Showing papers by "Renchao Che published in 2020"

Multidimension-Controllable Synthesis of MOF-Derived Co@N-Doped Carbon Composite with Magnetic-Dielectric Synergy toward Strong Microwave Absorption

Abstract: Metal-organic framework (MOF) is highly desirable as a functional material owing to its low density, tunable pore size, and diversity of coordination formation, but limited by the poor dielectric properties. Herein, by controlling the solvent and mole ratio of cobalt/linker, multidimension-controllable MOF-derived nitrogen-doped carbon materials exhibit tunable morphology from sheet-, flower-, cube-, dodecahedron- to octahedron-like. Tunable electromagnetic parameters of Co@N-doped carbon composites (Co@NC) can be obtained and the initial MOF precursor determines the distribution of carbon framework and magnetic cobalt nanoparticles. Carbonized Co@NC composites possess the following advantages: i) controllable dimension and morphology to balance the electromagnetic properties with evenly charged density distribution; ii) magnetic-carbon composites offer plenty of interfacial polarization and strong magnetic coupling network; iii) a MOF-derived dielectric carbon skeleton provides electronic transportation paths and enhances conductive dissipation. Surface-mediated magnetic coupling reflects the stray magnetic flux field, which is corroborated by the off-axis electron holography and micro-magnetic simulation. Optimized octadecahedral Co@NC sample exhibits the best microwave absorption (MA) of -53.0 dB at the thickness of 1.8 mm and broad effective frequency from 11.4 to 17.6 GHz (Ku-band). These results pave the way to fabricate high-performance MA materials with balanced electromagnetic distribution and controlled morphology.

Galvanic Replacement Reaction Involving Core–Shell Magnetic Chains and Orientation‐Tunable Microwave Absorption Properties

Abstract: Electromagnetic (EM) wave absorption materials have attracted considerable attention because of EM wave pollution caused by the proliferation of electronic communication devices. One-dimentional (1D) structural magnetic metals have potential as EM absorption materials. However, fabricating 1D core-shell bimetallic magnetic species is a significant challenge. Herein, 1D core-shell bimetallic magnetic chains are successfully prepared through a modified galvanic replacement reaction under an external magnetic field, which could facilitate the preparation of 1D core-shell noble magnetic chains. By delicately designing the orientation of bimetallic magnetic chains in polyvinylidene fluoride, the composites reveal the decreased complex permittivity and increased permeability compared with random counterparts. Thus, elevated EM wave absorption perfromances including an optimal reflection loss of -43.5 dB and an effective bandwidth of 7.3 GHz could be achieved for the oriented Cu@Co sample. Off-axis electron holograms indicate that the augmented magnetic coupling and remarkable polarization loss primarily contribute to EM absorption in addition to the antenna effect of the 1D structure to scatter microwaves and ohmic loss of the metallic attribute. This work can serve a guide to construct 1D core-shell bimetallic magnetic nanostructures and design magnetic configuration in polymer to tune EM parameters and strengthen EM absorption properties.

MOF-Derived Ni1-xCox@Carbon with Tunable Nano-Microstructure as Lightweight and Highly Efficient Electromagnetic Wave Absorber.

Abstract: . Intrinsic electric-magnetic property and special nano-micro architecture of functional materials have a significant effect on its electromagnetic wave energy conversion, especially in the microwave absorption (MA) field. Herein, porous Ni1−xCox@Carbon composites derived from metal-organic framework (MOF) were successfully synthesized via solvothermal reaction and subsequent annealing treatments. Benefiting from the coordination, carbonized bimetallic Ni-Co-MOF maintained its initial skeleton and transformed into magnetic-carbon composites with tunable nano-micro structure. During the thermal decomposition, generated magnetic particles/clusters acted as a catalyst to promote the carbon sp2 arrangement, forming special core-shell architecture. Therefore, pure Ni@C microspheres displayed strong MA behaviors than other Ni1−xCox@Carbon composites. Surprisingly, magnetic-dielectric Ni@C composites possessed the strongest reflection loss value − 59.5 dB and the effective absorption frequency covered as wide as 4.7 GHz. Meanwhile, the MA capacity also can be boosted by adjusting the absorber content from 25% to 40%. Magnetic–dielectric synergy effect of MOF-derived Ni1−xCox@Carbon microspheres was confirmed by the off-axis electron holography technology making a thorough inquiry in the MA mechanism.

