Thermally Induced Solid-State Syntheses of γ-Fe2O3 Nanoparticles and Their Transformation to α-Fe2O3 via ε-Fe2O3

The thermally induced solid-state syntheses of -Fe₂O₃ nanoparticles from iron-bearing materials (FeSO₄, Fe₂(C₂O₄)₃ and almandine garnet) are described. Magnetic properties, particles size and the mechanism of the structural change of -Fe₂O₃ nanoparticles have been investigated using ⁵⁷Fe Mössbauer spectroscopy, X-ray powder diffraction (XRD) and atomic force microscopy (AFM). -Fe₂O₃ nanoparticles are transformed into hematite via -Fe₂O₃ as the intermediate.     

Surface structure of γ-Fe2O3(111)

The surface structure of γ-Fe₂O₃(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10^? 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe₂O₃-like layer.     

Nanometric core-shell-shell γ-Fe2O3/SiO2/TiO2 particles

The preparation of core-shell-shell γ-Fe₂O₃/SiO₂/TiO₂ nanoparticles of few tens nanometers is performed by successively coating onto magnetic nanoparticles a SiO₂ layer and a TiO₂ layer, using sol–gel methods. The thickness of the two layers and the aggregation state of the particles can be controlled by the experimental conditions used for the two coatings. These composite nanoparticles may find application as magnetic photocatalysts, since they are characterized by their small diameters which allow a good accessibility to the TiO₂ shell. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mössbauer investigation of non-aggregated γ-Fe2O3 particles

Investigation by Mössbauer spectroscopy of non-aggregated nanometric -Fe₂O₃ particles dispersed in polymer is reported. Magnetic interactions between the particles were controlled by varying the particle concentration in the polymer. The results show that over the investigated range, the interactions make the relaxation time shorter. Infield experiments show spin canting which increases with decreasing particle size.     

Spectroscopy of Reflected Electron Energy Losses of γ-Fe2O3

Physics of the Solid State - The spectra of energy losses of reflected electrons, which are recorded with the energy of primary electrons in the range of 200–3000 eV, are investigated. From...     

Synthesis and application of the uniform particle size of nano-γ-Fe2O3: dispersed nanoparticles of γ-Fe2O3 for green synthesis of aminophosphonates

China started nanotechnology research from 1990s, and has paid much attention to nanoeducation since then. Scientific discoveries from nanoresearch have been introduced timely into high school textbooks, and more specific courses such as Nanomaterials and Nanofabrication are being taught at both college and graduate levels. In this paper, nanotechnology courses at the Graduate University of Chinese Academy of Sciences, Beijing University, and Tsinghua University are chosen to be overviewed, which could be a good indicator of Chinese nanoeducation because of high-quality science and engineering education in these schools. Research institutes also contribute in popularizing nanoscience knowledge through activities such as public open days. These combined efforts substantially promote the public acceptance and understanding of nanoscience and technology in China.     

Effect of contacts between γ-Fe2O3 nanoparticles on their phase-transition temperature

The effect of mechanical contacts between γ-Fe₂O₃ particles on the temperature of the γ-α structural transition in them is established by magnetic studies and differential thermal analysis (DTA). The sample in which γ-Fe₂O₃ particles had no mechanical contacts with one another remained in the ferromagnetic state up to T _C = 630°C and had two exothermal DTA peaks. The first peak almost coincided with the Curie temperature, while the second peak attributed to the γ → α structural transition corresponded to 760°C. The magnetic transition for particles with a larger number of contacts was shadowed by the γ → α structural transition with a temperature lowered to 550°C.     

Magnetic properties of γ-Fe2O3 nanoparticles encapsulated in surface-treated polymer spheres

Mössbauer and magnetic characterization of polymer-dispersed γ-Fe₂O₃ nanoparticles treated under different chemical processes are reported in this work. X-ray powder diffraction analysis provides a mean particle size of D ~ 8.0 nm. Whereas Mössbauer spectroscopy data suggest the presence of only Fe^3?+? ions, magnetization measurements indicate the occurrence of a freezing phenomenon in agreement with the thermal evolution of Mössbauer spectra. A core–shell model was used to determine a magnetically disordered layer (shell) of d ~ 1.0 nm covering a region of collinear magnetic moments (core). The chemical treatments with H₂O₂ and Na₂S₂O₈ modify notoriously the magnetic response of the polymer-dispersed nanoparticles.     

