Carbon Double Coated Fe3O4@C@C Nanoparticles: Morphology Features, Magnetic Properties, Dye Adsorption.

This work is devoted to the study of magnetic Fe3O4 nanoparticles doubly coated with carbon. First, Fe3O4@C nanoparticles were synthesized by thermal decomposition. Then these synthesized nanoparticles, 20-30 nm in size were processed in a solution of glucose at 200 °C during 12 h, which led to an unexpected phenomenon-the nanoparticles self-assembled into large conglomerates of a regular shape of about 300 nm in size. The morphology and features of the magnetic properties of the obtained hybrid nanoparticles were characterized by transmission electron microscopy, differential thermo-gravimetric analysis, vibrating sample magnetometer, magnetic circular dichroism and Mössbauer spectroscopy. It was shown that the magnetic core of Fe3O4@C nanoparticles was nano-crystalline, corresponding to the Fe3O4 phase. The Fe3O4@C@C nanoparticles presumably contain Fe3O4 phase (80%) with admixture of maghemite (20%), the thickness of the carbon shell in the first case was of about 2-4 nm. The formation of very large nanoparticle conglomerates with a linear size up to 300 nm and of the same regular shape is a remarkable peculiarity of the Fe3O4@C@C nanoparticles. Adsorption of organic dyes from water by the studied nanoparticles was also studied. The best candidates for the removal of dyes were Fe3O4@C@C nanoparticles. The kinetic data showed that the adsorption processes were associated with the pseudo-second order mechanism for cationic dye methylene blue (MB) and anionic dye Congo red (CR). The equilibrium data were more consistent with the Langmuir isotherm and were perfectly described by the Langmuir-Freundlich model.

Preparation and Magnetic Properties of Cobalt-Doped FeMn2O4 Spinel Nanoparticles.

Mixed-metal oxide nanoparticles have attracted great scientific interest since they find applications in many fields. However, the synthesis of size-controlled and composition-tuned mixed-metal oxide nanoparticles is a great challenge that complicates their study for practical application. In this study, Co-doped FeMn2O4 nanoparticles were synthesized by the solvothermal method in which the crystallization was carried out under autogenous pressure at temperatures of 190 °C for 24 h. The influence of Co doping on the evolution of the structural and magnetic properties was investigated by various methods. It was found from XRD data that crystallite size decreases from 9.1 to 4.4 nm with the increase in Co content, which is in good agreement with the results of TEM. Based on the results of magnetic measurements, it was found that the saturation magnetization first increases with an increase in the cobalt content and reaches its maximum value at x = 0.4, and a further increase in x leads to a decrease in the saturation magnetization. The influence of cation redistribution on the observed changes has been discussed.

Amino-Functionalized Fe3O4@SiO2 Core-Shell Magnetic Nanoparticles for Dye Adsorption.

Fe3O4@SiO2 core-shell nanoparticles (NPs) were synthesized with the co-precipitation method and functionalized with NH2 amino-groups. The nanoparticles were characterized by X-ray, FT-IR spectroscopy, transmission electron microscopy, selected area electron diffraction, and vibrating sample magnetometry. The magnetic core of all the nanoparticles was shown to be nanocrystalline with the crystal parameters corresponding only to the Fe3O4 phase covered with a homogeneous amorphous silica (SiO2) shell of about 6 nm in thickness. The FT-IR spectra confirmed the appearance of chemical bonds at amino functionalization. The magnetic measurements revealed unusually high saturation magnetization of the initial Fe3O4 nanoparticles, which was presumably associated with the deviations in the Fe ion distribution between the tetrahedral and octahedral positions in the nanocrystals as compared to the bulk stoichiometric magnetite. The fluorescent spectrum of eosin Y-doped NPs dispersed in water solution was obtained and a red shift and line broadening (in comparison with the dye molecules being free in water) were revealed and explained. Most attention was paid to the adsorption properties of the nanoparticles with respect to three dyes: methylene blue, Congo red, and eosin Y. The kinetic data showed that the adsorption processes were associated with the pseudo-second order mechanism for all three dyes. The equilibrium data were more compatible with the Langmuir isotherm and the maximum adsorption capacity was reached for Congo red.

Enhanced Photodegradation in Metal Oxide Nanowires with Co-Doped Surfaces under a Low Magnetic Field.