Rational design of 2D hierarchically laminated Fe3O4@nanoporous carbon@rGO nanocomposites with strong magnetic coupling for excellent electromagnetic absorption applications

Abstract: Developing electromagnetic absorption materials with a strong absorption ability and wide absorption bandwidth has attracted widespread attention in the field of electromagnetic shielding, but it still remains a great challenge. Herein, we successfully developed 2D hierarchically laminated Fe3O4@nanoporous carbon (NPC)@rGO magnetic/dielectric nanocomposites as high-performance microwave absorbers through a facile microwave-assisted approach. The rational design of the composition (Fe3O4, NPC and rGO) and the hierarchical microstructure provided the nanocomposite with a micro-scale 3D magnetic coupling network, a hierarchical dielectric carbon network and good impedance matching, which were identified by the off-axis electronic holography and electromagnetic characterization. As expected, the Fe3O4@NPC@rGO composites achieved a strong reflection loss of −72.6 dB, a matching thickness of 2.0 mm and a broad bandwidth of 5.5 GHz. Such excellent achievements encourage the development of hierarchical magnetic EMA absorbers and provide remarkable inspiration for designing high-performance microwave absorbers.

Covalent Assembly of MoS2 Nanosheets with SnS Nanodots as Linkages for Lithium/Sodium‐Ion Batteries

Abstract: Weak van der Waals interactions between interlayers of two-dimensional layered materials result in disabled across-interlayer electron transfer and poor layered structural stability, seriously deteriorating their performance in energy applications. Herein, we propose a novel covalent assembly strategy for MoS2 nanosheets to realize unique MoS2 /SnS hollow superassemblies (HSs) by using SnS nanodots as covalent linkages. The covalent assembly based on all-inorganic and carbon-free concept enables effective across-interlayer electron transfer, facilitated ion diffusion kinetics, and outstanding mechanical stability, which are evidenced by experimental characterization, DFT calculations, and mechanical simulations. Consequently, the MoS2 /SnS HSs exhibit superb rate performance and long cycling stability in lithium-ion batteries, representing the best comprehensive performance in carbon-free MoS2 -based anodes to date. Moreover, the MoS2 /SnS HSs also show excellent sodium storage performance in sodium-ion batteries.

Magnetized MXene Microspheres with Multiscale Magnetic Coupling and Enhanced Polarized Interfaces for Distinct Microwave Absorption via a Spray-Drying Method.

Abstract: As a typical 2D (two dimensional) material, Ti3C2Tx, has been used as a promising microwave absorber (MA) because of its massive interface architecture, abundant natural defects, and chemical surface functional groups. However, its single dielectric-type loss and excessive high conductivity seriously restrict the further enhancement of MA performance. Herein, we first describe a simple spray-drying routine to reshape the 2D MXene into a confined and magnetized microsphere with tightly embedded Fe3O4 nanospheres (designated as M/F), contributing to the enhanced specific interfaces and strong dielectric polarization. These Fe3O4 magnetic units are highly dispersed into the dielectric Mxene framework, leading to the optimized impedance balance and electromagnetic coordination capability. This composite way effectively exceeds the conventionally physical mixing, simple loading, and local phase separation method. Meanwhile, strong magnetic loss capability with significantly improved magnetic flux line density is achieved from microscale MXene and nanoscale Fe3O4, confirming our 3D multiscale magnetic coupling network. Accordingly, the M/F composites hold distinct microwave absorption property with the strong reflection loss (-50.6 dB) and effective absorption bandwidth (4.67 GHz) at the thickness as thin as only 2 mm. Our encouraging strategy provides important designable implications for MXene-based functional materials and high-performance absorbers.

MOF Induces 2D GO to Assemble into 3D Accordion-Like Composites for Tunable and Optimized Microwave Absorption Performance.

