Constructing three-dimensional Li-transport channels within the Fe3O4@SiO2@RGO composite to improve its electrochemical performance in Li-ion batteries

Magnetite Fe₃O₄ particles are usually pulverized when used as the anode material for Li-ion batteries and thus the solid electrolyte interface film grows on the surface progressively, leading to inferior cycling performance and poor rate capability. To solve these issues, core-shell Fe₃O₄@SiO₂ particles are wrapped by reduced graphene oxide (RGO), and meso-/micro-pores are produced not only in the SiO₂ layers but also in the RGO nanosheets by chemical etching, forming three-dimensional (3D) continuous channels for Li⁺ transportation. Benefiting from this unique structure, the as-prepared Fe₃O₄@mSiO₂@RGO composite can deliver a capacity of 1630 mA h g⁻¹ at 0.1 A g⁻¹ over the potential range of 0.01–3.00 V (vs. Li⁺/Li) in the first discharge along with an initial coulombic efficiency of 86%, and can retain the capacity of 514 mA h g⁻¹ at 5 A g⁻¹ after 1000 cycles, exhibiting an outstanding rate capability and a long-term span life. The results indicate that pseudocapacitive behavior enables this composite to charge/discharge fast while the porous SiO₂ shell and RGO nanosheets effectively accommodate the volume change of the Fe₃O₄ particles during cycling. Our findings provide a feasible strategy for improving the electrochemical properties of the Fe₃O₄ anode in Li-ion batteries.     

A novel photocatalyst AgBr/ZnO/RGO with high visible light photocatalytic activity

A novel ternary photocatalyst AgBr/ZnO/RGO, where AgBr/ZnO is supported on reduced graphene oxide, is synthesized via a facile hydrothermal–impregnation method. The resultant composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface. The photocatalytic experiment for methyl orange dye degradation under visible light irradiation shows that ternary composite AgBr/ZnO/RGO has an activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively. More importantly, the ternary composite also demonstrates a good photostability. Metallic Ag is produced during the photocatalytic process, which may serve as the electron transfer mediator in the vectorial Z-scheme transfer of photogenerated charge carriers at the interface of AgBr/ZnO/RGO. The effective separation of photogenerated electrons and holes was proposed to be responsible for the enhancement of visible light photoactivity.     

Microwave-assisted solution combustion synthesis of Fe3O4 powders

Magnetite (Fe₃O₄) powders were synthesized by solution combustion method at different fuel to oxidant ratios (ϕ = 0.5, 0.75, 1 and 1.5) using conventional and microwave ignition. The ignition method and fuel content affected the phase evolution, microstructure and magnetic properties of Fe₃O₄ powders as characterized by X-ray diffractometry, infrared spectroscopy, N₂ adsorption–desorption, electron microscopy and vibrating sample magnetometry techniques. Single phase Fe₃O₄ powders were only obtained using conventional ignition at ϕ value of 1, while the impurity phases such as α-Fe₂O₃ and FeO together with Fe₃O₄ phase were formed by microwave ignition. The bulky microstructure of conventionally combusted powders with specific surface area of 71.5 m²/g was transformed to disintegrated structure (76.5 m²/g) by microwave heating. The microwave combusted powders showed the highest saturation magnetization of 86.5 emu/g at ϕ value of 0.5 and the lower coercivity than that of conventionally combusted powders at all ϕ values, due to their larger particles.     

Intensifying the photocatalytic degradation of methylene blue by the formation of BiFeO3/Fe3O4 nanointerfaces

A novel highly efficient photocatalyst composite BiFeO₃/Fe₃O₄ has been synthesized by mechanosynthesis and applied to the degradation of Methylene Blue under visible light. Structural, optical and photocatalytic properties of the proposed photocatalyst composites are carefully investigated. The nanointerfaces, associated to ferrous Fe²⁺ ions of the Fe₃O₄ nanoparticles, improve the photocatalytic efficiency when compared with pure BiFeO₃ or Fe₃O₄. The time required to the complete degradation of Methylene Blue solution is 40 min for the sample with 20% of Fe₃O₄ which is more than 7 times faster than the time required using BiFeO₃ alone. Moreover, with the addition of H₂O₂ a complete degradation is achieved just after 10 min, which is faster than any other photocatalytic reaction reported for BiFeO₃-based materials. This enhancement is assumed to be related to an electron drain process due to the difference between energy levels of the conduction bands of BiFeO₃ and Fe₃O₄ combined with the direct Fenton-like process associated with the Fe²⁺ ions of the composites.     

