Characteristic Blocking Temperature Research Articles

α‐Fe2O3 Nanoparticles Embedded in a g‐C3N4 Nanocomposite: Optical, Magnetic and Electrochemical Study

AbstractThe development of low‐cost and high‐efficient electrocatalysts with multifunctionality by nanocomposite formation is an emerging route for addressing sustainable energy issues. This study reports the nanocomposite of α‐Fe2O3 nanoparticles embedded in g‐C3N4nanosheets synthesized by a facile chemical route. The synthesized nanocomposite was examined for structural, chemical, and morphological details. The optical study of the sample exhibits the potential capability of photon capture and electron‐hole separation towards photo‐excited applications. Magnetic study of the α‐Fe2O3/g‐C3N4 nanocomposite reveals that composite formation does not possess characteristic blocking temperature as present in bare α‐Fe2O3 nanoparticles at 62 K accompanying enhancement of remanent magnetization and coercivity at 5 K.The prepared nanocomposite was tested for oxygen reduction reaction (ORR) in an alkaline medium and it exhibited appreciable catalytic performance with considerable methanol tolerance and excellent durability. It also reveals that the composite catalyzes ORR by two steps: first by two‐electron transfer and then immediately by 4‐electron direct pathway resulting in complete reduction of oxygen to water.

Biogenic Ferrihydrite Nanoparticles: Synthesis, Properties In Vitro and In Vivo Testing and the Concentration Effect.

Biogenic ferrihydrite nanoparticles were synthesized as a result of the cultivation of Klebsiella oxytoca microorganisms. The distribution of nanoparticles in the body of laboratory animals and the physical properties of the nanoparticles were studied. The synthesized ferrihydrite nanoparticles are superparamagnetic at room temperature, and the characteristic blocking temperature is 23–25 K. The uncompensated moment of ferrihydrite particles was determined to be approximately 200 Bohr magnetons. In vitro testing of different concentrations of ferrihydrite nanoparticles for the functional activity of neutrophilic granulocytes by the chemiluminescence method showed an increase in the release of primary oxygen radicals by blood phagocytes when exposed to a minimum concentration and a decrease in secondary radicals when exposed to a maximum concentration. In vivo testing of ferrihydrite nanoparticles on Wister rats showed that a suspension of ferrihydrite nanoparticles has chronic toxicity, since it causes morphological changes in organs, mainly in the spleen, which are characterized by the accumulation of hemosiderin nanoparticles (stained blue according to Perls). Ferrihydrite can also directly or indirectly stimulate the proliferation and intracellular regeneration of hepatocytes. The partial detection of Perls-positive cells in the liver and kidneys can be explained by the rapid elimination from organs and the high dispersion of the nanomaterial. Thus, it is necessary to carry out studies of these processes at the systemic level, since the introduction of nanoparticles into the body is characterized by adaptive-proliferative processes, accompanied by the development of cell dystrophy and tension of the phagocytic system.

Temperature behavior of the antiferromagnetic susceptibility of nanoferrihydrite from the measurements of the magnetization curves in fields of up to 250 kOe

The cross-breeding problem of the temperature dependence of the antiferromagnetic susceptibility of ferrihydrite nanoparticles is considered. Iron ions Fe3+ in ferrihydrite are ordered antiferromagnetically; however, the existence of defects on the surface and in the bulk of nanoparticles induces a noncompensated magnetic moment that leads to a typical superparamagnetic behavior of ensemble of the nanoparticles with a characteristic blocking temperature. In an unblocked state, magnetization curves of such objects are described as a superposition of the Langevin function and the linear-in-field contribution of the antiferromagnetic “core” of the nanoparticles. According to many studies of the magnetization curves performed on ferrihydrite (and related ferritin) nanoparticles in fields to 60 kOe, dependence χAF(T) decreases as temperature increases, which was related before to the superantiferromagnetism effect. As the magnetic field range increases to 250 kOe, the values of χAF obtained from an analysis of the magnetization curves become lower in magnitude; however, the character of the temperature evolution of χAF is changed: now, dependence χAF(T) is an increasing function. The latter is typical for a system of AF particles with random orientation of the crystallographic axes. To correctly determine the antiferromagnetic susceptibility of AF nanoparticles (at least, ferrihydrite) and to search for effects related to the superantiferromagnetism effect, it is necessary to use in experiments the range of magnetic field significantly higher than that the standard value 60 kOe used in most experiments. The study of the temperature evolution of the magnetization curves shows that the observed crossover is due to the existence of small magnetic moments in the samples.

