ISSN : 0008-6223
Published by: Elsevier BV (10.1016)
Total articles ≅ 24,763
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The broadband electromagnetic wave absorption (EMWA) property of a material is governed by its electromagnetic impedance matching, but an effective regulation strategy to enhance the impedance matching is still lacking in the existing literature. A reasonable component composition and micro-structural design are very important for EMWA materials. In this work, through in situ pyrolysis of two polymetallic metal-organic frameworks (MOFs): CoMn-MOF and FeCoMn-MOF, Co and FeCo are subtly introduced into [email protected] carbon (NPC) composite to improve the permeability and the impedance matching, which leads to broadband EMWA performance. Co/[email protected] has an effective absorption bandwidth (EAB; reflection loss ≤ −10 dB) of 5.44 GHz from 12.56 to 18 GHz at 1.58 mm-thickness. FeCo/[email protected] has an EAB of 7.72 GHz from 10.28 to 18 GHz at 1.63 mm-thickness, and when the thickness is 2.37 mm, the reflection loss reaches −54.07 dB. The excellent broadband EMWA performances of the corn-like composites are attributed to the conductive network of carbon matrix, the optimized dielectric constant provided by MnO, the dramatically enhanced permeability due to the dispersed Co and FeCo, and the multi-polarization loss among multiple components. Overall, this work provides an effective strategy to improve the impedance matching and broaden the EAB of absorbers.
The N-doped carbon materials have natural advantages as the potential catalyst of oxygen reduction reaction (ORR). However, the lack of practical synthesis techniques for enabling highly efficient ORR performance hinders the go-ahead of robust carbocatalysts. Here, we design and synthesize nitrogen-doped carbon nanosheets (N-CN-Fd) with abundant activate sites from Sterculia lychnophora via a brief freeze-drying technology. This universal route promotes the specific surface areas, strengthens the degree of graphitization and increases the atom ratio of electron-donating groups of N-CN-Fd. The obtained N-CN-Fd displays an onset potential of 0.91 V, a half-wave potential of 0.79 V, and a limited-diffusion current density of 3.6 mA/cm2 for ORR under alkaline conditions, comparable to commercial Pt/C. Meanwhile, a four-electron ORR pathway is revealed and N-CN-Fd maintains higher stability than Pt/C in chronoamperometric response. Moreover, the advantage of the N-CN-Fd catalyst in increasing the discharge-charge capacity and improving the cycling stability of the Li-O2 battery is demonstrated. The theoretical calculation result illustrates that the abundant electron-donating surface groups on the samples able to accomplish the activation of ortho-C atoms, which endow N-CN-Fd the excellent ORR activity. Our findings provide guidelines for designing various carbon-based electrocatalysts with multiple active sites through freeze-drying crafts.
Twin graphene can introduce stable and extended defects to energy band engineering, giving unique electron transport properties, which is expected to have potential applications in the fields of magnetism, spin transport or photoluminescence. This study proposes a method of using electric-field-assisted temporally-shaped femtosecond laser ablation liquid (ETLAL) of graphene dispersion to prepare graphene quantum dots (GQDs) with an average particle size of 2-3 nm and oxygen-containing functional groups modified surface, which realizes the controllable preparation of single crystal and twin GQDs (5-fold twin). This method controls the crystallinity of GQDs from two aspects: (1) The intervention of an electric field can rapidly command the directional motion of the cavitation bubbles and the nanoparticles contained therein to collide and crystallize at higher temperatures and pressures, which is the key to the formation of twin GQD; (2) Adjusting temporally-shaped femtosecond laser pulse delay could control the proportion of the Coulomb explosion during the ablation process, which increases the carbon cluster supplied by the cavitation bubble which is the key of polyploid number in twin GQDs. This research provides a fast, green strategy for the preparation of unprecedented twin GQDs, which is of great significance to the applications in the field of 2D material defect engineering.
