A core–shell structured Si–Al@Al2O3 as novel support of Pd catalyst

Core–shell structured support is an effective way in eliminating mass transfer limitation for multiphase hydrogenation reactions. In this study, a downy Al₂O₃ layer with a specific surface area of 213.7 m²/g was generated in situ on Si–Al alloy via facile hot water etching technique. The downy surface is conducive in achieving a satisfactory dispersion of Pd particles with an average size of 5.5 nm. The core–shell structure and the moderate dispersion of Pd on Si–Al@Al₂O₃ support lead to a higher activity and selectivity in hydrogenation of styrene and consecutive hydrogenation of phenylacetylene than that on commercial Al₂O₃.     

Enhanced electrochemical properties of LiFePO4–silicon composites as positive electrode materials fabricated using a solid-state method

LiFePO₄–silicon composites were fabricated by using a solid-state method for applying positive electrodes in lithium ion batteries. The LiFePO₄–silicon composites were characterized with X-ray diffraction and field emission scanning electron microscopy. Their electrochemical properties were investigated with cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests. The added silicon not only suppressed the surface corrosion caused by the decreasing H⁺ concentration in the electrolyte, but it also acted as a barrier between the LiFePO₄ particles and LiPF₆ electrolyte, thereby preventing the dissolution of Fe²⁺ in the electrode and enhancing the electrolyte/active material interactions. This resulted in improved lithium-ion transfer kinetics and excellent positive electrode performance, especially at high current densities and different operating temperatures (0, 25, and 50 °C). At 25 °C, the LiFePO₄ composite containing 2 wt% of silicon delivered the best electrochemical performance with a lithium-ion diffusion coefficient of 1.81×10⁻⁹ cm² s⁻¹, a specific discharge capacity of 143 mA h g⁻¹ for the initial cycle, and a capacity retention of 98% after 100 cycles. In contrast, the corresponding values for the pure LiFePO₄ were 1.19×10⁻¹¹ cm² s⁻¹, 115 mA h g⁻¹, and a capacity retention of 76% after 100 cycles, respectively.     

Toughening polystyrene by core–shell grafting copolymer polybutadiene-graft-polystyrene with potassium persulfate as initiator

Core–shell polybutadiene-graft-polystyrene rubber particles with different ratios of polybutadiene core to polystyrene shell were synthesized by an emulsion polymerization using K₂S₂O₈ as an initiator. Then the core–shell rubber particles were blended with PS to prepare PS/PB-g-PS. The rubber particles with a size of 0.3–0.5 μm could toughen polystyrene significantly. The mechanical properties, morphologies and deformation mechanisms of samples were extensively investigated. The experimental results showed that the dispersion of rubber particles in a “cluster” state leads to better impact resistances. Crazing occurred from rubber particles and extended in a bridge-like manner to neighboring rubber particles parallel to the equatorial direction.     

A novel SiCN@TiO2 core–shell ceramic microspheres derived from a polymeric precursor

A novel core–shell ceramic microspheres, composed of a SiCN inner core and TiO₂ nanoparticles outer shell, were prepared via emulsion technique and polymer-derived ceramics (PDCs) method. The forming process of SiCN@TiO₂ core–shell ceramic microspheres were controlled by adjusting the ratio of raw material, curing temperature and pyrolysis temperature. The morphology, chemical composition and phase transformation were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). PVSZ@TiO₂ microspheres with good spherical structure and uniform-dispersed TiO₂ surface were fabricated at 200 °C with raw material ratio of 25%. After pyrolyzed at 1400 °C, the obtained SiCN@TiO₂ core–shell ceramic microspheres retained spherical structure. The XRD showed that the products were mainly composed of rutile TiO₂, SiC and Si₃N₄ crystalline phase, which were generated by polyvinylsilazane.     

Continuous synthesis of lithium iron phosphate (LiFePO4) nanoparticles in supercritical water: Effect of mixing tee

Continuous supercritical hydrothermal synthesis of olivine (LiFePO₄) nanoparticles was carried out using mixing tees of three different geometries; a 90° tee (a conventional Swagelok^® T-union), a 50° tee, and a swirling tee. The effects of mixing tee geometry and flow rates on the properties of the synthesized LiFePO₄, including particle size, surface area, crystalline structure, morphology, and electrochemical performance, were examined. It was found that, when the flow rate increased, the particle size decreased; however, the discharge capacity of the particles synthesized at the high flow rate was lower due to the enhanced formation of Fe³⁺ impurities. The use of a swirling tee led to smaller-sized LiFePO₄ particles with fewer impurities. As a result, a higher discharge capacity was observed with particles synthesized with a swirling tee when compared with discharge capacities of those synthesized using the 90° and 50° tees. After carbon coating, the order of initial discharge capacity of LiFePO₄ at a current density of 17 mA/g (0.1C) and at 25 °C was swirling tee (149 mAh/g) > 50° tee (141 mAh/g) > 90° tee (135 mAh/g). The carbon-coated LiFePO₄ synthesized using the swirling tee delivered 85 mAh/g at 20C-rate and at 55 °C.     

