which ultimately causes the variation in the values of the magnetic parameters such as saturation magnetization and coercivity. Infrared spectroscopy is employed to determine the local symmetry in the crystalline solids and to shed light on the ordering phenomenon under the influence of different pH values.     

Synthesis and magnetic properties of single-crystalline BaFe12O19 nanoparticles

Rod-like and platelet-like nanoparticles of simple-crystalline barium hexaferrite (BaFe₁₂O₁₉) have been synthesized by the molten salt method. Both particle size and morphology change with the reaction temperature and time. The easy magnetization direction (0 0 l) of the BaFe₁₂O₁₉ nanoparticles has been observed directly by performing X-ray diffraction on powders aligned at 0.5 T magnetic field. The magnetic properties of the BaFe₁₂O₁₉ magnet were investigated with various sintering temperatures. The maximum values of saturation magnetization (σ_s=65.8 emu/g), remanent magnetization (σ_r=56 emu/g) and coercivity field (H_ic=5251 Oe) of the aligned samples occurred at the sintering temperatures of 1100 °C. These results indicate that BaFe₁₂O₁₉ nanoparticles synthesized by the molten salt method should enable detailed investigation of the size-dependent evolution of magnetism, microwave absorption, and realization of a nanodevice of magnetic media.     

Magnetic properties of barium ferrite synthesized using a microemulsion mediated process

Ultrafine barium ferrite particles have been synthesized using a microemulsion mediated process. The aqueous cores (typically 10–25 nm in size) of water-in-oil microemulsions were used as constrained microreactors for the precipitation of precursor carbonates of Ba²⁺ and Fe³⁺. These precursors (5–15 nm in size) when heated at 950°C, transformed to the hexagonal ferrite BaFe₁₂O₁₉ as confirmed by X-ray diffraction. This barium ferrite powder had an intrinsic coercivity of 5089 Oe and a saturation magnetization of 60.1 emu/g.     

Synthesis of Co nanocapsules coated with BN layers by annealing of KBH4 and [Co(NH3)6]Cl3

Cobalt (Co) nanocapsules coated with boron nitride (BN) layers were synthesized by annealing of ammine complex. KBH₄ and [Co(NH₃)₆]Cl₃ were used as starting materials, and annealed these powders at 500–1000 °C with flowing nitrogen gas. Formation of fcc-Co nanocapsules coated with BN layers was observed from X-ray diffraction patterns and high-resolution electron microscopy. Particle size of fcc-Co prepared at 1000 °C with flowing 100 sccm N₂ gas was approximately 40 nm, and the values of saturation magnetization and coercivity were 74.5 emu/g and 88 Oe, respectively. Good oxidation- and wear-resistances were obtained by encapsulating Co nanoparticles with BN layers.     

Microwave-induced combustion synthesis of Ni–Zn ferrite powder and its characterization

Ni–Zn ferrite powders were successfully synthesized by microwave-induced combustion process. The process takes only a few minutes to obtain calcined Ni–Zn ferrite powders. The resultant powders were investigated by XRD, SEM, VSM, TG/DTA and surface area measurements. The as-received product shows the formation of cubic ferrite with saturation magnetization (M_s)≈23 emu/g, whereas upon annealing at 850°C for 4 h, the saturation magnetization (M_s) increased to ≈52 emu/g.     

Microwave-absorbing characteristics of epoxy resin composites containing nanoparticles of NiZn- and NiCuZn-ferrites

NiZn- and NiCuZn-ferrite nanoparticles (50–70 nm) with the chemical formula Ni_0.5 Zn_0.5Fe₂O₄ (NiZn) and Ni_0.35Cu_0.15Zn_0.5Fe₂O₄ (NiCuZn) were synthesized by a combustion synthesis method. The nanocrystallite of these materials was characterized by structural and magnetic methods. Saturation magnetization increases from 83 emu/g (NiZn) to 91 emu/g (NiCuZn). Magnetic permeability and dielectric permittivity were measured on sintered samples (pellets and toroids) in the frequency range of 1 MHz–1.8 GHz. Reflection losses (R_L) for both samples were calculated from complex permeability and permittivity. Cu substitution in NiZn-ferrite enhances permeability and R_L.In order to explore microwave-absorbing properties in X-band, magnetic nanoparticles were mixed with an epoxy resin to be converted into a microwave-absorbing composite and microwave behaviors of both materials were studied using a microwave vector network analyzer from 7.5 to 13.5 GHz. Cu substitution diminishes absorption intensity in the range 11.5–12.5 GHz.