Nanocrystalline γ-Fe2O3 Research Articles

Preparation of Nanocrystalline Fe<sub>2</sub>O<sub>3</sub> Powder by Cryomilling with Liquid Nitrogen and its Magnetic Property

Rapid preparation of nanocrystalline γ-Fe2O3 powder with superparamagnetism was realized by cryomilling commercial Fe2O3 powder using liquid nitrogen. The effects of milling temperature and duration on the grain size, phase and microstructure of the nanocrystalline Fe2O3 powder were analyzed. Magnetic property of the nanocrystalline γ-Fe2O3 powder was also tested by magnetometer at room temperature. The results demonstrate that nanocrystalline γ-Fe2O3 powder with single phase can be prepared rapidly by cryomilling with liquid nitrogen. The mean particle size of γ-Fe2O3 powder can be reduced from 300 nm to 13 nm by cryomilling at −130 °C within 3 hours. The nanocrystalline γ-Fe2O3 powder shows superparamagnetism at room temperature.

Silver ferrite/maghemite composites and mixtures: Impact of one-pot composite preparation on battery-relevant electrochemistry

A series of silver ferrite/maghemite composite materials (AgxFeOy, where AgxFeOy=composites of xAgFeO2+[(1−x)/2] γ-Fe2O3 and y=2−(1−x)/2) have been prepared via a non-stoichiometric, one-pot co-precipitation strategy. The impact of a one-pot composite preparation on electrochemistry was determined by comparison to a mechanical mixture of nanocrystalline AgFeO2 and γ-Fe2O3 powders, which were combined to afford an overall composition of Ag0.2FeO1.6. Notably, following 30 discharge-charge cycles, the one-pot Ag0.2FeO1.6 composite delivers an energy density (156mWh/g) profoundly higher than the mechanical mixture Ag0.2FeO1.6 (67mWh/g). AgxFeOy composites with the lowest Ag (x) content and smallest crystallite size, Ag0.2FeO1.6, deliver twice the discharge energy density (169mWh/g) of stoichiometric AgFeO2 with no maghemite component and exhibited good capacity retention over 50 cycles. The notable electrochemistry of one-pot AgxFeOy composites may involve reduced AgFeO2 and γ-Fe2O3 aggregate sizes and a concomitant increase in AgFeO2/γ-Fe2O3 crystallite contact not achievable through mechanical mixing.

γ-Fe2O3 nanoparticles prepared by combustion synthesis, followed by chemical oxidation of residual carbon with H2O2

The combustion reaction product between iron nitrate and glucose is a black powder containing nanocrystalline γ-Fe2O3 impurified with residual carbon. After the H2O2 treatment, the carbon residue is removed, so that the carbon content decreases from 32.7% to 0.4%, and the color of the sample changes from black to reddish brown. Simultaneously the BET surface area of γ-Fe2O3 increases dramatically from 72.6 m2 g−1 to 149.0 m2 g−1. The resulted γ-Fe2O3 particles have a round shape and an average size of 12 nm. The average crystallite size of γ-Fe2O3 calculated from XRD patterns was 5 nm. The resulted γ-Fe2O3 nanoparticles exhibit a superparamagnetic behavior and a saturation magnetization of 41.5 emu g−1. In addition to the removal of residual carbon, the H2O2 treatment also ensures the sterilization of γ-Fe2O3 particles, which is a key requirement in the field of biomedical applications. γ-Fe2O3 nanoparticles coated with a double layer of oleic acid were dispersed in saline solution, leading to a stable colloidal suspension. DLS measurements indicated a unimodal distribution of γ-Fe2O3 nanoparticles and a hydrodynamic diameter of 80 nm.

On the role of grain boundaries in nanocrystalline γ-Fe2O3 under high pressure

In situ impedance spectroscopy of nanophase and bulk γ-Fe2O3 were performed using a fabricated microcircuit on a diamond anvil cell. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The analysis establishes that the grain boundary conductance of nanocrystalline γ-Fe2O3 is higher than that of the bulk material. Both the grain and grain boundary resistances of γ-Fe2O3 smoothly decrease with pressure. A remarkable phenomenon of discontinuity appears at 7.4 GPa for the nanocrystals. This critical value is interpreted by the space charge model and changes in charge densities between the grain boundaries. The impedance analysis further reveals that the relaxation frequency gradually increased with the effect of pressure and the γ-Fe2O3 samples’ impedance imaginary part exhibits a typical capacitive characteristic. Within the given pressure range, the relaxation frequency of γ-Fe2O3 follows the Arrhenius behavior. This result was attributed to a dominant interfacial effect.

