Influence of Co doping on the magnetic properties of Bi2Fe4O9 powders

Co doped Bi₂Fe₄O₉ powders were synthesized via a simple sol–gel method. The crystal structure of the as-synthesized powders was characterized using X-ray diffraction and Raman spectra. The magnetic properties of the as-synthesized powders were investigated by a SQUID MPMS at 5 K. The results reveal that the calculated lattice parameters of Bi₂Fe₄O₉ decrease and the magnetic properties sharply increase with increasing the content of Co.     

Time-resolved spectroscopy of Bi3+ centers in Y4Al2O9

Steady-state and time-resolved emission and excitation spectra as well as luminescence decay kinetics are studied at 4.2–400 K under excitation in the 3–6 eV energy range for Bi³⁺ ions substituting for Y³⁺ ions in four inequivalent crystal lattice sites of Y₄Al₂O₉:Bi ceramics. Luminescence characteristics of Bi³⁺ centers of all the four types are identified and are shown to arise from the radiative decay of the triplet relaxed excited state (RES) of Bi³⁺ ions. The parameters of the triplet RES, namely, probabilities of the radiative and nonradiative transitions from the metastable and emitting levels as well as the energy distance between these levels, are determined. The influence of the nearest surroundings of Bi³⁺ ions on the luminescence characteristics and the parameters of the triplet RES of Bi³⁺ centers is discussed.     

Tunable morphology and magnetic properties of Bi2Fe4O9 nanocrystal synthesized by hydrothermal method

Bi2Fe4O9 nano and micron powders have been synthesized by a hydrothermal method. The as-obtained samples are pure phase and crystallize in the orthorhombic structure. Diverse particle morphologies, including nanoplates, nanospheres, microcubes, and microcylinders, are obtained under different synthesis conditions. The solvent N,N-Dimethylformamide (DMF), together with the mineralisers NaOH and NH4OH, are found to be the key factors for the formation of the particles with their diverse morphologies and sizes. The magnetization dependence of temperature (M-T), observed in a field of 1000 Oe from 10 to 340 K, and M-H loops measured at 10 K indicate that the Bi2Fe4O9 particles are paramagnetic at room temperature and undergo an antiferromagnetic transition at a Néel temperature (T(N)) of 250 K.     

Enhanced photocatalytic activity in Al-substituted Bi2Fe4O9 submicrocrystals

Bi₂(Fe_1−x Al _x )₄O₉ (x = 0, 0.1, 0.3, 0.5, and 0.7) samples were synthesized by a simple hydrothermal method. The samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier transformed infrared spectra, N₂-sorption, and UV–Vis diffuse reflectance spectra, and their photocatalytic activities were evaluated by the degradation of methyl orange under visible-light irradiation. The results reveal that the Al substitution can effectively improve photocatalytic performance, which is attributed to the increase of surface area, the improvement of energy band structure, and the distortion of the Fe–O octahedron in Al-substituted samples. In addition, it is found that the photocatalytic activity to decompose methyl orange under visible-light illumination increases monotonically as x increases from zero to 0.5 in Bi₂(Fe_1−x Al _x )₄O₉ and then decreases for x = 0.7, which is believed to be associated with the distribution of Fe³⁺ ions over the octahedral and tetrahedral sites.     

Synthesis and photocatalytic performance of the electrospun Bi2Fe4O9 nanofibers

Bi₂Fe₄O₉ nanofibers were successfully synthesized by an electrospinning method combined with a sol–gel process. The as-spun nanofibers were annealed at different temperatures ranging from 500 to 700 °C and a pure orthorhombic phase was obtained at 700 °C. The thermo-decomposition behavior, structure, morphology, optical property, and the specific surface area of the nanofibers were characterized by thermogravimetry and differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, UV-vis diffuse reflectance spectroscopy and photoluminescene spectroscopy, and specific surface analyzer, respectively. The results indicated that the diameter and morphology of the fiber changed with different calcination temperatures. Moreover, the results of UV-vis diffuse reflectance spectroscopy revealed that the Bi₂Fe₄O₉ nanofiber could be a photocatalyst under a visible light irradiation and the bandgap value was determined to be 2.1 eV based on the Kubelka–Munk theory. The photocatalytic activity of the obtained nanofibers was evaluated by the degradation of methyl orange. A favorable degradation rate of 45 % was obtained for the sample annealed at 600 °C under the illumination of visible light for 3 h and an enhanced efficiency up to 70 % with recycling stability could be obtained with the aid of H₂O₂ for the pure-phase sample annealed at 700 °C. These results demonstrated that the electrospun Bi₂Fe₄O₉ nanofibers could be a promising visible light photocatalyst.     