Abstract: Three-dimensional (3D) materials assembled by 2D layered lamella can provide abundant interfaces which are greatly advantageous for high-performance microwave absorbers. Herein, accordion-like CeO2- x /reduced graphene oxide (CeO2- x /RGO) hybrid materials can be successfully synthesized by a solvothermal and hydrothermal method, which are composed of laminated RGO sheets and confined CeO2- x nanoparticles (NPs). The multilayer structure is attributed to the process of Ce-MOF dissolving into NPs, then the NPs combining with graphene oxide (GO) to induce the 2D GO assembled into 3D accordion-like composites. The 3D accordion-like CeO2- x /RGO simultaneously utilizes the insulated CeO2- x and highly conductive RGO to assemble into the laminated structure with moderate electromagnetic parameters. The 3D-laminated lightweight CeO2- x /RGO composite exhibits excellent attenuation ability of an ultrabroad bandwidth (5.84 GHz) and a maximum reflection loss (-50.6 dB) which can be ascribed from the glorious impedance matching, synergistic effect between RGO sheets and the embedded CeO2- x NPs. An off-axis electron holography is carried out to visualize the spatial electrical potential and charge distribution around the CeO2- x /RGO heterojunction, which clarifies the dipole polarization and interfacial polarization. This work enlightens a simple strategy to fabricate an excellent 3D laminated RGO-based microwave absorber.

Engineering Phase Transformation of MoS2/RGO by N-doping as an Excellent Microwave Absorber

Abstract: As a hot two-dimensional (2D) material, molybdenum disulfide has been attracting extensive attention for electromagnetic wave response applications because of its unique structure. However, the electronic conductivity of nanostructured MoS2 needs to be optimized urgently. Here, nitrogen-doped 1T@2H-MoS2/reduced graphene oxide (RGO) composites are effectively constructed by hydrothermal reaction and consecutive calcination under an NH3 atmosphere. The prepared composites possess great microwave absorption (MA) performance with an expected absorption bandwidth (4.00 GHz) at the Ku band and a maximum reflection loss value (-67.77 dB), which is much better than the performance of conventional 2H-MoS2 or 2H-MoS2/RGO. The prominent absorption property is ascribed to the (i) unique self-assemble morphology of rose-like MoS2 supported on 2D RGO; (ii) controllable crystalline phase switch between 2H and 1T; and (iii) brilliant energy attenuation caused by the intense multipolarization. Furthermore, the dominant MA mechanism is described as the local polarization motivated by the interaction between RGO and MoS2. Thus, our novel structure design provides a necessary reference to achieve optimized absorption performance based on 2D materials.

Guided-formation of a favorable interface for stabilizing Na metal solid-state batteries

Abstract: The sodium (Na) anode suffers severe interfacial resistance and dendrite issues in a classic NASICON-type Na3Zr2Si2PO12 (NZSP) electrolyte, resulting in poor electrochemical performance for solid-state Na metal batteries. There has been little success in the reduction of interfacial resistance in recent years. The exact mechanism of this resistance has not been fully understood because of little information about the interface. In this work, we effectively address the large interfacial resistance issue and the metal dendrite problem between the Na anode and NZSP by introducing a TiO2 film as an active interphase. We employ quasiinsitu X-ray photoelectron spectroscopy (XPS) to uncover the interphase formation mechanism at the Na/TiO2–NZSP electrolyte interface. Our quasiinsitu XPS results confirm the formation of a sodiated-TiO2 interphase upon stepwise Na evaporation on the surface of the NZSP electrolyte. Further investigation by molten Na contact angle measurements, impedance spectroscopy and DFT calculations demonstrates that the sodiated-TiO2 interphase promotes Na ion transport between the Na anode and NZSP electrolyte. Moreover, the electrostatic potential formed at the NZSP/NaxTiO2 interface can effectively reduce electronic conductivity at the interface and hence prevent the growth of sodium dendrites. A representative paradigm for interphase design is provided to address the interface contact for developing stable solid-state batteries with high performance.

