Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

Influence of Fe2O3 Doping on TiO2 Electrode for Enhancement Photovoltaic Efficiency of Dye-Sensitized Solar Cells

In this work, we report for the first time the improvement of the photovoltaic characteristics of dye-sensitized solar cells (DSSCs) by doping TiO₂ with Fe₂O₃. DSSCs were fabricated using various percentages of Fe₂O₃-doped TiO₂ composite nanoparticles. The Fe₂O₃-doped DSSCs exhibited a maximum conversion efficiency of 5.76% because of the effective electron transport. DSSCs based on Fe₂O₃-doped TiO₂ films showed better photovoltaic performance than cells fabricated with only TiO₂ nanoparticles. This result was attributed to the prevention of recombination between electrons in the TiO₂ conduction band with the dye or electrolytes. A mechanism was suggested based on impedance results, which indicated improved electron transport at the interface of the TiO₂/dye/electrolyte.     

The study of interactions between iron and vanadium ions in semiconducting barium-vanadate glasses doped with Fe2O3

Semiconducting barium-vanadate glasses doped with Fe₂O₃ ranging from 0.1 to 10 wt% were studied. We made attempts to understand features of an incorporation of the impurity ions into the host matrix. EPR, magnetic susceptibility, dc-conductivity and the Mössbauer effect were investigated.It was established that iron entered into the host as Fe³⁺·Fe³⁺ and V⁴⁺ ions formed associates coupled by dipole-dipole interactions for low Fe₂O₃ contents in the glass. The V⁴⁺?Fe³⁺ and Fe³⁺?Fe³⁺ pairs co-existed for all glasses. The contribution of Fe³⁺?Fe³⁺ interactions increased with increasing Fe₂O₃ content. The deviation from paramagnetic behaviour was observed for glasses with 8–9 wt% Fe₂O₃. It was attributed to presence of fine crystalline magnetic particles.The iron impurity induces no considerable changes in the dc-conductivity of the glass. The concentration dependence of dc-conductivity exhibits a minimum of about 5–6 wt% Fe₂O₃.     

On the local coordination of Fe in Fe2O3-glass and Fe2O3-glass ceramic systems containing Pb, Na and Si

The effect of the composition and casting temperature on the structural role of Fe in a series of binary (Fe₂O₃-PbO and Fe₂O₃-Na₂O) and ternary (Fe₂O₃-PbO-SiO₂) glass and glass ceramic materials is studied by means of X-ray fluorescence (XRF) mapping, X-ray absorption fine structure (XAFS) and Mössbauer spectroscopies. According to the Mössbauer results the majority of Fe exists in the Fe⁺³ state. The XRF maps reveal that Fe-rich islands evolve into the vitreous matrix of ternary samples that contain more than 40 wt% Fe₂O₃. In these samples the XAFS results disclose that 40-43 at.% of the Fe atoms belong to the Fe-rich microcrystalline islands formed by Fe_xO_y oxides. Furthermore, the structural role of Fe⁺³ in the ternary glasses is found to depend on the Fe₂O₃ content. Finally in the binary Fe₂O₃-PbO systems the majority of Fe⁺³ is octahedrally coordinated in the Fe₂O₃ and/or PbFe₁₂O₁₉ crystalline phases.     

Electric conductivity of glasses in the system Na2O/CaO/SiO2/Fe2O3

The addition of polyvalent transition metal ions to the usually insulating traditional soda-lime-silica glasses can lead to semiconducting properties. We report on synthesis of glasses and glass-ceramics in a soda-lime-silicate based system containing Fe₂O₃ in the concentration range from 5 to 30 mol%. Two sub-systems were considered, in one of them the ratio [Na₂O]/[Fe₂O₃] was varied while in the other one, the ratio [SiO₂]/[Fe₂O₃] was changed. The phase composition of the synthesized products was characterized by X-ray diffraction and energy dispersive X-ray analysis, while the electrical properties were studied by impedance spectroscopy. Partially crystallized non-reduced samples are semiconducting even at room temperature while the glassy samples (both reduced and non-reduced) exhibit semiconducting properties at temperatures equal or larger than 100 °C. An attempt is done to predict the physical approximation explaining the conduction process in the glasses.     