Ag3PO4/BigFe4O9复合催化剂的制备及其光催化性能

利用水热法成功地制备得到具有高效光催化活性的Ag3PO4/Bi2Fe4O9复合型光催化剂.使用X-射线多晶粉末衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)对样品进行表征,并以罗丹明B为目标降解物对其光催化性能进行研究.结果表明:样品是由纳米Ag3 PO4颗粒负载在片状四边形的Bi2Fe4O9表面组成的,当Ag3PO4的负载量为4wt％时,复合材料的光催化效果最好,在可见光(波长＞420nm)照射下,1.5h内对100mL浓度为10-5mol.L-1罗丹明B溶液的脱色率可达98.7％.     

花瓣状Fe_3O_4/Bi_2O_3复合粒子的制备及光催化活性

用共沉淀法制备的Fe3O4纳米粒子作为种子,通过水热法获得了微米尺寸的Fe3O4/Bi2O3复合粒子。X射线衍射和X光电子能谱表征结果说明复合粒子是由Fe3O4和Bi2O3组成。扫描电子显微镜照片表明复合粒子形貌基本呈规则球形,并且具有花瓣状的三维多级结构。以罗丹明B的催化降解实验为模型考察了不同反应组成、不同反应介质、不同反应温度条件下制备的复合粒子的催化活性。结果表明,当反应条件中m(Bi(NO3)3·5H2O)/m(Fe3O4)为1.9 g∶0.2 g,水作反应介质在160℃时,所制备的复合粒子催化活性最高,对罗丹明B的降解率达95.4%。降解完成后,用磁铁吸附,Fe3O4/Bi2O3很快从体系中分离,可以重新催化降解罗丹明B,实现磁场控制的循环催化。实验发现,Fe3O4/Bi2O3经6次循环利用后,对罗丹明B的降解率仍达88.5%。     

Facile synthesis of pure BiFeO3 and Bi2Fe4O9 nanostructures with enhanced photocatalytic activity

Journal of Materials Science: Materials in Electronics - In the present work, pure BiFeO3 and pure Bi2Fe4O9 single phases were successfully synthesized by tailoring hydrothermal synthesis route....     

Characterization of new sorbent constructed from Fe3O4/chitin magnetic beads for the dynamic adsorption of Cd2+ ions

Novel magnetic chitin (CM) beads were successfully prepared by in situ synthesis of Fe₃O₄ nanoparticles in regenerated chitin beads (Ch beads) for the packing fixed-bed columns. The interpenetrated porous structure in the regenerated Ch beads at the swollen state served as templates for the inorganic nanoparticle preparation. The morphology and structure of the hybrid nanomaterials were characterized with scanning transmission electron microscopy, transmission electron microscopy, thermal gravimetry analysis, X-ray diffraction, and Fourier transform infrared spectroscopy, and the Cd²⁺ ion adsorption capacity of the CM beads was determined by UV–Vis spectrophotometry. The results revealed that the CM beads exhibited efficient adsorption of Cd²⁺ ions in the aqueous solution, as a result of the microporous structure, large surface area, and affinity for metal ions. The equilibrium process of this fixed-bed column was well described by Thomas and Bohart–Adams model, indicating that the external mass transfer was the rate-limiting process at the beginning of adsorption. The adsorption equilibrium was better described by the bed depth–service time model, indicating that the Cd²⁺ uptake could be controlled by external mass transfer at the beginning and intraparticle diffusion at a later stage of the adsorption. The CM beads loaded with the Cd²⁺ could be regenerated and reused easily. The CM beads should have potential applications in the chromatography packing and adsorbent both at the laboratory and industrial scales.     