Synthesis of magnetic γ-Fe2O3-based nanomaterial for ultrasonic assisted dyes adsorption: Modeling and optimization

γ-Fe₂O₃ nanoparticles were synthesized and loaded on activated carbon. The prepared nanomaterial was characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transforms infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The γ-Fe₂O₃ nanoparticle-loaded activated carbon (γ-Fe₂O₃-NPs-AC) was used as novel adsorbent for the ultrasonic-assisted removal of methylene blue (MB) and malachite green (MG). Response surface methodology and artificial neural network were applied to model and optimize the adsorption of the MB and MG in their individual and binary solutions followed by the investigation on adsorption isotherm and kinetics. The individual effects of parameters such as pH, mass of adsorbent, ultrasonication time as well as MB and MG concentrations in addition to the effects of their possible interactions on the adsorption process were investigated. The numerical optimization revealed that the optimum adsorption (>99.5% for each dye) is obtained at 0.02 g, 15 mg L⁻¹, 4 min and 7.0 corresponding to the adsorbent mass, each dye concentration, sonication time and pH, respectively. The Freundlich, Langmuir, Temkin and Dubinin–Radushkevich isotherms were studied. The Langmuir was found to be most applicable isotherm which predicted maximum monolayer adsorption capacities of 195.55 and 207.04 mg g⁻¹ for the adsorption of MB and MG, respectively. The pseudo-second order model was found to be applicable for the adsorption kinetics. Blank experiments (without any adsorbent) were run to investigate the possible degradation of the dyes studied in presence of ultrasonication. No dyes degradation was observed.     

Effect of interparticle interactions on the dynamical properties of γ-Fe2O3 nanoparticles

The dynamical properties of γ-Fe₂O₃ nanoparticles dispersed in a polymer have been investigated by means of AC susceptibility measurements in a large frequency range (10<ν<10⁴ Hz) and Mössbauer spectroscopy measurements. The frequency dependence of the blocking temperature, T_B, has been analysed for a series of samples with the same volume distribution and interparticle interactions of different strengths. An increase of the effective energy barrier with increasing interaction strength has been observed unambiguously, in agreement with our model, based on a statistical calculation of the interaction energy for a disordered assembly of particles with volume distribution and easy axes in random directions.     

Increased surface spin stability in γ-Fe2O3 nanoparticles with a Cu shell

γ-Fe(2)O(3) nanoparticles were coated with a Cu shell in situ during synthesis. An interfacial monolayer of CuO in the Cu-coated γ-Fe(2)O(3) nanoparticles was discovered that stabilized the disordered surface spins of γ-Fe(2)O(3) nanoparticles. Element-specific x-ray absorption spectroscopy at the L-edges for Cu and Fe indicated the magnetic moment of the Cu in the shell interacted with the γ-Fe(2)O(3) nanoparticle's surface magnetic moments. This exchange interaction between the Fe and Cu at the interface permitted an overall Cu moment in CuO (an antiferromagnet typically) that altered the γ-Fe(2)O(3) nanomagnetism. Increasing the Cu shell thickness also increased the total Fe magnetism of the nanoparticles.     

A Mössbauer and ESR study of LiNbO3-Fe2O3 for low Fe2O3 concentrations

Samples of the system LiNbO₃-Fe₂O₃ prepared by water quenching and by the double-roller quenching method in the range up to 24 mol% Fe₂O₃ were investigated by Mössbauer and ESR spectroscopy. In the water quenched samples up to 11 mol% Fe₂O₃ only the Fe³⁺ and the Fe²⁺ valence states could be detected. The Fe²⁺ concentration decreased with increasing Fe₂O₃ content. Above 11 mol% Fe₂O₃ magnetically split Mössbauer spectra indicated the presence of Fe₂O₃ clusters. The isomer shift values of Fe³⁺ as a function of Fe₂O₃ concentration showed jumps at 6 and 11 mol% Fe₂O₃, whereas no significant changes could be detected in the quadrupole splitting values. The ESR data already exhibited the existence of isolated Fe³⁺ ions and of clusters with Fe-Fe distances less than 8 Å for the lowest Fe₂O₃ concentration. The cluster signal intensity increased with increasing Fe₂O₃ content. The roller quenched samples showed increased Fe²⁺ concentration as compared to the water quenched samples, which suggests that slow quenching results in iron oxidation and cluster formation. For low Fe₂O₃ concentrations a valence state change Fe³⁺Fe²⁺ can easily be obtained by heat treatments in various atmospheres, whereas for higher Fe₂O₃ contents (9.8 mol%) precipitations of-Fe (in reducing atmosphere) and Fe₂O₃ (in air) could be observed in addition to the valence state changes of a remaining part of dissolved Fe ions. On the basis of the obtained results a model was suggested for the unusual behaviour of the lattice parameters observed in LiNbO₃-Fe₂O₃.     

