Facile synthesis of MnO2/CNT nanocomposite and its electrochemical performance for supercapacitors

A nanocomposite of manganese dioxide coated on the carbon nanotubes (MnO₂/CNTs) was synthesized by a facile direct redox reaction between potassium permanganate and carbon nanotubes without any other oxidant or reductant addition. The morphology, microstructure and crystalline form of this MnO₂/CNT nanocomposite were characterized by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical properties are characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD). The results show that the facile prepared MnO₂/CNTs nanocomposite shows specific capacitance of 162.2 F g⁻¹ at the current density of 0.2 A g⁻¹ and excellent charge/discharge property with 90% of its specific capacitance kept after 2000 cycles at the current density of 5 A g⁻¹.     

Structure and electrochemical performance of Fe3O4/graphene nanocomposite as anode material for lithium-ion batteries

Using hydrothermal method, Fe₃O₄/graphene nanocomposite is prepared by synthesizing Fe₃O₄ particles in graphene. The synthesized Fe₃O₄ is nano-sized sphere particles (100–200 nm) and uniformly distributed on the planes of graphene. Fe₃O₄/graphene nanocomposite as anode material for lithium ion batteries shows high reversible specific capacity of 771 mAh g⁻¹ at 50th cycle and good rate capability. The excellent electrochemical performance of the nanocomposite can be attributed to the high surface area and good electronic conductivity of graphene. Due to the high surface area, graphene can prevent Fe₃O₄ nanoparticles from aggregating and provide enough space to buffer the volume change during the Li insertion/extraction processes in Fe₃O₄ nanoparticles.     

Preparation of Fe3O4/PVA nanofibers via combining in-situ composite with electrospinning

A novel approach, combining in-situ composite method with electrospinning, was used to prepare high magnetic Fe₃O₄/poly(vinyl alcohol) (PVA) composite nanofibers. Fe₃O₄ magnetic fluids were synthesized by chemical co-precipitation method in the presence of 6 wt.% PVA aqueous solution. PVA was used as stabilizer and polymeric matrix. The resulting Fe₃O₄/PVA composite nanofibers were characterized with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray diffractometer (XRD), respectively. These composite fibers showed a uniform and continuous morphology, with the Fe₃O₄ nanoparticles embedded in the fibers. Magnetization test confirmed that the composite fiber showed a high saturated magnetization (M_s = 2.42 emµ·g^-1) although only 4 wt.% content.     

A true nanocomposite: Single crystalline Au nanoparticles on single crystalline Fe3O4 nanoparticles

An Au/Fe₃O₄ nanocomposite catalyst was fabricated through a simple deposition-precipitation method. The Au/Fe₃O₄ nanocomposite is a true nanocomposite that has single crystalline Au nanoparticles supported on single crystalline Fe₃O₄ nanoparticles. Lattice fringes from both Au and Fe₃O₄ single nanoparticles were simultaneously observed by transmission electron microscope (TEM). This nanocomposite catalyst showed much high activity in low temperature CO oxidation reaction. The Au/Fe₃O₄ nanocomposite catalyst reaches 100% CO conversion at 40 °C. In comparison, Au/commercial Fe₃O₄ catalyst needs 375 °C to convert CO. This Au/Fe₃O₄ nanocomposite is an ideal sample to study synergetic effect between the catalyst and the support at nanoscale.     

Electrochemical investigation of TiO2/carbon nanotubes nanocomposite as anode materials for lithium-ion batteries

Mechanically blended composite of nanosized TiO₂ and carbon nanotubes (CNTs) was investigated as potential anode materials for Li-ion batteries. It was found that the TiO₂/CNTs nanocomposite exhibits an improved cycling stability and higher reversible capacity than CNTs. The reversible capacity of the TiO₂/CNTs composite reaches 168 mAh g^− 1 at the first cycle and remains almost constant during long-term cycling. The electrochemical results show that the TiO₂ nanoparticles in the composite not only restrain the formation of surface film, but also make a contribution to the overall reversible capacity.     

