Zn-Fe-oxide nanostructures of different iron concentrations for multifunctional applications: properties and precursor influence

Zn-Fe-O nanoparticle systems (Z3F, Z20F and Z60F) were produced by changing the Zn:Fe ratio (0.97 : 0.03, 0.8 : 0.2 and 0.4 : 0.6 in at%, respectively) in Zn(ii)-Fe(iii)-carboxylate precursors. According to X-ray diffraction, Z60F is nearly single-phase ZnFe2O4 (5.9 nm crystallite size), Z20F is a ZnO/ZnFe2O4 nanocomposite consisting of 48.8% ZnFe2O4 (4.7 nm crystallite size), and Z3F is apparently pure ZnO (9.5 nm). We found evidence for a ZnFe2O4 spinel of high inversion degree (80-100%) and with superparamagnetic (SPM) behaviour at room temperature in all three samples by a remarkable correlation between HRTEM, FTIR, XPS, Mössbauer and magnetization analyses. Iron modifies the decomposition process of the precursor and enhances its viscosity, which appears to favour the separation of Zn- and Fe-rich phases. As a consequence, two-phase systems of individual nanocrystals/nanoparticles (ZnO and ZnFe2O4) are formed. The large anisotropy constant, 106-107 erg cm-3, of the ZnFe2O4 nanoparticles and the concentration dependence of their magnetic energy barrier are explained in terms of interparticle interactions interlinked with finite size effects and high inversion degree; these factors also control the other parameters of importance for applications, including the blocking temperature (13-111 K), saturation magnetization (1.08-17.7 emu g-1 at 300 K, 4.6-44.8 emu g-1 at 5 K) and coercivity (85.4-491 Oe at 5 K). Magnetic dynamic results, particularly modelled by the Néel-Brown and Vogel-Fulcher laws, yield fitting parameters which validate the presence of concentration-dependent dipole-like interactions between ZnFe2O4 nanoparticles. A fraction of iron was found in the Fe2+ state, presumably substituting for Zn2+ in zinc oxide; however, the samples behave like ZnFe2O4 SPM nanoclusters/nanoparticles dispersed in a nonmagnetic ZnO particle assembly, rather than Zn(Fe)O dilute magnetic semiconductors. The relevance of the properties of the investigated material for specific applications is highlighted throughout the manuscript.

Magnetic properties of γ-Fe2O3 nanoparticles in a porous SiO2 shell for drug delivery

A method is presented for synthesizing core-shell nanoparticles with a magnetic core and a porous shell suitable for drug delivery and other medical applications. The core contains multiple γ-Fe2O3 nanoparticles (∼15 nm) enclosed in a SiO2 (∼100-200 nm) matrix using either methyl (denoted TMOS-γ-Fe2O3) or ethyl (TEOS-γ-Fe2O3) template groups. Low-temperature Mössbauer spectroscopy showed that the magnetic nanoparticles have the maghemite structure, γ-Fe2O3, with all the vacancies in the octahedral sites. Saturation magnetization measurements revealed that the density of γ-Fe2O3 was greater in the TMOS-γ-Fe2O3 nanoparticles than TEOS-γ-Fe2O3 nanoparticles, presumably because of the smaller methyl group. Magnetization measurements showed that the blocking temperature is around room temperature for the TMOS-γ-Fe2O3 and around 250 K for the TEOS-γ-Fe2O3. Three dimensional topography analysis shows clearly that the magnetic nanoparticles are not only at the surface but have penetrated deep in the silica to form the core-shell structure.

