Superparamagnetic behavior and AC-losses in NiFe2O4 nanoparticles

Crystallographic, microstructural and magnetic properties of NiFe₂O₄ nanoparticles synthesized by precipitation from nonaqueous solutions have been studied in the work. The transmission electron microscopy studies reveal particle sizes ∼5 nm for the as-prepared particles which increase up to ∼20 nm upon annealing at 500 °C. Quasistatic magnetic measurements show superparamagnetic behavior with blocking temperature below room temperature for both the as-prepared and annealed particles. Characteristic magnetic parameters of the particles including average magnetic moment of an individual nanoparticle and effective anisotropy constant have been determined. The specific loss power which is released on the exposure of an ensemble of synthesized particles to an electromagnetic field is calculated and measured experimentally.     

Rapid synthesis of LiFePO4 nanoparticles by microwave-assisted hydrothermal method

Lithium iron phosphate (LiFePO₄) nanoparticles have been successfully prepared via microwave-assisted hydrothermal route within ultrashort synthesis-time condition. Electrochemical data reveal that the as-synthesized LiFePO₄ after a post-synthesis heat treatment in an inert atmosphere have excellent rate capability and cycling stability. Ultrashort synthesis-time and higher yield show a possibility of scaling up for industrial production.     

One pot facile hydrothermal synthesis of superparamagnetic ZnFe2O4 nanoparticles and their properties

Superparamagnetic ZnFe₂O₄ nanoparticles were synthesized by facile hydrothermal method using different amount of diethylamine (DEA) as a precipitating agent. The phase formation, morphological and magnetic properties were investigated using X-ray diffraction, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy and vibrating sample magnetometer. The amount of DEA required for forming single phase ZnFe₂O₄ was optimized. For 2 ml of DEA, a mixed phase of α-Fe₂O₃ and ZnFe₂O₄ was formed whereas single phase of ZnFe₂O₄ was formed with high crystalline quality for 4, 6, 8 and 10 ml of DEA. FE-SEM results indicated that all the synthesized products were in spherical shape with small aggregations. Particle size of 9 nm was obtained from TEM image for sample synthesized using 10 ml of DEA. Superparamagnetic property was observed for the samples synthesized using 4, 6, 8, and 10 ml of DEA.     

Facile synthesis and properties of ZnFe2O4 and ZnFe2O4/polypyrrole core-shell nanoparticles

We demonstrated that ZnFe₂O₄/polypyrrole core-shell nanoparticles could be facilely synthesized via in situ chemical oxidative polymerization of pyrrole monomers on the surface of ZnFe₂O₄ nanoparticles. The shell thickness of core-shell nanoparticles could be easily controlled by adjusting the amount of pyrrole monomers. The phase structures, morphologies and properties of the as-prepared products were investigated by XRD, TEM, SEM, VSM, and FTIR spectra. Magnetic studies revealed that the saturation magnetization (M_s) and coercivity (H_c) of ZnFe₂O₄/PPy core-shell nanoparticles is 17.8 emu/g and 130 Oe, respectively. The electromagnetic characteristics of products showed that ZnFe₂O₄/PPy core-shell nanoparticles exhibit excellent microwave absorption performance than ZnFe₂O₄ nanoparticles, such as more powerful absorbing property and wider electromagnetic wave absorbing frequency band due to the proper matching of the permittivity and the permeability of ZnFe₂O₄/PPy core-shell nanoparticles.     

Time-resolved spectroscopic behavior of Fe2O3 and ZnFe2O4 nanocrystals

Using nanosecond (ns) and femtosecond (fs) time-resolved absorption spectroscopies (pump-probe technique), the carrier dynamics in transition metal oxide nanocrystals of alpha-Fe2O3 and ZnFe2O4 was studied during the photolysis process. For Fe2O3 and ZnFe2O4 nanocrystals, the fs measurements detect similar profiles of a positive nonlinear absorption in their capped nanocrystals, whereas much weak signals in the naked particles. In the nanosecond measurements Fe2O3 and ZnFe2O4 nanocrystals show obvious excitation-power dependent absorption properties and at the low pump power they show weak photobleaching, but at high pump power they produce positive nonlinear absorptions. For Fe2O3 nanocrystals, the threshold power of negative absorption (bleach) to positive absorption increases with reducing size, whereas for the ZnFe2O4 samples, the threshold powers reach minimum at a critical size of 11 nm, grow for both the bigger and the smaller nanocrystals. These results reflect the influences of their microscopic magnetic couplings and carrier correlation on biexciton absorption in Fe2O3 and ZnFe2O4 nanocrystals. All the results indicate that the time resolved photoabsorption techniques are useful to study the microscopic spin interactions and carrier correlations in transition metal oxide nanocrystals.     

