Tuning optical or magnetic properties of nanoparticles, by addition of impurities, for specific applications is usually achieved at the cost of band gap and work function reduction. Additionally, conventional strategies to develop nanoparticles with a large band gap also encounter problems of phase separation and poor crystallinity at high alloying degree. Addressing the aforementioned trade-offs, here we report Ni–Zn nanoferrites with energy band gap (E_g) of ≈3.20 eV and a work function of ≈5.88 eV. While changes in the magnetoplasmonic properties of the Ni–Zn ferrite were successfully achieved with the incorporation of bismuth ions at different concentrations, there was no alteration of the band gap and work function in the developed Ni–Zn ferrite. This suggests that with the addition of minute impurities to ferrites, independent of their changes in the band gap and work function, one can tune their magnetic and optical properties, which is desired in a wide range of applications such as nanobiosensing, nanoparticle based catalysis, and renewable energy generation using nanotechnology.

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.

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.

Core–shell Fe _x O _y @C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). Fe _x O _y @C NPs synthesized by the pyrolysis process of the mixture of Fe(NO ₃ ) ₃ · 9H ₂ O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO ₃ in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe ₃ O ₄ and γ-Fe ₂ O ₃ . Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid Fe _x O _y @C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe ₃ O ₄ @C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the Fe _x O _y @C-Ag NPs is discussed.