Physical properties of lead iron phosphate glasses containing Cr2O3

The influence of Cr₂O₃ on glass forming characteristics and physical properties of PbO-Fe₂O₃-P₂O₅ glasses has been investigated by Raman and Mössbauer spectroscopies, X-ray diffraction analysis (XRD), Differential Thermal Analysis (DTA), Scanning Electron Microscopy (SEM) and impedance spectroscopy. Glasses of the general composition xCr₂O₃-(28.3-x)PbO-28.7Fe₂O₃-43.0P₂O₅, 0 ≤ × ≤ 10, (mol%) were prepared by conventional melt-quenching technique. The compositions containing up to 4 mol% Cr₂O₃ formed fully amorphous samples and their Raman spectra show systematic increase in the fraction of orthophosphate Q⁰ units with increasing Cr₂O₃ content and O/P ratio.On the other hand, compositions containing 8 and 10 mol% Cr₂O₃ partially crystallized during cooling and annealing to Fe₇(PO₄)₆, Fe₂Pb₃(PO₄)₄ and Cr₂Pb₃(PO₄)₄. A high tendency for crystallization of these melts is related to the high O/P (> 4) and Fe²⁺/Fe_tot (≈ 0.60) ratios.Electrical conductivity of xCr₂O₃-(28.3-x)PbO-28.7Fe₂O₃-43.0P₂O₅, 0 ≤ × ≤ 10, (mol%) compositions is independent of Cr₂O₃ and controlled entirely by the polaron transfer between Fe²⁺ and Fe³⁺ ions.     

Evolution from (110) Fe to (111) Fe3O4 thin films grown by magnetron sputtering using Fe2O3 target

Fe and Fe₃O₄ thin films were grown by radio frequency magnetron sputtering. Fe₂O₃ was used as the target and hydrogen was introduced together with Argon gas to provide a certain reducing atmosphere. By varying H₂/Ar flow ratio, the changes in composition and structure of the thin films from (110) Fe to (111) Fe₃O₄ were observed by X-ray diffraction. The valence states of Fe in the thin films were analyzed by X-ray photoelectron spectroscopy. Magnetization measurements indicate that the Fe thin films grown with low H₂/Ar flow ratios possess large coercive force. It was ascribed to the increasing boundary density and the increasing amount of Fe oxides such as FeO distributed at the boundary.     

Structural properties of Cr2O3–Fe2O3–P2O5 glasses,Part I

The structural properties of xCr₂O₃–(40 − x)Fe₂O₃–60P₂O₅, 0 ⩽ x ⩽ 10 (mol%) glasses have been investigated by Raman and Mössbauer spectroscopies, X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The Raman spectra show that the addition of up to 5.3 mol% Cr₂O₃ does not produce any changes in the glass structure, which consists predominantly of pyrophosphate, Q¹, units. This is in accordance with O/P ≈ 3.5 for these glasses. The increase in glass density and T_g that occurs with increasing Cr₂O₃ suggests the strengthening of glass network. The Mössbauer spectra indicate that the Fe²⁺/Fe_tot ratio increases from 0.13 to 0.28 with increasing Cr₂O₃ content up to 5.3 mol%, which can be related to an increase in the melting temperature from 1423 to 1473 K. After annealing, the 10Cr₂O₃–30Fe₂O₃–60P₂O₅ (mol%) sample was partially crystallized and contained crystalline β-CrPO₄ and Fe₃(P₂O₇)₂. The SEM and AFM micrographs of the partially crystallized sample revealed randomly distributed crystals embedded in a homogeneous glass matrix. EDS analysis indicated that the glass matrix was rich in Fe₂O₃ (39.6 mol%) and P₂O₅ (54.9 mol%), but contained only 5.5 mol% of Cr₂O₃. These results suggest that the maximum solubility of chromium in these iron phosphate melts is 5.5 mol% Cr₂O₃.     