Synthesis,characteristics and sonocatalytic activities of calcined γ-Fe2O3 and TiO2 nanotubes/γ-Fe2O3 magnetic catalysts in the degradation of Orange G

In this work, γ-Fe₂O₃ and TiO₂ NTs/γ-Fe₂O₃ composites with good magnetism and sonocatalytic activity were prepared by a facile polyol method and utilize the principle of isoelectric point method, respectively. The structural and magnetic features of the prepared calcined γ-Fe₂O₃ and composite catalysts were investigated by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), surface analysis, UV–Vis diffuse reflectance spectra (UV–Vis DRS), vibrating sample magnetometry (VSM) and zeta potential analysis. The effects of calcination temperature on γ-Fe₂O₃ phase variation, physical properties and sonocatalytic properties were investigated. The porosity, specific surface area, band gap energy and sonocatalytic activity of γ-Fe₂O₃ were gradually decreased with calcination temperature increased. TiO₂ NTs/γ-Fe₂O₃ with appropriate composition and specific structural features possess synergetic effects such as efficient separation of charge carriers and hydroxyl radicals produced by heterogeneous fenton and fenton-like reactions. This enhanced the sonocatalytic activity for the degradation of Orange G under ultrasonic irradiation. The sonocatalytic reactions obeyed pseudo first-order kinetics. All these information provide insight into the design and development of high-efficiency catalyst for wastewater treatment.     

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

-Fe₂O₃ nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10^–2–10⁴ Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.     

The modification of the initial susceptibility of dilute binary ferrofluids based on γ-Fe2O3/ZnFe2O4 nanoparticles

This paper presents a multi-band metamaterial absorber comprising three multi-gap split-ring resonators (SRRs) with different radii and ring widths, designed in combinatorial approach. Experiments demonstrate that it can perform absorption peaks at three resonant frequencies 7.10 GHz, 10.04 GHz, and 17.44 GHz with the absorption of 99.90%, 99.91%, and 99.68%, respectively. The physical mechanism of metamaterial absorber was explained through numerical calculation and simulation, which showed that three absorption peaks were caused respectively by the three four-gap SRRs. The absorber is insensitive to incident angles and polarization states, so it has broad prospect of application.     

α-Fe2O3 and Fe3O4 hollow nanospheres as high-capacity anode materials for rechargeable Li-ion batteries

α-Fe₂O₃ and Fe₃O₄ hollow nanospheres of size 30?±?2 nm were synthesized by using polymeric micelles as a soft template for the first time. The hollow nanospheres were thoroughly characterized by transmission electron microscope, superconducting quantum interference device, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry and differential thermal analysis, cyclic voltammogram, and nitrogen sorption analyses. The α-Fe₂O₃ and Fe₃O₄ hollow nanospheres were used as anode materials in lithium-ion rechargeable batteries to investigate their electrochemical properties. The hollow particle-based electrodes exhibit high capacity, stable cycling performance, and good rate capability at different current densities. The α-Fe₂O₃ and Fe₃O₄ hollow nanospheres with nanosized shell domain favors fast lithium insertion/extraction processes during the repeated charge/discharges.     