Highly stable dielectric frequency response of chemically synthesized Mn-substituted ZnFe2O4

The tenacity of the present study was to develop a material using an economical chemical route, having balance between magnetic and dielectric order parameters for maximum transmittance of electromagnetic waves in order to use them in shielding materials. In this context, Mn-Zn ferrites were prepared using a wet chemical based sol-gel auto-combustion technique. X-ray diffraction confirmed the pure phase formation of samples, while some impurity peaks were also present for the higher value of Mn substitution. Field emission scanning electron microscopy revealed a decrease in grain size with increasing Mn substitution. While energy dispersive X-ray spectroscopy confirmed the elemental composition of pure and Mn substituted samples, the dielectric constant, dielectric loss and tangent loss were decreased with increasing frequency and increasing Mn substitution. The complex electric modulus was found to be a function of frequency and values of complex electric modulus were increased with increasing frequency and Mn substitution. The complex impedance of RC series circuit and RC parallel circuit was also decreased with increasing both the parameters while AC conductivity was increased in the series. Dielectric frequency response was also studied for the prepared samples and the best match was found with expected results. The Nyquist and Cole-Cole plots revealed the semi-conductive behavior at higher frequency and Mn substitution also yielded the same results. The relative stability of the samples to be used as dielectric materials was also studied using Bode and Nichols plots, and a comparatively high gain margin was observed, well suitable for potential applications in electromagnetic shielding.     

Comparison of reduction behavior of Fe2O3, ZnO and ZnFe2O4 by TPR technique

Advanced integrated gasification combined cycle (IGCC) power generation systems require the development of high-temperature,regenerable,desulfurization sorbents capable of removing hydrogen sulfide from coal gasifier gas to very low levels.As a sort of effective desufurizer,such as Fe2O3,ZnO and ZnFe2O4,it will endure strong reducing atmosphere in desulfurization process.The reduced degree of desufurizer can have an effect on its desulfurization reactivity.In this paper,Fe2O3,ZnO and ZnFe2O4 were synthesized by precipitation or co-precipitation at constant pH.After aging,washing and drying,the solids were calcined at 800℃.The reduction behaviors of sample were characterized by temperature-programmed reduction (TPR).It is found that there are two reduction peaks for Fe203 in TPR,and whereas no reduction peaks for ZnO are found.The reduction process of ZnFe2O4 prepared by co-precipitation is different from that of Fe2O3.ZnFe2O4 is easier to be reduced than Fe2O3.The activation energy of reduction process for Fe2O3 and ZnFe2O4 is obtained at different reduction periods.     

Synthesis and characterization of pure and Li2O doped ZnFe2O4 nanoparticles via glycine assisted route

Nanoparticles of zincl ferrite have been synthesized by the combustion method and the effect of lithia doping on their structural, morphological and magnetic properties have been studied in detail. The samples were characterized by IR, XRD, SEM, TEM and VEM. The powder XRD patterns confirm the single phase spinel structure for the synthesized materials. Transmission electron microscopy analysis revealed spherical Zn-ferrite particles with a narrow size distribution. 0.69 wt% Li₂O-doping results in a decrease of the magnetization and increase of coercivity of the nanoparticles. This observation implies that, besides size, doping causes also significant structural rearrangements which in turn induce changes in the strength and the number of Fe³⁺(A)–O–Fe³⁺(B) superexchange interactions.     

Characterization of bis-Schiff bases synthesized using ZnFe2O4 nanoparticles as catalyst,and examination of their antibacterial and in vitro cytotoxic properties

In this study, ZnFe₂O₄ nanoparticles (NPs) were prepared by a hydro/solvothermal method, and their structures were characterized by infrared, scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction analyses. These ZnFe₂O₄ NPs were used in the synthesis of new bis-Schiff bases by condensation of hydroxyl-substituted aldehydes with various aromatic diamines in aqueous media. The antimicrobial activities of the synthesized bis-Schiff bases were tested against gram-negative as well as gram-positive bacteria. The MTT method was used to assess the anticancer activity of the bis-Schiff bases towards human lung and breast cancer cell lines. The results indicated that these compounds could be considered as new anti-tumor candidates.     

