Heterogeneous Fenton-like discoloration of methyl orange using Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/MWCNTs as catalyst: combination mechanism and affecting parameters

Multi-walled carbon nanotubes (MWCNTs) can act not only as a support for Fe₃O₄ nanoparticles (NPs) but also as a coworker with synergistic effect, accordingly improving the heterogeneous Fenton-like efficiency of Fe₃O₄ NPs. In this study, Fe₃O₄ NPs were in situ anchored onto MWCNTs by a moderate co-precipitation method and the as-prepared Fe₃O₄/MWCNTs nanocomposites were employed as the highly efficient Fenton-like catalysts. The analyses of XRD, FTIR, Raman, FESEM, TEM and HRTEM results indicated the formation of Fe₃O₄ crystals in Fe₃O₄/MWCNTs nanocomposites prepared at different conditions and the interaction between Fe₃O₄ NPs and MWCNTs. Over a wide pH range, the surface of modified MWCNTs possessed negative charges. Based on these results, the possible combination mechanism between Fe₃O₄ NPs and MWCNTs was discussed and proposed. Moreover, the effects of preparation and catalytic conditions on the Fenton-like catalytic efficiency were investigated in order to gain further insight into the heterogeneous Fenton-like reaction catalyzed by Fe₃O₄/MWCNTs nanocomposites.     

Heterogeneous Fenton-like discoloration of methyl orange using Fe3O4/MWCNTs as catalyst: process optimization by response surface methodology

Fe₃O₄/MWCNTs nanocomposites were prepared by chemical oxidation coprecipitation method and developed as highly efficient heterogeneous Fenton-like catalyst. XRD results revealed that Fe₃O₄ nanoparticles deposited onto MWCNTs surface remained the inverse spinel crystal structure of cubic Fe₃O₄ phase. The FTIR characteristic peaks of MWCNTs weakened or disappeared due to the anchor of Fe₃O₄ nanoparticles and Fe–O peak at 570 cm^–1 was indicative of the formation of Fe₃O₄. TEM observation revealed that Fe₃O₄ nanoparticles were tightly anchored by MWCNTs. The Fenton-like catalytic activity of Fe₃O₄/MWCNTs nanocomposites for the discoloration of methyl orange (MO) was much higher than that of Fe₃O₄ nanoparticles. The process optimization of this heterogeneous Fenton-like system was implemented by response surface methodology (RSM). The optimum conditions for MO discoloration were determined to be of 12.3 mmol/L H₂O₂ concentration, 2.9 g/L catalyst dosage, solution pH 2.7 and 39.3 min reaction time, with the maximum predicted value for MO discoloration ratio of 101.85%. The corresponding experimental value under the identical conditions was obtained as 99.86%, which was very close to the predicted one with the absolute deviation of 1.99%.     

<Emphasis Type="Italic">In situ</Emphasis> anchor of magnetic Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles onto natural maifanite as efficient heterogeneous Fenton-like catalyst

In situ anchor of magnetic Fe₃O₄ nanoparticles (NPs) onto the surface of natural maifanite was realized by chemical oxidation coprecipitation in hot alkaline solution. The Fe₃O₄/maifanite composites were characterized by XRD, FTIR, SEM, and TEM. These results indicated that polycrystalline Fe₃O₄ NPs with inverse spinel structure were formed and tightly dispersed on maifanite surface. Based on the measurement of surface Zeta potential of maifanite at different medium pHs, the possible combination mechanism between natural maifanite and Fe₃O₄ NPs was proposed. Then, the asobtained composites were developed as highly efficient heterogeneous Fenton-like catalyst for the discoloration of an azo dye, Methyl Orange (MO). The comparative tests on MO discoloration in different systems revealed that Fe₃O₄/maifanite composite exhibited much higher Fenton-like catalytic activity than Fe₃O₄ NPs and the heterogeneous Fentonlike reaction governed the discoloration of MO. Kinetic results clearly showed that MO discoloration process followed the second-order kinetic model. Fe₃O₄/maifanite composites exhibited the typical ferromagnetic property detected by VSM and could be easily separated from solution by an external magnetic field.     

