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The functionalization of nanoparticles is important to improve their oxidation resistance, prevent agglomeration and improve stability of colloids. However, capping magnetic nanoparticles with a stable ultrathin layer of organic or inorganic materials, without compromising magnetic properties, is still a challenge. In this paper, we report for the first time a novel approach to prepare magnetite (Fe 3 O 4) nanoparticles with superior thermal stability by capping them with a thin shell of silicon carbide (SiC), using sonochemical degradation of dimethyldichloro silane (DMDCSi). The synthesized silicon carbide capped Fe 3 O 4 nanoparticles are characterized using Fourier transform infrared (FT-IR) spectroscopy, and thermogravimetric analyzer. FTIR analysis confirms the hydrophilic SiC capping over the Fe 3 O 4 nanoparticles. The influence of silicon carbide capping over magnetic property and crystal structure was analyzed by vibrating sample magnetometer (VSM) and X-ray diffractometer (XRD) measurements, respectively. The possible mechanism of SiC capping over the particles is explained. SiC capped particles are hydrophilic in nature, where water molecules dissociate spontaneously on the surface of the SiC layer and form Si–OH and electropositive Si atom stabilizes the surface dangling bonds and reduces surface energy. The SiC-capped particles showed better thermal stability with an increased magnetic γ-Fe 2 O 3 to the non-magnetic α-Fe 2 O 3 phase transition temperature of >167 °C as compared to bare particles. The enhancement of phase transition temperature in SiC capped Fe 3 O 4 is due to the increased activation energy by effective binding of SiC with the surface oxidized γ-Fe 2 O 3 layer and the formation of an oxidized passive layer that acts as a thermal barrier. The results suggest that the surface oxidized maghemite (γ-Fe 2 O 3) content over the Fe 3 O 4 particles surface can improve SiC adsorption and enhance their phase stability significantly. [Display omitted] [ABSTRACT FROM AUTHOR]

To minimize the exposure time in magnetic fluid hyperthermia based cancer therapy, it is important to improve the heating efficiency. We demonstrate a 62% enhancement in heating efficiency through in situ orientation of magnetic nanoparticles (MNP) along the direction of the radio frequency alternating magnetic field using a static external magnetic field of 80 G in phosphate coated superparamagnetic Fe3O4 MNP dispersions. The heating efficiency increase in oriented samples is due to the enhanced effective anisotropy energy density along the linear chainlike structures of the MNPs, as confirmed from atomic force microscopy and the larger dynamic hysteresis loop area. [ABSTRACT FROM AUTHOR]

We study the influence of dynamic capping of Fe3O4 nanoparticles with phosphate and oleic acid, on their structure, magnetic properties and thermal stability of magnetic nanoparticles. It is observed that the phosphate coating on iron oxide lowers the dipole-dipole interaction significantly, as compared to oleic acid capping. The Mössbauer results show that the spin canting order of oxidized shell and the mean hyperfine field values follow the order Fe0 (uncoated) > FeOA (oleic acid capped) > FP1 (phosphate capped). The uncoated Fe3O4 nanoparticle is non-stoichiometric in nature due to oxidation, whereas FP1 and FeOA are of the correct stoichiometry. Mössbauer and photoacoustic spectroscopic studies on air-annealed phosphate-coated magnetite nanoparticles confirm that the magnetic iron oxide phase is preserved up to 833 K and a complete conversion of Fe3O4 into the non-magnetic hematite phase occurs at 1173 K. The iron oxide air annealed at 833 K is found to have a shell of orthorhombic α-Fe2O3 over the magnetite core. However, in oleic acid-coated nanoparticles, the magnetic to non-magnetic phase transformation commences at 623 K and the conversion was complete at 823 K. The photoacoustic spectra of the air-annealed phosphate-coated Fe3O4 particles showed a flipping of the absorption intensity between 500−700 nm and 800−1000 nm, due to the conversion of Fe3O4 to γ-Fe2O3 at 923 and γ-Fe2O3 to α-Fe2O3 at 1173 K, respectively. The γ-Fe2O3 showed an intense absorption peak above 750 nm, whereas the α-Fe2O3 showed a peak broadening in the wavelength range of 600-700 nm, in addition to the strong peaks at a wavelength above 750 nm. This study suggests that the photoacoustic spectroscopy can distinguish clearly the three polymorphs of iron oxide i.e., Fe3O4, γ-Fe2O3 and α-Fe2O3. Our results confirm the ability of phosphate-capped iron oxide particles to retard the oxidation of Fe2+ contents during the crystal growth process. [ABSTRACT FROM AUTHOR]

