Showing papers by "Renchao Che published in 2017"

Dipolar-Distribution Cavity γ-Fe 2 O 3 @C@α-MnO 2 Nanospindle with Broadened Microwave Absorption Bandwidth by Chemically Etching

Abstract: Developing microwave absorption materials with ultrawide bandwidth and low density still remains a challenge, which restricts their actual application in electromagnetic signal anticontamination and defense stealth technology. Here a series of olive-like γ-Fe2 O3 @C core-shell spindles with different shell thickness and γ-Fe2 O3 @C@α-MnO2 spindles with different volumes of dipolar-distribution cavities were successfully prepared. Both series of absorbers exhibit excellent absorption properties. The γ-Fe2 O3 @C@α-MnO2 spindle with controllable cavity volume exhibits an effective absorption (<-10 dB) bandwidth as wide as 9.2 GHz due to the chemically dipolar etching of the core. Reflection loss of the γ-Fe2 O3 @C spindle reaches as high as -45 dB because of the optimized electromagnetic impedance balance between polymer shell and γ-Fe2 O3 core. Intrinsic ferromagnetism of the anisotropy spindle is confirmed by electron holography. Strong coupling of magnetic flux stray lines between spindles is directly imaged. This unique morphology and facile etching technique might facilitate the study of core-shell type microwave absorbers.

Nanoporous TiNb2O7/C Composite Microspheres with Three-Dimensional Conductive Network for Long-Cycle-Life and High-Rate-Capability Anode Materials for Lithium-Ion Batteries.

Abstract: On the basis of the advantages of ideal cycling stability, high discharge voltage (1.65 V), and excellent reversibility, more and more attention has been focused on TiNb2O7 (marked as TNO) as an anode material candidate for lithium-ion batteries. However, the poor electronic conductivity and low ionic diffusion rate intrinsically restrict its practical use. Herein, we first synthesize the TNO/C composite microspheres with three-dimensionally (marked as 3D) electro-conductive carbon network and abundant nanoporous structure by a simple spray-drying method. The microspheres are constructed by irregularly primary cubic nanoparticle units with size of 100-200 nm. The nanopores throughout the microspheres range from 1 to 50 nm. As an anode material, the prepared TNO/C composite microspheres demonstrate a prominent charge/discharge capacity of 323.2/326 mA h g-1 after 300 cycles at 0.25 C (1 C = 388 mA g-1) and 259.9/262.5 mA h g-1 after 1000 long cycles at a high current density of 5 C, revealing the ideal reversible capacity and long cycling life. Meanwhile, the TNO/C composite microspheres present ideal rate performance, showing the discharge capacity of 120 mA h g-1 at 30 C after 10 cycles. The super electrochemical performance could be attributed to the 3D electro-conductive carbon network and nanoporous structure. The nanopores facilitate the permeation of electrolyte into the intercontacting regions of the anode materials. Carbon layers disperse uniformly throughout the 3D microspheres, effectively improving the electrical conductivity of the electrode. Hence, the prepared TNO/C composite microspheres have great potential to be used as an anode material for lithium-ion batteries.

Insight into the atomic structure of Li2MnO3 in Li-rich Mn-based cathode materials and the impact of its atomic arrangement on electrochemical performance

Abstract: Li-rich Mn-based cathode materials have been considered as promising candidates for next generation Li-ion batteries due to their high-energy density, low cost and non-toxicity. However, the atomic arrangement of such materials and the relationship between the microstructure and electrochemical performance are still not fully understood. In this paper, local heterogeneity in the crystal lattice is directly observed in synthesized Li2MnO3/LiMO2 (M = Ni and Mn) cathode materials. With SAED application, for the first time, we accordingly uncover that the lattice heterogeneity is induced by different Li2MnO3 atomic arrangements coexisting in the same crystal domain. The co-growth of Li2MnO3 with different orientations is proved to be a defective feature, which would induce atomic vacancy concentration in the lattice and increase the risk of layered structure collapse in the cycling process. The electrochemical test results also suggest that the composition with a relatively uniform Li2MnO3 arrangement exhibits better cycling performance (the capacity retention is as high as 95.1% after 50 cycles at 0.1C), oppositely, the coexistence of multiple complex Li2MnO3 arrangements results in poor cycling performance (the capacity retention is below 70% after 50 cycles at 0.1C). The crystal lattice structure comparison between primary and cycled is shown to manifest the effect of Li2MnO3 arrangement on the electrochemical performance and structural stability, providing one possible explanation for the capacity degradation of the Li-rich materials.