Synthesis of a CoTiO3/BiOBr heterojunction composite with enhanced photocatalytic performance

A novel heterojunction CoTiO₃/BiOBr nanocomposite with enhanced photocatalytic performance was synthesized by a precipitation-deposition method. The samples were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrophotometry. Moreover, the photocatalytic activities were evaluated by decomposing the dye molecule Rhodamine B under visible light irradiation. The results showed that high photocatalytic performance can be achieved on the heterojunction photocatalysts, with the 0.15CoTiO₃/0.85BiOBr composite displaying the highest activity. The results of the study concluded that it was the introduction of CoTiO₃ into the catalyst that mainly enhanced the activity of the photocatalyst by promoting the separation of the electron-hole group on the interface between BiOBr and CoTiO₃.     

Synthesis of Fe3O4 nanowire@CeO2/Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure

Ternary magnetic Fe₃O₄ nanowire@CeO₂/Ag nanocomposites have been firstly synthesized by means of hydrothermal and co–precipitation techniques, and their ability to adsorb, photocatalytic degradation organic pollutants, methylene blue present in water, and separate, has been demonstrated. The results show that CeO₂ and Ag nanoparticles are uniformly deposited on the surface of Fe₃O₄ nanowires. The photocatalytic experiments demonstrate that the Fe₃O₄@CeO₂/Ag nanocomposites exhibit remarkably enhanced photocatalytic properties and stability compared to CeO₂, CeO₂/Ag, Fe₃O₄@CeO₂, Fe₃O₄ under natural sunlight exposure. Moreover, excellent photocatalytic degradation efficiency for phenol and MO are also observed. The enhanced photocatalytic performance may be attributed to the synergetic effect of Fe₃O₄ nanowire, CeO₂ and Ag nanoparticles, which lead to the enhanced light harvesting, the promoted charge separation and enhanced adsorption capacity. In addition, the Fe₃O₄@CeO₂/Ag photocatalyst can be easily collected and separated by an external magnet. These results suggest that the nanocomposites could be exploited as potential candidates for solar photocatalysis.     

Synthesis of cross-linked graphene aerogel/Fe2O3 nanocomposite with enhanced supercapacitive performance

In this work, cross-linked graphene aerogel (CL-GA) and its composite with Fe₂O₃ nanoparticles (NPs) were synthesized through a one-step hydrothermal procedure by using p-phenylenediamine (PPD). Structural characterizations revealed that in the preparation of the composite PPD acts as a cross-liker and provides high surface area by decreasing restacking of graphene sheets and functions as nitrogen source simultaneously. The electrochemical characteristics of the nanocomposite were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS) and Fast Fourier transform continues cyclic voltammetry (FFTCCV). The results show that cross-linked graphene aerogel/Fe₂O₃ (CL-GA/Fe₂O₃) nanocomposite displays enhanced supercapacitive performance, where it has capacitance of 445 at 1 A g⁻¹, high energy density of 63 W h Kg⁻¹, and 89% capacitance retention after 5000 cycles in 3 M KOH. Presence of PPD considerably improved supercapacitive performance of nanocomposite as a result it could be promising material in synthesis of efficient graphene/metal oxide-based electrode material for high performance supercapacitors.     

Effects of pH value on the microstructure and magnetic properties of solution combusted Fe3O4 powders

Magnetite (Fe₃O₄) powders were prepared by solution combustion synthesis method using conventional and microwave ignition at various pH values of starting solution, adjusted by NH₄OH. The chelated species in dried gels were predicted by theoretical calculations and Fourier transform infrared spectroscopy. The combustion reaction rate strongly depended on pH values as investigated by thermal analysis. Phase evolution and structure characterized by X-ray diffraction method showed single phase and well-crystalline Fe₃O₄ powders which were achieved using conventional ignition at pH ≥ 7. However, the microwave ignition led to the formation of impure FeO phase together with Fe₃O₄. The microwave combusted powders exhibited the disintegrated structure in comparison with the bulky microstructure for conventionally combusted powders, as observed by scanning electron microscopy. Magnetic properties of the as-combusted powders studied by vibration sample magnetometry showed the highest saturation magnetization of 81.3 emu/g for conventional ignition at pH of 7, due to the high purity and large crystallite size.     