Tuning strain effect and surface composition in PdAu hollow nanospheres as highly efficient ORR electrocatalysts and SERS substrates

Abstract: Although palladium-gold (PdAu) bimetals have been extensively investigated as oxygen reduction reaction (ORR) electrocatalysts, the comprehensive mechanism studies on the strain and electronic effect are in their infancy. Herein, the strain effect and surface composition of PdAu hollow nanospheres (PdAu HSs) can be tuned by Pd-to-Au ratio. The optimal ORR performance can deliver the half-wave potential of 0.866 V versus reversible hydrogen electrode (vs. RHE) even superior to commercial Pt/C, which can be attributed to sufficient strain effect, local micro-electric field and the adsorption strength of oxygen-containing species in moderation. In addition, surface-enhanced Raman scattering (SERS) performances further supported the variation trend of surface composition. The enhancement factor (EF) of Au HSs can reach 3.8 × 105, further indicating its great potential as efficient SERS substrates for analysis and detection applications.

Self-transforming ultrathin α-Co(OH) 2 nanosheet arrays from metal-organic framework modified graphene oxide with sandwichlike structure for efficient electrocatalytic oxygen evolution

Abstract: Developing efficient and low-cost electrocatalysts for oxygen evolution reaction (OER) with high electrochemical activity and durability for diverse renewable and sustainable energy technologies remains challenging. Herein, an ultrasonic-assisted and coordination modulation strategy is developed to construct sandwich-like metal-organic framework (MOF) derived hydroxide nanosheet (NS) arrays/graphene oxide (GO) composite via one-step self-transformation route. Inducing from unsteady state, the dodecahedral ZIF-67 with Co2+ in tetrahedral coordination auto-converts into defect-rich ultrathin layered hydroxides with the interlayered ion NO3-. The self-transforming a-Co(OH)2/GO nanosheet arrays from ZIF-67 (Co(OH)2-GNS) change the coordination mode of Co2+ and bring about the exposure of more metal active sites, thereby enhancing the spatial utilization ratio within the framework. As monometal-based electrocatalyst, the optimized Co(OH)2-GNS exhibits remarkable OER catalytic performance evidenced by a low overpotential of 259 mV to achieve a current density of 10 mA·cm-2 in alkaline medium, even exceeding commercial RuO2. During the oxygen evolution process, electron migration can be accelerated by the interfacial/in-plane charge polarization and local electric field, corroborated by the off-axis electron holography. Density functional theory (DFT) calculations further studied the collaboration between ultrathin Co(OH)2 NS and GO, which leads to lower energy barriers of intermediate products and greatly promotes electrocatalytic property.

Dandelion-like carbon nanotube assembly embedded with closely separated Co nanoparticles for high-performance microwave absorption materials

Abstract: Enhancing the magnetic loss capacity by microstructure design remains a considerable challenge in the microwave absorption field. Herein, a high-performance microwave absorbent is developed by dispersing a considerable amount of magnetic nanoparticles within the dandelion-like carbon nanotube assembly. A controllable fabrication method is further exploited to adjust the distribution feature of these embedded nanomagnets. In such a hierarchical composite, parts of the interaction network among the coupled closely spaced nanomagnets can be frequently broken and rebuilt to intensively dissipate the microwave energy, which is confirmed by electron holography and micromagnetic simulation for the first time. By virtue of this dynamic magnetic coupling network mechanism, the hierarchical C/Co composite acquires the first-rate microwave absorption performance. The maximum reflection loss value reaches as much as −52.9 dB (absorbance >0.99999) and the effective absorption bandwidth (absorbance >0.9) occupies the entire X band. It is believed that the above insightful mechanism provides a new opportunity to lower the density of the magnet-based microwave absorbent as much as possible. Besides, the unique method for dispersing magnetic nanoparticles also broadens the pathway to assemble the hierarchical architecture.

Highly Compressible Polymer Composite Foams with Thermal Heating-Boosted Electromagnetic Wave Absorption Abilities.