Enhanced Stability of the Magnetic Skyrmion Lattice Phase under a Tilted Magnetic Field in a Two-Dimensional Chiral Magnet.

Abstract: The magnetic skyrmion is a topologically stable vortex-like spin texture that offers great promise as information carriers for future spintronic devices. In a two-dimensional chiral magnet, it was generally considered that a tilted magnetic field is harmful to its formation and stability. Here we investigated the angular-dependent stability of magnetic skyrmions in FeGe nanosheets by using high-resolution Lorentz transmission electron microscopy (Lorentz TEM). Besides the theoretically predicted destruction of skyrmion lattice state by an oblique magnetic field as the temperature closes to its magnetic Curie temperature Tc ∼ 278 K, we also observed an unexpected reentry-like phenomenon at the moderate temperatures near the border between conical and skyrmion phase, Tt ∼ 240 K. This behavior is completely beyond the theoretical prediction in a conventional two-dimensional (2D) system. Instead, a three-dimensional (3D) model involving the competition between conical phase and skyrmions is likely to play a c...

High-temperature annealing of an iron microplate with excellent microwave absorption performance and its direct micromagnetic analysis by electron holography and Lorentz microscopy

Abstract: To meet the demand of electromagnetic interference shielding, cheap and easily available microwave absorbers are urgently required Recently, most of the related research has been focussed on a number of complicated absorbers comprising multi-components because of their better electromagnetic match However, it is still a great challenge to develop an absorber that simultaneously possesses the advantages of easy fabrication, low-cost, ultra-wide bandwidth, and strong absorption Hence, development of a simple and convenient absorber with efficient performance is attracting significant attention because of the urgent requirement of this type of absorbers Herein, a series of single-component iron-based absorbers with different morphologies and grain sizes was successfully prepared Strong absorption intensity (∼−434 dB) was found in plate-like samples, which could even match those of some multi-component absorbers Electron holography and Lorentz microscopy analysis were used for the further comprehension of the relationships among the microstructure, electromagnetic property, and microwave absorption performance The primary grain size of the present iron microplate was found fundamentally important for microwave absorption performance This cheap and available absorber is believed to be an optimal choice for single-component absorbers and useful in the research of absorption mechanism

Gold nanoparticles decorated Ag(Cl,Br) micro-necklaces for efficient and stable SERS detection and visible-light photocatalytic degradation of Sudan I

Abstract: In this paper, we report an air-exposed and room-temperature immersion reaction for synthesis of novel Au nanoparticles decorated Ag(Cl,Br) [Ag(Cl,Br)–Au] micro-necklaces from the AgBr template for efficient and stable photocatalytic degradation and SERS detection of food contaminant Sudan I (SDI) molecules. Amazingly, as the photocatalyst, the partial substitution of bromine atoms by chlorine in crystalline lattices and decoration of Au nanoparticles on the surface have synergistically ensured these Ag(Cl,Br)–Au micro-necklaces of enhanced degradation efficiency of SDI from 65.1% achieved by AgBr to 100% after 18 min of visible light irradiation, along with significantly promoted efficiency maintenance after 12 cycles of the photocatalytic reaction. Meanwhile, due to the designed decoration of Au nanoparticles on surfaces of semiconducting micro-necklaces, these Ag(Cl,Br)–Au micro-necklaces also exhibited the ability to offer sensitive SERS signals for trace detection of SDI molecules with the limit of detection as low as 10−10 M being achieved. Hence, in consideration of the novel structures, facile preparation as well as attractive applications in both SERS detection and photocatalytic degradation of SDI dye of these Ag(Cl,Br)–Au micro-necklaces, it is believable that such bifunctional substrate materials hold great potential for various environmental and health-related applications.