A new photocatalyst: Bi2TiO4F2 nanoflakes synthesized by a hydrothermal method

A pure phase of Bi(2)TiO(4)F(2) nanoflakes with layered Aurivillius structure are synthesized by a simple hydrothermal method. The as-prepared sample is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and UV-vis diffuse reflectance spectroscopy. The OH radicals produced during the photocatalytic reaction is detected by a photoluminescence (PL) technique. The electronic structure is investigated by DFT calculations. The photocatalytic properties of Bi(2)TiO(4)F(2) are explored by degradation of Rhodamine B (RhB) and phenol. The results show that Bi(2)TiO(4)F(2) exhibits much higher photocatalytic performances than Bi(4)Ti(3)O(12) due to the unique layered structure and the existence of F. F acts as an electron trapper, which enhances the separation of photogenerated electron-hole pairs, and lead to higher photocatalytic activity. Reaction with terephthalic acid demonstrates that OH radicals are formed as a result of UV irradiation of Bi(4)TiO(4)F(2) in solution, in agreement with the proposed mechanism. Thus, Bi(2)TiO(4)F(2) can be used as a new efficient photocatalyst.     

High-energy-density LiMn0.7Fe0.3PO4 nanorods synthesized by microwave-assisted solvothermal method

Microwave-assisted solvothermal method has been developed for synthesizing LiMn_0.7Fe_0.3PO₄ cathode materials with an olivine structure. The obtained LiMn_0.7Fe_0.3PO₄ nanorods were characterized by X-ray diffraction, scanning and transmission electron microscope, Brunauer-Emmett-Teller surface area measurements, and electrochemical tests. Electrochemical tests clearly indicate that the as-made LiMn_0.7Fe_0.3PO₄ nanorods exhibit two redox activities of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ couple at galvanostatic charge-discharge process, which is due to the coexistence of Mn²⁺ with Fe²⁺ at 4c sites. The as-synthesized materials have high energy density, excellent rate capability and cycling stability.     

Graphene anchored with ZnFe2O4 nanoparticles as a high-capacity anode material for lithium-ion batteries

Heterostructured ZnFe₂O₄–graphene nanocomposites are synthesized by a facile hydrothermal method. The as-prepared ZnFe₂O₄–graphene nanocomposites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis and galvanostatic charge and discharge measurements. Compared with the pure ZnFe₂O₄ nanoparticles, the ZnFe₂O₄–graphene nanocomposites exhibit much larger reversible capacity up to 980 mAh g⁻¹, greatly improved cycling stability, and excellent rate capability. The superior electrochemical performance of the ZnFe₂O₄–graphene nanocomposites could be attributed to the synergetic effect between the conducting graphene nanosheets and the ZnFe₂O₄ nanoparticles.     

Investigation of photocatalytic effect of SnO2 nanoparticles synthesized by hydrothermal method on the decolorization of two organic dyes

Tin oxide nanoparticles about 4 nm in size were successfully synthesized using hydrothermal method. The photocatalytic activity of the particles was determined by the decolorization of malachite green (MG) and titanium yellow (TY) under UV light. 12 ppm of MG and TY were used for the solution with an initial volume of 100 mL. The amounts of catalysts were 10, 30 and 50 mg. The effect of the catalyst loading on the reaction kinetic parameters and the decolorization rate constants (k) were determined. In order to reveal the photocatalytic efficiency of the nano particles, further experiments were conducted with bulk SnO(2). The oxygen species registered no observable effect on the reaction mechanism as nitrogen bubbling leads to no change in decolorization rates. Results showed that the synthesized nano tin oxide particles represent excellent photocatalytic activity for the decolorization of 12 ppm MG under UV light with 150 min of irradiation time. The energies of the highest occupied molecular orbital (HOMO) E(HOMO) of the dyes were also calculated by using the quantum chemical software in order to discuss the differences for the decolorization of two dyes. Electrical energy efficiency values for the decolorization of two dyes were also calculated.     

纳米铁酸锌的水热合成

利用水热合成法制备纳米铁酸锌粉体，使用XRD、TEM研究了铁酸锌晶相组成、粒度大小和形貌，并对影响合成工艺的因素如反应前驱物的pH、反应温度、反应时间与产物的关系进行系统的考察研究。结果表明：前驱物的最佳pH为11－12，反应温度为448K，反应时间为360min，可制得结晶良好、纯度高、分散性好、粒度为纳米级的ZnFe2O4粉体。     