Synthesis and performance of LiMn0.7Fe0.3PO4 cathode material for lithium ion batteries

Pure and carbon-containing olivine LiMn_0.7Fe_0.3O₄ were synthesized at 600 °C by the method of solid-state reaction. Structure, surface morphology and charge/discharge performance of LiMn_0.7Fe_0.3O₄ were characterized by X-ray diffraction, scanning electron microscopy, and electrochemical measurement, respectively. The prepared materials with and without carbon both show the single olivine structure. The morphologies of primary particles are greatly affected by the addition of carbon. Large particles (500-1000 nm) and densely sintered blocks were observed in pure LiMn_0.7Fe_0.3PO₄, which made the insertion and extraction of lithium ions difficult. Battery made from this sample can not charge and discharge effectively. The carbon-containing LiMn_0.7Fe_0.3PO₄ has a small particle size (100-200 nm) and a regular appearance. This material demonstrates high reversible capacity of about 120 mAh g⁻¹, perfect cycling performance, and excellent rate capability. It is obvious that the addition of carbon plays an important role in restricting the particle size of the material, which helps to prepare LiMn_0.7Fe_0.3PO₄ with excellent electrochemical performance. The electrochemical reaction resistance is much lower in the partly discharged state than in the fully charged or fully discharged state by the measurement of ac impedance for carbon-containing LiMn_0.7Fe_0.3PO₄. It is indicated that the mixed-valence of Fe³⁺/Fe²⁺ or Mn³⁺/Mn²⁺ is beneficial to the transfer of electron which happens between the interface.     

Preparation, characterization and microwave absorption properties of polyaniline/Co0.5Zn0.5Fe2O4 nanocomposite

The polyaniline (PAni)/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was prepared by an in situ polymerization in an aqueous solution. The products were characterized by Fourier transform infrared (FT-IR) spectrometer, ultraviolet-visible (UV-vis) spectrometer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The average particle size of the PAni/Co_0.5Zn_0.5Fe₂O₄ was estimated to be about 70 nm by TEM. The reflection loss (dB) of the nanocomposite was measured at different microwave frequencies in X-band (8.2-12.4 GHz), U-band (12.4-18 GHz) and K-band (18-26.5 GHz) by radar cross-section (RCS) method according to the national standard GJB-2038-94. The results showed the reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was higher than that of the PAni. The maximum reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was about −39.9 dB at 22.4 GHz with a bandwidth of 5 GHz (full frequency width at about a half of the peak response). In conclusion, this sample is a good microwave shielding and absorbing materials at higher frequency.     

Chemical vapor deposition growth of Fe3O4 thin films and Fe/Fe3O4 bi-layers for their integration in magnetic tunnel junctions

A possible route for the synthesis of Fe₃O₄, Fe, and Fe/Fe₃O₄ bi-layers with chemical vapor deposition by employing the same Fe₃(CO)₁₂ carbonyl precursor is presented. The comprehensive structural, chemical, and morphological investigation of the as-deposited thin single films and bi-layers is performed by X-ray diffraction, X-ray reflectivity, Raman spectroscopy, and time-of-flight secondary ion mass spectrometry depth profiling. We present the possibility of performing the deposition of pure metallic Fe and Fe₃O₄/γ-Fe₂O₃ by adjusting the deposition pressure from 10^- 3/^- 4 Pa to 1 Pa, respectively. The integration of Fe₃O₄ thin films in a magnetic tunnel junction stack fully synthesized by in situ atomic layer and chemical vapor deposition processes is also presented, showing good stack stability and marginal interdiffusion.     

Synthesis of Bi2Fe4O9 via PVA sol-gel route

Bi₂Fe₄O₉ powders were synthesized by a sol-gel process using polyvinyl alcohol (PVA) as a complexing agent. Differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Field emission scanning electron microscope (FSEM) techniques were used to characterize precursor and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of Bi₂Fe₄O₉ was investigated. XRD analysis showed that single-phase Bi₂Fe₄O₉ was obtained from the Mⁿ⁺/-OH = 2:1 and Mⁿ⁺/-OH = 1:1 precursors at the temperature of 700 °C. For the precursor with Mⁿ⁺/-OH = 4:1, pure Bi₂Fe₄O₉ formed at the temperature of 800 °C. Bi₂Fe₄O₉ powders clacined at 700 °C from Mⁿ⁺/-OH = 2:1 precursor shows weak ferromagnetism.     

Fabrication of transparent conductive carbon nanotubes/polyurethane-urea composite films by solvent evaporation-induced self-assembly (EISA)

Transparent and conductive carbon nanotubes (CNTs)/polyurethane-urea (PUU) composite films were prepared by solvent evaporation-induced self-assembly (EISA). Pristine CNTs were treated with acids (H₂SO₄/HNO₃ = 3:1, v:v), acylated with thionyl chloride, and purified after filtration. These acylated CNTs (0.05 wt.% in dimethylformamide, DMF) were deposited onto the 3-aminopropyl triethoxysilane (APTES)-modified glass substrate by DMF EISA at 100 °C with the withdrawal rate of 3 cm/h. The CNT layers of 200–400 nm thicknesses were transferred to the PUU films by solution casting or resin transfer molding (RTM) at ambient temperature. Optical transmittances of the composite films were 60–75% at 550 nm wavelength and their sheet resistances were 5.2 × 10⁰–2.4 × 10³ kΩ/square, and which varied significantly with type of CNTs and the transferring methods of CNT layers.     