Broadening microwave absorption via a multi-domain structure

Abstract: Materials with a high saturation magnetization have gained increasing attention in the field of microwave absorption; therefore, the magnetization value depends on the magnetic configuration inside them. However, the broad-band absorption in the range of microwave frequency (2-18 GHz) is a great challenge. Herein, the three-dimensional (3D) Fe/C hollow microspheres are constructed by iron nanocrystals permeating inside carbon matrix with a saturation magnetization of 340 emu/g, which is 1.55 times as that of bulk Fe, unexpectedly. Electron tomography, electron holography, and Lorentz transmission electron microscopy imaging provide the powerful testimony about Fe/C interpenetration and multi-domain state constructed by vortex and stripe domains. Benefiting from the unique chemical and magnetic microstructures, the microwave minimum absorption is as strong as −55 dB and the bandwidth (<−10 dB) spans 12.5 GHz ranging from 5.5 to 18 GHz. Morphology and distribution of magnetic nano-domains can be facilely re...

Simultaneous Ni Doping at Atom Scale in Ceria and Assembling into Well-Defined Lotuslike Structure for Enhanced Catalytic Performance

Abstract: Oxide materials with redox capability have attracted worldwide attentions in many applications. Introducing defects into crystal lattice is an effective method to modify and optimize redox capability of oxides as well as their catalytic performance. However, the relationship between intrinsic characteristics of defects and properties of oxides has been rarely reported. Herein, we report a facile strategy to introduce defects by doping a small amount of Ni atoms (∼1.8 at. %) into ceria lattice at atomic level through the effect of microstructure of crystal on the redox property of ceria. Amazingly, a small amount of single Ni atom-doped ceria has formed a homogeneous solid solution with uniform lotuslike morphology. It performs an outstanding catalytic performance of a reduced T50 of CO oxidation at 230 °C, which is 135 °C lower than that of pure CeO2 (365 °C). This is largely attributed to defects such as lattice distortion, crystal defects and elastic strain induced by Ni dopants. The DFT calculation has...

Efficient photodegradation of dye pollutants using a novel plasmonic AgCl microrods array and photo-optimized surface-enhanced Raman scattering

Abstract: A novel arrayed AgCl micro-rods have in situ grown on an Ag foil successfully for the first time. The preparation process is consisted of two facile steps: (1) immersed oxidation and (2) directional ions exchange. The structure of the as-synthesized arrayed substrate has been characterized comprehensively, and the relevant growth mechanism is proposed. This highly aligned AgCl arrays show a remarkable visible-light-driven photocatalytic activity towards degrading 10 −5 mol/L rhodamine 6G (R6G) aqueous solution. The ultra-stable catalytic performance of the plasmonic arrays was revealed by the recycled tests in the neutral and acidic conditions. Moreover, a facile SERS substrate based on the AgCl arrays was obtained with the optimal enhancement factor (EF) of ∼3.25 × 10 7 , by directly putting the substrate under a Xe lamp in 7.5 min. Amazingly, the photo-optimized surface-enhanced Raman scattering (SERS) substrate still shows a stable activity for photodegrading R6G. The photocatalytic and SERS mechanism are proposed in this study.

Ultrafast self-assembly of silver nanostructures on carbon-coated copper grids for surface-enhanced Raman scattering detection of trace melamine.

Abstract: Structurally well-defined assemblies of silver nanoparticles, including the dendritic nano-flowers (NFs), planar nano-spheres (NSs) and nano-dendrites (NDs) were obtained by a surfactant-free and ultrafast (≈15min) self-assembly process on as-purchased carbon-coated copper TEM grids. The silver nano-assemblies, especially the NFs modified TEM grids, when serving as surface-enhanced Raman spectroscopy (SERS) substrates for detecting melamine molecules, demonstrated a long-lived limit of detection (LOD) of as low as 10-11M, suggesting the potential of these silver-assemblies modified carbon-coated copper grids as novel potable and cost-effective SERS substrates for trace detection toward various food contaminants like melamine.