Quasi-isothermal decomposition: a way to nanocrystalline mesoporous-like Fe2O3 catalyst for rapid heterogeneous decomposition of hydrogen peroxide

Mesoporous-like heterogeneous catalyst, consisting of ultrasmall (<5 nm) amorphous Fe2O3 and nanocrystalline γ-Fe2O3 possessing surface area of 440 m2 g−1 and reaching an enormous rate constant of 70 × 10−3 min−1 in hydrogen peroxide decomposition at room temperature, pH of 6.3–6.5 and the catalyst concentration of 1 g L−1 was easily prepared by a one-step quasi-isothermal decomposition (an isothermal decomposition accompanied with a controlled time-limited exotherm) of iron(III) oxalate in air. The non-equilibrium temperature conditions proceeding at the very beginning of the oxalate decomposition proved to be the key factor responsible for the formation of highly efficient combination of nanocrystalline character, large surface area and mesoporous-like nature of the catalyst. The material and catalytic properties of the as-prepared catalyst are discussed and compared to those obtained for our iron(III) oxide-based catalyst (26 × 10−3 min−1) prepared earlier by a slow, purely isothermal decomposition of iron(II) oxalate.

Structure and photocatalytic performance of magnetically separable titania photocatalysts for the degradation of propachlor

A magnetic photocatalyst was prepared by modification of TiO2 nanoparticles (Degussa P25) with nanocrystalline γ-Fe2O3 nanoparticles through a protective lining made up of two oppositely charged polyelectrolytes. As-prepared magnetically separable photocatalysts differing in γ-Fe2O3 loading (3, 8, 13, 20 and 30wt.%) were characterized by XRD, TEM, thermal analysis, Mössbauer and magnetic measurements. The photocatalytic efficiency of the nanocomposite catalysts was evaluated using a chloroacetanilide herbicide (propachlor) in water as model compound. The primary degradation of propachlor followed pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. Generally, all magnetic photocatalysts exhibit good catalytic activity towards organic pollutants, do not suffer from photodissolution and can be reused several times without any decrease in their photocatalytic activity.

Enhancement of the phase transformation temperature of γ-Fe2O3by Zn2+doping

The phase transformation temperature of γ-Fe2O3 to α-Fe2O3 is increased with Zn2+ doping and enhancement of the transition temperature by ∼100 °C is obtained for doping of ≥15 mol% Zn in nanocrystalline γ-Fe2O3 synthesized by an autocombustion method.

Oriented Growth of Nanocrystalline Gamma Ferric Oxide in Electrophoretically Deposited Films

Films of nanocrystalline γ-Fe2O3 were deposited on silicon substrates by using the technique of electrophoretic deposition. The precursor powder was nanocrystalline γ-Fe2O3, which was synthesized, using DC arc plasma in the oxygen ambient by vapour–vapour interaction in gas phase condensation; at a stabilized arc current of 40 A. This powder was characterized by X-ray diffraction, Transmission Electron Microscopy, Vibrating Sample Magnetometer and Mossbauer Spectroscopy. An increase in directional coercivity was observed in case of films deposited on silicon substrates, which is dramatically significant. Preferred orientation of almost similar sized nanocrystalline magnetic domains in deposited films is evident from the results of X-ray diffraction and Transmission Electron Microscopy results. The preferred alignment of the nanocrystallites seems to be responsible for the significant changes observed in magnetic properties of films.

Magnetism of amorphous Fe 2 O 3 nanopowders synthesized by solid‐state reactions

Monodispersed amorphous Fe2O3 nanoparticles with the size ranging from 1 to 4 nm have been prepared by thermally induced oxidative decomposition of Prussian Blue, Fe4[Fe(CN)6]3, in air. Amorphous nanopowders exhibit specific magnetic behaviour strongly affected by interparticle interactions and extremely low particle size with a high contribution of the surface anisotropy as illustrated by temperature dependent and external field Mossbauer spectroscopy and magnetization measurements (4–300 K). Some of the properties are in close similarity to those of spin glasses including the fast thermal variation of superparamagnetic fraction in the Mossbauer spectra, or the particle size independence of the blocking temperature, and non-saturation of magnetization even at 5 K and magnetic field of 10 T. The reduced value of internal magnetic hyperfine field (∼49 T at 5 K) and no indications of the tetrahedral Fe3+ sites in the 5T/5K external field Mossbauer spectra are the principal found differences in comparison with the magnetic behaviour of nanocrystalline γ-Fe2O3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mössbauer Studies on Nanocrystalline Diol Capped -Fe2O3