Amino-functionalized Fe3O4/SiO2 magnetic submicron composites and In3+ ion adsorption properties

Five kinds of amino-functionalized (polyaniline, poly(1,2-diaminobenzene), poly(1,3-diaminobenzene), poly(diphenylamine), and poly(o-toluidine)) Fe₃O₄/SiO₂ submicron composites (SCs) were prepared. The SEM and TEM results showed that these SCs possessed a sphere-like core/shell structure with an average diameter of ~500 nm. The XRD results indicated good crystallinity of Fe₃O₄ core, the amorphous SiO₂, and amino-functionalized shells. The XPS results confirmed that amino groups were plentiful rich outside the surface of these SCs which acted as the effective groups for adsorbing the metal ions. These SCs showed a good thermal stability at 20–250 °C. The high saturation magnetization of 60–70 emu/g is better than other similar reports. In³⁺ adsorption coefficients from aqueous solution by these SCs were higher than 10⁶ mL/g, indicating the higher selectivity and affinity to In³⁺ compared with Cd²⁺ and Hg²⁺ ions. In addition, these SCs could be magnetically reclaimed within 30 s and regenerated with acid after adsorption. The adsorption capabilities only decreased by 6 % after five cycles. The present work indicates that the amino-functionalized Fe₃O₄/SiO₂ SCs are promising for removal of In³⁺ ions in field application.     

Single Unit-Cell Layered Bi2Fe4O9 Nanosheets: Synthesis,Formation Mechanism,and Anisotropic Thermal Expansion

As an important multiferroic material, pure and low-dimensional phase-stable bismuth ferrite has wide applications. Herein, one-pot hydrothermal method was used to synthesize bismuth ferrite. Almost pure Bi₂Fe₄O₉, BiFeO₃, and their mixture were successfully obtained by controlling the KOH concentration in the hydrothermal solutions. The as-prepared Bi₂Fe₄O₉ products were crystalline with Pbam space group, had nanosheet morphology, and tended to aggregate into nanofloret or random stacking. Each Bi₂Fe₄O₉ nanosheet was a single crystal with (001) plane as its exposed surface. Single unit-cell layered Bi₂Fe₄O₉ nanosheets had a uniform thickness of 1 nm. The surface energies of various (100), (010), and (001) planes were 3.6–4.0, 5.6–15.1, and 1.7–3.0 J m⁻², respectively, in the Bi₂Fe₄O₉ crystal. The formation mechanism and structural model of the as-prepared single unit-cell layered Bi₂Fe₄O₉ nanosheets have been given. The growth of Bi₂Fe₄O₉ nanosheets was discussed. Thermal analysis showed that the Bi₂Fe₄O₉ phase was stable up to 1260 K. The thermal expansion behavior of the Bi₂Fe₄O₉ nanosheet was nonlinear. The thermal expansion coefficients of the ultrathin Bi₂Fe₄O₉ nanosheets on the a-, b-, c-axes, and on the unit-cell volume V were determined, showing an anisotropic thermal expansion behavior. This study is helpful for the controllable synthesis of ultrathin Bi₂Fe₄O₉ nanosheets.     

Fe3O4缓冲层对CoFe2O4薄膜磁性能的影响

采用射频磁控溅射在基片Si（100）和Fe3O4（20nm）/Si（100）上制备了钴铁氧体（CoFe2O4）薄膜，制备的薄膜在空气气氛中进行300～1000℃的退火处理，采用XRD、VSM分析了薄膜的微结构以及磁性能。结果表明，制备的钴铁氧体薄膜均具有尖晶石结构，Fe3O4缓冲层薄膜促进了钴铁氧体薄膜的结晶，但降低了钴铁氧体薄膜的垂直各向异性和垂直于膜面方向的矫顽力，而钴铁氧体薄膜的磁化强度和矩形度得到了一定的提高。     

