A mössbauer investigation of non-bridging oxygens in Na2O-Fe2O3-B2O3 glasses

Mössbauer measurements of sodium borate glasses 4 Fe₂O₃· x Na₂O· (96-x) B₂O₃ (x=6, 10, 15, 20, 25, 30) have been carried out. It was found that Fe³⁺ is present in both octahedral and tetrahedral sites, whereby the tetrahedral sites are preferred. In the range of Na₂O contents higher than about 15 mol%, the isomer shifts and quadrupole splittings for Fe³⁺ decrease continuously with increasing Na₂O concentration. These changes could be attributed to the formation of non-bridging oxygens (NBO) in the neighborhood of ferric ions. Increasing Na₂O content thus gives rise to more NBO increasing the symmetry of the iron polyhedra.     

Study of structural modification of sodium aluminophosphate glasses with TiO2 addition through Raman and NMR spectroscopy

Structural modification of sodium aluminophosphate (NAP) glasses with TiO₂ addition has been investigated using Raman and MAS-NMR (³¹P and ²⁷Al) spectroscopy. TiO₂ incorporated NAP glasses having composition (mol%): 40Na₂O-10Al₂O₃-xTiO₂-(50−x)P₂O₅ (x=0-20), are prepared by conventional melt quench method. The low-frequency Raman spectrum suggests an increase in the average ionic character of phosphate glass network with addition of TiO₂. Raman and ³¹P MAS-NMR revealed that the glasses without TiO₂, consist mainly metaphosphate (Q²) structural units. These are gradually converted into pyrophosphate (Q¹) and orthophosphate (Q⁰) structural units along with the formation of P-O-Ti/P-O-Al linkages. ²⁷Al MAS-NMR revealed the change in coordination of Al from octahedral (AlO₆) to tetrahedral (AlO₄) for TiO₂ above 10 mol%. Raman spectra indicate that TiO₂ enters the network in the form of octahedral (TiO₆) and tetrahedral (TiO₄) structural units and at high concentration of TiO₂, tetrahedral structural units are more favourable. Various thermo-physical properties e.g. density (ρ), molar volume (V_m), glass transition temperature (T_g), microhardness (MH), and thermal expansion coefficient (TEC) have been measured as a function of TiO₂ content. Variations in the thermo-physical properties are correlated with these structural modifications in the phosphate structural units and consequently changes in the average ionic character of phosphate glass network.     

Cobalt ion as a structural probe in PbO–As2O3 glass ceramics

PbO–As₂O₃ glasses mixed with different concentrations of CoO (ranging from 0 to 1.0 mol%) were crystallized. The samples were characterized by X-ray diffraction, scanning electron microscopy, EDS and differential scanning calorimetric techniques. The X-ray diffraction studies have indicated the presence of Pb(As₂O₆), Pb₃(AsO₄)₂, Co₆As₂O₁₁, Co₃O₄ crystalline phases in these samples. Optical absorption, IR and photoluminescence studies of these samples have been carried out. The analysis of the results of these studies has indicated that the cobalt ions exist in Co²⁺ and Co³⁺ states in the glass matrix. The studies have further revealed that as the concentration of the CoO is increased, there is a gradual transformation of cobalt ions from tetrahedral to octahedral positions.     

A conversion electron Mössbauer spectroscopy study of pulsed laser treatment at α-Fe2O3/H2O interface

Magnetite (Fe₃O₄) has been synthesized for the first time by using pulsed ruby laser induced reactive quenching process at α-Fe₂O₃/H₂O interface. Iron foils (99.99% pure) were oxidised at 450° C for four hours to form a thick layer of α-Fe₂O₃ on it. These oxidised samples were immersed in water and then treated with ruby laser pulses (λ=0.694 μm, pulse width = 30 ns, energy density = 10 J/cm²). The conversion electron Mössbauer spectroscopy (CEMS) has been used to characterize the laser induced surface modifications. It is shown that laser treated samples show the formation of Fe₃O₄ phase along with FeO. The stability of magnetite phase in laser treated sample against thermal treatment is also studied by investigating the changes in hyperfine interaction parameters upon vacuum annealing at 300° C.     

