Fabrication of magnetic nanosized γ-FeNi-coated ceramic core–shell microspheres by heterogeneous precipitation and thermal reduction

Precursors with NiCO₃·2Ni(OH)₂·2H₂O- and Fe₂O₃·nH₂O-coated alumina, graphite and cenosphere were synthesized by precipitation using ferrous sulfate, nickel sulfate, ammonium bicarbonate, alumina, graphite and cenosphere as the main starting materials. Magnetic γ-FeNi-coated alumina, graphite and cenosphere core–shell structural microspheres were subsequently prepared by thermal reduction of the as-prepared precursors at 600 °C for 2 h. Precipitation parameters, e.g. concentration of ceramic micropowders (10 g/L), sulfate solution (0.2 mol/L), rate of adding reactants (3 mL/min) and pH value were optimized by a trial-and-error method. Powders of the precursors and the resulting coating of γ-FeNi with grain size below 40 nm on alumina, graphite and cenosphere microspheres were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The magnetic properties of the nanosize γ-FeNi-coated alumina, graphite and cenosphere microspheres were measured by vibrating sample magnetometer (VSM). The results show that the core–shell structural γ-FeNi-coated ceramic microspheres exhibited higher coercivity than pure γ-FeNi powders, indicating that these materials can be used for high-performance functional materials and devices.     

Preparation of nanosized hollow silica spheres from Na2SiO3 using Fe3O4 nanoparticles as templates

Nanosized hollow silica spheres with average diameters from 43 to 70 nm were prepared by removal of Fe₃O₄ templates with hydrochloric acid from silica-coated Fe₃O₄ core–shell composites. The shells of the hollow silica spheres had nanopores with average diameters of 0.92–1.25 nm. When the silica-coated Fe₃O₄ core–shell composites were prepared at a high pH value or with a low mole ratio of Na₂SiO₃ to Fe₃O₄, the resulting hollow silica spheres consisted of highly porous shells. When the silica-coated Fe₃O₄ core–shell composites were prepared with a high mole ratio of Na₂SiO₃ to Fe₃O₄, the resulting hollow silica spheres had large diameters and thick shells. The release rate of herbicide, ammonium glyphosate, could be tuned by using hollow silica spheres with different shell thicknesses.     

Solid-state synthesis of Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode materials for lithium-ion batteries

Layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials were synthesized via a solid-state reaction for Li-ion batteries, in which lithium hydroxide monohydrate, manganese dioxide, nickel monoxide, and cobalt monoxide were employed as metal precursors. To uncover the relationship between the structure and electrochemical properties of the materials, synthesis conditions such as calcination temperature and time as well as quenching methods were investigated. For the synthesized Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ materials, the metal components were found to be in the form of Mn⁴⁺, Ni²⁺, and Co³⁺, and their molar ratio was in good agreement with stoichiometric ratio of 0.56:0.16:0.08. Among them, the one synthesized at 800 °C for 12 h and subsequently quenched in air showed the best electrochemical performances, which had an initial discharge specific capacity and coulombic efficiency of 265.6 mAh/g and 84.0%, respectively, and when cycled at 0.5, 1, and 2 C, the corresponding discharge specific capacities were 237.3, 212.6, and 178.6 mAh/g, respectively. After recovered to 0.1 C rate, the discharge specific capacity became 259.5 mAh/g and the capacity loss was only 2.3% of the initial value at 0.1 C. This work suggests that the solid-state synthesis route is easy for preparing high performance Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials for Li-ion batteries.     

