Qianwang Chen

High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework

Theoretical and experimental results have revealed that the lithium-ion storage capacity for nitrogen-doped graphene largely depends on the nitrogen-doping level. However, most nitrogen-doped carbon materials used for lithium-ion batteries are reported to have a nitrogen content of approximately 10 wt% because a higher number of nitrogen atoms in the two-dimensional honeycomb lattice can result in structural instability. Here we report nitrogen-doped graphene particle analogues with a nitrogen content of up to 17.72 wt% that are prepared by the pyrolysis of a nitrogen-containing zeolitic imidazolate framework at 800 °C under a nitrogen atmosphere. As an anode material for lithium-ion batteries, these particles retain a capacity of 2,132 mA h g(-1) after 50 cycles at a current density of 100 mA g(-1), and 785 mAh g(-1) after 1,000 cycles at 5 A g(-1). The remarkable performance results from the graphene analogous particles doped with nitrogen within the hexagonal lattice and edges.

CoMn2O4 Spinel Hierarchical Microspheres Assembled with Porous Nanosheets as Stable Anodes for Lithium-ion Batteries

Herein, we report the feasibility to enhance the capacity and stability of CoMn2O4 anode materials by fabricating hierarchical mesoporous structure. The open space between neighboring nanosheets allows for easy diffusion of the electrolyte. The hierarchical microspheres assembled with nanosheets can ensure that every nanosheet participates in the electrochemical reaction, because every nanosheet is contacted with the electrolyte solution. The hierarchical structure and well interconnected pores on the surface of nanosheets will enhance the CoMn2O4/electrolyte contact area, shorten the Li+ ion diffusion length in the nanosheets, and accommodate the strain induced by the volume change during the electrochemical reaction. The last, hierarchical architecture with spherical morphology possesses relatively low surface energy, which results in less extent of self-aggregation during charge/discharge process. As a result, CoMn2O4 hierarchical microspheres can achieve a good cycle ability and high rate capability.

Fabrication Based on the Kirkendall Effect of Co3O4 Porous Nanocages with Extraordinarily High Capacity for Lithium Storage

Herein we report a novel facile strategy for the fabrication of Co(3)O(4) porous nanocages based on the Kirkendall effect, which involves the thermal decomposition of Prussian blue analogue (PBA) Co(3)[Co(CN)(6)](2) truncated nanocubes at 400 °C. Owing to the volume loss and release of internally generated CO(2) and N(x) O(y) in the process of interdiffusion, Co(3)O(4) nanocages with porous shells and containing nanoparticles were finally obtained. When evaluated as electrode materials for lithium-ion batteries, the as-prepared Co(3)O(4) porous nanocages displayed superior battery performance. Most importantly, capacities of up to 1465 mA h g(-1) are attained after 50 cycles at a current density of 300 mA g(-1). Moreover, this simple synthetic strategy is potentially competitive for scaling-up industrial production.

Metal-free catalytic reduction of 4-nitrophenol to 4-aminophenol by N-doped graphene

The metal-free catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) mediated by N-doped graphene (NG) was reported. Nip could be reduced to Amp completely without any by-product generation. The activity of the NG is comparable with some of the previously reported metallic catalysts. Interestingly, the NG sheet catalyzed reaction follows pseudo-zero-order kinetics, while all the metallic catalysts follow pseudo-first-order kinetics. The in situ FTIR experiment demonstrated that Nip ions will combine with NG via the O atoms of their hydroxyl groups. Theoretical calculations verified this adsorption model, and confirmed that the adsorption of Nip ions is the critical step, leading to the pseudo-zero-order kinetics. Moreover, only the carbon atoms next to the doped N atoms on NG surface can be activated, serving as the active sites. As expected, all four kinds of the doped N atoms are beneficial to the adsorption and activation of Nip, contributing to the catalytic reduction reaction.

Non-precious alloy encapsulated in nitrogen-doped graphene layers derived from MOFs as an active and durable hydrogen evolution reaction catalyst

Non-precious metal based catalysts are emerging as the most promising alternatives to Pt-based ones for hydrogen evolution reaction (HER) due to their low cost and rich reserves. However, their low efficiency and stability due to inherent corrosion and oxidation in acid media are the main barriers blocking sustainable hydrogen production. Metal–organic frameworks, with both designable metal ion centers and organic ligands, are promising precursors for the one-step synthesis of metal/alloy@carbon composites for HER. Herein, we synthesized FeCo alloy nanoparticles encapsulated in highly nitrogen-doped (8.2 atom%) graphene layers by direct annealing of MOF nanoparticles at 600 °C in N2. The catalyst shows a low onset overpotential (88 mV) and an overpotential of only 262 mV at 10 mA cm−2. Besides, it exhibits an excellent long-term durability performance even after 10 000 cycles due to the protection of the graphene layers. Our density functional theory calculations reveal that the nitrogen dopants can provide adsorption sites for H* and the appropriate increase of nitrogen will decrease ΔGH* for HER. Besides, the unique structure of the metal and graphene composites derived from MOFs can also decrease ΔGH* thereby promoting the catalytic activity.

