Jiri Plocek

Preparation of ZnFe2O4/SiO2 and CdFe2O4/SiO2 nanocomposites by sol–gel method

Abstract The purpose of this paper is to characterise the phase relations in the system ZnFe2O4/SiO2 and CdFe2O4/SiO2 by dependence on the thermal treatment. Both ferrites have the structure of normal spinels. The samples were prepared by the sol–gel method, heated at a temperature of 800–1100 °C and characterised by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and Mossbauer spectroscopy. HRTEM revealed nanocrystals of the size of about 5–10 nm, depending on the heat treatment temperature. The spinel structure of ZnFe2O4 in the amorphous silica matrix proved to be very stable up to 1100 °C without decomposition to the zinc silicate and iron (III) oxide. On the other hand, CdFe2O4 already starts to decompose at 900 °C and complete decomposition to the α-Fe2O3 and cadmium silicate can be observed at 1100 °C. This decomposition is also accompanied with the recrystallisation of the silica matrix to quartz and cristobalite.

Surface spin effects in La-doped CoFe2O4 nanoparticles prepared by microemulsion route

A comparative study of pure CoFe2O4 nanoparticles and La-doped CoFe2O4 nanoparticles, prepared by microemulsion route has been performed. The samples were characterized using x-ray diffraction and transmission electron microscopy in order to obtain average particle size. The doping of small amount of La3+ ions (up to 3 molar %) causes significant reduction of the particle size using the identical preparation route. The samples were investigated by magnetization measurements, which revealed the coercivity values strongly dependent on particle size, but not significantly on level of La3+ doping. Detailed in-field Mossbauer spectroscopy studies were performed in order to determine spin canting angles and cation distribution within the spinel network. The non-negligible canting angles up to 40° in the La-doped samples were observed. The presence of the spin surface effects was also supported by magnetic measurement as the magnetization did not saturate even in considerably high magnetic fields (7 T). Moreover...

Nanocomposites NiFe2O4/SiO2 and CoFe2O4/SiO2-Preparation by Sol-Gel Method and Physical Properties

This paper aims to characterise the systems NiFe2O4/SiO2 and CoFe2O4/SiO2 prepared by the sol-gel method. After heat treatment, the various samples have been studied by means of X-ray diffraction, Mössbauer spectroscopy, magnetic measurements and transmission electron microscopy (HR TEM).X-ray diffraction and Mössbauer spectra confirmed the presence of the spinel phase. HR TEM observations revealed the nanocrystals with the size in the range of 2–25 nm. Magnetic measurements showed a superparamagnetic behaviour of the samples heated at lower temperature (800°C) and ferrimagnetic character for the samples heated at higher temperature (900, 1000°C).The final phase composition of the heated samples depends on the preparation conditions. The samples, treated up to 300°C in vacuum and then subsequently heated at 800°C or 900°C, do not contain hematite (the most stable phase at higher temperatures). On the contrary, the samples heated at 1000°C or 1250°C display certain content of hematite.

Nanocomposites of monodisperse nanoparticles embedded in high-K oxide matrices – a general preparation strategy

We present a general approach, which enables preparation of multifunctional nanocomposites of monodisperse nanoparticles embedded in oxide matrices. The two-step route has been successfully applied to nanocomposites composed of CoFe2O4 nanoparticles embedded in high-K oxide matrices (ZrO2, Al2O3 and TiO2). First, hydrophobic CoFe2O4 nanoparticles were produced by hydrothermal synthesis and then their incorporation in the oxide matrix was completed by the sol–gel method using the corresponding alcoxides. The as-prepared samples were subjected to annealing at temperatures ranging from 200 to 700 °C, and characterized in detail by the Powder X-Ray Diffraction (PXRD), Energy Dispersive Analysis (EDX), Mossbauer Spectroscopy (MS) and magnetic measurements. The particle size does not change with the annealing temperature, while the amorphous matrices crystallize at temperatures above 400 °C. At much higher annealing temperatures, partial decomposition of the CoFe2O4 occurs accompanied by formation of additional phases. The magnetic measurements also confirmed presence and stability of the uniform CoFe2O4 nanoparticles in the matrices. Thus the proposed method allows preparation of new types of nanocomposites constituted of uniform nanoparticles of the desired type (magnetic, luminescent etc.) embedded in the favored oxide matrix.

