Abstract
The comparison between NiFe2O4 (co-precipitation) and NiFe2O4@SiO2 (co-precipitation and microemulsion) ferrite nanoparticles in their as-received and annealed form is presented. The structural characterization revealed the gradual crystallization of as-received samples induced by thermal treatment. The existence of cubic inverse spinel ferrite structure with tetrahedral and octahedral iron occupancy is confirmed in all samples by the comprehensive study. The Fourier-transform infrared (FTIR) spectroscopy confirmed the typical spinel structure and other Fe-based states, whereas the presence of nonstoichiometric hematite is detected in the annealed NiFe2O4 sample. In the case of nanoparticles embedded into the silica matrix, the crystallization of initially amorphous silica is revealed in structural and microstructural characterization. As shown by FTIR, the applied thermal treatment reduces the water molecules and hydroxyl units compared to the initial material. The separation of the rhombohedral hematite α-Fe2O3 phase in the NiFe2O4 ferrite evidenced during the annealing process is demonstrated in structural and magnetic studies. The analysis of saturation magnetization pointed to the spin canting phenomenon in the surface layer with a slight change of the so-called dead layer upon heating. The room temperature superparamagnetic state (SPM) is modified in the NiFe2O4 sample across annealing as an effect of ferrite crystallization and grain growth as well as hematite separation. For as-received NiFe2O4, with temperature decrease, the blocking process preceded by the freezing process is observed. The silica shell is recognized as the sustaining cover for the SPM state. The electronic structure studies confirmed the complex nature of the Fe-based states.
Highlights
In the first, (AB2O4), the Fe3+ cations are only located in the octahedral site as follows [A2+]A[Fe3+]2BO42−, while in the second (B)(AB)O4, the trivalent iron cations are moved into the tetrahedral site adopting the formula [Fe3+]A[A2+Fe3+]2BO42−.[21,28,32]
The NFO nanoferrites crystallize in the inverse spinel ferrite structure in the following formula [Ni1−x2+ Fex3+]A[Nix2+ Fe2−x3+]BO42−,[17,27,32] where generally x = 1.0 and the occupation within (A) tetrahedral and (B) octahedral sites may be assigned as A = Fe and B = NiFe.[28]
The average crystallite size was determined based on the Scherrer formula: dXcrRysDt where: (i) K is the Scherrer constant which for a spherical particle equals 0.89; (ii)qλffiffiffiiffisffiffiffiffitffiffihffiffieffiffiffiffiincident Xray; (iii) b 1⁄4 b2m À b2s is the full width at half-maximum (FWHM) containing the experimental βm and the βS = 0.08 instrumental line broadening determined from the standard silicon sample; (iv) θ is the diffraction angle corresponding to the most intense (311) peak in the inverse spinel structure
Summary
ULTRAFINE spinel nickel ferrite nanoparticles (SF-NPs) adopting the general Ni2+Fe23+O4 formula[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17] have been widely studied over the years as materials that can be used for various potential technological applications, e.g., as memory and energy storage devices, permanent magnets, power transformers, telecommunications devices and magnetic fluids as well as having photocatalytic and biomedical applications, e.g., as targeted drug delivery, hyperthermia or cancer treatment.[1,2,16] Among various spinel nanoferrite types, nickel ferrite is one of the most versatile because of its magnetic properties, catalytic behavior, chemical stability, low conductivity, high electrochemical stability and relatively low cost.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34] NiFe2O4 nanoparticles were tested as highly reproducible gas and humidity sensors and as microwave devices.[1]. The silica matrix is used in active catalytic applications, whereas a porous shell allows the transport of gases from and to the ferrite core
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