Magnetic mesoporous silica nanostructures: investigation of magnetic properties

Magnetic mesoporous silica (MS) nanocomposites provide the possibility of generating multi-functional objects for application in different technological areas. This paper focuses on the magnetic properties of nanocomposites constituted by spinel iron oxide nanoparticles (magnetic nanoparticles (MNPs), < D > ≈ 8-9 nm) embedded in an MS matrix. The mesoporous structure of the silica matrix and the presence of the nanoparticles inside clearly emerge from transmission electron microscopy (TEM) measurements. Low temperature (5 K) field-dependent magnetization measurements reveal saturation magnetization (M_S ) close to bulk value (M _S bulk ∼ 90 emu g^-1) for both MNPs and MNP/MS nanocomposites, indicating that the presence of silica does not affect the magnetic features of the single MNPs. Moreover, the dependence of the remanent magnetization on field (i.e. δM plots) at low temperature has shown a small but evident decrease of interaction in an MNP/MS sample with respect to MNP samples A m² Kg^-1. Finally, a partial orientation of the easy axis is observed when the MNPs are embedded in the silica matrix.

Magnetism from Fe2O3 nanoparticles embedded in amorphous SiO2 matrix

Fe2O3 nanoparticles are embedded in amorphous SiO2 matrix by coprecipitation method with varying concentrations. Conditions are optimized to get almost monodispersed Fe2O3 nanoparticles with high chemical stability. Microstructure of synthesized nanoparticles is well characterized and found that Fe2O3 is in nanocrystalline form and embedded uniformly in amorphous SiO2 matrix. Enhanced surface reactivity is found for nanoparticles which influences physical properties of the SiO2 supported Fe2O3 system due to adsorption. In oxide nanoparticles, significant number of defect sites at the surface is expected but when supported medium such as SiO2 it reduces this defect concentration. Field- and temperature-dependent magnetisation studies on these samples show superparamagnetic behaviour. Superparamagnetic behaviour is seen in all the concentration systems but the coercivity observed in the lower concentration systems is found to be anomalous compared to that of higher concentrations. The observed magnetic behaviour comes from either unsaturated bond existing due to the absence of anions at the surface of nanoparticles or reconstruction of atomic orbitals taking place at interface of Fe2O3–SiO2 system.

Synthesis of MnO2 nanoparticles and their effective utilization as UV protectors for outdoor high voltage polymeric insulators used in power transmission lines

The mechanism of the UV protection of polymers by MnO₂ where UV radiation is absorbed by MnO₂ particles present in the PDMS/EVA–MnO₂ composite used in high voltage outdoor insulators for power transmission lines.

High coercivity α-Fe2O3 nanoparticles prepared by continuous spray pyrolysis

Surfactant free growth of high coercivity (H_C) α-Fe₂O₃ nanoparticles by continuous spray pyrolysis (CoSP).

ZnFe2O4 nanoparticles dispersed in a highly porous silica aerogel matrix: a magnetic study

We report the detailed structural characterization and magnetic investigation of nanocrystalline zinc ferrite nanoparticles supported on a silica aerogel porous matrix which differ in size (in the range 4-11 nm) and the inversion degree (from 0.4 to 0.2) as compared to bulk zinc ferrite which has a normal spinel structure. The samples were investigated by zero-field-cooling-field-cooling, thermo-remnant DC magnetization measurements, AC magnetization investigation and Mössbauer spectroscopy. The nanocomposites are superparamagnetic at room temperature; the temperature of the superparamagnetic transition in the samples decreases with the particle size and therefore it is mainly determined by the inversion degree rather than by the particle size, which would give an opposite effect on the blocking temperature. The contribution of particle interaction to the magnetic behavior of the nanocomposites decreases significantly in the sample with the largest particle size. The values of the anisotropy constant give evidence that the anisotropy constant decreases upon increasing the particle size of the samples. All these results clearly indicate that, even when dispersed with low concentration in a non-magnetic and highly porous and insulating matrix, the zinc ferrite nanoparticles show a magnetic behavior similar to that displayed when they are unsupported or dispersed in a similar but denser matrix, and with higher loading. The effective anisotropy measured for our samples appears to be systematically higher than that measured for supported zinc ferrite nanoparticles of similar size, indicating that this effect probably occurs as a consequence of the high inversion degree.