Synthesis of mesoporous γ-AlOOH@Fe3O4 magnetic nanomicrospheres

Mesoporous γ-AlOOH@Fe₃O₄ magnetic nanomicrospheres were synthesized using superparamagnetic Fe₃O₄ nanoparticles as the core and aluminum isopropoxide (AIP) as the aluminum source. The obtained magnetic nanomicrospheres were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N₂ adsorption–desorption and vibrating sample magnetometry (VSM). The effects of preparation parameters such as hydrolysis time of AIP, concentration of AIP and coating layer number on microspheres were investigated. The results indicated that the mesoporous γ-AlOOH@Fe₃O₄ magnetic nanomicrospheres consisted of a mesoporous γ-AlOOH shell and a Fe₃O₄ magnetic core. The diameter of γ-AlOOH@Fe₃O₄ nanomicrospheres was about 200 nm, the thickness of mesoporous γ-AlOOH shell was about 5 nm and the average pore size was 3.8 nm. The thickness of the mesoporous γ-AlOOH shell could be controlled via layer-by-layer coating times. The formation mechanism of the mesoporous γ-AlOOH shell involved a “chemisorption–hydrolysis” process.     

2D reduced graphene oxide–titania nanocomposites synthesized under different hydrothermal conditions for treatment of hazardous organic pollutants

Two-dimensional reduced graphene oxide–titania (RGO–TiO₂) composites were prepared using a single-step hydrothermal method under various hydrothermal reaction conditions. The morphological and surface characteristics of the RGO–TiO₂ composites and reference materials were determined. The RGO–TiO₂ composites showed photocatalytic activity for the decomposition of two target pollutants that was superior to both pure TiO₂ and RGO under fluorescent daylight lamp illumination. The photocatalytic activity of the RGO–TiO₂ composite increased as the hydrothermal treatment time increased from 1 to 24 h, but then it decreased as the time increased to 36 h, which indicated the presence of an optimal treatment time. RGO–TiO₂ composites activated by violet light-emitting diodes (LEDs) displayed lower decomposition efficiency than those activated by a daylight lamp, likely because of the lower light intensity of violet LEDs (0.2 mW/cm²) when compared with that of the daylight lamp (1.4 mW/cm²). However, the photocatalytic decomposition of the target pollutants using the RGO–TiO₂ composite was more energy-efficient using the violet LEDs. The photocatalytic reaction rates increased as the residence time decreased, whereas the reverse was true for the decomposition efficiency.     

Synthesis of layered cathode materials Li[CoxNiyMn1−x−y]O2 from layered double hydroxide precursors

Cathode materials Li[Co_xNi_yMn_1?x?y]O₂ for lithium secondary batteries have been prepared by a new route using layered double hydroxides (LDHs) as a precursor. The resulting layered phase with the α-NaFeO₂ structure crystallizes in the rhombohedral system, with space group R-3m having an interlayer spacing close to 0.47 nm. X-ray photoelectron spectroscopy (XPS) was used to measure the oxidation states of Co, Ni and Mn. The effects of varying the Co/Ni/Mn ratio on both the structure and electrochemical properties of Li[Co_xNi_yMn_1?x?y]O₂ have been investigated by X-ray diffraction and electrochemical tests. The products demonstrated a rather stable cycling behavior, with a reversible capacity of 118 mAh/g for the layered material with Co/Ni/Mn = 1/1/1.     

Preparation and electrochemical performance of carbon-coated LiFePO4/LiMnPO4-positive material for a Li-ion battery

Lithium iron phosphate (LiFePO₄)/lithium manganese phosphate (LiMnPO₄)-positive material was successfully prepared through ball milling and high-temperature sintering using manganese acetate, lithium hydroxide, ammonium dihydrogen phosphate, and ferrous oxalate as raw materials. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, a constant current charge–discharge test, cyclic voltammetry, and electrochemical impedance spectroscopy. The effects of lithium iron phosphate coating were also discussed. Because of its special core–shell structure, the as-prepared LiMn_0.7Fe_0.3PO₄–LiFePO₄–C exhibits excellent electrochemical performance. The discharge capacity reached 136.6 mAh/g and the specific discharge energy reached 506.9 Wh/kg at a rate of 0.1 C.     