General strategy for embedding high quality Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> quantum dots and ZnS:Mn<Superscript>2+</Superscript> quantum dots in a silica matrix

In this paper, ZnS:Mn²⁺ quantum dots (QDs) Fe₃O₄ quantum dots (QDs)/SiO₂ nanocomposites were successfully synthesized by reverse microemulsion method. The average diameter of ZnS:Mn²⁺ QDs, Fe₃O₄ QDs and ZnS:Mn²⁺ QDs Fe₃O₄ QDs/SiO₂ nanocomposites was about 5.8, 9 and 29 nm, respectively. As the mass ratio of ZnS:Mn²⁺ to Fe₃O₄ QDs increased from 2.5:4 to 7.5:4, the intensity of the yellow–orange emission coming from Mn²⁺ ions was increased. The superparamagnetic property of ZnS:Mn²⁺ QDs Fe₃O₄ QDs/SiO₂ nanocomposites was observed at room temperature, and the saturation magnetization was decreased as the amount of ZnS:Mn²⁺ QDs increased.     

Synthesis of magnetic Cu/CuFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite as a highly efficient Fenton-like catalyst for methylene blue degradation

A magnetic Cu/CuFe₂O₄ nanocomposite was synthesized by a facile one-pot solvothermal method and characterized as an excellent Fenton-like catalyst for methylene blue (MB) degradation. The content of zero-valent copper (Cu⁰) in Cu/CuFe₂O₄ composite could be simply controlled by changing the dosage of sodium acetate in the synthetic process, and the Fenton-like catalytic performance of Cu/CuFe₂O₄ composite enhanced with increasing the Cu⁰ content. In the presence of H₂O₂ (15 mM), the as-synthesized 3-Cu/CuFe₂O₄ nanocomposite could remove 99% of MB (50 mg/L) after only 4 min at pH 2.50, greatly higher than that of pure CuFe₂O₄ and Cu⁰ under the same condition. The enhancement activity of Cu/CuFe₂O₄ nanocomposite was due to the synergistic effect between Cu⁰ and CuFe₂O₄. The radical capture experiments and coumarin fluorescent probe technique confirmed that MB was degraded mainly by the attack of OH^· radicals in Cu/CuFe₂O₄–H₂O₂ system.     

Synthesis and applications of fluorescent-magnetic-bifunctional dansylated Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@SiO<Subscript>2</Subscript> nanoparticles

Bifunctional magnetic-luminescent dansylated Fe₃O₄@SiO₂ (Fe₃O₄@SiO₂-DNS) nanoparticles were fabricated by the nucleophilic substitution of dansyl chloride with primary amines of aminosilane-modified Fe₃O₄@SiO₂ core–shell nanostructures. The morphology and properties of the resultant Fe₃O₄@SiO₂-DNS nanoparticles were investigated by transmission electron microscopy, FT–IR spectra, UV–vis spectra, photoluminescence spectra, and vibrating sample magnetometry. The Fe₃O₄@SiO₂-DNS nanocomposites exhibit superparamagnetic behavior at room temperature, and can emit strong green light under the excitation of UV light. They show very low cytotoxicity against HeLa cells and negligible hemolysis activity. The T ₂ relaxivity of Fe₃O₄@SiO₂-DNS in water was determined to be 114.6 Fe mM⁻¹ s⁻¹. Magnetic resonance (MR) imaging analysis coupled with confocal microscopy shows that Fe₃O₄@SiO₂-DNS can be uptaken by the cancer cells effectively. All these positive attributes make Fe₃O₄@SiO₂-DNS a promising candidate for both MR and fluorescent imaging applications.     

Magnetically separable Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@TiO<Subscript>2</Subscript> nanospheres: preparation and photocatalytic activity

A facile and efficient approach for the fabrication of Fe₃O₄@TiO₂ nanospheres with a good core–shell structure has been demonstrated. Products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The results showed that Fe₃O₄@TiO₂ nanocomposites exhibited high degree of crystallinity, excellent magnetic properties at room temperature. Furthermore, the as-prepared Fe₃O₄@TiO₂ nanocomposites exhibited good photocatalytic activity toward the degradation of Rhodamine B (RhB) solution. Additionally, the recycling experiment of Fe₃O₄@TiO₂ nanocomposites had been done, demonstrating that Fe₃O₄@TiO₂ nanocomposites have high efficiency and stability.     