The thermal stability and magnetic properties of iron oxide nanomaterials under different environmental conditions are important for technological applications. Herein, we report the influence of dynamic capping of iron oxide nanoparticles with phosphate and oleic acid, on their structure, magnetic properties and thermal stability and compare the results with that of uncoated nanoparticles. The composition, structure, morphology, quality of capping, thermal stability and magnetic properties of prepared magnetite nanoparticles are evaluated by using X-Ray Diffractometer (XRD), Small angle X-ray scattering (SAXS), High resolution transmission electron microscopy (HRTEM), Energy dispersive X-ray (EDX) analysis, Fourier transform infrared (FTIR) spectrometer, Thermo gravimetric analyzer (TGA), Differential scanning calorimetry (DSC) and Vibrating Sample Magnetometer (VSM). Our results suggest that the rapid adsorption of phosphate ions onto specific crystal planes of nucleated magnetite enabled the growth of magnetite core into nearly cubic shape while the oleic acid capping enabled a homogeneous growth of magnetite core along all the crystallographic directions, enabling the formation of nearly spherical particles with narrow size distribution. On the contrary, the phosphate adsorbs as multilayer islands on nanoparticles, leading to a broader size distribution, identical to the uncoated particles. The HRTEM analysis showed dark areas with asymmetric contrast in uncoated nanoparticles, which was not observed in phosphate and oleic acid capped magnetite. The FTIR study on phosphate coated magnetite particles confirms that the surface capping is a mixture of mono and non-protonated monodentate phosphate complexes. The DSC studies confirm a 100 °C enhancement in the magnetic phase thermal stability in phosphate-coated particles, as compared to uncoated and oleic acid coated magnetite. The temperature dependant magnetization results showed a lower blocking temperature for phosphate capped nanoparticles due to the denser multilayer islands of phosphate over magnetite as compared to a uniform close packed monolayer coverage of oleic acid. These results are useful for tailoring stable superparamagnetic nanoparticles for technological applications. [ABSTRACT FROM AUTHOR]

We investigate the effect of hydrodynamic particle size on the magnetic field induced light transmission and transmitted speckle pattern in water based ferrofluids containing functionalized Fe 3 O 4 nanoparticles of size ranging from 15 to 46 nm. Three water-based magnetic nanofluids, containing Fe 3 O 4 nanoparticles functionalized with poly-acrylic acid (PAA), tetra-methyl ammonium hydroxide (TMAOH) and phosphate, are used in the present study. In all three cases, the transmitted light intensity starts decreasing above a certain magnetic field (called first critical field) and becomes a minimum at another field (second critical field). These two critical fields signify the onset of linear aggregation process and zippering transitions between fully grown chains, respectively. Both these critical fields shift towards a lower magnetic field with increasing hydrodynamic diameter, due to stronger magnetic dipolar interactions. The first and the second critical fields showed a power law dependence on the hydrodynamic diameters. The dipolar resonances occurring at certain values of the scatterer size, leads to the field induced extinction of light. Both the onset of chaining and zippering transitions were clearly evident in the time dependent transmitted light intensity. Above the first critical field, the lobe part of the transmitted intensity and the lobe speckle contrast values increase with increasing external magnetic field due to reduced Brownian motion of the field induced aggregates. The speckle contrast was highest for nanoparticle with the largest hydrodynamic diameter, due to reduced Brownian motion. These results provide better insight into field dependent light control in magnetic colloids, which may find interesting applications in magneto-optical devices. [ABSTRACT FROM AUTHOR]