Boosting oxygen reduction activity of spinel CoFe 2 O 4 by strong interaction with hierarchical nitrogen-doped carbon nanocages

Abstract: The unique hierarchical nitrogen-doped carbon nanocages (hNCNC) are used as a new support to homogeneously immobilize spinel CoFe2O4 nanoparticles by a facile solvothermal method. The so-constructed hierarchical CoFe2O4/hNCNC catalyst exhibits a high oxygen reduction activity with an onset potential of 0.966 V and half-wave potential of 0.819 V versus reversible hydrogen electrode, far superior to the corresponding 0.846 and 0.742 V for its counterpart of CoFe2O4/hCNC with undoped hierarchical carbon nanocages (hCNC) as the support, which locates at the top level for spinel-based catalysts to date. Consequently, the CoFe2O4/hNCNC displays the superior performance to the CoFe2O4/hCNC, when used as the cathode catalysts in the home-made Al-air batteries. X-ray photoelectron spectroscopy characterizations reveal the more charge transfer from CoFe2O4 to hNCNC than to hCNC, indicating the stronger interaction between CoFe2O4 and hNCNC due to the nitrogen participation. The enhanced interaction and hierarchical morphology favor the high dispersion and modification of electronic states for the active species as well as the mass transport during the oxygen reduction process, which plays a significant role in boosting the electrocatalytic performances. In addition, we noticed the high sensitivity of O 1s spectrum to the particle size and chemical environment for spinel oxides, which is used as an indicator to understand the evolution of ORR activities for all the CoFe2O4-related contrast catalysts. Accordingly, the well-defined structure-performance relationship is demonstrated by the combination of experimental characterizations with theoretical calculations. This study provides a promising strategy to develop efficient, inexpensive and durable oxygen reduction electrocatalysts by tuning the interaction between spinel metal oxides and the carbon-based supports.

The Deformations of Carbon Nanotubes under Cutting

Abstract: The determination of structural evolution at the atomic level is essential to understanding the intrinsic physics and chemistries of nanomaterials. Mechanochemistry represents a promising method to trace structural evolution, but conventional mechanical tension generates random breaking points, which makes it unavailable for effective analysis. It remains difficult to find an appropriate model to study shear deformations. Here, we synthesize high-modulus carbon nanotubes that can be cut precisely, and the structural evolution is efficiently investigated through a combination of geometry phase analysis and first-principles calculations. The lattice fluctuation depends on the anisotropy, chirality, curvature, and slicing rate. The strain distribution further reveals a plastic breaking mechanism for the conjugated carbon atoms under cutting. The resulting sliced carbon nanotubes with controllable sizes and open ends are promising for various applications, for example, as an anode material for lithium-ion bat...

Self-Assembled 3D Hierarchical Copper Hydroxyphosphate Modified by the Oxidation of Copper Foil as a Recyclable, Wide Wavelength Photocatalyst

Abstract: In this work, three-dimensional flower-like and petal-like copper hydroxyphosphate Cu5(OH)4(PO4)2 (CHP) based on the self-assembly of numerous nanosheets has been successfully fabricated on a copper foil by a mild one-pot wet-chemical method without ligand assistance. This research contributes to the development of the method to change the morphology of the CHP active material by varying the degree of substrate oxidation. The two different CHP architectures were used to photocatalytically degrade rhodamine 6G (Rh 6G) under solar light, which can absorb wide-range light wavelength from the UV to the near-infrared region. They all exhibit high photocatalytic activity and good durability, which are potential candidates for high performance and recyclable wide wavelength photocatalysis.