Precipitation of zinc ferrite nanoparticles in the Fe2O3–ZnO–SiO2 glass system

Glass materials in the ZnO–Fe₂O₃–SiO₂ system, containing zinc ferrite nanoparticles, were prepared by the sol–gel method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, AC- and DC-magnetization techniques. The gel samples, dried at 130 °C, were further heat treated in air at 500 and 800 °C. At 500 °C zinc ferrite and hematite nanoparticles, with an average size of approximately 24 nm, were precipitated in the brown and opaque 10ZnO–10Fe₂O₃–80SiO₂ and in the ruby colored transparent 5ZnO–5Fe₂O₃–90SiO₂ and 2.5ZnO–2.5Fe₂O₃–95SiO₂ glass matrices. In the 5ZnO–5Fe₂O₃–90SiO₂ sample the nanoparticles exhibited ferro or ferrimagnetic interactions combined with superparamagnetism with a blocking temperature of approximately 14 K. Heating at 800 °C seems to cause partial dissolution of the zinc ferrite and hematite particles in all the investigated compositions. Accordingly at 800 °C the 5ZnO–5Fe₂O₃–90SiO₂ glass shows a paramagnetic behavior down to 2 K.     

α‐Fe2O3 nanospindles loaded with ZnO nanocrystals: Synthesis and improved gas sensing performance

ZnO/α‐Fe₂O₃ nanocomposites were fabricated through a two‐step hydrothermal method. The morphology and composition of the as‐synthesized products were characterized by X‐ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), energy‐dispersive X‐ray spectroscopy (EDS), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The gas sensing properties of the fabricated products were investigated towards ethanol, acetone, propanol, isopropanol, formaldehyde, chloroform and so on. The results demonstrated that the ZnO/α‐Fe₂O₃ nanocomposites exhibited excellent sensing properties and showed remarkably higher sensing responses and much lower optimum operating temperature compared to individual ZnO and α‐Fe₂O₃. In addition, the ZnO/α‐Fe₂O₃ nanocomposites have some selectivity for ethanol, propanol and isopropanol. The possible gas sensing mechanism was also proposed. Our studies demonstrate that our fabricated materials could be widely used in the future.     

Rapid synthesis of dendrite‐ and platelet‐like α‐Fe2O3 via a hydrothermal oxidation route

Dendrite and platelet‐like α‐Fe₂O₃ microcrystals were synthesized by the oxidation reaction of K₄Fe(CN)₆and NaClO₃ through a simple hydrothermal method. The structures and morphologies of the as‐prepared samples were characterized in detail by X‐ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The experiment results show that NaOH played an important role in controlling the morphology of the final products. The possible mechanism was discussed to elucidate the formation of different morphologies of the α‐Fe₂O₃ microstructures. Besides, the magnetic property of the dendrite α‐Fe₂O₃ microstructure was characterized by a vibrating sample magnetometer (VSM). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and magnetic investigations of Fe2O3–TeO2 glasses

A series of tellurite glasses containing Fe₂O₃ with the nominal composition x(Fe₂O₃)–(1−x)(TeO₂), where x = 0.05, 0.10, 0.15, and 0.20, have been synthesized and investigated using X-ray photoelectron spectroscopy (XPS) and magnetization techniques. The Te 3d core level spectra for all glass samples show symmetrical peaks at essentially the same binding energies as measured for TeO₂ indicating that the chemical environment of the Te atoms in these glasses does not vary significantly with the addition of Fe₂O₃. Furthermore, the full-width at half-maximum (FWHM) of each peak does not vary with increasing Fe₂O₃ content which suggests that the Te ions exist in a single configuration, namely TeO₄ trigonal bipyramid (tbp). The O 1s spectra are narrow and symmetric for all compositions such that oxygen atoms in the Te–O–Te, Fe–O–Fe and Te–O–Fe configurations must have similar binding energies. The analysis of the Fe 3p spectra indicates the presence of Fe³⁺ ions only, which is consistent with the valence state of the Fe ions determined from magnetic susceptibility measurements.     