Flexible electrochemical biosensors based on O2 plasma functionalized MWCNT

A flexible glucose sensor is fabricated using O₂ plasma-functionalized multiwalled carbon nanotube (MWCNT) films on polydimethylsiloxane (PDMS) substrates and its performance is electrochemically characterized. After enzyme immobilization, the GOD/ MWCNT/Au/PDMS electrode exhibits a sensitivity of 18.15 μA mm^− 2mM^− 1 and a detection limit of 0.01 mM (signal to noise ratio was about 3). This high sensitivity may be attributed to a large enzyme loading and a higher electrocatalytic activity and electron transfer exhibited by O₂ plasma-functionalized CNTs than the pristine CNT, due to some oxygen-contained groups present on the O₂ plasma-functionalized CNT surface, which has been verified by XPS spectrum.     

One-step synthesis of Fe-phthalocyanine/Fe3O4 hybrid microspheres

Fe-phthalocyanine/Fe₃O₄ hybrid microspheres were synthesized from bis-phthalonitrile and FeCl₃·6H₂O through a simple and effective solvent-thermal route. The hybrids were monodispersed solid microspheres with diameter of ~ 400 nm. The ferromagnetic signature emerged with the saturated magnetization of ~ 55.7 emu g⁻¹, and the coercive force of ~ 93.7 Oe at 300 k. The addition of bis-phthalonitrile oligomer brought Fe₃O₄ nanoparticles novel dielectric property: a new dielectric loss peak appeared at ~ 8 GHz. Considering the microwave magnetic loss properties, two microwave magnetic loss peaks were presented at ~ 1.5 GHz and ~ 10 GHz, the former peak was attributed to the natural properties of the Fe₃O₄, and the latter originated from the interface effects between the bis-phthalonitrile oligomer and Fe₃O₄.     

Effects of Fe2O3 doping on the microstructure and piezoelectric properties of 0.55Pb(Ni1/3Nb2/3)O3-0.45Pb(Zr0.3Ti0.7)O3 ceramics

0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr_0.3Ti_0.7)O₃(PNN-PZT) ceramics with different concentration of xFe₂O₃ doping (where x = 0.0, 0.8, 1.2, 1.6 mol%) were synthesized by the conventional solid state sintering technique. X-ray diffraction analysis reveals that all specimens are a pure perovskite phase without pyrochlore phase. The density and grain size of Fe-doped ceramics tend to increase slightly with increasing concentration of Fe₂O₃. Comparing with the undoped ceramics, the piezoelectric, ferroelectric and dielectric properties of the Fe-doped PNN-PZT specimens are significantly improved. Properties of the piezoelectric constant as high as d₃₃ ~ 956 pC/N, the electromechanical coupling factor k_p ~ 0.74, and the dielectric constant ε_r ~ 6095 are achieved for the specimen with 1.2 mol% Fe₂O₃ doping sintered at 1200 °C for 2 h.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Synthesis and electrochemical performances of spinel LiCr0.1Ni0.4Mn1.5O4 compound

The spinel compound LiCr_0.1Ni_0.4Mn_1.5O₄ was synthesized by a solid reaction method and a sol-gel method using citric acid as chelating agent. The pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ was obtained by the wet method. The electrochemical performances of the pure phase sample were measured at different current rates. There were three voltage plateaus at about 4.9, 4.7 and 4.0 V in the charge-discharge curves, which were attributed to the oxidation/reduction of chromium, nickel and manganese respectively. In the range of 3.5-5.0 V, its first discharge capacity was 143, 118 and 111 mAh/g corresponding to current densities of 1.0, 4.0 and 5.0 mA/cm², respectively. After 50 cycles, the capacity retention remained well at the current densities of 1.0, 4.0 and 5.0 mA/cm². The electrochemical performances of pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ at 55 °C was also measured, and the results were discussed.     