Metallic iron nanoparticles: Flame synthesis,characterization and magnetic properties

Metallic iron (Fe) nanoparticles (NPs) with a typical core–shell structure have been prepared by a simple and continuous flame spray pyrolysis (FSP) method, which are stabilized by the corresponding Fe₃O₄ shell with a thickness of 4–6 nm. The size of metallic Fe cores is about 30–80 nm. The core–shell structured iron NPs show an air stability as long as one month as a result of the protection of oxide shell. Through the control of the residence time of materials in flame and flame atmosphere, metallic Fe and iron oxides are obtained, showing a better external magnetic field responsibility. It is concluded that the evolution of morphology and composition of flame-made magnetic NPs could be attributed to the competition mechanism between reduction and oxidation reactions of in situ flame combustion, which offers more choices and better effective design strategy for the synthesis of advanced functional materials via FSP techniques.     

Controllable hydrothermal synthesis of star-shaped Sr3Fe2(OH)12 assemblies and their thermal decomposition and magnetic properties

Pure phase star-shaped hydrogarnet Sr₃Fe₂(OH)₁₂ assemblies were synthesized by a mild hydrothermal method (210 °C, 12 h), and the effects of the preparation conditions on the phase composition of the product were investigated. It was found that the impurity phases could be decreased or eliminated by increasing the molar ratio of Sr²⁺ to Fe³⁺, and that high temperatures favored the formation of Sr₃Fe₂(OH)₁₂ and reduced the concentration of CO₃^2–-containing byproducts. The thermal decomposition of the star-shaped Sr₃Fe₂(OH)₁₂ assemblies was examined, and the results showed that the dehydration process at higher temperatures is accompanied by the formation of SrFeO_3–δ. Above 655 °C, a solid state reaction between the SrFeO_3–δ and Sr(OH)₂ or SrCO₃ results in the formation of Sr₄Fe₃O_10–δ.The magnetic properties of the as-synthesized Sr₃Fe₂(OH)₁₂ and of samples calcined at different temperatures were assessed. A sample calcined at 575 °C exhibited greatly enhanced ferromagnetic properties, with a remanent magnetization of 1.28 emu/g and a coercivity of 4522.1 Oe at room temperature.     

Emission of organic carbon,elemental carbon and water-soluble ions from crop straw burning under flaming and smoldering conditions

Emissions from major agricultural residues were measured using a self-designed combustion system. Emission factors (EFs) of organic carbon (OC), elemental carbon (EC), and water-soluble ions (WSIs) (K⁺, NH₄⁺, Na⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃⁻, SO₄^2–) in smoke from wheat and rice straw were measured under flaming and smoldering conditions. The OC₁/TC (total carbon) was highest (45.8% flaming, 57.7% smoldering) among carbon fractions. The mean EFs for OC (EF_OC) and EC (EF_EC) were 9.2 ± 3.9 and 2.2 ± 0.7 g/kg for wheat straw and 6.4 ± 1.9 and 1.1 ± 0.3 g/kg for rice straw under flaming conditions, while they were 40.8 ± 5.6 and 5.8 ± 1.0 g/kg and 37.6 ± 6.3 and 5.0 ± 1.4 g/kg under smoldering conditions, respectively. Higher EC ratios were observed in particulate matter (PM) mass under flaming conditions. The OC and EC for the two combustion patterns were significantly correlated (p < 0.01, R = 0.95 for wheat straw; p < 0.01, R = 0.97 for rice straw), and a higher positive correlation between OC₃ and EC was observed under both combustion conditions. WSIs emitted from flaming smoke were dominated by Cl⁻ and K⁺, which contributed 3.4% and 2.4% of the PM mass for rice straw and 2.2% and 1.0% for wheat straw, respectively. The EFs of Cl⁻ and K⁺ were 0.73 ± 0.16 and 0.51 ± 0.14 g/kg for wheat straw and 0.25 ± 0.15 and 0.12 ± 0.05 g/kg for rice straw under flaming conditions, while they were 0.42 ± 0.28 and 0.12 ± 0.06 g/kg and 0.30 ± 0.27 and 0.05 ± 0.03 g/kg under smoldering conditions, respectively. Na⁺, Mg²⁺, and NH₄⁺ were vital components in PM, comprising from 0.8% (smoldering) to 3.1% (flaming) of the mass. Strong correlations of Cl⁻ with K⁺, NH₄⁺, and Na⁺ ions were observed in rice straw and the calculated diagnostic ratios of OC/EC, K⁺/Na⁺ and Cl⁻/Na⁺ could be useful to distinguishing crop straw burning from other sources of atmospheric pollution.     