Self-diffusion in α-Fe2O3 natural single crystals

Measurements of¹⁸O self-diffusion in hematite (Fe₂O₃) natural single crystals have been carried out as a function of temperature at constant partial pressure a_{O 2}=6.5·10^?2 in the temperature range 890 to 1227 °C. The a_{O 2} dependence of the oxygen self-diffusion coefficient at fixed temperature T=1150 °C has also been deduced in the a_{O 2} range 4.5·10^?4 - 6.5·10^?1. The concentration profiles were established by secondary-ion mass spectrometry; several profiles exhibit curvatures or long tails; volume diffusion coefficients were computed from the first part of the profiles using a solution taking into account the evaporation and the exchange at the surface. The results are well described by $$D_O \left( {{{cm^2 } \mathord{\left/ {\vphantom {{cm^2 } s}} \right. \kern-\nulldelimiterspace} s}} \right) = 2.7 \cdot 10^8 a_{O_2 }^{ - 0.26} \exp \left( { - \frac{{542\left( {{{kJ} \mathord{\left/ {\vphantom {{kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}}} \right)}}{{RT}}} \right)$$ From fitting a grain boundary diffusion solution to the profile tails, the oxygen self-diffusion coefficient in sub-boundaries has been deduced. They are well described by $$D''_O \left( {{{cm^2 } \mathord{\left/ {\vphantom {{cm^2 } s}} \right. \kern-\nulldelimiterspace} s}} \right) = 3.2 \cdot 10^{25} a_{O_2 }^{ - 0.4} \exp \left( { - \frac{{911\left( {{{kJ} \mathord{\left/ {\vphantom {{kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}}} \right)}}{{RT}}} \right)$$ Experiments performed introducing simultaneously¹⁸O and⁵⁷Fe provided comparative values of the self-diffusion coefficients in volume: iron is slower than oxygen in this system showing that the concentrations of atomic point defects in the iron sublattice are lower than the concentrations of atomic point defects in the oxygen sublattice. The iron self-diffusion values obtained at T>940 °C can be described by $$D_{Fe} \left( {{{cm^2 } \mathord{\left/ {\vphantom {{cm^2 } s}} \right. \kern-\nulldelimiterspace} s}} \right) = 9.2 \cdot 10^{10} a_{O_2 }^{ - 0.56} \exp \left( { - \frac{{578\left( {{{kJ} \mathord{\left/ {\vphantom {{kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}}} \right)}}{{RT}}} \right)$$ The exponent - 1/4 observed for the oxygen activity dependence of the oxygen self-diffusion in the bulk has been interpreted considering that singly charged oxygen vacancies V _O ^? are involved in the oxygen diffusion mechanism. Oxygen activity dependence of iron self-diffusion is not known accurately but the best agreement with the point defect population model is obtained considering that iron self-diffusion occurs both via neutral interstitals Fe _x ⁱ and charged ones.     

溶剂热法制备α-Fe2O3纳米材料

采用溶剂热法,以硝酸铁为铁源,以聚乙烯吡咯烷酮（PVP）为表面活性剂,合成了立方体α-Fe2O3纳米材料,并用X-射线衍射仪（XRD）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）对其进行了表征.结果表明：溶剂热法所制备的立方体α-Fe2O3纳米材料,其平均边长约为30nm.     

From pure superparamagnetic regime to glass collective state of magnetic moments in γ-Fe2O3 nanoparticle assemblies

Studies of the frequency dependence of the temperature of the AC susceptibility peak, of the thermal variation of the nonlinear DC susceptibility, and of ageing effects on the magnetization relaxation in γ-Fe₂O₃ 4.7 nm nanoparticle assemblies with interparticle interactions of varying strength, give evidence of three magnetic regimes: pure superparamagnetic, superparamagnetic modified by the interactions, and collective. The properties of the latter regime, called glass collective state, are close to those of a canonical spin glass.     

Mechanochemical Transformations in α-Fe2O3 – ICEMS Study

Integral conversion electron Mössbauer spectroscopy has been used to investigate the phases and transformations which occur when -Fe₂O₃ is wet-milled in vacuum for up to 144 h. In addition to the transformation to off-stoichiometric Fe_3–x O₄ observed previously by transmission Mössbauer spectroscopy, the room temperature ICEMS spectra reveal the presence of 10–15% -Fe₂O₃ on the surfaces of particles milled for more than 24 h. Time-of-flight neutron diffraction of an -Fe₂O₃ sample wet-milled for 200 h also provides evidence of the occurrence of a small fraction (4%) of -Fe₂O₃ in the milled product. The -Fe₂O₃ is likely to occur as a surface oxide layer and does not appear to play a significant role in the mechanochemical transformation of -Fe₂O₃ to Fe_3–x O₄.