Core–shell BaMoO4@SiO2 nanospheres: Preparation,characterization, and optical properties

Core–shell BaMoO₄@SiO₂ nanospheres were prepared in reverse microemulsions and exhibited enhanced photoluminescence (PL) intensity as compared to that of the uncoated BaMoO₄. Characterization was performed using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX), and X-ray powder diffraction (XRD). It was found that the silica shell could increase the PL intensity, but the shell is not the thicker the better. The PL emission can be decomposed into three individual Gaussian components: two UV emissions at 308 nm and 369 nm and a visible emission at 448 nm. Such short emission wavelengths can be attributed to quantum size effect of the small BaMoO₄ cores (～16 nm).     

Comparison of the effects of FePO4 and FePO4·2H2O as precursors on the electrochemical performances of LiFePO4/C

Carbon-coated LiFePO₄/C composite as cathode materials is synthesized by solid-state method using anhydrous FePO₄ and hydrous FePO₄·2H₂O as precursors.The effects of sintering temperature and carbon content on the properties of LiFePO₄/C composite are compared by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and charging–discharging test. The crystallinity, morphology, and particle size distribution of these two precursors are compared to investigate their effect on the electrochemical performances of LiFePO₄/C composite. Compared with hydrous FePO₄·2H₂O, anhydrous FePO₄ has good crystallinity, uniform particle morphology and symmetrical size distribution, contributing to LiFePO₄/C composite have excellent electrochemical performances. Due to the dehydration of hydrous FePO₄·2H₂O during synthesis, uneven distribution of carbon content and carbon layer is coated on LiFePO₄ surface, deteriorating the electrochemical performance of LiFePO₄/C composite. When anhydrous FePO₄ was used as the precursor, the LiFePO₄/C composite sintered at 700 °C with carbon content of 0.4 by molar ratio show high discharge capacity and stable cycling performance, with discharge capacity of 106.3 mA h g⁻¹ at 10 C, and a capacity retention rate of 99.2% after 200 cycles at 1 C.     

Core/shell-structured nickel cobaltite/onion-like carbon nanocapsules as improved anode material for lithium-ion batteries

Core/shell-structured nanocapsules consisting of a nickel cobaltite (NiCo₂O₄) nanoparticle core encapsulated in an onion-like carbon (C) shell are synthesized by arc-discharge and air-annealing methods. Void spaces between NiCo₂O₄ core and the carbon shell are observed in the NiCo₂O₄/C nanocapsules. Lithium-ion batteries fabricated using the nanocapsules as the anode material exhibit enhanced initial coulombic efficiency of 82.3% and specific capacity of 1197.2 mA h/g after 300 cycles at 0.2 A g⁻¹ current density. Varying the rate of charge/discharge current from 0.2 to 4 A/g does not show negative effects on the recycling stability of the nanocapsules and a recoverable specific capacity as high as 1270.4 mA h/g is obtained. The introduction of the onion-like C shell and the presence of the void spaces are found to increase the contact areas between the electrolyte and the nanocapsules for improved electrolyte diffusion, to enhance the electronic conductivity and ionic mobility of the NiCo₂O₄ nanoparticle cores, and to accommodate the change in volume during the lithium-ion insertion/extraction process.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.     

Optimized synthesis of Cu-doped LiFePO4/C cathode material by an ethylene glycol assisted co-precipitation method

LiFePO₄/C and cupric ion doped LiFePO₄/C cathode materials were synthesized via an ethylene glycol assisted co-precipitation method. We assessed the influence of different parameters on electrochemical performance including calcination conditions, the amount of cupric ions added, doping ways, and drying methods. The microstructure of the materials was characterized by XRD, SEM, TEM, and EA. The results indicated the optimized Cu-doped LiFePO₄/C shows enhanced electrochemical performance with excellent high-rate capacity and cycle stability compared with LiFePO₄/C. The optimized Cu-doped LiFePO₄/C exhibited a high specific capacity of 148 mA h g⁻¹ at 0.1 C. Even at a rate of 10 C, it still achieved a specific capacity of 111 mA h g⁻¹ and its capacity retention ratio remained at 99.9% after 100 cycles at 1 C. These enhanced electrochemical properties were mainly due to a lesser extent of particle aggregation and more uniform carbon coating. Importantly, the synthesis process of this study is simple, fast, and economical thus it is promising to apply in industrialization.     

