Bingjun Yang

Facile Preparation of One-Dimensional Wrapping Structure: Graphene Nanoscroll-Wrapped of Fe3O4 Nanoparticles and Its Application for Lithium-Ion Battery

Graphene nanoscroll (GNS) is a spirally wrapped two-dimensional (2D) graphene sheet (GS) with a 1D tubular structure resembling that of a multiwalled carbon nanotube (MWCNT). GNS provide open structure at both ends and interlayer galleries that can be easily intercalated and adjusted, which show great potential applications in energy storage. Here we demonstrate a novel and simple strategy for the large-scale preparation of GNSs wrapping Fe3O4 nanoparticles (denoted as Fe3O4@GNSs) from graphene oxide (GO) sheets by cold quenching in liquid nitrogen. When a heated aqueous mixed suspension of GO sheets and Fe3O4 nanoparticles is immersed in liquid nitrogen, the in-situ wrapping of Fe3O4 nanoparticles with GNSs is easily realized. The structural conversion is closely correlated with the initial temperature of mixed suspension, the zeta potential of Fe3O4 nanoparticles and the immersion way. Remarkably, such hybrid structure provides the right combination of electrode properties for high-performance lithium-ion batteries. Compared with other wrapping structure, such 1D wrapping structure (GNSs wrapping) effectively limits the volume expansion of Fe3O4 nanoparticles during the cycling process, consequently, a high reversible capacity, good rate capability, and excellent cyclic stability are achieved with the material as anode for lithium storage. The results presented here may pave a way for the large-scale preparation of GNS-based materials in electrochemical energy storage applications.

Enhanced field emission properties from aligned graphenes fabricated on micro-hole patterned stainless steel

The graphene emitters on micro-hole patterned stainless steel (SUS304) were prepared using electrophoresis method. The field emission property of three-dimensional graphene emitters was enhanced remarkably compared to that of graphene on flat substrates. The turn-on and threshold fields of the patterned emitter were, respectively, 4.8 and 5.6 V μm−1 lower than those of graphene on flat SUS304 (turn on field is 5.6 V μm−1). The micro-hole patterned cathode provides 10 times higher current density due to vertical aligned sharp edges of graphene in micro holes, and this design may open a potential way to layered-nanomertial-based cold cathodes.

Controllable synthesis of graphene nanoscroll-wrapped Fe3O4 nanoparticles and their lithium-ion battery performance

In this work, a series of graphene nanoscroll (GNS)-wrapped Fe3O4 nanoparticles (NPs) composites (denoted as Fe3O4@GNS) are prepared by cold quenching of mixed suspensions of water-dispersible Fe3O4 NPs and graphene oxide (GO) with different mass ratios in liquid nitrogen followed by a low-temperature thermal reduction. In all samples, it is interesting that Fe3O4 NPs are able to be in situ encapsulated completely in GNSs, forming a three-dimensional network consisting of a fiber-like structure. The amount of Fe3O4 NPs wrapped with GNSs is in proportion to the mass ratio between Fe3O4 NPs and GO in the initial mixed suspension. As a new anode material of lithium ions batteries (LIBs), these Fe3O4@GNSs exhibit outstanding Li-ion storage characteristics. Among them, the Fe3O4@GNS (Fe3O4 : GO = 2 : 1) electrode shows the best electrochemical properties, including excellent cycling stability with a reversible capacity of 1172 mA h g−1 over 200 cycles at 100 mA g−1 and 525 mA h g−1 over 1000 cycles at 2 A g−1, as well as superior rate performance with a reversible capacity of 648 and 480 mA h g−1 at 2 and 5 A g−1, respectively. Such high performance is very close to the novel hybrid structure of Fe3O4@GNS as well as the optimal ratio between Fe3O4 and GO.

Enhanced photocurrent of a ZnO nanorod array sensitized with graphene quantum dots

In this communication, we demonstrate a facile method to prepare a graphene quantum dots (GQDs) decorated ZnO nanorod array (ZNRA) ultraviolet detector. The characterization of I–V and time-dependent current behaviors under UV light illumination show that, although GQDs have little influence on the dark conductivity of ZNRA, the coating of GQDs has a remarkable sensitization effect on the photocurrent of ZNRA. This enhancement of the photocurrent was due to the interfacial charge transfer from GQDs to ZNRA.

Silica-grafted ionic liquids for revealing the respective charging behaviors of cations and anions in supercapacitors

Supercapacitors based on activated carbon electrodes and ionic liquids as electrolytes are capable of storing charge through the electrosorption of ions on porous carbons and represent important energy storage devices with high power delivery/uptake. Various computational and instrumental methods have been developed to understand the ion storage behavior, however, techniques that can probe various cations and anions of ionic liquids separately remain lacking. Here, we report an approach to monitoring cations and anions independently by using silica nanoparticle-grafted ionic liquids, in which ions attaching to silica nanoparticle cannot access activated carbon pores upon charging, whereas free counter-ions can. Aided by this strategy, conventional electrochemical characterizations allow the direct measurement of the respective capacitance contributions and acting potential windows of different ions. Moreover, coupled with electrochemical quartz crystal microbalance, this method can provide unprecedented insight into the underlying electrochemistry.Quantifying the individual capacitance contributions of in-pore ions during charging remains a challenge. Here the authors design silica-grafted ionic liquids to reveal the charging behaviors of cations and anions separately, providing fresh insight into the storage mechanism of supercapacitors.