One-step hydrothermal process to prepare highly crystalline Fe3O4 nanoparticles with improved magnetic properties

Hydrothermal process was successfully used to synthesize Fe3O4 powder using ferrous chloride (FeCl2) and diamine hydrate (H4N2·H2O) as starting materials by carefully controlling the reaction conditions. The as-prepared Fe3O4 sample was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and its magnetic properties were evaluated on a vibrating sample magnetometer (VSM). The nanoscale (40 nm) Fe3O4 powder obtained at 140 °C for 6 h possessed a saturation magnetization of 85.8 emu/g, a little lower than that of the correspondent bulk Fe3O4 (92 emu/g). It is suggested that the well-crystallized Fe3O4 grains formed under appropriate hydrothermal conditions should be responsible for the increased saturation magnetization in nanosized Fe3O4.

Magnetic field-induced growth and self-assembly of cobalt nanocrystallites

The growth and assembly behavior of cobalt magnetic nanocrystallites under an external magnetic field were studied. Co polycrystalline wires with an average length of 2 mm and diameter of 13 µm were formed by the self-assembly of Co nanocrystallites (15 nm on average) under the induction of a 0.25 T external magnetic field. The wires were nearly parallel because their axes were all parallel to the magnetic line of force. The Ms and Hc values of the sample, 111 emu g−1 and 389 Oe, are higher than those of the sample prepared without an external magnetic field applied (91 emu g−1 and 375 Oe), which might be associated with the special nanostructure in which Co nanocrystallites were arranged in polycrystalline wires acting as permanent magnetic dipoles. The process could be used to fabricate large arrays of uniform wires of some magnetic materials and improve the magnetic properties of nanoscale magnetic materials.

Magnetic-Field-Induced Formation of One-Dimensional Magnetite Nanochains

Magnetite nanoclusters with an average size of about 120 nm have been prepared and allowed to self-assemble into one-dimensional (1D) nanochain structures with the average length about of 2 mum by a simple magnetic-field-induced (MFI) assembly approach (0.20 T). The constituent, phase, and morphology of these 1D nanochains have been characterized by X-ray diffraction and transmission electron microscopy. Magnetic measurement reveals that these 1D nanochains are weakly ferromagnetic at room temperature. In this paper, we discuss the influence of magnetization time and strength of external magnetic field on the formation of 1D nanochains. We also show that, by changing the amount of hydrogen peroxide in the starting materials, 1D nanochains with different interparticle spacing can be obtained. This 1D nanochain structure with different interparticle spacing would be an ideal system for the further study of magnetization properties of 1D ordered magnetic nanostructures.

Facile Fabrication of Porous NixCo3–xO4 Nanosheets with Enhanced Electrochemical Performance As Anode Materials for Li-Ion Batteries

Herein, we report a novel and facile route for the large-scale fabrication of 2D porous NixCo3-xO4 nanosheets, which involves the thermal decomposition of NixCo1-x hydroxide precursor at 450 °C in air for 2 h. The as-prepared 2D porous NixCo3-xO4 nanosheets exhibit an enhanced lithium storage capacity and excellent cycling stability (1330 mA h g(-1) at a current density of 100 mA g(-1) after 50 cycles). More importantly, it can render reversible capacity of 844 mA h g(-1), even at a high current density of 500 mA g(-1) after 200 cycles, indicating its potential applications for high power LIBs. Compared to pure Co3O4, the reduction of Co in NixCo3-xO4 is of more significance because of the high cost and toxicity of Co. The improved electrochemical performance is attributed to the 2D structure and large amounts of mesopores within the nanosheets, which can effectively improve structural stability, reduce the diffusion length for lithium ions and electrons, and buffer volume expansion during the Li(+) insertion/extraction processes.

Fundamental magnetic parameters from pure synthetic greigite (Fe3S4)

Pure ferrimagnetic greigite (Fe3S4) has been synthesized by reacting ferric chloride with thiourea and formic acid at 170°C. Sample purity was confirmed by X-ray diffraction, neutron diffraction and Mossbauer spectroscopy, coupled with magnetic measurements. Scanning electron microscope observations indicate clear cubo-octahedral and polyhedral crystal morphologies. The grain sizes are as large as 44 ?m. Detailed low- and high-temperature magnetic measurements document the previously poorly known magnetic properties of greigite. The synthetic greigite samples are dominated by pseudo-single-domain and multi-domain behavior. The saturation magnetization (M s ) at room temperature is ?59 Am2kg?1 (3.13 ? B per formula unit), which is higher than any value previously reported for greigite in the literature largely because of the high purity of this sample compared to others. No low-temperature magnetic transition has been detected; however, a local coercivity minimum is observed at around 130 K, which is probably associated with domain walls present in the studied samples. The high-temperature magnetic properties of greigite are dominated by chemical decomposition above around 250°C, which precludes determination of the Curie temperature, but our evidence indicates that it must exceed 350°C. On the basis of the Bloch spin wave expansion, the spin wave stiffness of greigite was determined for the first time as ?193 meV·A2 from low-temperature M s measurements, with the corresponding exchange constant J AB of ?1.03 meV.