Sono-assisted preparation of highly-efficient peroxidase-like Fe3O4 magnetic nanoparticles for catalytic removal of organic pollutants with H2O2

Fe₃O₄ magnetic nanoparticles (Fe₃O₄ MNPs) with much improved peroxidase-like activity were successfully prepared through an advanced reverse co-precipitation method under the assistance of ultrasound irradiation. The characterizations with XRD, BET and SEM indicated that the ultrasound irradiation in the preparation induced the production of Fe₃O₄ MNPs possessing smaller particle sizes (16.5 nm), greater BET surface area (82.5 m² g^?1) and much higher dispersibility in water. The particle sizes, BET surface area, chemical composition and then catalytic property of the Fe₃O₄ MNPs could be tailored by adjusting the initial concentration of ammonia water and the molar ratio of Fe²⁺/Fe³⁺ during the preparation process. The H₂O₂-activating ability of Fe₃O₄ MNPs was evaluated by using Rhodamine B (RhB) as a model compound of organic pollutants to be degraded. At pH 5.4 and temperature 40 °C, the sonochemically synthesized Fe₃O₄ MNPs were observed to be able to activate H₂O₂ and remove ca. 90% of RhB (0.02 mmol L^?1) in 60 min with a apparent rate constant of 0.034 min^?1 for the RhB degradation, being 12.6 folds of that (0.0027 min^?1) over the Fe₃O₄ MNPs prepared via a conventional reverse co-precipitation method. The mechanisms of the peroxidase-like catalysis with Fe₃O₄ MNPs were discussed to develop more efficient novel catalysts.     

Synthesis and characterization of TiO2-coated magnetite clusters (nFe3O4@TiO2) as anode materials for Li-ion batteries

TiO₂-coated magnetite clusters (nFe₃O₄@TiO₂) were facilely prepared through the sol–gel reaction between Ti alkoxides (TEOT) and magnetite clusters (nFe₃O₄) with terminated alkoxy groups. The composite particles represented a core–shell nanostructure (nFe₃O₄@TiO₂) consisting of a Fe₃O₄ cluster core and a TiO₂ capsule layer. The capsule layer of nFe₃O₄@TiO₂ was increased with increasing amounts of TEOT (150, 300, 500 μl) in sol–gel reaction. The Fe₃O₄@TiO₂ (150 μl of TEOT) with a thin TiO₂ layer (ca. 10 nm) exhibited two kinds of cathodic (0.79 V and 1.61 V) and anodic (1.78 and 2.1 V) peaks attributed to the reduction and oxidation process by Fe₃O₄ core and TiO₂ layer, respectively. The thin nFe₃O₄@TiO₂ (150 μl of TEOT) exhibited the enhanced capacity retention by ca. 40% probably due to the buffering effect of TiO₂ capsule layer. However, the thick nFe₃O₄@TiO₂ (300–500 μl of TEOT) exhibited a rapid capacity fading due to the disintegrated core–shell nanostructure, i.e., unfavorable hetero-junction between TiO₂ matrix and magnetite clusters.     

Study on the mechanisms of photoinduced carriers separation and recombination for Fe-TiO2 photocatalysts

The iron(III)-ion doped TiO₂ (Fe³⁺-TiO₂) with different doping Fe³⁺ content were prepared via a sol-gel method. The as-prepared Fe³⁺-TiO₂ nanoparticles were investigated by means of surface photovoltage spectroscopy (SPS), field-induced surface photovoltage spectroscopy (FISPS), and the photoelectrochemical properties of Fe³⁺-TiO₂ catalysts with different Fe³⁺ content are performed by electrical impedance spectroscopy (EIS) as well as photocatalytic degradation of RhB are studied under illuminating. Based on the experiment results, the mechanism of photoinduced carriers separation and recombination of Fe³⁺-TiO₂ was revealed: that is, the Fe³⁺ captures the photoinduced electrons, inhibiting the recombination of photoinduced electron-hole pairs, this favors to the photocatalytic reaction at low doping concentration (Fe/Ti ≤ 0.03 mol%); while Fe³⁺ dopant content exceeds 0.03 mol%, Fe₂O₃ became the recombination centers of photoinduced electrons and holes because of that the interaction of Fe₂O₃ with TiO₂ leads to that the photoinduced electrons and holes of TiO₂ transfer to Fe₂O₃ and recombine quickly, which is unfavorable to the photocatalytic reaction.     