Inducing multiferroic behaviour in the diamagnetic Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The synthesis and characterization of Y_2−xFe_xO₃ (where x = 0–0.3) compounds has been carried out for their importance in the field of multiferroic materials. The powder X-ray diffraction reveal that the compounds Y_1.95Fe_0.05O₃, Y_1.9Fe_0.1O₃, Y_1.85Fe_0.15O₃ and Y_1.8Fe_0.2O₃ crystallize in tetragonal structure whereas Y_1.75Fe_0.25O₃ and Y_1.7Fe_0.3O₃ compounds crystallize in orthorhombic structure. The change in crystal system with respect to the concentration of Fe may be attributed to the variation in occupancy position of Fe³⁺ into the Y³⁺ site of Y₂O₃ system. Variation in crystal structure, surface morphology and composition was studied by micro-Raman analysis, SEM and EDX analysis. The shift in intense Raman signals from 426 to 385 cm⁻¹ confirms the change in the crystal structure of the prepared compounds. Further it is also identified that the E_g mode of vibration is the dominant in the Fe substituted compounds. The substitution of Fe in the Y₂O₃ system leads to the increase in the intensity of resonance band, which indicates a large polarisability variation in the Y_2−xFe_xO₃ compounds. Diffused reflectance studies show a red shift in energy gap values while increasing the concentration of Fe. The room temperature magnetization and electron paramagnetic resonance studies reveal that the incorporation of Fe in the Y₂O₃ system leads to magnetic phase change from diamagnetic to ferromagnetic. The electric polarization studies imply that the substitution of lower ionic radii element Fe³⁺ in the Y³⁺ site leads to distortion in the lattice and show the way to spontaneous dipole moment and it was found that the Y_1.8Fe_0.2O₃ compound exhibits the possibility of multiferroic behaviour. Therefore this paper explores the possibility of inducing ferromagnetic and ferroelectric behaviour in the Fe substituted yttrium oxide system.     

Redox behavior and reduction mechanism of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–CeZrO<Subscript>2</Subscript> as oxygen storage material

Fe₂O₃–CeZrO₂ is a suitable oxygen storage material for the production of pure hydrogen by a cyclic water gas shift (CWGS) process which is based on the reduction of the material by syngas followed by the re-oxidation of the reduced material with water vapor. For identification of the reduction kinetics H₂-temperature programmed reduction experiments were performed. Several kinetic models were tested and the activation energy of reduction was calculated by the Kissinger method, by model-based curve fitting and by the isoconversional analysis method. The reduction of Fe₂O₃–CeZrO₂was found to occur in a four-step process including the reduction of Fe₂O₃,Fe₃O₄, and CeZrO₂. The overall process can be interpreted as phase-boundary controlled reduction of Fe₂O₃ to Fe₃O₄, and two-dimensional nucleation controlled reduction of Fe₃O₄ to Fe and of CeO₂ to Ce₂O₃. At higher oxygen conversion, the reduction of Fe₃O₄ and CeO₂ are significantly influenced by volume-diffusion in the solid bulk.     

Direct synthesis of mesoporous Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> through citric acid-assisted solid thermal decomposition

A simple and inexpensive approach to synthesizing mesoporous Fe₃O₄ is developed by using citric acid-assisted solid thermal decomposition of ferric nitrate. The structure and magnetic property of mesoporous Fe₃O₄ were characterized by XRD, FT–IR, N₂ adsorption–desorption isotherms, TEM, and vibrating sample magnetometer. It was shown that the decomposition of citric acid results in the formation of the mesoporous structure and narrow pore-size distribution. The reducing atmosphere which created by the decomposition of the ferric nitrate–citric acid complex caused the partial reduction of Fe(III) to Fe(II) and in turn the formation of Fe₃O₄. Moreover, the strength of the coordination between carboxyl group and Fe³⁺ also affected the phase composition of the iron oxides.     

Synthesis of magnetic nanocomposite MgO/MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> from Mg-Fe layered double hydroxides precursors

The present letter reported the synthesis of MgO-containing magnetic nanocomposites by calcinations of tailored hydrotalcite-like layered double hydroxides (LDHs) of the type [Mg_{1 – x – y}Fe²⁺_yFe^{3 +}_x(OH)₂]^{x +}(A)_x/2·mH₂O (y 0; A = CO₃^{2 –} or SO₄^{2 –}) precursors at 900°C for 2 h. The results indicate that calcination of LDHs gives rise to the formation of magnetic nanocomposites of MgO and MgFe₂O₄ spinel ferrite, where MgO formed could disperse and separate MgFe₂O₄ particles, and MgO itself was also dispersed or embedded uniformly in the MgFe₂O₄ spinel matrix. Furthermore, initial studies on the bactericidal properties with staphylococcus aureus show that these as-synthesized nanometer-sized materials have significant bactericidal effect, which increases with the increasing volume fraction of MgO.     