Abstract: Polymer composite foams are desirable materials for electromagnetic (EM) energy attenuation. However, a number of challenges limit improvement in the EM energy attenuation properties of foams. In this study, a simple microcellular injection molding method was used to fabricate highly compressible thermoplastic urethane (TPU)/carbon nanotube (CNTs) composite foams, which also had increased conductivity with an increase in CNT content. Compared to unfoamed composites, foamed composites exhibited higher conductivity and EM attenuation properties because of the presence of a microcellular structure. Moreover, the TPU/CNT foam with 4 wt % CNTs (F(4)) demonstrated strong EM dissipation and an optimal reflection loss (RL) value of -30.4 dB. Furthermore, stimulated by thermal heating and cyclic compression, EM attenuation was observed to increase because of the higher conductivity. Note that F(4) foam having a small thickness of 1.3 mm when treated at 333 K had the highest EM dissipation and the lowest RL value of -51.8 dB. Enhanced polarization and ohmic losses and multiscattering were responsible for the increased EM absorption. This behavior is attributed to the movement of CNTs within the TPU elastomer walls via thermal or compression stimulation. For designing stimulation-dependent multifunctional materials, composite foams with response to thermal heating were proved to be an alternative approach.

Hierarchical coupling effect in hollow Ni/NiFe2O4-CNTs microsphere via spray-drying for enhanced oxygen evolution electrocatalysis

Abstract: Design and fabrication of cost-effective transition metal and their oxides-based nanocomposites are of paramount significance for metal-air batteries and water-splitting. However, the traditional optimized designs for nanostructure are complicated, low-efficient and underperform for wide-scale applications. Herein, a novel hierarchical framework of hollow Ni/NiFe2O4-CNTs composite microsphere forcibly-assembled by zero-dimensional (0D) Ni/NiFe2O4 nanoparticle (< 16 nm) and one-dimensional (1D) self-supporting CNTs was fabricated successfully. Benefitted from the unique nanostructure, such monohybrids can achieve remarkable oxygen evolution reaction (OER) performance in alkaline media with a low overpotential and superior durability, which exceeds most of the commercial catalysts based on IrO2/RuO2 or other non-noble metal nanomaterials. The enhanced OER performance of Ni/NiFe2O4-CNTs composite is mainly ascribed to the increased catalytic activity and the optimized conductivity induced by the effects of strong hierarchical coupling and charge transfers between CNTs and Ni/NiFe2O4 nanoparticles. These effects are greatly boosted by the polarized heterojunction interfaces confirmed by electron holography. The density functional theory (DFT) calculation indicates the epitaxial Ni further enriches the intrinsic electrons contents of NiFe2O4 and thus accelerates absorption/desorption kinetics of OER intermediates. This work hereby paves a facile route to construct the hollow composite microsphere with excellent OER electrocatalytic activity based on non-noble metal oxide/CNTs.

Improved microwave absorption performance of a multi-dimensional Fe2O3/CNTCM@CN assembly achieved by enhanced dielectric relaxation

Abstract: The large-scale preparation of a homogeneous multi-dimensional assembly for microwave absorbers is still believed to be a huge challenge. Herein, a novel class of Fe2O3/CNTCM@CN hybrids featuring a three-dimensional (3D) hollow structure (d < 6.88 μm) was successfully fabricated by a facile spray-drying process, followed by an annealing treatment. Magnetic-dielectric composite microspheres were obtained by forcibly assembling 0D γ-Fe2O3 nanoparticles, 1D carbon nanotubes (CNTs), and 2D N-doped carbon layers. Due to the unique design of its interface structure and abundant electron conduction paths, Fe2O3/CNTCM@CN-2 exhibited excellent microwave absorption (MA) performance. Impressively, the MA value was enhanced to −51.5 dB for Fe2O3/CNTCM@CN-2 with a thickness of only 2 mm, and the efficient absorption bandwidth (<10 dB) was broadened to 5.4 GHz with only 10% loading mass; these values have surpassed those of most of the state-of-art MA materials. In this heterogeneous system, remarkable multi-interface regions were built by compressive contact among CNTs, nanoparticles, and N-doped carbon layers, which played a key role in promoting the interfacial polarization. Electron charge density redistribution observed by electron holography further supports the presence of dielectric relaxation locally around these contact interfaces. The special 3D CNT networks and bridged N-doped carbon layers enriched the electron transport routes and optimized the conductivity. In addition, spray-dried Fe2O3/CNTCM@CN absorbers at micron-size exhibited a better electromagnetic and size matching. Therefore, this study provides deep insights into a novel strategy for the construction of multi-dimensional hollow assemblies as MA candidates.