Atomic Mechanism of Interfacial-Controlled Quantum Efficiency and Charge Migration in InAs/GaSb Superlattice

Abstract: A series of systematic electron microscopy imaging evidence are illustrated to prove that a high-quality interface is vital for enhancing quantum efficiency from 23 to 50% effectively, because improved crystal quality of each layer can suppress the disordered atom arrangement and enhance the carrier lifetime via decreasing the overall residual strain. The distribution width of charge rises and then falls as bias increasing, revealing the existence of an optimum operating voltage, which could be attributed to the proper energy band bending. Our results provide new insights into the understanding of the association between macro-property and microstructure of the superlattice system.

Facile preparation of 3D hierarchical coaxial-cable-like Ni-CNTs@beta-(Ni, Co) binary hydroxides for supercapacitors with ultrahigh specific capacitance.

Abstract: A facile chemical method for Co doping Ni-CNTs@α-Ni(OH)2 combining with an in situ phase transformation process is successfully proposed and employed to synthesize three-dimensional (3D) hierarchical Ni-CNTs@β-(Ni, Co) binary hydroxides. This strategy can effectively maintain the coaxial-cable-like structure of Ni-CNTs@α-Ni(OH)2 and meanwhile increase the content of Co as much as possible. Eventually, the specific capacitances and electrical conductivity of the composites are remarkably enhanced. The optimized composite exhibits high specific capacitances of 2861.8F g-1 at 1A g-1 (39.48F cm-2 at 15mAcm-2), good rate capabilities of 1221.8F g-1 at 20A g-1 and cycling stabilities (87.6% of capacitance retention after 5000cycles at 5A g-1). The asymmetric supercapacitor (ASC) constructed with the as-synthesized composite and activated carbon as positive and negative electrode delivers a high specific capacitance of 287.7F g-1 at 1A g-1. The device demonstrates remarkable energy density (96Whkg-1) and high power density (15829.4Wkg-1). The retention of capacitance remains 83.5% at the current density of 5A g-1 after 5000cycles. The charged and discharged samples are further studied by ex situ electron energy loss spectroscopy (EELS) analysis, XRD and SEM to figure out the reasons of capacitance fading. Overall, it is believable that this facile synthetic strategy can be applied to prepare various nanostructured metal hydroxide/CNT composites for high performance supercapacitor electrode materials.

Quantum efficiency optimization by maximizing wave function overlap in type-II superlattice photodetectors.

Abstract: Quantum efficiency (QE) is a crucial parameter that determines the final performance of photodetector devices. Herein, by fitting the charge distribution fluctuation under a series of bias voltages, revealed by groups of in situ electron holography experiments, a simple model based on modulus square of wave function (MSWF) is qualitatively built to shed new light on the relationship between QE and wave function overlap (WFO). It is found that there exists a competition of WFO between the potential well regions and the interface regions, and a peak value of the overall WFO can be obtained under an appropriate voltage. On combining such competition with the measured QE results from actual infrared photodetectors, the positive correlation between QE and WFO is manifested, and the QE can be boosted to 51% from 34%. Our results offer a new perspective to the understanding of the carrier transportation within superlattice (SL) structures and the design on photoelectric devices with enhanced performance.

Controllable one-pot synthesis of FeSe2 nanooctahedra embedded microtubes by a sacrificial self-template method

Abstract: Iron diselenium (FeSe2) with rational design and tailored synthesis is a promising transition metal chalcogenide because of its suitable optical and electronic properties. Despite these prospects, controllable synthesis of FeSe2 with a special hierarchical heterostructure has hardly been paid attention to previously. Here, we report a successful synthesis of FeSe2 microtubes in a system of water glycol mixture using a sacrificial self-template method. This method leads to the formation of a FeSe2 microtube embedded with nanooctahedra in the remaining Se template, which can be regarded as a special hetero-junction morphology. Moreover, a growth mechanism of the FeSe2 microtube was discussed. This unique morphology and facile synthesis might facilitate the study of transition metal chalcogenides.