Mössbauer spectra and electrical conductivity ofxFe2O3−(40-x)V2O5−60P2O5 glasses

Mössbauer spectra and electrical conductivities were measured for the purpose of studying on the conduction mechanism of xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses. The ratios Fe²⁺/Fe²⁺+Fe³⁺ in xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses were determined by Mössbauer spectroscopy. On the composition dependence of D. C. conductivities in these glasses, the minimum of log σ at each temperature was obtained at 25Fe₂O₃?15V₂O₅?60P₂O₅. The conductivities of ternary glassses in Fe rich region could be explained only by the changes of carrler (Fe²⁺) concentration and the hopping conduction between Fe²⁺ ions and Fe³⁺ ions in binary glasses. In V rich region, the saturation tendency of D. C. conductivities are observed. It was suggested to be explained by increasing of V⁴⁺ ions due to the influence of Fe ions.     

Antiferromagnetic resonance and magnetic anisotropy in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system

The antiferromagnetic resonance in single crystals of the YFe₃(BO₃)₄-GdFe₃(BO₃)₄ system is studied in the frequency range 25–140 GHz and the temperature range 4.2–50.0 K. It is established that the YFe₃(BO₃)₄ crystal containing only the magnetic subsystem of Fe³⁺ ions is an antiferromagnet with an easy anisotropy plane. The temperature dependences of the gaps in the antiferromagnetic resonance spectra of GdFe₃(BO₃)₄ and Y _x Gd_{1 ? x} Fe₃(BO₃)₄ are used to calculate the contributions of the Fe³⁺ and Gd³⁺ subsystems to the magnetic anisotropy of these crystals. The contributions are found to be close in magnitude and have opposite signs. This leads to a relatively weak uniaxial anisotropy field in the crystals under investigation. Since the exchange interaction between the Gd³⁺ and Fe³⁺ ions magnetizes the magnetic subsystem of gadolinium, both subsystems start to contribute simultaneously at the Néel temperature of the iron subsystem.     

57Fe-Mössbauer study of electrically conducting barium iron vanadate glass after heat treatment

Local structure and thermal durability of semiconducting xBaO·(90?? x)V₂O₅ · 10Fe₂O₃ glasses (x = 20, 30 and 40), NTA glass ^TM, before and after isothermal annealing were investigated by ⁵⁷Fe-Mössbauer spectroscopy and differential thermal analysis (DTA). An identical isomer shift ( $\mathit{\delta}$ ) of 0.39 ± 0.01 mm s^???1 and a systematic increase in the quadrupole splitting (Δ) were observed from 0.70 ± 0.02 to 0.80 ± 0.02 mm s^???1 with an increasing BaO content, showing an increase in the local distortion of Fe^IIIO₄ tetrahedra. From the slope of the straight line in the T _g–Δ plot of NTA glass ^TM, it proved that Fe^III plays a role of network former. Large Debye temperature (Θ _D) values of 1000 and 486 K were respectively obtained for 20BaO · 70V₂O₅ · 10Fe₂O₃ glass before and after isothermal annealing at 400°C for 60 min, respectively. This result also suggests that Fe^III atoms constitute the glass network composed of tetrahedral FeO₄, tetrahedral VO₄ and pyramidal VO₅ units. The electric conductivity of 20BaO · 70V₂O₅ · 10Fe₂O₃ glass increased from 1.6 × 10^???5 to 5.8 × 10^???2 S cm^???1 after isothermal annealing at 450°C for 2,000 min. These results suggest that the drastic increase in the electric conductivity caused by heat treatment is closely related to the structural relaxation of the glass network structure.     