Using density functional theory with the generalized gradient approximation, we explored the geometric and electronic structure of polymerized spiro[4,4]nonatetraene (spiro-graphene) as a possible two-dimensional carbon allotrope comprising sp2 and sp3 C atoms. By reflecting a shape of the hydrocarbon molecule, this two-dimensional allotrope has a covalent network of fused pentagons with nanometer-scale structural rippling. The covalent network is thermally and dynamically stable with a relatively high total energy, higher than graphene by 0.6 eV/atom. The spiro-graphene is a metal where two linear dispersion bands cross each other at the Fermi level. In addition to these linear dispersion bands, the covalent network possesses the Dirac nodal line just above the Fermi level and along the Brillouin zone boundary. Accordingly, the ribbons with zigzag edges derived from spiro-graphene possess edge states like those of graphene nanoribbons with zigzag edges.
Single-atom catalysts have been extensively studied due to the high atom utilization efficiency. However, the atomically dispersed dual-metal-site material for bifunctional catalysis in zinc-air battery is still rare. Herein, taking advantage of the trapping ability of graphene oxide to anchor metal ions, a dual-metal-site bifunctional catalyst with atomically dispersed FeN4 and NiN4 in the nitrogen doped graphene (Fe/Ni(1:3)-NG) is developed. Benefiting from the synergy effect between Fe and Ni, the catalytic performance can be greatly enhanced. The resultant catalyst exhibits highly efficient bifunctional catalytic activity, with the half-wave potential of 0.842 V for oxygen reduction reaction (ORR) and an overpotential of 480 mV at the current of 10 mA cm-2 for oxygen evolution reaction (OER). On the basis of bifunctional catalytic activities, the Fe/Ni(1:3)-NG based zinc-air battery shows an excellent power density (164.1 mW cm-2), outstanding specific capacity (824.3 mAh g-1) along with the prominent durability.
Strengthening effect of carbon nanotubes (CNTs) reinforced metal matrix composites (MMCs) mainly depends on the interfacial bonding strength which has a decisive influence on comprehensive properties of the composites. However, in CNTs reinforced Cu matrix composites, poor interface bonding due to the lack of wettability between CNTs and Cu matrix has become a bottleneck problem restricting the load-transfer capacity of CNTs. Different from the conventional thoughts of producing interfacial carbides by in-situ reaction with CNTs which will lead to deterioration of structural integrity of CNTs, a novel strategy is used by forming TiC on the surface of intact CNTs through solid phase reaction between the pre-milled CNTs and the Ti powders. The mechanical properties of the composites can be regulated by tailoring the morphology and amount of TiC decorated at the interface. Strong interfacial bonding achieved due to the specially designed interface structure which contributes to the dramatic improvement of strengthening efficiency of CNTs and benefits the plastic deformation of Cu matrix. Consequently, a simultaneous enhancement of tensile strength (281 MPa) and tensile elongation (20.1%) is achieved in the composite with TiC decorated at the interface, and a super high strengthening efficiency (135) is realized. The tactic used in our study can provide a reference for the preparation of other CNTs/MMCs.
Single-layer carbon dots (CDs) are used as capping agents to synthesize silver nanoparticles (AgNPs). The obtained nanohybrids (AgNPs/CDs) can be self-assembled into nanostructures like nanochains (1D-AgNPs/CDs), nanoflats (2D-AgNPs/CDs) or nanobodys (3D-AgNPs/CDs) by simply tuning the amount of added CDs. It is found that CDs play a key role in controlling the aggregation AgNPs/CDs. The formation mechanisms of the AgNPs/CDs aggregates have been discussed. On the basis, the effects of aggregation dimension on the surface plasmon absorption and surface enhanced Raman spectroscopy (SERS) activity of AgNPs/CDs are investigated and discussed. It is found that the aggregation of AgNPs/CDs creates strong localized surface plasmon resonance. Furthermore, the aggregated AgNPs/CDs of different dimension have similar absorption intensity in the range from 500 to 800 nm, which is most commonly used in the surface enhanced Raman spectroscopy (SERS) measurement. The outstanding local electromagnetic field of the aggregated AgNPs and the enrichment effect of the CDs towards the analytes make all the obtained materials to be SERS substrates. In particular, 2D-AgNPs/CDs exhibit an ultrahigh enhancement factor of 4.02×1014 and a good reproducibility during the SERS test, using crystal violet as the model target molecule. The applicability of 2D-AgNPs/CDs in SERS detection is further confirmed by the measurement of trace thiram residual in apple.