Facile synthesis of Fe@Fe2O3 nanochains exhibiting high heating efficiency in magnetic hyperthermia

Ferromagnetic nanoparticles (NPs), with high heating efficiency, have great potential as heating elements for use in magnetic hyperthermia and thermoablation therapies. In this work, core@shell structured Fe@Fe2O3 nanochains were prepared by facile chemical reduction of Fe2+ ions in aqueous solution. The resulting nanochains were characterized by X-ray powder diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), elemental mapping, Raman analysis, and magnetization measurements. It was found that Fe nanochains form through connection of Fe nanospheres one by one and are coated by amorphous Fe2O3 layer. The chains are about 50–100 nm in diameter and several micrometers in length. Magnetic measurements revealed that as-prepared nanoconstructs possess typical ferromagnetism with a characteristic blocking temperature above 400 K. The saturation magnetization of the samples prepared at 25 °C, 40 °C and 80 °C are 118.6, 81.9, 22.8 emu/g and the coercivity are 136, 259 and 411Oe, respectively. It was interesting to find that these nanochains, dispersed in water at physiological pH, present higher heating efficiency than that of superparamagnetic ferrite NPs. The enhanced heating performance of Fe@Fe2O3 nanochains may be attributed to the activation of a magnetic hysteresis loss mechanism. This study indicates that the resulting Fe@Fe2O3 nanochains are promising materials in magnetic hyperthermia and thermoablation therapies.

Magnetic properties of heat treated bacterial ferrihydrite nanoparticles

The magnetic properties of ferrihydrite nanoparticles, which are products of vital functions of Klebsiella oxitoca bacteria, have been studied. The initial powder containing the nanoparticles in an organic shell was subjected to low-temperature (T=160°C) heat treatment for up to 240h. The bacterial ferrihydrite particles exhibit a superparamagnetic behavior. Their characteristic blocking temperature increases from 26 to 80K with the heat treatment. Analysis of the magnetization curves with regard to the magnetic moment distribution function and antiferromagnetic contribution shows that the low-temperature heat treatment enhances the average magnetic moment of a particle; i.e., the nanoparticles coarsen, probably due to their partial agglomeration during heat treatment. It was established that the blocking temperature nonlinearly depends on the particle volume. Therefore, a model was proposed that takes into account both the bulk and surface magnetic anisotropy. Using this model, the bulk and surface magnetic anisotropy constants KV≈1.7×105erg/cm3 and KS≈0.055erg/cm2 have been determined. The effect of the surface magnetic anisotropy of ferrihydrite nanoparticles on the observed magnetic hysteresis loops is discussed.

Large-Scale Synthesis of Colloidal Fe3O4 Nanoparticles Exhibiting High Heating Efficiency in Magnetic Hyperthermia