Li2O-(LiCl)2-B2O3-Al2O3系统玻璃的Raman光谱研究

我们用Raman光谱研究了Li₂O(LiCl)₂B₂O₃-Al₂O₃系玻璃的结构,着重研究了Al₂O₃的影响。对于Li₂O-B₂O₃系玻璃,Li₂O含量增加使玻璃中存在的BO₃三角体转变为BO₄四面体, 关键词：     

A novel non-thermal process of TiO2-shell coating on Fe3O4-core nanoparticles

In this work magnetite (Fe₃O₄) nanoparticles coated with titanium dioxide (TiO₂) were prepared by a novel non-thermal method. In this method, magnetite and pure TiO₂ (anatase) nanoparticles were individually prepared by the sol–gel method. After modifying the surface of magnetite nanoparticles by sodium citrate, titanium dioxide was coated on them without using conjunction or heat treatment to obtain Fe₃O₄:TiO₂ core–shell nanoparticles. XRD, EDX, SEM, TEM and VSM were used to investigate the structure, morphology and magnetic properties of the samples. The average crystallite size of the powders was measured by Scherrer's formula. The results obtained from different measurements confirm the formation of Fe₃O₄:TiO₂ core–shell nanoparticles with a decrease in saturation magnetization. Hysteresis loops of the core–shell nanoparticles show no exchange bias effects, which confirms that there is no interaction or interdiffusion at the interface.     

Chemical reactions induced by 40 keV He+ ions in Fe3O4 and α-Fe2O3

The chemical reactions induced by 40 keV He⁺ ions in α-Fe₂O₃ and Fe₃O₄ were investigated by the conversion electron Mössbauer spectroscopy(CEMS). Magnetite(Fe₃O₄) was formed upon the bombardment of α-Fe₂O₃, whereas no change was observed in Fe₃O₄. The initial G value for Fe₃O₄ formation is estimated to be 3.5×10^?4 for 100 nm depth from the surface. The application of CEMS and sputtering technique to ion bombardment chemistry is discussed.     

Preparation and characterization of visible light responsive Fe2O3-TiO2 composites

In this study we present the effects of iron oxide (Fe₂O₃) on titanium dioxide (TiO₂) in synthesising visible-light reactive photocatalysts. A Fe₂O₃-TiO₂ composite photocatalyst was synthesized from Fe₂(SO₄)₃ and Ti(SO₄)₂ by a ethanol-assisted hydrothermal method. The preparation conditions were optimized through the investigation of the effects of hydrothermal temperature and time as well as molar ratio of Ti to Fe on the photocatalytic activity. The visual, physical and chemical properties of the Fe₂O₃-TiO₂ composites were investigated. The results showed that α-Fe₂O₃ and anatase TiO₂ were present in the composites. The Fe₂O₃-TiO₂ synthesized under optimum condition consisted of mesoporous structure with an average pore size of 4 nm and a surface area of 43 m²/g. Under visible and solar light irradiation, the photocatalytic activity of optimized sample was significantly higher than that of pure TiO₂. This sample led to a photodegradation efficiency of 90% and 40% of auramine under visible light and solar light, respectively.     

Structural and crystallization behavior of (Ba,Sr)TiO3 borosilicate glasses

Various glass samples were prepared by melt quench technique in the glass system [(Ba_{1? x} Sr _x ) TiO₃]–[2SiO₂–B₂O₃]–[K₂O] doped with 1?mole% of La₂O₃. Infrared spectra show the number of absorption peaks with different spliting in the wave number range from 450 to 4000?cm^?1. Absorption peaks occurs due to asymetric vibrational streching of borate by relaxation of the bond B–O of trigonal BO₃. Raman spectra show the Raman bands due to ring-type metaborate anions, symmetric breathing vibrations BO₃ triangles replaced by BO₄ tetrahedra, and symmetric breathing vibrations of six-member rings. The differential thermal analysis of a glass sample corresponding to composition x?=?0.0 shows crystallization temperature at 847°C and glass transition temperature at 688°C. X-ray diffraction (XRD) pattern of glass ceramic samples shows the major crystalline phase of BaTiO₃ whereas pyrochlore phases of barium titanium silicate. Scanning electron micrographs confirm the results of XRD as barium titanate is major crystalline phase along with pyrochlore phase of barium titanium silicate.     