Synthesis and properties of α‐Fe2O3 nanorods

We report synthesis of α‐Fe₂O₃ (hematite) nanorods by reverse micelles method using cetyltrimethyl ammonium bromide (CTAB) as surfactant and calcined at 300 °C. The calcined α‐Fe₂O₃ nanorods were characterized by X‐ray diffraction (XRD), high‐resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM). The result showed that the α‐Fe₂O₃ nanorods were hexagonal structure. The nanorods have diameter of 30‐50 nm and length of 120‐150 nm. The weak ferromagnetic behavior was observed with saturation magnetization = 0.6 emu/g, coercive force = 25 Oe and remanant magnetization = 0.03 emu/g. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mössbauer and optical spectroscopic study of temperature and redox effects on iron local environments in a Fe-doped (0.5 mol% Fe2O3) 18Na2O-72SiO2 glass

Local environments of ferric and ferrous irons were systematically studied with Mössbauer (at liquid helium temperature) and ultraviolet-visible-near infrared spectroscopic methods for various 18Na₂O-72SiO₂ glasses doped with 0.5 mol% Fe₂O₃. These were prepared at temperatures of 1300-1600 °C in ambient air or at 1500 °C under reducing conditions with oxygen partial pressures from 12.3 to 0.27×10⁻⁷ atmospheres. The Mössbauer spectroscopic method identified three types of local environments, which were represented by the Fe³⁺ sextet, the Fe³⁺ doublet, and the Fe²⁺ doublet. The Fe³⁺ sextet ions were assigned to ‘isolated’ octahedral ions. Under reducing conditions, the octahedral Fe³⁺ ions were readily converted into octahedral ferrous ions. The Fe³⁺ doublet exists both in octahedral and tetrahedral environment, mainly as tetrahedral sites in the reduced samples. The tetrahedral ions were found stable against reduction to ferrous ions. The Fe²⁺ doublet sites existed in octahedral coordination. Combining results from both spectroscopic studies, the 1120- and 2020-nm optical bands were assigned to octahedral ferrous ions with a different degree of distortion rather than different coordinations. Further, we assigned the 375-nm band to the transition of octahedral ferric ions that are sensitive to the change of oxygen partial pressure in glass melting and 415-, 435-, and 485-nm bands to the transitions of the tetrahedral ferric ions that are insensitive to oxidation states of the melt. The effect of ferric and ferrous ions with different coordination environments on the glass immiscibility was elucidated.     

The influence of Fe2O3 and B2O3 additions on the thermal properties,crystallization kinetics and durability of a sodium aluminum phosphate glass

A sodium aluminum phosphate glass, identified as a potential encapsulating phase for radioactive wastes, has been noted to be thermally unstable, crystallizing upon heat treatment to yield a variety of phases which may be detrimental to the longer term properties of the final wasteform. In an attempt to improve the thermal stability and overall resistance of this glass to crystallization, the influence of various oxide additions including Fe₂O₃ and B₂O₃ has been investigated. It has been shown that quite small additions of B₂O₃ are extremely effective in improving the stability, with ⩾2 mol% B₂O₃ almost completely suppressing crystallization whilst only marginally affecting the excellent chemical durability of the ternary glass, at least at the lower B₂O₃ concentrations. In contrast, addition of Fe₂O₃ or B₂O₃/Fe₂O₃ mixtures does not lead to significant improvements in the thermal stability, although durability relative to that of the ternary composition is improved significantly with increasing Fe₂O₃ content.     