Effect of gravity on the mechanical properties of lunar regolith tested using a low gravity simulation device

Simulations of the bearing capacity and shear strength of regolith under Earth’s gravity produce different results from those under low gravity. A low-gravity simulation device was developed in this study, and an internal stress model of regolith simulant was established to correct the errors. The model revealed additional force on both shear plane in the shear test and the press plate area in the pressure–sinkage test. The sinkage and shear test results showed that low gravity decreased the deformable index n, frictional modulus k_φ and cohesion c, whereas there were no obvious changes to the cohesive modulus k_c and internal friction angle φ. The sinkage generally increased as the gravity decreased under a consistent normal load larger than 50 N, but when the wheel load was lower than 50 N, the sinkage of the TYII-1 simulant was larger under 1 G than 1/6 G. Gravity had little effect on the shear strength of the regolith. However, the tractive thrust of the TYII-1 simulant was lower under 1/6 G than 1 G. The smaller difference was due to differences in the way the soils responded to changes in the gravity level for the TYII-2 simulant.     

Microwave-hydrothermal preparation of a graphene/hierarchy structure MnO2 composite for a supercapacitor

Graphene/hierarchy structure manganese dioxide (GN/MnO₂) composites were synthesized using a simple microwave-hydrothermal method. The properties of the prepared composites were analyzed using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) measurements. The electrochemical performances of the composites were analyzed using cyclic voltammetry, electrochemical impedance spectrometry (EIS), and chronopotentiometry. The results showed that GN/MnO₂ (10 wt% graphene) displayed a specific capacitance of 244 F/g at a current density of 100 mA/g. An excellent cyclic stability was obtained with a capacity retention of approximately 94.3% after 500 cycles in a 1 mol/L Li₂SO₄ solution. The improved electrochemical performance is attributed to the hierarchy structure of the manganese dioxide, which can enlarge the interface between the active materials and the electrolyte. The preparation route provides a new approach for hierarchy structure graphene composites; this work could be readily extended to the preparation of other graphene-based composites with different structures for use in energy storage devices.     

Spatiotemporal variations of PM2.5 and PM10 concentrations between 31 Chinese cities and their relationships with SO2, NO2, CO and O3

The variations of mass concentrations of PM_2.5, PM₁₀, SO₂, NO₂, CO, and O₃ in 31 Chinese provincial capital cities were analyzed based on data from 286 monitoring sites obtained between March 22, 2013 and March 31, 2014. By comparing the pollutant concentrations over this length of time, the characteristics of the monthly variations of mass concentrations of air pollutants were determined. We used the Pearson correlation coefficient to establish the relationship between PM_2.5, PM₁₀, and the gas pollutants. The results revealed significant differences in the concentration levels of air pollutants and in the variations between the different cities. The Pearson correlation coefficients between PMs and NO₂ and SO₂ were either high or moderate (PM_2.5 with NO₂: r = 0.256–0.688, mean r = 0.498; PM₁₀ with NO₂: r = 0.169–0.713, mean r = 0.493; PM_2.5 with SO₂: r = 0.232–0.693, mean r = 0.449; PM₁₀ with SO₂: r = 0.131–0.669, mean r = 0.403). The correlation between PMs and CO was diverse (PM_2.5: r = 0.156–0.721, mean r = 0.437; PM₁₀: r = 0.06–0.67, mean r = 0.380). The correlation between PMs and O₃ was either weak or uncorrelated (PM_2.5: r = −0.35 to 0.089, mean r = −0.164; PM₁₀: r = −0.279 to 0.078, mean r = −0.127), except in Haikou (PM_2.5: r = 0.500; PM₁₀: r = 0.509).     