Magnetocaloric phenomena in Mg-ferrite nanoparticles

A comparative study of magnetocaloric effect (MCE) in superparamagnetic (SPM) regime is reported in two different types of magnesium ferrite nanostructures. The samples were prepared either by microemulsion method as MgFe2O4 nanoparticles encapsulated in amorphous SiO2, or as matrix-less nanoparticles using hydrothermal synthesis in supercritical water conditions. The particle diameter in all prepared samples was obtained from XRD measurements and TEM analysis. All samples show a SPM behavior above the blocking temperature, TB. The entropy change, ΔS was finally derived from the measurements of magnetization, M(H,T) curves at defined temperature intervals. We observed, that all samples show a broad peak of ΔS in the temperature range that is fairly above the TB. The values of the ΔS also depend on the particle size, and they are of about two orders lower than those reported in the famous giant magnetocaloric materials.

ACr2O4 /SiO2 (A = Zn, Cu, Cd) nanocomposites, their preparation and physical properties

This article presents preparation and characterization of zinc, copper and cadmium chromites nanocrystals embedded in a silica matrix. The ZnCr2O4/SiO2, CuCr2O4/SiO2 and CdCr2O4/SiO2 samples were prepared by a conventional sol-gel method using HNO3 as an acid catalyst, formamide as a modifier, methanol as a solvent and TEOS. Final heat treatment of the nanocomposites was carried out at temperatures in the range of 900 – 1100°C. The resulting samples were characterized by X-ray diffraction, High Resolution Transmission Electron Microscopy, and magnetic measurements.

Nanocomposite of CeO2 and High-Coercivity Magnetic Carrier with Large Specific Surface Area

We succeeded in the preparation of CoFe2O4/CeO2 nanocomposites with very high specific surface area up to 264 g/m2. First, highly crystalline nanoparticles NPs of CoFe2O4 4.7 nm were prepared by hydrothermal method in water-alcohol-oleic acid system. The oleate surface coating was subsequently modified by ligand exchange to citrate. Then the NPs were embedded in CeO2 using heterogeneous precipitation from diluted Ce3+ sulphate solution. Dried samples were characterized by Powder X-Ray Diffraction, Energy Dispersive X-Ray Analysis, Scanning and Transmission Electron Microscopy, Mossbauer Spectroscopy, and Brunauer-Emmett-Teller method. Moreover, detailed investigation of magnetic properties of the bare NPs and final composite was carried out. We observed homogeneous embedding of the magnetic NPs into the CeO2 without significant change of their size and magnetic properties. We have thus demonstrated that the proposed synthesis method is suitable for preparation of extremely fine CeO2 nanopowders and their nanocomposites with NPs. The morphology and magnetic nature of the obtained nanocomposites make them a promising candidate for magnetoresponsive catalysis.

Observation of surface effects in La-doped CoFe2O4/SiO2 nanocomposites

In magnetism of nanoparticles, surface spin effects play a key role. The so-called spin canting at the surface leads either to lowering of saturation magnetization, or even to lack of saturation in high magnetic fields. The value of the saturation magnetization depends on cation distribution within the spinel network and on the spin canting angles. We have investigated structure and magnetic behavior of CoFe2O4 nanoparticles doped with La (Co1−xLaxFe2O4, x = 0.05 − 0.5 and CoLaxFe2−xO4, x = 0.05 − 0.2) embedded in amorphous silica matrix prepared by a sol-gel method. The samples were characterized using X-ray diffraction and TEM. The zero-field cooled and field cooled measurements of magnetization showed that the blocking temperature is well above the room temperature. The high coercivity was observed (up to 2 T at 10 K) and no saturation of the magnetization even in applied field of 7 T was obtained, suggesting considerable spin canting effects. Finally, the Mossbauer spectroscopy under magnetic field and down to low temperature was performed to determine the cation distribution and the spin canting angles.