Exceptional magnetic properties of magnetite, Fe3O4, nanoparticles make them one of the most intensively studied inorganic nanomaterials for biomedical applications. We report successful gram-scale syntheses, via hydrothermal route or controlled coprecipitation in an automated reactor, of colloidal Fe3O4 nanoparticles with sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm. To investigate structure–property relationships as a function of the synthetic procedure, we used multiple techniques to characterize the structure, phase composition, and magnetic behavior of these nanoparticles. For the iron oxide cores of these nanoparticles, powder X-ray diffraction and electron microscopy both confirm single-phase Fe3O4 composition. In addition to the core composition, the magnetic performance of nanoparticles in the 13–20 nm size range can be strongly influenced by the surface properties, which we analyzed by three complementary techniques. Raman scattering and X-ray photoelectron spectroscopy (XPS) measurements indicate overoxidation of nanoparticle surfaces, while transmission electron microscopy (TEM) shows no distinct core–shell structure. Considered together, Raman, XPS, and TEM observations suggest that our nanoparticles have a gradually varying nonstoichiometric Fe3O4+δ composition, which could be attributed to the formation of Fe3O4–γ-Fe2O3 solid solutions at their outermost surface. Detailed analyses by TEM reveal that the hydrothermally produced samples include single-domain nanocrystals coexisting with defective twinned and dimer nanoparticles, which form as a result of oriented-attachment crystal growth. All our nanoparticles exhibit superparamagnetic-like behavior with a characteristic blocking temperature above room temperature. We attribute the estimated saturation magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the relatively large size of the nanoparticles (13–20 nm) coupled with the syntheses under elevated temperature; alternative explanations, such as surface-mediated effects, are not supported by our spectroscopy or microscopy measurements. For these colloids, the heating efficiency in magnetic hyperthermia correlates with their saturation magnetization, making them appealing for therapeutic and other biomedical applications that rely on high-performance nanoparticle-mediated hyperthermia.

Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH2O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe3+ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT 2.

MCD spectroscopy of hexanuclear Mn(iii) salicylaldoxime single-molecule magnets

The hexanuclear cages [Mn(6)O(2)(R-sao)(6)L(2)(EtOH)(x)(H(2)O)(y)] "Mn(6)" behave as single-molecule magnets (SMMs) below a characteristic blocking temperature. As with [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] "Mn(12)" the electronic absorption spectra are rather featureless, yielding little information on the electronic structure of the magnetic ions. Low temperature Magnetic Circular Dichroism (MCD) spectra afford greater resolution of the optical transitions and also probe the magnetic properties of the system. Both the ground state spin and blocking temperature of the Mn(6) cages are determined by subtle structural perturbations of a generic Mn(6)O(2) core. Absorbance and MCD spectra are reported for [Mn(6)O(2)(Et-sao)(6){O(2)CPh(Me)(2)}(2)(EtOH)(6)] (1), [Mn(6)O(2)(Et-sao)(6){O(2)CPh}(2)(EtOH)(4)(H(2)O)(2)] (2), [Mn(6)O(2)(sao)(6){O(2)CPh}(2)(EtOH)(4)]·EtOH (3) and the trinuclear precursor [Mn(3)O(Et-sao)(3)(MeOH)(3)](ClO(4)) (4) cast into polymer film. SMM behaviour has previously been observed using magnetic susceptibility measurements on powder and single-crystal samples. The ligand field environment of the magnetic ions is assumed to be similar in (1) and (2) and their different blocking temperatures are attributed to the magnitude of the effective exchange constant. The MCD spectra of (1) and (2), in which the ground state spin S = 12, show that the ligand field environments of the Mn ions are almost identical and that magnetic hysteresis persists for isolated molecules when crystal packing forces are removed. The subtle structural differences between (1) and (2) are manifested in the field dependence of the MCD response at different wavelengths that reflect changes in band polarisation. The MCD spectrum of (3) contains features not apparent in those of (1) and (2). These are attributed to 5-coordinate Mn(iii), which is unique to (3) among the compounds studied. (3) has ground state spin S = 4, a lower blocking temperature and consequently no observable hysteresis in the MCD down to 1.7 K. Comparison of the MCD spectra of (1)-(3) to that of (4) confirms the integrity of the Mn(6)O(2) core when these materials are cast into polymer film.

Akaganeite polymer nanocomposites

An “in situ” method for the production of akaganeite polymer nanocomposites is described. A controlled precipitation is achieved by using a polymer matrix, polyvinylpyridine, containing N-base functional groups that form coordination bonds with iron ions. The resulting materials have permitted the observation of two sources of magnetic moment in akaganeite nanoparticles: (1) finite size effects with a characteristic blocking temperature below 2K; and (2) a deficient Cl− occupancy, with a characteristic blocking temperature around 18K. Moreover, the nanocomposites can be dissolved in slightly acidic media to obtain stable aqueous nanoparticle dispersions that could be useful in biomedical applications.