Real-time measurements of PM2.5, PM10–2.5, and BC in an urban street canyon

A continuous dichotomous beta gauge monitor was used to characterize the hourly content of PM_2.5, PM_10–2.5, and Black Carbon (BC) over a 12-month period in an urban street canyon of Hong Kong. Hourly vehicle counts for nine vehicle classes and meteorological data were also recorded. The average weekly cycles of PM_2.5, PM_10–2.5, and BC suggested that all species are related to traffic, with high concentrations on workdays and low concentrations over the weekends. PM_2.5 exhibited two comparable concentrations at 10:00–11:00 (63.4 μg/m³) and 17:00–18:00 (65.0 μg/m³) local time (LT) during workdays, corresponding to the hours when the numbers of diesel-fueled and gasoline-fueled vehicles were at their maximum levels: 3179 and 2907 h⁻¹, respectively. BC is emitted mainly by diesel-fueled vehicles and this showed the highest concentration (31.2 μg/m³) during the midday period (10:00–11:00 LT) on workdays. A poor correlation was found between PM_2.5 concentration and wind speed (R = 0.51, P-value > 0.001). In contrast, the concentration of PM_10–2.5 was found to depend upon wind speed and it increased with obvious statistical significance as wind speed increased (R = 0.98, P-value < 0.0001).     

Capture of phosphates in surface water by TiO2 nanoparticles under UV irradiation

The capture of orthophosphates and total phosphorus from the Pudong Canal river in the Pudong District of Shanghai by TiO₂ nanoparticles is studied using a rotating photoreactor and the nano-TiO₂ photocatalyst Degussa P25. The effects of UV irradiation intensity in a range of 20–74 mW/cm², the loading of the TiO₂ nanoparticles in a range of 0.05–0.1 g/L, irradiation time up to 4 h, and pH values in a range of 2–10.5 on the capture efficiency are investigated. The results show that the capture of orthophosphates and total P are significantly enhanced by UV irradiation; at a loading of 0.1 g/L and an irradiation intensity above 36 mW/cm², orthophosphates and total phosphorus are rapidly captured by TiO₂ nanoparticles, causing an observed reduction from 0.4 mg/L down to 0.02 mg/L. pH values in a range of 2–10.5 have little effect on the capture efficiency of orthophosphates and total phosphorus.     

Large-scale synthesis of hierarchical MnO2 for benzene catalytic oxidation

Hierarchical sea-urchin-shaped manganese oxide microspheres were synthesized via a facile method based on the reaction between KMnO₄ and MnSO₄ in HNO₃ solution at 50 °C. The average diameter of the microspheres is ∼850 nm. The microspheres consist of a core of diameter of ∼800 nm and nanorods of width ∼50 nm. The nanorods exist at the edge of the core. The Brunauer–Emmett–Teller surface area of the sea-urchin-shaped microspheres is 259.4 m²/g. A possible formation mechanism of the hierarchical sea-urchin-shaped microspheres is proposed. The temperature for 90% conversion of benzene (T_90%) on the hierarchical urchin-shaped MnO₂ microspheres is about 218 °C.     

SiO2-coated pure anatase TiO2 catalysts for enhanced photo-oxidation of naphthalene and anthracene

Our current efforts reveal the preparation of SiO₂@TiO₂ nanocomposites having different thicknesses of silica shell and the relationship to photocatalytic activity (PCA) for the photo-oxidation of naphthalene and anthracene. The presence of SiO₂ coating over TiO₂ surface was demonstrated by FT-IR analysis, with peaks corresponding to SiOSi (1081 cm⁻¹) and SiOTi (950 cm⁻¹) bonds observed. High-resolution transmission electron microscopy analysis confirmed the presence of SiO₂ in the as-prepared nanocomposites and the amount of Si, Ti, and O was determined by energy dispersive X-ray spectroscopy analysis. Increasing the SiO₂ shell thickness increases the surface area of the nanocomposites (69–235 m²/g), which enhances naphthalene/anthracene adsorption. However, the observed PCA trend presents an inverse correlation to the adsorption studies, where the as-prepared samples possessing the highest surface areas exhibited the least PCA, while catalysts having lower surface areas (among silica coated samples) displayed the highest PCA in the degradation of naphthalene and anthracene to CO₂. Despite complete degradation of naphthalene and anthracene, incomplete mineralization occurred, ascribed to the formation of various intermediates, identified by GC–MS analysis.     