Facile synthesis of Co0.5Zn0.5Fe2O4 nanoparticles decorated reduced graphene oxide hybrid nanocomposites with enhanced electromagnetic wave absorption properties

Herein, reduced graphene oxide/cobalt-zinc ferrite (RGO/Co_0.5Zn_0.5Fe₂O₄) hybrid nanocomposites were fabricated by a facile hydrothermal strategy. Results revealed that the contents of RGO could affect the micromorphology, electromagnetic parameters and electromagnetic wave absorption properties. As the contents of RGO increased in the as-synthesized hybrid nanocomposites, the dispersibility of the particles was improved. Meanwhile, numerously ferromagnetic Co_0.5Zn_0.5Fe₂O₄ particles were evenly anchored on the wrinkled surfaces of flaky RGO. Besides, the obtained hybrid nanocomposites exhibited superior electromagnetic absorption in both X and Ku bands, which was achieved by adjusting the RGO contents and matching thicknesses. Significantly, when the content of RGO was 7.4 wt%, the binary nanocomposites showed the optimal reflection loss of -73.9 dB at a thickness of 2.2 mm and broadest effective absorption bandwidth of 6.0 GHz (12.0–18.0 GHz) at a thin thickness of merely 2.0 mm. The enhanced electromagnetic absorption performance was primarily attributed to the multiple polarization effects, improved conduction loss caused by electron migration, and magnetic loss derived from ferromagnetic Co_0.5Zn_0.5Fe₂O₄ nanoparticles. Our results could provide inspiration for manufacturing graphene-based hybrid nanocomposites as high-efficient electromagnetic wave absorbers.     

Synthesis of magnetically recyclable porous CoFe2O4/Rh composites with large BET surface area for enhanced photocatalytic degradation of organic pollutant

The organic pollutants in water have been a great environment challenges to human beings, and photocatalytic degradation is an effective method to solve this problem. In this paper, the Rh-loaded cobalt ferrite CoFe₂O₄ (CFO) nanoparticles have been successfully synthesized by in situ photodeposition of Rh nanoparticles onto the porous CFO particles as the photocatalysts. After incorporating Rh nanoparticles, the CFO/Rh composite has a higher specific surface area and is more efficient in charge separation than the bare CFO. The photocatalytic efficiency of decomposing Malachite Green (MG) is improved from 70% over the bare CFO to 97% over the optimized CFO/Rh in 60 min. The CFO/Rh sample also demonstrates its durability for the degradation of MG in 5 photocatalytic reaction cycles. Additionally, hydroxyl radicals (?OH) and superoxide radicals (?O₂^?) are proved to be the crucial reactive species during the photocatalytic degradation of MG with CFO/Rh, evidenced by the active species capture experiments. This work provides a useful approach to enhance the photocatalytic activity of semiconductors for degrading organic dyes.     

Synthesis of butterfly-like BiVO4/RGO nanocomposites and their photocatalytic activities

A simple and high efficient method was proposed for the synthesis of uniform three dimensional(3D) BiVO_4/reduced graphene oxide(RGO) nanocomposite photocatalyst by adopting the microwave assistant and using Bi(NO_3)_3·5H_2O, graphene oxide(GO) and NH_4VO_3 as precursor. The as-obtained composites were well characterized with the aid of various techniques to study the morphology, structure, composition, optimal and electrical property. In the as-obtained composites, the GO sheets were fully reduced into RGO, and monoclinic structure BiVO_4 crystallized completely into butterfly-like BiVO_4 lamellas and well bonded with the RGO lamellas. The length and the width of the butterfly-like BiVO_4 particle were about 1.5 μm, and the thickness of the flake was about 20 nm. Photocatalytic performances of BiVO_4/RGO composite and pure BiVO_4 particle have been evaluated by investigating the reduction of Cr(Ⅵ) ion-contained wastewater under simulated solar light irradiation, where the BiVO_4/RGO composite displayed enhanced photocatalytic activity. It is found that the pseudo-first-order rate constants(k) for the photocatalytic reduction of Cr(VI) by BiVO_4/RGO composite was about 4 times as high as that of the pure BiVO_4. The present work suggested that the combination of BiVO_4 and RGO displayed a remarkable synergistic effect, which led to enhanced photo-catalytic activity on Cr(Ⅵ) reduction.     