Effective Pd/C catalyst for chlorobenzene and hexachlorobenzene hydrodechlorination by direct pyrolysis of sawdust impregnated with palladium nitrate

Two Pd/C catalysts were prepared by pyrolysis of Pd(NO₃)₂ impregnated sawdust. At equal pyrolysis time slow ramping with shorter isothermal heating resulted in 0.9 wt.% Pd/C-S1 sample comprising carbon support with some oxygen-containing moieties and Pd⁰ with 2.6 nm average particle size (APS) partially decorated with carbon shell, whereas fast temperature ramping and long isothermal heating provided 0.6 wt.% Pd/C-S2 containing Pd⁰ with 3.7 nm APS, with larger fraction of carbon decorated particles. Pd/C-S1 is slightly more efficient than Pd/C-S2 in gas phase chlorobenzene hydrodechlorination to benzene at 100–250 °C. Only Pd/C-S1 provides hexachlorobenzene hydrodechlorination in liquid phase due to lower APS and probably smaller PdC_x content.     

Synthesis and ethanol sensing properties of CuO nanorods coated with In2O3

CuO/In₂O₃ core–shell nanorods were fabricated using thermal evaporation and radio frequency magnetron sputtering. X-ray diffraction and transmission electron microscopy showed that both the cores and shells were crystalline. The multiple networked CuO/In₂O₃ core–shell nanorod sensors showed responses of 382–804%, response times of 36–54 s and recovery times of 144–154 s at ethanol (C₂H₅OH) concentrations ranging from 50 to 250 ppm at 300 °C. These responses were 2.3–2.8 times higher than those of the pristine CuO nanorod sensor over the same C₂H₅OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.     

Hollow core–shell ZnMn2O4 microspheres as a high-performance anode material for lithium-ion batteries

The hollow core–shell ZnMn₂O₄ microspheres are successfully prepared by a solvothermal carbon templating method and then a annealing process. The crystal phase and particle morphology of resultant ZnMn₂O₄ microspheres are characterized by XRD and TEM. The electrochemical properties of the ZnMn₂O₄ microspheres as an anode material are investigated for lithium ion batteries. The results show that the ZnMn₂O₄ microspheres exhibit a reversible capacity of 855.8 mA h g⁻¹ at a current density of 200 mA g⁻¹ after 50 cycles. Even at 1000 mA g⁻¹, the reversible capacity of the ZnMn₂O₄ microspheres is still kept at 724.4 mA h g⁻¹ after 60 cycles. The enhanced electrochemical performance suggests the promising potential of the hollow core–shell ZnMn₂O₄ microspheres in lithium-ion batteries.     

ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Control of the carbon shell thickness in Sn70Ge30@carbon core–shell nanoparticles using alkyl terminators: Its implication for high-capacity lithium battery anode materials

This paper describes the synthesis and electrochemical characterization of Sn₇₀Ge₃₀@carbon core–shell nanoparticles prepared by vacuum annealing of the alkyl-capped Sn₇₀Ge₃₀ nanoparticles obtained from the reaction of SnCl₄ and GeCl₄ with sodium naphthalide in ethylene glycol dimethyl ether (glyme) and RLi (R = butyl, ethyl, methyl). The Sn₇₀Ge₃₀@carbon core–shell nanoparticles have different core sizes and shell thicknesses depending on the alkyl terminator. The annealed nanoparticles that terminate with butyl and ethyl groups have core sizes of ～14 and ～17 nm with carbon shell thicknesses of ～16 and ～8 nm, respectively. On the other hand, annealed nanoparticles that terminate with methyl groups have core sizes of 40 nm with a very thin carbon shell without uniform coverage of the core. Electrochemical characterization shows that nanoparticles prepared using butyl terminators have the highest capacity retention out to 40 cycles (95%) and a first charge capacity of 1040 mAh/g. On the other hand, ethyl- and methyl-capped nanoparticles show 82 and 64% capacity retention after 40 cycles.     

Flame-made MoO3/SiO2–Al2O3 metathesis catalysts with highly dispersed and highly active molybdate species