Ni doped Fe3O4 magnetic nanoparticles

In this work, the effect of nickel doping on the structural and magnetic properties of Fe3O4 nanoparticles is analysed. Ni(x)Fe(3-x)O4 nanoparticles (x = 0, 0.04, 0.06 and 0.11) were obtained by chemical co-precipitation method, starting from a mixture of FeCl2 x 4H2O and Ni(AcO)2 x 4H2O salts. The analysis of the structure and composition of the synthesized nanoparticles confirms their nanometer size (main sizes around 10 nm) and the inclusion of the Ni atoms in the characteristic spinel structure of the magnetite Fe3O4 phase. In order to characterize in detail the structure of the samples, X-ray absorption (XANES) measurements were performed on the Ni and Fe K-edges. The results indicate the oxidation of the Ni atoms to the 2+ state and the location of the Ni2+ cations in the Fe2+ octahedral sites. With respect to the magnetic properties, the samples display the characteristic superparamagnetic behaviour, with anhysteretic magnetic response at room temperature. The estimated magnetic moment confirms the partial substitution of the Fe2+ cations by Ni2+ atoms in the octahedral sites of the spinel structure.     

Electronic and spin configurations of Co3+ and Ni3+ ions in oxides of K2NiF4 structure: A magnetic susceptibility study

In La₄LiCoO₈, Li⁺ and Co³⁺ ions are ordered in two dimensions and Co³⁺ ions undergo transitions from the low-spin to the intermediate as well as the high-spin states. Both Sr₄TaCoO₈ and Sr₄NbCoO₈ exhibit low to intermediate-spin state transitions of Co³⁺ ions. In the system LaSr_1?xBa_xNiO₄, the e_g electrons are essentially in extended states forming a band. With increase in x, the band width decreases accompanying an increase in unit cell volume; high-spin Ni³⁺ ions are formed to a small extent with increasing x, but there is no spin-state transition. In LaSrAl_1?xNi_xO₄, at small x, there is a small proportion of high-spin Ni³⁺; when x ≈ 0.6, there is an abrupt decrease in the $\text{c}$/$\text{a}$ ratio, signalling the formation of the band. In LnSrNiO₄, the $\text{c}$/$\text{a}$ ratio decreases sharply between Ln = La and Nd; this is likely to be accompanied by a broadening of the band.     

Effect of Ultrasonic Treatment on the Magnetization Enhancement of Fe3O4 Nanocrystallines

The effect of ultrasonic treatment on the morphology changes and magnetization enhancement of Fe₃O₄ nanocrystallines (NCs) is reported. The samples were characterized by various techniques such as X-ray diffraction, scanning transmission electron microscopy, etc. The crystallinity and the morphology of Fe₃O₄ NCs are obviously changed when the as-grown Fe₃O₄ NCs are irradiated by ultrasonic for 30 min. The magnetization of these Fe₃O₄ NCs after ultrasonic radiation is found to be 20 % enhanced compared to the sample without this treatment. A well dispersive magnetic fluid with the chemisorbed –OH as the surfactant is composed after ultrasonic irradiation. The mechanism of the morphology changes, magnetization enhancement and dispersibility of Fe₃O₄ NCs are also discussed in detail.     

Fe3O4磁性纳米颗粒的合成及其调控

利用水解共沉淀法制备了Fe3O4纳米颗粒,研究了温度和pH值对Fe3O4纳米颗粒粒径、形貌的影响关系。研究结果表明,反应温度从30℃升高到90℃,Fe3O4颗粒的粒径从6～8nm增大到10～12nm;同时,Fe3O4颗粒的饱和磁矩也随着Fe3O4颗粒粒径的增加而升高。溶液pH值会影响Fe3O4纳米颗粒的形状,高pH值易使合成的Fe3O4纳米颗粒为四方形,随着pH值的降低,Fe3O4纳米颗粒向球形转变。Fe3O4纳米颗粒的粒径和形状的可控性为进一步合成、调控Fe3O4电磁功能复合材料奠定了良好基础。