A smart platform for hyperthermia application in cancer treatment: cobalt-doped ferrite nanoparticles mineralized in human ferritin cages

Magnetic nanoparticles, MNPs, mineralized within a human ferritin protein cage, HFt, can represent an appealing platform to realize smart therapeutic agents for cancer treatment by drug delivery and magnetic fluid hyperthermia, MFH. However, the constraint imposed by the inner diameter of the protein shell (ca. 8 nm) prevents its use as heat mediator in MFH when the MNPs comprise pure iron oxide. In this contribution, we demonstrate how this limitation can be overcome through the controlled doping of the core with small amount of Co(II). Highly monodisperse doped iron oxide NPs with average size of 7 nm are mineralized inside a genetically modified variant of HFt, carrying several copies of α-melanocyte-stimulating hormone peptide, which has already been demonstrated to have excellent targeting properties toward melanoma cells. HFt is also conjugated to poly(ethylene glycol) molecules to increase its in vivo stability. The investigation of hyperthermic properties of HFt-NPs shows that a Co doping of 5% is enough to strongly enhance the magnetic anisotropy and thus the hyperthermic efficiency with respect to the undoped sample. In vitro tests performed on B16 melanoma cell line demonstrate a strong reduction of the cell viability after treatment with Co doped HFt-NPs and exposure to the alternating magnetic field. Clear indications of an advanced stage of apoptotic process is also observed from immunocytochemistry analysis. The obtained data suggest this system represents a promising candidate for the development of a protein-based theranostic nanoplatform.

The influence of precursors on phase evolution of nano iron oxides/oxyhydroxides: optical and magnetic properties

The present investigation describes the evolution of nanoiron oxide/oxyhydroxide phases synthesized under identical conditions of precipitation using different starting reagents.

Anomalous magnetic properties of nanoparticles arising from defect structures: topotaxial oxidation of Fe(1-x)O|Fe(3-δ)O4 core|shell nanocubes to single-phase particles

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe(1-x)O|Fe(3-δ)O4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of antiphase boundaries. Dark-field transmission electron microscopy and powder X-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe(1-x)O core is removed but that the antiphase boundaries remain. The antiphase boundaries result in anomalous magnetic behavior, that is, a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

Seeded Growth of Asymmetric Binary Nanocrystals Made of a Semiconductor TiO2 Rodlike Section and a Magnetic γ-Fe2O3 Spherical Domain

Asymmetric binary nanocrystals (BNCs), comprising one c-axis elongated anatase TiO2 section and one gamma-Fe2O3 spherical domain attached together, are synthesized by heterogeneous nucleation of iron oxide onto the longitudinal facets of TiO2 nanorods in a ternary surfactant mixture. The topologically controlled composition of the BNCs is ascertained by a combination of powder X-ray diffraction, Raman and Mössbauer spectroscopy, high-angle annular dark-field imaging, and high-resolution transmission electron microscopy lattice fringe mapping, while their size-dependent magnetic behavior is demonstrated by ac susceptibility measurements. The heteroepitaxial growth proceeds through a mechanism never observed before for colloidal nanoheterostructures: the two domains share a restricted and locally curved junction region, which accommodates efficiently the interfacial strain and retards the formation of misfit dislocations. It is believed that these BNCs, which combine the properties of two technologically relevant oxide materials, can pave the way to reinforced applications in several fields of nanoscience, such as in photocatalysis, in malignant cell treatments, and in nanocrystal assembly.

Near Equiatomic FeCo Nanocrystalline Alloy Embedded in an Alumina Aerogel Matrix: Microstructural Features and Related Magnetic Properties

FeCo-Al2O3 nanocomposite aerogels were studied by high-resolution electron microscopy, energy filtered transmission electron microscopy, Mössbauer spectroscopy, and measurements of static magnetizations and hysteretic behavior. The combined use of such techniques provided insights on the formation of bcc FeCo nanocrystalline particles inside the alumina matrix, which is promoted by thermal treatment under hydrogen flow of the parent aerogel. Sample characteristics such as alloy composition and crystallinity, influence of the matrix on the structural evolution, and resulting magnetic properties were investigated as a function of the temperature and time of the reduction treatment.