Carbon fibre electrodes can enable a solid-state battery to carry mechanical load as normal construction materials. The multifunctionality is promising for most lightweight applications. Like all electrode materials, both volume and elastic moduli of the carbon fibre electrodes change during battery cycling. Such changes jeopardize the mechanical integrity of the battery. Due to the challenging corrosion problem of the lithiated component in air, the effect of lithiation on the carbon fibre’s elastic moduli has yet to be explored. Also, robust data on the expansion of carbon fibres from lithiation are lacking. In the present work, we demonstrate a method and perform tests of corrosion protected carbon fibres in scanning electron microscope. The volume, and longitudinal and transverse moduli of a carbon fibre at three states of lithiation are determined and compared. The transverse modulus of the lithiated fibre is found to be more than double that of the pristine and delithiated fibres.
Carbon nanotubes (CNTs) are promising for realizing ultrafast membranes with implications to molecular separations and beyond. However, it is a big challenge to harness the potential of CNTs for designing scalable yet high-performance membranes. Here we systematically explore the role of loading and vacuum-assisted alignment of CNTs for improving the desalination performance of polyamide (PA) based thin-film composites. To rule out the dispersion instability issues, we focused on carboxylated single-walled CNTs (SWCNTs) commercially available in the market. After applying a pre-treatment for cleaning, we deposited SWCNTs on porous polysulfone supports by vacuum filtration and coated a PA layer on top via interfacial polymerization. Morphological assessments supported by polarized Raman microspectroscopy allowed the quantification of SWCNT alignment. At an optimum SWCNT loading, which we found critical for alignment, the water permeability of resulting membranes significantly improved without compromising NaCl selectivity. Also, we achieved an improved boric acid selectivity, arguably owing to the hydrophobic nature of nanotube channels. Moreover, nanotubes promoted resistance against chlorine degradation and improved mechanical strength. Vacuum deposition is instrumental for infiltrating SWCNTs into the support layer, but a mat layer forms between the support and PA layers when SWCNT loading exceeds the limit that the support pores can accommodate. Given that we use ordinary SWCNTs and a scalable methodology (vacuum-assisted infiltration), the developed membranes are promising for practical applications.
The temperature-dependent (T-dependent) linewidth (ΓG) and frequency shift (ΔωG) of the G mode provide valuable information on the phonon anharmonicity of graphene-based materials. In contrast to the negligible contribution from electron-phonon coupling (EPC) to the linewidth of a Raman mode in semiconductors, ΓG in pristine graphene is dominated by EPC contribution at room temperature due to its semimetallic characteristics. This leads to difficulty in resolving intrinsic contribution from phonon anharmonicity to ΓG. Here, we probed the intrinsic phonon anharmonicity of heavily-doped graphene by T-dependent Raman spectra based on FeCl3-based stage-1 graphite intercalation compound (GIC), in which the EPC contribution is negligible due to the large Fermi level (EF) shift. The ΔωG and ΓG exhibit a nonlinear decrease and noticeable broadening with increasing temperature, respectively, which are both dominated by phonon anharmonicity processes. The contribution of phonon anharmonicity to ΓG of heavily-doped graphene decreases as the EF approaches to the Dirac point. However, the T dependence of ΔωG is almost independent on EF and qualitatively agrees with the theoretical result of pristine graphene. These results provide a deeper understanding of the role of phonon anharmonicity on the Raman spectra of heavily doped graphene.