Tribological and tribochemical properties of magnetite nanoflakes as additives in oil lubricants

This detailed the tribological and tribochemical properties of magnetite (Fe₃O₄) nanoflakes used as additives in ^#40 base oil in a four-ball tribo-tester. The average friction coefficient of the friction pair for lubricant containing the Fe₃O₄ nanoflakes of 1.5 wt% as a lubricant additive in the base oil is decreased by 18.06% compared to that of solely base oil. The chemical composition of base oil with the Fe₃O₄ nanoflake additives did not change during the 48-h friction assessment. The decreased saturation magnetization and increased coercivity of magnetite nanoflakes occurred due to the distortion of the basal planes and the presence of hematite (α-Fe₂O₃) generated by the tribochemical reactions during the friction process. The multi-layer low-shear-stress tribochemical lubrication films on the surface of the friction pair could form because the nanoflake particles arrange and adhere onto the surface of the friction pair in an orderly manner, and the tribochemical reactions of the friction pair in the presence of the nanoflakes occur as Fe → FeO → Fe₃O₄ → γ-FeOOH → γ-Fe₂O₃ → α-Fe₂O₃. The formation of the films can improve the tribological properties.     

Mesoporous composite of LiFePO4 and carbon microspheres as positive-electrode materials for lithium-ion batteries

Mesoporous LiFePO₄/C microspheres consisting of LiFePO₄ nanoparticles are successfully fabricated by an eco-friendly hydrothermal approach combined with high-temperature calcinations using cost-effective LiOH and Fe³⁺ salts as raw materials. In this strategy, pure mesoporous LiFePO₄ microspheres, which are composed of LiFePO₄ nanoparticles, were uniformly coated with carbon (∼1.5 nm). Benefiting from this unique architecture, these mesoporous LiFePO₄/C microspheres can be closely packed, having high tap density. The initial discharge capacity of LiFePO₄/C microspheres as positive-electrode materials for lithium-ion batteries could reach 165.3 mAh/g at 0.1 C rate, which is notably close to the theoretical capacity of LiFePO₄ due to the large BET surface area, which provides for a large electrochemically available surface for the active material and electrolyte. The material also exhibits high rate capability (∼100 mAh/g at 8 C) and good cycling stability (capacity retention of 92.2% after 400 cycles at 8 C rate).     

Advanced microemulsion synthesis and characterization of wollastonite (CaSiO3)/polystyrene one-dimensional nanorods with core–shell structures

In this work, one-dimensional core–shell nanorods (CSNRs; 185–250 nm wide and 1–1.5 μm long) consisting of triethoxyvinylsilane-modified wollastonite (CaSiO₃) nanorods (MWNRs) as a core and polystyrene as a shell with uniform size were successfully synthesized using an advanced microemulsion technique. The effect of varying the surfactant cetyltrimethylammonium bromide (producing CSNRs_CTAB) or sodium dodecyl sulphate (producing CSNRs_SDS) upon the size and morphology of the CSNRs was investigated by field-emission scanning electron microscopy (FE-SEM). X-ray diffractometry and Fourier transform infrared spectrophotometer revealed the existence of a strong interaction between the MWNRs and polystyrene, which implies that the polymer chains were successfully grafted onto the surface of the MWNRs. The CSNRs were blended with polypropylene by melt processing, and the effect of the CSNRs upon the morphological properties of the polypropylene matrix was investigated by FE-SEM and atomic force microscopy. It was observed that the polystyrene chains that grafted onto the CaSiO₃ nanorods interfered with the aggregation of CaSiO₃ nanorods in the polypropylene matrix and thus improved the compatibility of the CaSiO₃ nanorods with the polypropylene matrix. Furthermore, the compatibility of CaSiO₃ nanorods with polypropylene of CSNR_SDS/polypropylene was superior to that of CSNR_CTAB/polypropylene.