Synthesis and thermoelectric properties of defect-containing PbSe–PbTe heterojunction nanostructures

Abstract: A facile and controllable approach has been devised to synthesize PbSe–PbTe heterogeneous nanostructures (HNSs). The defects could be modulated simply by controlling the addition of Se and Te precursors. High-resolution transmission electron microscopy and electron energy-loss spectroscopy show the defect structures in the interface and the distribution of PbSe and PbTe, respectively. Geometric phase analysis based on HRTEM imaging reveals the strain distribution in the defect-free and defect-containing PbSe–PbTe HNSs. The strain distribution and defects in the interface of the PbSe–PbTe HNSs affect the Seebeck coefficient and the electrical conductivity of the PbSe–PbTe HNSs.

Insight into the split and asymmetry of charge distribution in biased M-structure superlattice

Abstract: The charge distribution in real space of an insertion variant based on an InAs/GaSb superlattice for an infrared detector is illustrated by in situ electron microscopy. The localization split of positive charge can be directly observed in the InAs/GaSb/AlSb/GaSb superlattice (M-structure) rather than in the InAs/GaSb superlattice. With the applied bias increasing from 0 to 4.5 V, the double peaks of positive charge density become asymmetrical gradually, with the peak integral ratio ranging from 1.13 to 2.54. Simultaneously, the negative charges move along the direction of the negative electric field. Without inserting the AlSb layer, the charge inversion occurs in both the hole wells and the electron wells of the InAs/GaSb superlattice under high bias. Such a discrepancy between the M-structure superlattice and the traditional superlattice suggests an effective reduction of tunneling probability of the M-structure design. Our result is of great help to understand the carrier immigration mechanism of the superlattice-based infrared detector.

Tailorable coaxial carbon nanocables with high storage capabilities

Abstract: The preparation of nanomaterials into required sizes, structures and components is critical but remains challenging for practical applications. Here, coaxial nanocables were designed with the ability to be tailored into desired diameters, lengths and components through a low-cost, high-efficiency slicing process. The nanocables were radially grown from carbon nanotube cores with coaxial graphene or heteroatom-doped graphene sheet sheaths. This approach allowed the thickness to be fine-tuned, and the increased modulus from the sheath made the nanocables tailorable. The nanocables exhibited an aligned structure and were able to be cut into thin films with accurately controlled thicknesses ranging from tens of nanometers to micrometers while the two ends were left open. These nanocables are promising for the storage of materials and ions, and their incorporation into lithium-ion batteries demonstrates their high specific capacities.

Method for preparing iron di-selenide microtubes and iron di-selenide nanosheets

Abstract: The invention belongs to the technical field of nano functional materials, and particularly relates to a method for preparing iron di-selenide microtubes and iron di-selenide nanosheets with two different types of morphology. Selenium oxide and iron chloride are used as raw materials, ethylene glycol is used as a reducing agent, and polyvinylpyrrolidone is used as a surfactant. Primarily reduced selenium microrods are used as mold plates, and hollow tubular iron di-selenide multi-level structures can be formed. Obtained end products can be transformed into the iron di-selenide nanosheets if dimethyl sub-amide is used as a reducing agent. The method has the advantages that processes for preparing the iron di-selenide microtubes and the iron di-selenide nanosheets are simple, the iron di-selenide microtubes and the iron di-selenide nanosheets are short in preparation period and suitable for industrial mass production, and accordingly the method has a broad application prospect.

Heterogeneous growth of KDP/KT crystal

Abstract: The growth of heterogeneous crystal has aroused a great deal of interest in recent years. In this study, KH2PO4 (KDP)/KTaO3 (KT) heterogeneous crystal is acquired based on the KT substrate. Here, we report the observation of the oriented layer-by-layer structure in KDP/KT composite crystal by scanning electron microscopy (SEM). The structure of KDP/KT composite crystal is accurately identified by transmission electron microscopy (TEM) for the first time and we find that the KT crystals dope into KDP crystal in the growth process with the mode of doping. It can be obtained from the analysis of crystal structure that the structure difference leads to the doping growth mode. Our research demonstrates a facile method to fabricate a composite nonlinear optical crystal based on KDP/KT heterostructure, and might shed light on potential applications of the composite nonlinear optical crystal.