Cobalt–poly(amido amine) superparamagnetic nanocomposites

Metallic cobalt–dendrimer nanocomposites were prepared using generation 5 poly(amido amine) dendrimers with primary amino termini. Cobalt loading of ∼ 38 atoms per dendrimer was determined by atomic absorption spectrophotometry. Magnetic properties of the cobalt–dendrimer nanocomposites were investigated across the temperature range from 2 to 300 K by SQUID magnetometry. Magnetization as a function of temperature and applied field strength was studied in zero field cooled samples. Magnetization–demagnetization curves (hysteresis loops) were also acquired at temperatures between 10 and 300 K. These results clearly indicate superparamagnetism for the nanocomposites with a characteristic blocking temperature of ∼ 50 K.

Low coercive field and conducting nanocomposite formed by Fe 3O 4 and poly(thiophene)

Magnetite and poly(thiophene) composites have been produced by in situ monomer oxidation. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) confirmed the presence of Fe 3O 4 as particle agglomerates ranging from 15 to ca. 54 nm in size. Transmission electron micrographs (TEMs) revealed a face-to-face structure in both the pure magnetite and the nanocomposite. Typical superparamagnetic (ferrimagnetic) curves have been observed, whereas the relatively weak magnetic field employed in measurement, 200 Oe, was sufficient to split the curves completely. Zero field cooling (ZFC) and field cooling (FC) curves coincide only above room temperature, indicating that the characteristic blocking temperature ( T B) for superparamagnetic particles in this assembly is above room temperature.

Cobalt nanoparticles formed in polysiloxane copolymer micelles: effect of production methods on magnetic properties

The effect of production method on the chemical and magnetic stability of a range of cobalt nanoparticles formed in polysiloxane copolymer micelles was studied. The nanoparticles were suspended in polydimethylsiloxane (PDMS) carrier fluids and are prototypes for use in biomedical applications. Three families of particles were investigated: cobalt particles in triblock copolymer micelles and silica-coated cobalt particles where the silica coating is formed either in toluene or PDMS carrier fluid. The silica coating effectively protects the cobalt particles from ongoing oxidation, with little change in the saturation magnetization of the particles observed over time and only a small shift in the centre of the field-cooled hysteresis loop observed. Temperature-dependent magnetization measurements show that the silica-coated particles have superparamagnetic blocking temperatures of 100–130 K compared with the non-silica-coated particles, which do not reach their characteristic blocking temperature below the melting point of the frozen fluids. This result is interpreted as a smaller magnetic core size in the silica-coated particles. The zero-field-cooled temperature-dependent magnetization of the silica-coated particles has two apparent superparamagnetic blocking temperatures, with a second, lower blocking temperature of approximately 15 K. This second apparent blocking temperature may be due to unreacted cobalt clusters in the suspensions or, alternatively, due to surface states on the nanoscale cobalt particles. The prototype silica-coated cobalt particles show promise for potential biomedical applications.

Magnetization of the Layer Compounds AFeIIFeIII(C2O4)3 (A = Organic Cation), in Low and High Magnetic Fields: Manifestation of Néel N and Q Type Ferrimagnetism in a Molecular Lattice

Detailed bulk magnetization and magnetic susceptibility measurements are reported for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFeIIFeIII(C2O4)3 (A = quaternary ammonium, phosphonium, or arsonium). The compounds all crystallize in two-dimensional hexagonal honeycomb lattices, but while some show conventional low field positive magnetization at low temperature (Néel type Q), others show a large negative magnetization below a compensation temperature Tcomp (Néel type N) under the same measurement protocol. Isothermal and temperature-dependent magnetizations are measured after zero field and field cooling. Temperature-dependent hysteresis measurements are also presented. In compounds that exhibit negative magnetization a discontinuity in the magnetization is observed at Tt (Tcomp < Tt < Tc) which correlates with the onset of a giant anisotropy and is indicative of a magnetostrictive transition. In compounds that exhibit positive magnetization, hysteresis behavior and frequency-dependent ac susceptibility data indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a characteristic blocking temperature or imbalance and disorder of Fe(II) and Fe(III).