Stabilization of the high coercivity epsilon-Fe 23 O phase in the CeO2 - Fe2O3-SiO 2 nanocomposites

We have investigated the processes leading to the formation of the Fe2O3 and CeO2 nanoparticles in the SiO2 matrix in order to stabilize the ϵ-Fe2O3 as the major phase. The samples with two different concentrations of the Fe were prepared by sol-gel method, subsequently annealed at different temperatures up to 1100 °C, and characterized by the Mössbauer spectroscopy, Transmission Electron Microscopy (TEM), Powder X-ray Diffraction (PXRD), Energy dispersive X-ray analysis (EDX) and magnetic measurements. The evolution of the different Fe2O3 phases under various conditions of preparation was investigated, starting with the preferential appearance of the γ-Fe2O3 phase for the sample with low Fe concentration and low annealing temperature and stabilization of the major ϵ-Fe2O3 phase for high Fe concentration and high annealing temperature, coexisting with the most stable α-Fe2O3 phase. A continuous increase of the particle size of the CeO2 nanocrystals with increasing annealing temperature was also observed. The graphical abstract displays the most important results of our work. The significant change of the phase composition due to the variation of preparation conditions is demonstrated. As a result, significant change of the magnetic properties from superparamagnetic γ-Fe2O3 phase with negligible coercivity to the high coercivity ϵ-Fe2O3 phase has been observed. ⺠Research of the stabilization of the high coercivity ϵ-Fe2O3 in CeO2-Fe2O3/SiO2. ⺠Samples with two different concentrations of Fe and three annealing temperatures. ⺠Phase transition γ→ϵ→(β)→α with increasing annealing temperature and particle size. ⺠Elimination of the superparamagnetic phases in samples with higher content of Fe. ⺠Best conditions for high coercivity ϵ-Fe2O3 - higher Fe content and

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, Mössbauer 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

The internal structure of magnetic nanoparticles determines the magnetic response

This work aims to emphasize that the magnetic response of single-domain magnetic nanoparticles (NPs) is driven by the NPs' internal structure, and the NP size dependencies of magnetic properties are overestimated. The relationship between the degree of the NPs' crystallinity and magnetic response is unambiguously demonstrated in eight samples of uniform maghemite/magnetite NPs and corroborated with the results obtained for about 20 samples of spinel ferrite NPs with different degrees of crystallinity. The NP samples were prepared by the thermal decomposition of an organic iron precursor subjected to varying reaction conditions, yielding variations in the NP size, shape and relative crystallinity. We characterized the samples by using several complementary methods, such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), high resolution TEM (HR-TEM) and Mössbauer spectroscopy (MS). We evaluated the NPs' relative crystallinity by comparing the NP sizes determined from TEM and PXRD and further inspecting the NPs' internal structure and relative crystallinity by using HR-TEM. The results of the structural characterization were put in the context of the NPs' magnetic response. In this work, the highest saturation magnetization (M) was measured for the smallest but well-crystalline NPs, while the larger NPs exhibiting worse crystallinity revealed a lower M. Our results clearly demonstrate that the NP crystallinity level that is mirrored in the internal spin order drives the specific magnetic response of the single-domain NPs.This work was supported by the Czech Science Foundation (Project 15-01953S), the Spanish Ministry of Economy and Competitiveness (Project MAT2015-71806-R), the Madrid regional government (NANOFRONTMAG, S2013/MIT-2850), the Spanish government project MAGO (under Grant MAT2014-52069-R). Magnetic measurements were performed in the MLTL