Nanocrystalline γ-Fe2O3 was prepared by refluxing ferric nitrate in 1,4-butanediol. Characterisation was done by XRD, FTIR and Raman spectroscopy. TEM showed that the average particle size of the two samples were 9 nm and 6 nm with a small particle size distribution. FTIR and TG showed that the γ-Fe2O3 particles were capped with the diol. Mossbauer spectra of the 6 nm sample showed that the sample is superparamagnetic with a blocking temperature around 225 K. With increase in particle size, blocking temperature increases. 20 K MS of 9 nm sample was fitted to 2 sextets. IS, QS and Hf parameters indicate presence of Fe3+ on A and B sites of the spinel lattice. MS of 6 nm sample were fitted to 3 sextets: Fe3+ ions on A and B sites and on the surface. As the coating is attached to the surface Fe3+ ions, EFG of these ions will be larger than the bulk Fe3+ ions. Hence the QS and Hf values are different for the 3rd sextet. These results indicate that although both the samples are nanocrystalline and both are coated, the effect of coating on the MS is only shown by the sample with smaller particle size. Thus when the particles are large the EFG of the Fe3+ ions in the bulk and surface are same and hence good fitting of the spectra can be obtained with 2 sextets only. From these results it may be concluded that synthesis of γ-Fe2O3 in diol results in a diol coated sample and the coating can be detected by MS when the particle size is very small.

Free Rotation of Magnetic Nanoparticles in a Solid Matrix

Magnetic data for a composite material consisting of nanocrystalline γ-Fe2O3 dispersed in an alginate methanol matrix indicate the presence of rotationally free magnetic particles. The decoupling of the particles from the matrix was achieved by subjecting the sample to a magnetic field applied in alternating directions. Before field treatment, a magnetic blocking process is observed at low temperature, which is characteristic of nanoscale magnetic materials in the presence of barriers for the magnetic moment of the particles. After the field treatment, no magnetic hysteresis is observed and the magnetization follows a Curie−Weiss law. This behavior can only be interpreted as due to magnetic moments that follow the external magnetic field without delay, implying, therefore, the existence of free particles.

Anomalous dielectric behavior in nanocrystalline γ-Fe2O3

The measurements of electric capacities and dielectric loss angles for nanocrystalline γ-Fe2O3, under air and vacuum atmospheres have been conducted by using ac LRC method, and the conductivity and the polarization relaxation time have also been calculated from the frequency spectra of real and imaginary parts of the dielectric constants. The anomalous dielectric behavior implies the existence of two kinds of polarization mechanisms with different relaxation times, which are caused by the defects and the dangling bonds in the interfaces, respectively. The experimental and calculated results indicated that the polarization loss and the conductance loss are dominant in air atmosphere and vacuum, respectively.

Size quantization and interfacial effects on a novel γ-Fe2O3/SiO2 magnetic nanocomposite via sol-gel matrix-mediated synthesis

Novel isolated magnetic single-domain γ-Fe2O3 nanoclusters have been prepared by coprecipitation of ferrous and ferric salts encapsulated within sol-gel derived silica (SiO2). The nonmagnetic SiO2 coating formed by hydrolysis and polycondensation of tetraethoxysilane on the surface of the Fe2O3 nanoclusters provides a means for thermally stable dispersion of Fe2O3 clusters. The precipitated particles coated with SiO2 are spherical with 4–5 nm diameters. Surface and strain effects played a critical role in determining the overall magnetic behavior of the spherical single-domain particles. Superparamagnetic behavior was observed by superconducting quantum interference device magnetometry and Mössbauer spectroscopy. Superparamagnetic barrier energies and the low-temperature coercivities were modified through cluster/support interface microstructure manipulation. The optical studies showed the absorption edge of the nanocomposites to be slightly blue shifted in the UV–VIS spectrum range when compared to that of bulk γ-Fe2O3. This was attributed to the combined effects of the quantum confinement of the nanocrystalline γ-Fe2O3 clusters and the stress present at the particle/support interface. The magnetic properties can be manipulated via the matrix microstructure, synthesis conditions and thermal treatment.

Magnetic relaxation and quantum tunneling in nanocrystalline particles

Low-temperature magnetic relaxation phenomena have been studied in two samples consisting of nanocrystalline γ-Fe2O3 and polymer in either water (as ferrofluid) or in potassium silicate (as glass). The relaxation of the magnetization follows a logarithmic law and the magnetic viscosity, S, tends to be independent of temperature below 2.1 K for the glass sample, giving evidence of macroscopic quantum tunneling of the magnetization.

Magnetic relaxation in nanocrystalline iron-oxides

Nanocrystalline Fe oxides are compacted solids of fine particles. By compacting interfaces are formed between adjacent particles. Since collective magnetic relaxation and superparamagnetism depend on the anisotropic energy, it is expected that the existence of interfaces and related interface anisotropy will change the relaxation behavior of nanocrystalline Fe oxides. Measurements on nanocrystalline α-Fe2O3 and γ-Fe2O3 have shown that the exchange interaction in the interfaces is more effective in suppressing superparamagnetic relaxation upon compacting than the dipole interaction.