Reduced graphene oxide as co-catalyst for enhanced visible light photocatalytic activity of BiOBr

BiOBr-reduced graphene oxide (RGO) composites were successfully synthesized via a simple hydrothermal method. Their morphology, structure and photocatalytic activity in the degradation of nitrobenzene were characterized by scanning electron microscopy, X-ray diffraction, nitrogen adsorption-desorption, UV–vis absorption spectroscopy, photoluminescence spectra, electrochemical impedance spectra and total organic carbon, respectively. The results showed that the introduction of RGO could enhance the visible light photocatalytic activity of BiOBr. The BiOBr-RGO composite with 0.6 wt% RGO exhibited an optimal photocatalytic activity, and the maximum degradation rate of nitrobenzene was about 2.16 times that of pure BiOBr due to the increased light absorption and the reduced electron-hole pair recombination in BiOBr with the introduction of RGO.     

Photo-induced electric polarizability of Fe3O4 nanoparticles in weak optical fields

Using a developed co-precipitation method, we synthesized spherical Fe₃O₄ nanoparticles with a wide nonlinear absorption band of visible radiation. Optical properties of the synthesized nanoparticles dispersed in an optically transparent copolymer of methyl methacrylate with styrene were studied by optical spectroscopy and z-scan techniques. We found that the electric polarizability of Fe₃O₄ nanoparticles is altered by low-intensity visible radiation (I ≤ 0.2 kW/cm²; λ = 442 and 561 nm) and reaches a value of 10⁷ ų. The change in polarizability is induced by the intraband phototransition of charge carriers. This optical effect may be employed to improve the drug uptake properties of Fe₃O₄ nanoparticles.

PACS

33.15.Kr78.67.Bf42.70.Nq     

Construction of g-C3N4/TiO2/Ag composites with enhanced visible-light photocatalytic activity and antibacterial properties

In this study, the multifunctional carbon nitride based composite graphitic-C₃N₄ (g-C₃N₄)/TiO₂/Ag was prepared through a simple and efficient vacuum freeze-drying route. TiO₂ and Ag nanoparticles were demonstrated to decorate onto the surface of g-C₃N₄ sheet. In the ultraviolet–visible absorption test, a narrower band gap and red-shift of light absorption edge were observed for g-C₃N₄/TiO₂/Ag compared to pristine g-C₃N₄ and single-component modified g-C₃N₄/TiO₂. The photodegradation property of g-C₃N₄/TiO₂/Ag was investigated toward the degradation of methylene blue (abbreviated as MB) under the irradiation of visible light. These results indicated that the degradation performance of organic dyes for g-C₃N₄/TiO₂/Ag was obviously improved compared with g-C₃N₄/TiO₂ and g-C₃N₄. The reaction rate constant of MB degradation for g-C₃N₄/TiO₂/Ag was 4.24 times higher than that of pristine g-C₃N₄. In addition, such rationally constructed nanocomposite presented evidently enhanced antibacterial performance against the Gram-negative Escherichia coli. Concentration dependent antibacterial performance was systematically investigated. And 84% bacterial cell viability loss had been observed at 500 μg/mL g-C₃N₄/TiO₂/Ag within 2 h visible light irradiation.     

Electrodeposition of flexible stainless steel mesh supported ZnO nanorod arrays with enhanced photocatalytic performance

Large scale well oriented ZnO nanorod arrays (ZNRAs) were electrodeposited on flexible stainless steel mesh (SSM) substrate pre-treated by Al doped ZnO (AZO) seed layers. The effects of substrate pre-treatment conditions such as Al doping and spin coating times of the colloid on the morphology characteristics and photocatalytic properties of as-prepared ZNRAs were systematically studied. The results showed that by introducing Al into ZnO colloid solution, well aligned ZNRAs with relatively higher specific surface area (higher growth density and smaller rod diameter) could be obtained on the premodified SSM substrate. In addition, increasing spin coating times of AZO colloid solution would decrease the average diameter of ZNRAs. Under the optimum preparing conditions, the formed flexible SSM supported ZNRAs exihibited enhanced photocatalytic performance of 93.42% and remarkable photocatalytic stability under the UV-lamp for degradation of Rhodamine B.     

Effect of atmosphere control on magnetic properties of CaO-Al2O3-SiO2-Fe3O4 glass ceramics

The influences of atmosphere during processes of melting and heat treatment, heat treatment temperature, Fe₃O₄ content and basicity on the magnetic properties of magnetite-based glass ceramics were investigated. For sample containing 20 % Fe₃O₄ melted in different atmospheres, the highest saturation magnetisation was realized in 20vol% air + 80 vol% Ar, due to the fact that ratio of Fe³⁺ to Fe²⁺ in melt obtained in this atmosphere was close to 2. However, it was found that the coercivity of glass ceramics was not affected by the melting atmosphere. A high sintering temperature led to the decrease of saturation magnetisation and the increase of coercivity. As increasing Fe₃O₄ content, the main crystal phase transformed from CaSiO₃ to CaFe_0.6Al_1.3Si_1.08O₆ and finally to magnetite phase, accompanied by the increase of saturation magnetisation and coercivity. In addition, the increase of basicity caused the decrease of saturation magnetisation and the increase of coercivity.     