In situ dynamics response mechanism of the tunable length-diameter ratio nanochains for excellent microwave absorber

Abstract: Faster response benefits the high-performance of magnetic material in various live applications. Hence, enhancing response speed toward the applied field via engineering advantages in structures is highly desired. In this paper, the precise synthesis of Co nanochain with the tunable length-diameter ratio is realized via a magnetic-field-guided assembly approach. The Co nanochain exhibits enhanced microwave absorption performance (near to -60 dB, layer thickness 2.2 mm) and broader effective absorption bandwidth (over 2/3 of total S, C, X, Ku bands). Furthermore, the simulated dynamic magnetic response reveals that the domain motion in 1D chain is faster than that in 0D nanoparticle, which is the determining factor of magnetic loss upgrade. Meanwhile, based on the controllable magnetic field experiment via in situ transmission electron microscopy, the association between magnetic response and microstructure is first present at the nanometer-level. The real and imaginary parts of relative complex permeability are determined by the domain migration confined inside Co nanochain and the magnetic flux field surrounded outside Co nanochain, respectively. Importantly, these findings can be extended to the novel design of microwave absorbers and promising candidates of magnetic carriers based on 1D structure.

Rutile TiO2 Nanoparticles Encapsulated in a Zeolitic Imidazolate Framework-Derived Hierarchical Carbon Framework with Engineered Dielectricity as an Excellent Microwave Absorber.

Abstract: Aiming to solve the poor response of titanium dioxide (TiO2) in the microwave frequency, versatile series of N-doped carbon (NC) components are employed to improve the conductivity and polarization...

Giant Topological Hall Effect and Superstable Spontaneous Skyrmions below 330 K in a Centrosymmetric Complex Noncollinear Ferromagnet NdMn2Ge2.

Abstract: Skyrmions with topologically nontrivial spin textures are promising information carriers in next-generation ultralow power consumption and high-density spintronic devices. To promote their further development and utilization, the search for new room temperature skyrmion-hosting materials is crucial. Considering that most of the previous skyrmion-hosting materials are noncollinear magnets, here, the detection of the topological Hall effect (THE) and the discovery of skyrmions at room temperature are first reported in a centrosymmetric complex noncollinear ferromagnet NdMn2Ge2. Below 330 K, the compound can host stable Bloch-type skyrmions with about 75 nm diameter in a wide window of magnetic field and temperature, including zero magnetic field and room temperature. Moreover, the skyrmions can induce a giant topological Hall effect in a wide temperature range with a maximum value of -2.05 μΩ cm. These features make the compound attractive for both fundamental research and potential application in novel spintronic devices.

Polarization-enhanced three-dimensional Co3O4/MoO2/C flowers as efficient microwave absorbers

Abstract: Three-dimensional (3D) hierarchical structures have been widely employed to enhance the microwave absorption performance due to their large surface area, satisfied impedance matching and polymorphic morphology. However, to achieve multiphase and multifunctional performance only within a primary unit remains a tremendous challenge. Herein, special carbon-based C/MoO2/Co3O4 nanosheets were assembled into a flower-like hierarchical architecture by a facile coordination-polymerization method. Abundant Co3O4 magnetic nanoparticles were anchored onto carbon nanoflake substrates, leading to both enhanced polarization and enhanced magnetic coupling. Meanwhile, a large number of heterogeneous junction interfaces among MoO2 nanoparticles and carbon substrates enhanced the interface polarization. Therefore, the C/MoO2/Co3O4 microflower achieved a minimum reflection loss of −59.7 dB at 17.7 GHz with a thickness of only 1.2 mm, indicative of its great potential as a lightweight microwave absorption material.