This work demonstrated the enhanced photodegradation (PD) resulting from Co-rich doping of ZnO nanowire (NW) surfaces (Co2+/ZnO NWs) prepared by combining Co sputtering on ZnO NWs and immersion in deionized water to exploit the hydrophilic-hydrophobic transitions on the ZnO surfaces resulting from Co atom diffusion. Because of the controllable spin-dependent density of states (DOS) induced by Co2+, the PD of methylene blue dye can be enhanced by approximately 90% (when compared with bare ZnO NWs) by using a conventional permanent magnet with a relatively low magnetic field strength of approximately 0.15 T. The reliability of spin polarization-modulation attained through surface doping, based on the magnetic response observed from X-ray absorption measurements and magnetic circular dichroism, provides an opportunity to create highly efficient catalysts by engineering surfaces and tailoring their spin-dependent DOS.

Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe2-xCrxO4 Nanoparticles Synthesized by Chemical Combustion.

A series of nickel-chromium-ferrite NiFe2-xCrxO4 (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mössbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (TN is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mössbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4. The saturation magnetization MS in the largest NPs is about (0.98-0.95) µB, which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O4. At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about Tcom = 360-365 K. This value significantly exceeds the point Tcom in bulk ferrites NiFexCr2-xO4 (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFe0.75Cr1.25O4 with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure.

Structural, magnetic and electronic properties of Fe1+xGa2-xO4 nanoparticles synthesized by the combustion method.

The combustion method was used to prepare a precursor powder of an iron-gallium oxide compound which was further heat-treated in order to obtain a set of Fe1+xGa2-xO4 nanoparticles. All samples have a cubic spinel-type structure (space group Fd3[combining macron]m) and the particle size varies from 1.8 to 28.0 nm depending on the treatment conditions. From the comparative analysis by XRD, EDS, and Raman and Mössbauer spectroscopy the creation of a new spinel phase γ-FeGaO3, which was mainly located on the particle surface, was established. As a result, the composition consists of a FeGa2O4 core covered by a FeGaO3 shell. The relative content of FeGa2O4/FeGaO3 compounds in the composites can be varied by heat treatment. The maximum in the ZFC magnetization curves appeared in all samples at about 20-30 K corresponding to the spin-freezing temperature Tsg, which is much higher than in the bulk compound with a pure inverse spinel structure (Ga)[FeGa]O4. The values of effective Curie temperature ΘC for the Fe1+xGa2-xO4 nanoparticles are rather high and positive, indicating a ferromagnetic interaction between iron ions. The high values of the magnetic frustration parameter f = ΘC/Tsg (up to 7) indicate a high degree of magnetic frustration. The low temperature Mössbauer data reveal the magnetic ordering of Fe ions in all samples with the magnetic transition at about 20-26 K depending on the particle size. The specific features of the Mössbauer parameters indicate the properties of non-homogeneous magnetic systems with frustrated interactions specific to spin-glasses. The magnetic system behaves as a spin-glass below Tsg and it is superparamagnetic above Tsg. Such a system is called a "super-spin-glass". The anisotropy energy Eanis strongly depends on the content of Fe(2+) and Fe(3+) ions which contribute to the magnetocrystalline Ecryst and exchange Eex anisotropies, respectively. The anisotropy energy can be tuned by variation of the content of the (FeGaO3)-(FeGa2O4) phases in these complex composites.

Synthesis and magnetic properties of the chromium-doped iron sulfide Fe1-xCrxS single crystalline nanoplates with a NiAs crystal structure.

Single crystalline iron sulfide nanoparticles doped with chromium Fe1-xCrxS (0 ≤x≤ 0.15) have been successfully prepared by a thermal decomposition method. The particles are self-organized into the single crystalline plates with the accurate hexagonal shape and dimensions up to 1 µ in plane and about 30-40 nm in thickness. The samples have the NiAs-type crystal structure (P63/mmc) at all Cr concentrations up to x = 0.15. Fe(57)-Mössbauer spectroscopy data reveal four nonequivalent iron sites in these nanocrystals related to the different number of cation vacancies in neighboring of the iron atoms. A 2C-type superstructure or a mixture of 2C and 3C superstructures of vacancy ordering can appear in these samples. It was established that in the Fe1-xCrxS series chromium prefers to replace iron in the cation layers containing vacancies at 0.00 < x < 0.10 and Cr atoms occupy both iron and vacant sites at x > 0.10. The specific magnetic properties, which can be tuned by chromium doping, enable potential applications of these nanoparticles in technical devices using the material with thermally activated magnetic memory, for example, switches or storages.