MoO₃/SiO₂–Al₂O₃ catalysts are prepared via flame spray pyrolysis and evaluated in the self-metathesis of propene to ethene and butene. Their specific surface area ranges between 100 and 170 m² g^?1 depending on the MoO₃ loading (1–15 wt.%, corresponding to Mo surface density between 0.3 and 6.1 Mo atoms per nm²). The catalysts were characterized by N₂-physisorption, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). The silica–alumina matrix condenses first in the flame and forms non-porous spherical particles of 5–20 nm, followed by the dispersion of Mo oxide at their surface. Depending on the MoO₃ loading, different MoO_x species are stabilized: dispersed and amorphous molybdates (mono- and oligomeric) at low loadings (<5 wt.%, <1.5 Mo nm^?2) and crystalline MoO₃ species at higher loadings. Raman spectroscopy suggests the presence of monomeric species for surface densities of 0.3, 0.5 and 0.8 Mo nm^?2. The formation of MoOMo bonds is, however, clearly established by ToF-SIMS from surface densities as low as 0.5 Mo nm^?2. At 1.5 Mo nm^?2, crystallites of β-MoO₃ (2–3 nm) are detected and further increasing the loading induces the formation of bigger α- and β-MoO₃ crystals (around 20 nm). The speciation of Mo proves to have a marked impact on the metathesis activity of the catalysts. Catalysts with high Mo loading and exhibiting MoO₃ crystals are poorly active, whereas catalysts with low Mo loading (<5 wt.%) perform well in the reaction. The catalyst loaded with only 1 wt.% of MoO₃ (0.3 Mo nm^?2) is the most active, reaching turn over frequencies seven times higher than reference catalysts reported in the literature. Moreover, the specific metathesis activity is clearly inversely correlated to the degree of condensation of the molybdenum oxide phase (as evaluated by ToF-SIMS). The latter finding indicates that monomeric MoO_x species are the main active centres in the olefin metathesis.     

Synthesis of size controlled phase pure KNbO3 fine particles via a solid-state route from a core–shell structured precursor

From a core–shell structured precursor, comprising Nb₂O₅ core enveloped by KHCO₃ in an equimolar proportion, phase pure KNbO₃ (KN) fine particles were obtained by calcining in air at 600 °C for 1 h. Disintegrating the large agglomerated particles of KHCO₃ prior to the precursor preparation enabled the micronization of the KN particle size down to 240 nm, close to that of the starting Nb₂O₅, due to increased mixing homogeneity and consequent thorough enveloping of individual Nb₂O₅ particles. Based on these findings, together with the known coupling diffusion mechanism of potassium and oxygen into Nb₂O₅, it was concluded that the core–shell particles in the precursor serve as a separated reaction space to complete the formation of KN without appreciable coalescence or local sintering, as far as the firing temperature is low enough like those employed in the present study. Superiority of KHCO₃ over K₂CO₃ or KNO₃ as a potassium source was also discussed.     

Microstructure characterization of ZrB2–SiC composite fabricated by spark plasma sintering with TaSi2 additive

Dense ZrB₂–SiC composite was synthesized by spark plasma sintering with 10 vol.% TaSi₂ additive. When sintered at 1600 °C, core–shell structure was found existing in the sample. The core was ZrB₂ and the shell was (Zr,Ta)B₂ solid solution. This result was ascribed to the decomposition of TaSi₂ and the solid solution of Ta atoms into ZrB₂ grains. The solid solution process probably decreased the boride grain boundary active energy, contributing to the formation of coherent structure of grain boundaries. Additionally, the existence of dislocations in the boride grains indicated that the applied pressure also imposed an important effect on the densification of composite. When sintered at 1800 °C, owing to the atom diffusion, Ta atoms homogeneously distributed in the boride grains, leading to the disappearance of core–shell structure. The boundaries between (Zr,Ta)B₂ grains, as well as between boride grains and SiC particles, were still clear without amorphous phase existing.     

Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering

Silicon carbide (SiC) layers were deposited on silica (SiO₂) glass powder by rotary chemical vapor deposition (RCVD) to form SiO₂ glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO₂ glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO₂ glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO₂ glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO₂ glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m^1/2, respectively.     

Limiting factors for low-temperature performance of electrolytes in LiFePO4/Li and graphite/Li half cells

Low-temperature performance of LiBF₄ and LiPF₆-based electrolytes in LiFePO₄/Li and graphite/Li half cells was investigated. In the temperature range from 0 °C to ?40 °C, electrochemical impedance spectroscopy (EIS) results show that the charge-transfer resistance (R_ct) of graphite/Li cell decreases, the R_ct of LiFePO₄/Li cell increases, and sum resistance of LiFePO₄/Li and graphite/Li cell decreases when replacing LiPF₆ with LiBF₄. In the temperature range from 25 °C to ?40 °C, energy barrier (W) for Li-ion jump at the solid electrolyte interface (SEI) alters slightly from 16.04 kJ/mol to 13.60 kJ/mol in LiFePO₄/Li cells, but declines greatly from 46.47 kJ/mol to 19.81 kJ/mol in graphite/Li cells when using LiBF₄ instead of LiPF₆, meanwhile, activation energy (ΔG) of electrode reaction is approximately the same (～60 kJ/mol). The above results indicate that the ionic conductivity is the main limiting factor for low-temperature performance of electrolytes in LiFePO₄/Li cell, while factors related with electrolyte-interface are more crucial in graphite/Li cell than in LiFePO₄/Li cell.