Nuclear57Fe relaxation in Fe2+-containing NiFe2O4 and CoFe2O4 single crystals

Spin-lattice (T ₁) and spin-spin (T ₂) relaxation times of ⁵⁷Fe nuclei in the single-crystal NiFe₂O₄ and CoFe₂O₄ ferrites containing Fe²⁺ ions have been studied in the temperature range of 4.2–100 K by a spin-echo technique. The peaks of relaxation rates T ₁ ^?1 and T ₂ ^?1 caused by the presence of Fe²⁺ ions were observed for both ferrites in the ranges 38–42 and 28–32 K, respectively. The analysis of the results obtained with invocation of the data on ferromagnetic resonance and the measurements of the temperature dependence of resistivity shows that the mechanism of nuclear relaxation responsible for “impurity” peaks and is a slow relaxation process caused by electron exchange Fe²⁺ ? Fe³⁺, characterized by a low activation energy.     

Study of cation ordering and magnetic phase transitions in ternary Fe-containing perovskite oxides by Mössbauer spectroscopy

Nuclear gamma-resonance (NGR) spectra were studied in ceramics of ternary perovskite-type oxides Pb(Fe_2/3W_1/3)O₃, Pb(Fe_1/2Ta_1/2)O₃, and Pb(Fe_1/2Nb_1/2)O₃ and in the Pb(Fe_1/2Sb_1/2)O₃ oxide synthesized under pressure. A singlet corresponding to compositional-ordering regions was observed in the NGR spectra for Pb(Fe_1/2Sb_1/2)O₃. Weak doublets associated with paramagnetic regions (in which the degree of cation ordering is higher than the volume average) were observed in the spectra for Pb(Fe_2/3W_1/3)O₃ and Pb(Fe_1/2Nb_1/2)O₃ at temperatures 30–70 K below the antiferromagnetic phase transition point.     

Fabrication of Fe3O4 octahedra by a triethanolamine‐assisted hydrothermal process

Octahedral Fe₃O₄microcrystals were synthesized using a triethanolamine‐assisted route under hydrothermal conditions. The chemical compositions and morphologies of the as‐prepared samples were characterized in detail by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). During the hydrothermal process for the preparation of Fe₃O₄ octahedra, the possible mechanism was discussed to elucidate the formation of the octahedral Fe₃O₄microcrystals. Triethanolamine and hydrazine hydrate play important roles in the formation of the final products. The magnetic property of sample was evaluated on a vibrating sample magnetometer (VSM) at room temperature. The values of saturation magnetization and coercivity of octahedral Fe₃O₄are about 103 emu/g and 157 Oe, respectively. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric behavior and impedance spectroscopy of bismuth iron phosphate glasses

The electrical and dielectrical properties of Bi₂O₃–Fe₂O₃–P₂O₅ glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 4 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe₂O₃ content and Fe(II)/Fe_tot ratio. With increasing Fe(II) ion content from 17% to 34% in the bismuth-free 39.4Fe₂O₃–59.6P₂O₅ and 9.8Bi₂O₃–31.7Fe₂O₃–58.5P₂O₅ glasses, the dc conductivity increases. On the other hand, the decrease in dc conductivity for the glasses with 18.9 mol% Bi₂O₃ is attributed to the decrease in Fe₂O₃ content from 31.7 to 23.5 mol%, which indicates that the conductivity for these glasses depends on Fe₂O₃ content. The conductivity for these glasses is independent of the Bi₂O₃ content and arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electronic conduction. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions. The Raman spectra show that the addition of up to 18.9 mol% of Bi₂O₃ does not produce any changes in the glass structure which consists predominantly of pyrophosphate units.     

Crystallization of Ba<Subscript>3</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> and Ba<Subscript>2</Subscript>Fe<Subscript>2</Subscript>GeO<Subscript>7</Subscript> in oxide-fluoride fluxes

Crystallization of trigonal Ba₃Fe₂Ge₄O₁₄ and tetragonal Ba₂Fe₂GeO₇ barium ferrogermanates is investigated in their own fluxes diluted by B₂O₃-BaF₂ and B₂O₃-PbF₂ mixtures, respectively. The ratios of the components at which the properties of the fluxes satisfy the requirements for controlled crystallization of these incongruently melting compounds are determined. Single crystals of Ba₂Fe₂GeO₇ barium ferrogermanate are grown for the first time.