Effects of alkaline-earth dopants on structural,optical and magnetic properties of Bi<Subscript>2</Subscript>Fe<Subscript>4</Subscript>O<Subscript>9</Subscript> powders

The pure Bi₂Fe₄O₉ and Bi_{2 (1?x)} A_2xFe₄O₉ (A?=?Ca, Ba, x?=?0.03) powders are synthesized via a modified solid-state reaction method to study the effects of alkaline-earth metal ions doping on crystal structural, optical and magnetic properties. Both X-ray diffraction and Raman spectroscopy data reveal that all the powders are Mullite-type Bi₂Fe₄O₉ orthorhombic single phase without any impurities. Much greater structural distortion in Bi_1.94Ba_0.06Fe₄O₉ than that of Bi_1.94Ca_0.06Fe₄O₉ is observed. The chemical compositions of Ba²⁺ and Ca²⁺ doped powders have been investigated with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy results indicate that oxygen vacancies could be found in all doped powders. The ratio of Fe²⁺ in the total Fe ions is almost unchanged by Ca doping and increases a little with Ba substitution. Compared with that of pure Bi₂Fe₄O_9, the band gap values decrease slightly in Bi_1.94Ca_0.06Fe₄O₉ but drop dramatically in Bi_1.94Ba_0.06Fe₄O₉. A clear and obvious ferromagnetic behavior is found in Bi_1.94Ba_0.06Fe₄O₉ at 10 K. However, Bi_1.94Ca_0.06Fe₄O₉ shows a weak ferromagnetism with enhanced magnetization and Bi₂Fe₄O₉ exhibits antiferromagnetism with a linear M–H relationship. The varied bandgap and magnetization resulting from the alkaline-earth metal ionic species are discussed in terms of structural distortion due to the ionic radius size effect.     

Magnetic and ferroelectric properties of SmBi<Subscript>4</Subscript>Fe<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>15</Subscript> compounds prepared with different synthesis methods

The SmBi₄Fe_0.5Co_0.5Ti₃O₁₅ compounds were prepared by the insertion of the SmFe_0.5Co_0.5O₃ into the Bi₄Ti₃O₁₂ host and the conventional solid state reaction method, respectively. X-ray diffraction analysis indicates that the conventional solid state reaction method favors the formation of a single phase four-layer Aurivillius phase of SmBi₄Fe_0.5Co_0.5Ti₃O₁₅ more easily than that prepared by the insertion method. Magnetic and ferroelectric measurements demonstrate that the samples prepared by both methods exhibit coexistence of strong ferromagnetic and weak ferroelectric behaviors at room temperature. Compared with the Bi₅FeTi₃O₁₅, the ferromagnetism of the SmBi₄Fe_0.5Co_0.5Ti₃O₁₅ was dramatically enhanced by the partial substitution of Co for Fe and Sm for Bi. The SmBi₄Fe_0.5Co_0.5Ti₃O₁₅ samples exhibit large magnetic responses (2M _r ?~?643 memu/g and coercive fields 2H _c ?~?344 Oe) at room temperature.     

Fast microwave synthesis of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Ag magnetic nanoparticles using Fe<Superscript>2+</Superscript> as precursor

A simple and quick microwave method to prepare high performance magnetite nanoparticles (Fe₃O₄ NPs) directly from Fe has been developed. The as-prepared Fe₃O₄ NPs product was fully characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the as-prepared Fe₃O₄ NPs are quite monodisperse with an average core size of 80 × 5 nm. The microwave synthesis technique can be easily modified to prepare Fe₃O₄/Ag NPs and these NPs possess good magnetic properties. The formation mechanisms of the NPs are also discussed. Our proposed synthesis procedure is quick and simple, and shows potential for large-scale production and applications for catalysis and biomedical/biological uses.     

Characterization and electrical properties of semiconducting Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-K<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> glasses

Semiconducting glasses of the Fe₂O₃-Bi₂O₃-K₂B₄O₇ system were prepared by the press-quenching method and their dc conductivity in the temperature range 223–393 K was measured. The glass transition temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The density for these glasses was consistent with the ionic size, atomic weight and amount of different elements in the glasses. Mössbauer results revealed that the relative fraction of Fe increases with increasing Fe₂O₃ content. Electrical conductivity showed a similar composition dependency as the fraction of Fe. The glasses had conductivities ranging from 10 to 10 Scm at temperatures from 223 to 393 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Microwave assisted fast fabrication of Fe3O4-MWCNTs nanocomposites and their application as MRI contrast agents

Uniform Fe₃O₄ nanoparticles with diameters of 3-5 nm are successfully decorated onto the external walls of multiwall carbon nanotubes (MWCNTs) by in situ high-temperature decomposition of Fe(acac)₃ in polyol solution under the irradiation of microwave. With this method, reaction time of forming Fe₃O₄-MWCNTs nanocomposites has been significantly shortened to 15 min. The resulting Fe₃O₄-MWCNTs nanocomposites show superparamagnetic property at room temperature and can be remained as stable aqueous dispersion for 2 months. Longitudinal relaxivity (r₁) and transverse relaxivity (r₂) of the magnetic MWCNTs are 8.34 Fe mM⁻¹ S⁻¹ and 146 Fe mM⁻¹ S⁻¹ respectively. The much higher r₂ value and the obvious change in the gray scale of MR images confer the Fe₃O₄-MWCNTs nanocomposites as potential candidates for T₂-weighted MRI contrast agents.     