Magnetically recyclable Ni1-xCdxCeyFe2-yO4-rGO nanocomposite photocatalyst for visible light driven photocatalysis

In this study, a magnetically recyclable Ni_1-xCd_xCe_yFe_2-yO₄-rGO (x, y = 0.05) (NCCF-rGO) nanocomposite photocatalyst has been prepared by following a facile in-situ co-precipitation method combined with ultra-sonication means. The as-synthesized magnetically separable NCCF-rGO nanocomposite photocatalyst efficiently degrades methylene blue (MB) dye in comparison to bare Ni_1-xCd_xCe_yFe_2-yO₄ (x, y = 0.05) (NCCF) nanoparticles (NPs) under visible light irradiation. The photo-degradation rate of MB with NCCF-rGO was ~9 times higher than NCCF nanoparticles (NPs). This enhanced photocatalytic performance of NCCF-rGO photocatalyst was due to the presence of reduced graphene oxide, which greatly help in production of photoactive species by reducing the rate of electro-hole pair recombination. The role of photoactive species that were responsible for the photocatalytic degradation of methylene blue has also been investigated. The as-synthesized NCCF-rGO photocatalyst expressed superb chemical stability and photocatalytic activity even after seven cycle runs. Moreover, the NCCF-rGO nanocomposite worked at all pH values and showed good acid resistance. In particular, the as-synthesized NCCF-rGO photocatalyst could be collected for the next cycle run by simply applying an external magnetic field. Hence, the NCCF-rGO nanocomposite could have potential use in organic dyes contained wastewater treatment.     

One-pot synthesis of 3D porous Bi7O9I3/N-doped graphene aerogel with enhanced photocatalytic activity for organic dye degradation in wastewater

The compound Bi₇O₉I₃ has been considered as a promising candidate for organic dye degradation in wastewater, but it has relatively low photocatalytic activity and difficulties in the recycling processes. In this work, a novel floating 3D porous Bi₇O₉I₃/N-doped graphene aerogel (Bi₇O₉I₃/NGA) composite was successfully synthesized through a facile hydrothermal route. The Bi₇O₉I₃/NGA composite exhibited highly enhanced photocatalytic performance toward degrading rhodamine B under visible-light irradiation, which increased 6.0 and 2.3 times compared with the Bi₇O₉I₃ and Bi₇O₉I₃/GA, respectively. The enhancement of photocatalytic degradation activity could be ascribed to the extensively promoted charge generation and migration efficiency, visible light utilization ability and reactive oxygen species production. Besides, the special 3D macroscopic block structure of Bi₇O₉I₃/NGA allowed it to float, making it easy to recycle. The photocatalytic degradation efficiency of Bi₇O₉I₃/NGA composite still could reach up to 92.7% after four consecutive cycles and presented satisfactory stability and reusability. Moreover, a possible photocatalytic degradation mechanism was revealed by radical species trapping and semi-quantitative analyses experiments.     

Hollow ZnSnO3 cubes@carbon/reduced graphene oxide ternary composite as anode of lithium ion batteries with enhanced electrochemical performance

The ternary composite, carbon coated hollow ZnSnO₃ (ZS@C) cubes encapsulated in reduced graphene oxide sheets (ZS@C/rGO), was synthesized via low-temperature coprecipitation and colloid electrostatic self-assembly. The uniform carbon-coating layer not only plays a role in buffering the volume change of ZnSnO₃ cubes in the charging/discharging processes, but also forms three-dimensional network with the cooperation of graphene to maintain the structural integrity and improve the electrical conductivity. The results show that the reduced graphene oxide sheets encapsulated ZS@C microcubes with a typical core-shell structure of ~700 nm in size exhibit an improved electrochemical performance compared with bare ZS@C microcubes. The ZS@C/rGO electrode delivered an initial discharge capacity of 1984 mA h g⁻¹ at a current density of 0.1 A g⁻¹ and maintained a capacity of 1040 mA h g⁻¹ after 45 cycles. High specific capacity and superior cycle stability indicate that the ZS@C/rGO composite has a great potential for the application of lithium-ion anode material.