The gas sensing properties of TiO_2 nanotubes synthesized by hydrothermal method

In this paper,the TiO₂ nanotubes were synthesized by hydrothermal method using a 10 mol/L NaOH aqueous solution at 150℃. The structure of prepared materials was characterized by X-ray diffraction（XRD）,transmission electron microscopy（TEM）. scanning electron microscope（SEM） and Brunauer-Emmett-Teller（BET）.The prepared TiO₂ nanotubes were used to prepare thick film gas sensors and the gas sensing properties to various gases were tested.Results show the prepared TiO₂ nanotube gas sensors responses to ethanol under dry condition at 450℃.This could be attributed to the fact that it had high porous morphology and a higher pore volume,which can promote the diffusion of ethanol deep inside the films and improve the sensor response. Moreover,the gas sensor made with nanotubes exhibit high selective response towards ethanol gas compared with H₂,CO,acetone.     

Structural and optical properties of KTa0.77Nb0.23O3 nanoplates synthesized by hydrothermal method

Two-dimensional KTa(0.77)Nb(0.23)O(3) (KTN) nanoplates with edge sizes of about 100 nm and thickness of about 10 nm have been fabricated by hydrothermal method at 200 degrees C using Ta(2)O(5), Nb(2)O(5), and KOH served as the precursors. Detailed structural studies indicate that the synthesized products are made up of large quantity single crystalline nanoplates with quadrilateral shape, which have a cubic perovskite structure without any other impurity phase and (011) growth direction. The stacking faults may be the key in the formation and growth of the (011) plate. Room temperature photoluminescence spectra excited at different wavelengths exhibit a strong emission band centered at 470 nm (2.63 eV) as well as two weak emission peaking at 423 nm (3.06 eV) and 505 nm (2.46 eV), respectively. The electron-hole recombination of localized excitons should responsible for the light emissions at 423 and 470 nm. Room temperature Raman spectrum of KTN nanoplates reveal that the frequencies of vibration mode are lower slightly than that of KTN bulk materials, and the scattering profile is to be more diffused and enlarged, which may be induced by the crystal structure defects, such as stacking faults.     

ZnFe2O4纳米微晶的自控合成及其气敏性

纳米晶;AnFe2O4;ZnFe2O4纳米微晶的自控合成及其气敏性     

Enhanced visible-light absorption and dopant distribution of iodine-TiO2 nanoparticles synthesized by a new facile two-step hydrothermal method

In order to prepare visible-light responsive iodine-doped TiO₂, a new facile synthetic approach was proposed, which started with the cost-efficient and environmentally friendly precursor of undoped anatase TiO₂ to form nanotube structures as templates that collapsed and recrystallized into I-TiO₂ nanopowders in HIO₃ solution, followed by annealing at different temperatures. The modification of TiO₂ to incorporate iodine and form titanium dioxide with significantly enhanced absorption in the visible range of the spectrum was investigated. The extent of iodine dopant incorporation was determined by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX) and was found to be homogenously distributed on each nanostructure as determined by electron energy-loss spectroscopy (EELS) elemental mapping and EDX spectroscopy. The modified TiO₂ exhibits a dramatically extended absorption edge beyond 800 nm as compared to the original and unmodified TiO₂.     

一步水热合成Y-Co3O4复合氧化物催化剂及催化分解N2O

用一步水热、分步水热、浸渍等方法分别制备Y-Co₃O₄复合氧化物,用于催化分解N₂O的反应,其中,一步水热法制备的催化剂活性较高。再用一步水热法制备了不同Y/Co物质的量比的Y-Co₃O₄复合氧化物,在优化出的催化剂（0.03Y-Co₃O₄）表面浸渍K₂CO₃溶液,制备K改性催化剂（0.02K/0.03Y-Co₃O₄）。用X射线衍射（XRD）、N₂物理吸附、H₂程序升温还原（H₂-TPR）、O₂程序升温脱附（O₂-TPD）、扫描电镜（SEM）、X射线光电子谱（XPS）等技术表征催化剂结构。研究发现,Co₃O₄和Y-Co₃O₄同为尖晶石结构,但Y-Co₃O₄的催化活性显著高于Co₃O₄。K改性增加了催化剂表面的活性位（Co²⁺）,还有利于吸附氧的脱除,从而提高了催化剂活性。在无氧无水、有氧无水、有氧有水气氛中,K改性催化剂上的N₂O全分解温度分别为325、350、375 ℃,催化剂活性较高。有氧有水气氛350 ℃连续反应50 h,K改性催化剂上N₂O分解率保持90%以上,稳定性较高。研究发现,Y-Co₃O₄及K改性催化剂上N₂O分解反应的E_a和lnA之间存在动力学补偿效应。