Dissolution of UO<Subscript>2</Subscript> in the N<Subscript>2</Subscript>O<Subscript>4</Subscript>-H<Subscript>2</Subscript>O system

The kinetics of the UO₂ dissolution in the N₂O₄-H₂O system was studied. At 25°C, the process is kinetically controlled, whereas at 55°C the process occurs initially under kinetic control (3 min) and then under diffusion-kinetic control. At 80°C, the process occurs exclusively under diffusion-kinetic control. The apparent activation energy was estimated at ∼39 kJ mol⁻¹.     

Morphological control of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> particles via glycothermal process

Acicular magnetite (Fe₃O₄) powders were synthesized through new glycothermal dehydration by using crystalline α-FeOOH as precursor and glycols as solvent. When ethylene glycol was used as solvent, the phase was in-situ transformed from acicular α-FeOOH to α-Fe₂O₃ and finally to Fe₃O₄ at 270 °C for 6 h without morphological change. When water was added as a co-solvent in glycothermal reaction, Fe₃O₄ powders were synthesized through dissolution–recrystallization process at 230 °C for 3 h. The volume ratio of ethylene glycol to water (E/W) in the reaction has a strong effect on the morphology of the synthesized Fe₃O₄ particles. The particle shape of Fe₃O₄ particles changed from needle to sphere when the water content in E/W volume ratio increased from 0.5 to 1 mL in mixed glycothermal condition. When the water were added by more than 10 ml, the particle shape of Fe₃O₄ changed from sphere to octahedron truncated with the {100} faces and finally distinct octahedron with only {111} faces. Also, it is demonstrated that the size of Fe₃O₄ particles can be controlled from 1–2 μm to 100–200 nm by varying the reaction conditions such as the volume ratio of water to ethylene glycol and additive in glycothermal reaction.     

Temperature-stable and low loss Fe-containing dielectrics in BaO-Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Polycrystalline samples of Ba₄Ln₂Fe₂Ta₈O₃₀ (Ln = La and Nd) were prepared by a high temperature solid-state reaction technique. The formation, structure, dielectric and ferroelectric properties of the compounds were studied. Both compounds are found to be paraelectrics with filled tetragonal tungsten bronze (TB) structure at room temperature. Dielectric measurements revealed that the present ceramics have exceptional temperature stability, a relatively small temperature coefficient of dielectric constant (τ _ε ) of −25 and −58 ppm/°C, with a high dielectric constant of 118 and 96 together with a low dielectric loss of 1.2 × 10⁻³ and 2.8 × 10⁻³ (at 1 MHz) for Ba₄La₂Fe₂Ta₈O₃₀ and Ba₄Nd₂Fe₂Ta₈O₃₀, respectively. The measured dielectric properties indicate that both materials are possible candidates for the fabrication of discrete multilayer capacitors in microelectronic technology.     

Synthesis of sillenite-type Bi<Subscript>36</Subscript>Fe<Subscript>2</Subscript>O<Subscript>57</Subscript> and elemental bismuth with visible-light photocatalytic activity for water treatment

With Fe(NO₃)₃·9H₂O and Bi(NO₃)₃·5H₂O as raw materials, different sillenite-type compounds and elemental bismuth were prepared by a facile one-pot solvothermal method using H₂O, C₂H₅OH, (CH₂OH)₂ and C₃H₈O₃ as solvents, respectively. The structure, morphology, elemental compositions and properties of samples were examined by XRD, SEM, TEM, ICP, XPS, N₂ adsorption and desorption, UV-vis DRS and PL. The photocatalytic activities of different samples were evaluated by the photodegradation of RhB under visible-light irradiation (λ > 400 nm), and results show that Bi₃₆Fe₂O₅₇ prepared using C₂H₅OH as the solvent owns the optimum performance. In order to explore the reaction mechanism, an additional experiment was designed to investigate the main active species during the photodegradation process via dissolving different trapping agents in the reaction solution before light irradiation. The results show that superoxide radical anions play a major role in this system since the RhB degradation was significantly suppressed after the addition of benzoquinone.