Organic/inorganic nanocomposite films based on poly(3-methoxythiophene) and WO3

Organic/inorganic nanocomposite films based on poly(3-methoxythiophene) (PMOT) and WO₃ were prepared by a consecutive two-step electrochemical method. The products were characterized in detail by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDS) and Fourier-transform infrared spectroscopies (FTIR). The results show that the PMOT/WO₃ nanocomposite films consist of two layers, the substrate WO₃ with 30 nm grains and superstratum PMOT, which average grain size is 60 nm. The obtained PMOT/WO₃ nanocomposite films were also characterized by cyclic voltammetry to investigate their electrochemistry properties which display significant enhancement of electrochemical activity than that of pure PMOT and WO₃ films.     

Effect of Fe on the synthesis and electrochemical performance of nanostructured MnO2

Different MnO₂ nanostructures were synthesized in stoichiometric KMnO_4/MnSO₄ aqueous solutions in the absence/presence of Fe³⁺ at temperature ranging from 30 °C to 180 °C. The phase structures, morphologies and electrochemical properties of the as-prepared MnO₂ products were investigated using X-ray powder diffraction, scanning electron microscope, N₂ physical adsorption and cyclic voltammetry techniques. The results showed that the presence of Fe³⁺ addition had a significant effect on the phase structural evolution, morphological features and electrochemical properties of the MnO₂ products. Fe³⁺ was found to greatly prevent the epitaxial growth and crystallization of MnO₂ nucleus, which in turn influenced textual characteristics. The electrochemical performance of the nanostructured MnO₂ products had a complex relationship with the phase structures, specific surface area as well as pore characteristics. MnO₂ prepared in the presence of Fe³⁺ (KMF-MnO₂) showed relatively higher specific capacitance compared to that of MnO₂ prepared in the absence of Fe³⁺ (KM-MnO₂). Maximum capacitance of 214 F g⁻¹ was obtained for KMF-MnO₂ prepared at 30 °C at a scan rate of 2 mV s⁻¹ in 0.1 M Na₂SO₄ electrolyte.     

TG/DSC analysis of Fe8(OOH)16Cl1.3 nanospindles

The thermal analysis of Fe₈(OOH)₁₆Cl_1.3 (Akaganeite-M) nanospindles prepared by the hydrolysis of FeCl₃ solutions are determined by thermogravimetric analyses and differential scanning calorimetry (TG/DSC), in conjunction with field-emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD). Different products are formed after Fe₈(OOH)₁₆Cl_1.3 nanospindles are calcined at different temperatures for 30 min in N₂ atmosphere: Fe_1.833(OH)_0.5O_2.5 and magnetite obtained at 250 °C; pure magnetite (Fe₃O₄) obtained at 630 °C; and magnetite containing some iron nitrides (Fe₂N and Fe₄N) obtained at 800 °C. The calcination of Fe₈(OOH)₁₆Cl_1.3 provides a new method to prepare pure magnetite.     

A new kind of mesoporous Fe7Co3/carbon nanocomposite and its application as magnetically separable adsorber

A new kind of magnetic mesoporous Fe₇Co₃/carbon nanocomposite has been successfully synthesized for the first time via a simple cocasting method. By introducing Fe₇Co₃ alloy nanoparticles into mesoporous carbon, the magnetic property of the nanocomposite is greatly improved compared to using a single metal Fe or Co nanoparticles, which makes the porous carbon easily separated from solution by an external magnetic field. The nanocomposite is successfully applied to magnetically separable adsorber and exhibits excellent performance of adsorption for bulk dyes due to its high surface area (up to 1429 m²/g) and pore volume (up to 1.93 cm³/g).     

Low temperature synthesis of Fe3O4 nanoparticles and its application in lithium ion batteries

Well dispersed Fe₃O₄ nanoparticles with mean size about 160 nm are synthesized by a simple chemical method at atmosphere pressure. The products are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and Raman spectrum. Electrochemical properties of the as-synthesized Fe₃O₄ nanoparticles as anode electrodes of lithium ion batteries are studied by conventional charge/discharge tests, showing initial discharge and charge capacities of 1140 mAh g⁻¹ and 1038 mAh g⁻¹ at a current density of 0.1 mA cm⁻². The charge and discharge capacities of Fe₃O₄ electrode decrease along with the increase of cycle number, arriving at minimum values near the 70th cycle. After that, the discharge and charge capacities of Fe₃O₄ electrode begin to increase along with the increase of cycle number, arriving at 791 and 799 mAh g⁻¹ after 393 cycles. The morphology and size of the electrode after charge and discharge tests are characterized by SEM, which exhibits a large number of dispersive particles with mean size about 150 nm.