Influence of Electron Delocalization on the Magnetic Properties of Iron Ludwigite Fe3O2BO3

⁵⁷Fe Mössbauer spectroscopic studies of iron ludwigite Fe₃O₂BO₃ performed between 4 and 450 K allow the discussion of magnetic spin arrangements and the dynamics of electronic configurations of iron. The observed magnetic transitions are related to charge ordering.     

Influence of nanocrystalline phases on the electrical properties of lithium titanate phosphate glass ceramics mixed with Ga2O3 nanocrystals

Several glass ceramic compositions dispersed with Ga₂O₃ nanocrystals, in the series samples (100???x)[0.4Li₂O–0.1TiO₂–0.5P₂O₅]?+?xGa₂O₃ with x?=?0, 2, 4, 6, 8, and 10?mol% of Ga₂O₃ were synthesized via high-energy ball milling technique and labeled as lithium gallium titanate phosphate glass (LTPG _x ) (x is the mol% of Ga₂O₃ nanocrystals). The compositions have been selected on the basis of thermal stability data obtained from differential thermal analysis. X-ray diffraction studies indicate nanocrystalline phase formation in the controlled crystallized glasses. The variation of electrical conductivity was explained in the light of growth of nanocrystalline phases. The best bulk conductivity (σ?=?7.03?×?10^?4?S?cm^?1, at 303?K) was achieved by the sample containing 8?mol% of Ga₂O₃ nanocrystals content, labeled as LTPG₈ sample. The activation energy for conduction (E_{a σ} ) is obtained from the temperature dependent of conductivity data, which is fitted to Arrhenius equation. The single super curve in the scaling spectra suggested the temperature-independent relaxation phenomenon.     

An optimal low-temperature tartrate precursor method for the synthesis of monophasic nanosized ZnFe2O4

In this study, the synthesis of monophasic nanocrystalline zinc ferrite (ZnFe₂O₄) was achieved by controlling the thermal decomposition conditions of a zinc–iron tartrate precursor method. Differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Fe²⁺ content analysis, transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) techniques were used to investigate the effect of heat treatment conditions on the calcined powders. The thermal decomposition of the precursor led to an intermediate phase formation of ZnO, Fe₃O₄, and γ-Fe₂O₃. It was found that the Fe₃O₄ → γ-Fe₂O₃ oxidation reaction is the key step in producing monophasic nanosized ZnFe₂O₄. The monophasic nanoparticles of ZnFe₂O₄ can be obtained when the precursor is heat treated under a low temperature (300–400 °C) and long residence time (4 h) process that can prompt the Fe₃O₄ oxidation and prevent the formation of α-Fe₂O₃.     

Features of the local structural disorder in Li2O-CaF2-P2O5 glass-ceramics with Cr2O3 as nucleating agent

Li₂O-CaF₂-P₂O₅ glasses mixed with different concentrations of Cr₂O₃ (ranging from 0 to 1.0 mol%) were crystallized. The samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS), differential thermal analysis and conventional spectroscopic techniques. The X-ray diffraction and scanning electron microscopic studies reveal the presence of lithium phosphate, calcium phosphate and chromium phosphate (complexes of Cr³⁺, Cr⁵⁺ and Cr⁶⁺ ions) crystal phases. The study on DTA suggests that the crystallization is predominantly due to the surface crystallization when the concentration of nucleating agent Cr₂O₃ is around 0.8 mol%. The IR and Raman spectral studies of these samples indicate that the sample crystallized with 0.8 mol% Cr₂O₃ is more compact and possesses high rigidity due to the presence of chromium ions largely in tetrahedral positions.     

Effect of mechanical activation on the properties of the working mixture for BaB2O4 and Ba2Na3[B3O6]2F crystal growth

A comparative study is made regarding the effect of the mechanical activation of carbonate acidic (BaCO₃-HBO₂/H₃BO₃)₂ and carbonate-acidic fluoride (BaCO₃-Na₂CO₃-NaF-(HBO₂/H₃BO₃)₂) systems on the thermal synthesis of barium metaborate β-BaB₂O₄ and fluoroborate Ba₂Na₃[B₃O₆]₂F.