Large field emission current and density from robust carbon nanotube cathodes for continuous and pulsed electron sources

Highly adhesive cold cathodes with high field emission performance are fabricated by using a screen-printingmethod. The emission density of carbon nanotube (CNT) cold cathode reaches 207.0 mA cm−2 at an electric field of 4.5 V μm−1 under continuous driving mode, and high peak current emission of 315.8 mA corresponding to 4.5 A cm−2 at the electric field of 10.3 V μm−1 under pulsed driving mode. The emission patterns of the cold cathodes are of excellent uniformity that was revealed by vivid luminescent patterns of phosphor coated transparent indium tin oxide (ITO) anode. The cold cathodes also exhibit highly stable emission under continuous and pulsed driving modes. The high adhesion of CNTs tomolybdenum substrates results in robust cold cathodes and is responsible for the high field emission performance. This robust CNT emitter could meet the operating requirements of continuous and pulsed electron sources, and it provides promising applications in various vacuummicro/nanoelectronic devices.摘要本文采用丝网印刷技术制备了具有高粘接性能的碳纳米管冷阴极. 该碳纳米管冷阴极在直流连续及脉冲场下均具有优异的场发射 性能, 同时具有高发射电流密度与发射总电流, 可以满足高功率器件对冷阴极电子源的使用需求. 在直流连续场下, 该冷阴极的电流发射 密度可达到207.0 mA cm−2 (电场强度为4.5 V μm−1);在脉冲场(200 Hz, 10 μs)激发下, 峰值电流密度最高可达4.5 A cm−2(电场强度为10.3 V μm−1), 同时具有高的峰值发射电流(315.8 mA). 为观察阴极发射均匀性, 采用荧光板为阳极进行实时监测, 发现此印刷阴极发射较均一; 稳 定性测试表明该阴极在连续及脉冲场下均具有良好的发射稳定性. 该冷阴极同时具有高电流密度及高发射电流, 可以满足高功率真空电 子器件的使用要求, 在真空微纳电子器件中显示出巨大的应用前景.

First-principles study of perpendicular magnetic anisotropy in ferrimagnetic D022-Mn3X (X = Ga, Ge) on MgO and SrTiO3

The magnetic anisotropy energy (MAE) of bulk D022-Mn3X (X = Ga, Ge), Mn3X/MgO, and Mn3X/STiO3(STO) heterostructures is calculated from first principles calculations. The main source of the large perpendicular magnetic anisotropy (PMA) of bulk Mn3X is identified as Mn atoms in the Mn-Mn layer. In the four heterostructures, the magnetic moment of interfacial Mn atoms was reversed when Mn3X was epitaxially grown on MgO and STO substrates. More importantly, a large in-plane tensile strain induced by lattice mismatch between Mn3X and MgO significantly changes the MAE, explaining the difficulty in experiments to obtain PMA in epitaxial Mn3X/MgO. Furthermore, interface and surface Mn atoms also help to enhance the PMA of Mn3X/STO (MgO) heterostructures due to dxy and d z 2 states changing from occupied states in bulk Mn3X to unoccupied states in the interface (surface) Mn of the heterostructures. These results suggest that the PMA of manganese compound heterostructures can be produced by decreasing the lattice mismatch with substrates and will guide the search for ultrathin manganese compound films with high PMA epitaxially grown on substrates for the application of spintronic devices.The magnetic anisotropy energy (MAE) of bulk D022-Mn3X (X = Ga, Ge), Mn3X/MgO, and Mn3X/STiO3(STO) heterostructures is calculated from first principles calculations. The main source of the large perpendicular magnetic anisotropy (PMA) of bulk Mn3X is identified as Mn atoms in the Mn-Mn layer. In the four heterostructures, the magnetic moment of interfacial Mn atoms was reversed when Mn3X was epitaxially grown on MgO and STO substrates. More importantly, a large in-plane tensile strain induced by lattice mismatch between Mn3X and MgO significantly changes the MAE, explaining the difficulty in experiments to obtain PMA in epitaxial Mn3X/MgO. Furthermore, interface and surface Mn atoms also help to enhance the PMA of Mn3X/STO (MgO) heterostructures due to dxy and d z 2 states changing from occupied states in bulk Mn3X to unoccupied states in the interface (surface) Mn of the heterostructures. These results suggest that the PMA of manganese compound heterostructures can be produced by decreasing the lat...

Enhanced capacities of carbon nanosheets derived from functionalized bacterial cellulose as anodes for sodium ion batteries

We studied carbon nanosheets prepared from bacterial cellulose (BC) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized-BC through carbonization at temperatures ranging from 900 to 1100 °C. Based on experimental results, we propose a comprehensive perspective of Na storage for BC based anode material. The pyrolysis TEMPO-oxidized BC delivers the highest capacity at annealing temperature of 1000 °C under different current densities, and displays excellent rate capability and cyclability. The superior electrochemical performance is attributed to the increasing interlayer distance, rich porous structure and oxygen-containing functional groups. The experimental studies reveal that the introduction of carboxyl is an effective strategy to enhance the specific capacity and cycling stability for Na-ion storage.

Determination of spin relaxation times in heavy metals via second-harmonic spin injection magnetoresistance

In tunnel junctions between ferromagnets and heavy elements with strong spin orbit coupling the magnetoresistance is often dominated by tunneling anisotropic magnetoresistance (TAMR). This makes conventional DC spin injection techniques impractical for determining the spin relaxation time (

Enhanced tunneling electroresistance in multiferroic tunnel junctions due to the reversible modulation of orbitals overlap

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