Showing papers on "Carbon nanotube published in 2017"

Conductive MOF electrodes for stable supercapacitors with high areal capacitance

Abstract: Using MOFs as active electrodes in electrochemical double layer capacitors has so far proved difficult. An electrically conductive MOF used as an electrode is now shown to exhibit electrochemical performance similar to most carbon-based materials. Owing to their high power density and superior cyclability relative to batteries, electrochemical double layer capacitors (EDLCs) have emerged as an important electrical energy storage technology that will play a critical role in the large-scale deployment of intermittent renewable energy sources, smart power grids, and electrical vehicles1,2,3. Because the capacitance and charge–discharge rates of EDLCs scale with surface area and electrical conductivity, respectively, porous carbons such as activated carbon, carbon nanotubes and crosslinked or holey graphenes are used exclusively as the active electrode materials in EDLCs4,5,6,7,8,9. One class of materials whose surface area far exceeds that of activated carbons, potentially allowing them to challenge the dominance of carbon electrodes in EDLCs, is metal–organic frameworks (MOFs)10. The high porosity of MOFs, however, is conventionally coupled to very poor electrical conductivity, which has thus far prevented the use of these materials as active electrodes in EDLCs. Here, we show that Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2), a MOF with high electrical conductivity11, can serve as the sole electrode material in an EDLC. This is the first example of a supercapacitor made entirely from neat MOFs as active materials, without conductive additives or other binders. The MOF-based device shows an areal capacitance that exceeds those of most carbon-based materials and capacity retention greater than 90% over 10,000 cycles, in line with commercial devices. Given the established structural and compositional tunability of MOFs, these results herald the advent of a new generation of supercapacitors whose active electrode materials can be tuned rationally, at the molecular level.

Ion sieving in graphene oxide membranes via cationic control of interlayer spacing

Abstract: Graphene oxide membranes-partially oxidized, stacked sheets of graphene-can provide ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution. These materials have shown potential in a variety of applications, including water desalination and purification, gas and ion separation, biosensors, proton conductors, lithium-based batteries and super-capacitors. Unlike the pores of carbon nanotube membranes, which have fixed sizes, the pores of graphene oxide membranes-that is, the interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane)-are of variable size. Furthermore, it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of graphene oxide membranes to swell when immersed in aqueous solution. These challenges hinder the potential ion filtration applications of graphene oxide membranes. Here we demonstrate cationic control of the interlayer spacing of graphene oxide membranes with angstrom precision using K+, Na+, Ca2+, Li+ or Mg2+ ions. Moreover, membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes. First-principles calculations and ultraviolet absorption spectroscopy reveal that the location of the most stable cation adsorption is where oxide groups and aromatic rings coexist. Previous density functional theory computations show that other cations (Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) should have a much stronger cation-π interaction with the graphene sheet than Na+ has, suggesting that other ions could be used to produce a wider range of interlayer spacings.

Tailoring Graphene Oxide-Based Aerogels for Efficient Solar Steam Generation under One Sun.

Abstract: Graphene oxide-based aerogels with carefully tailored properties are developed to enable efficient solar steam generation. Aerogels, with inherent porous structures, are excellent thermal insulators and provide channels for water supply and vapor escape. With enhanced absorption and hydrophilicity by incorporation of carbon nanotubes and sodium alginate, the resulting aerogels can enable efficient (≈83%) solar steam generation under one-sun illumination.

General Oriented Formation of Carbon Nanotubes from Metal–Organic Frameworks

Abstract: Carbon nanotubes (CNTs) are of great interest for many potential applications because of their extraordinary electronic, mechanical and structural properties. However, issues of chaotic staking, high cost and high energy dissipation in the synthesis of CNTs remain to be resolved. Here we develop a facile, general and high-yield strategy for the oriented formation of CNTs from metal–organic frameworks (MOFs) through a low-temperature (as low as 430 °C) pyrolysis process. The selected MOF crystals act as a single precursor for both nanocatalysts and carbon sources. The key to the formation of CNTs is obtaining small nanocatalysts with high activity during the pyrolysis process. This method is successfully extended to obtain various oriented CNT-assembled architectures by modulating the corresponding MOFs, which further homogeneously incorporate heteroatoms into the CNTs. Specifically, nitrogen-doped CNT-assembled hollow structures exhibit excellent performances in both energy conversion and storage. On the ...

Atomic Modulation of FeCo–Nitrogen–Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All-Solid-State Zinc–Air Battery

Abstract: Rational design and exploration of robust and low-cost bifunctional oxygen reduction/evolution electrocatalysts are greatly desired for metal–air batteries. Herein, a novel high-performance oxygen electrode catalyst is developed based on bimetal FeCo nanoparticles encapsulated in in situ grown nitrogen-doped graphitic carbon nanotubes with bamboo-like structure. The obtained catalyst exhibits a positive half-wave potential of 0.92 V (vs the reversible hydrogen electrode, RHE) for oxygen reduction reaction, and a low operating potential of 1.73 V to achieve a 10 mA cm−2 current density for oxygen evolution reaction. The reversible oxygen electrode index is 0.81 V, surpassing that of most highly active bifunctional catalysts reported to date. By combining experimental and simulation studies, a strong synergetic coupling between FeCo alloy and N-doped carbon nanotubes is proposed in producing a favorable local coordination environment and electronic structure, which affords the pyridinic N-rich catalyst surface promoting the reversible oxygen reactions. Impressively, the assembled zinc–air batteries using liquid electrolytes and the all-solid-state batteries with the synthesized bifunctional catalyst as the air electrode demonstrate superior charging–discharging performance, long lifetime, and high flexibility, holding great potential in practical implementation of new-generation powerful rechargeable batteries with portable or even wearable characteristic.

Interwoven MXene Nanosheet/Carbon-Nanotube Composites as Li-S Cathode Hosts.

Abstract: The complex surface chemistry that dictates the interaction between MXene and polysulfides - the formation of thiosulfate via consumption of -OH surface groups, followed by Lewis acid-base interaction between the exposed Ti atoms and polysulfides - is unravelled. Interweaving carbon nanotubes between the MXene layers creates a porous, conductive network with high polysulfide adsorptivity, enabling sulfur hosts with excellent performance even at high loading (5.5 mg cm-2 ).

Highly selective and active CO 2 reduction electrocatalysts based on cobalt phthalocyanine/carbon nanotube hybrid structures

Abstract: Electrochemical reduction of carbon dioxide with renewable energy is a sustainable way of producing carbon-neutral fuels. However, developing active, selective and stable electrocatalysts is challenging and entails material structure design and tailoring across a range of length scales. Here we report a cobalt-phthalocyanine-based high-performance carbon dioxide reduction electrocatalyst material developed with a combined nanoscale and molecular approach. On the nanoscale, cobalt phthalocyanine (CoPc) molecules are uniformly anchored on carbon nanotubes to afford substantially increased current density, improved selectivity for carbon monoxide, and enhanced durability. On the molecular level, the catalytic performance is further enhanced by introducing cyano groups to the CoPc molecule. The resulting hybrid catalyst exhibits >95% Faradaic efficiency for carbon monoxide production in a wide potential range and extraordinary catalytic activity with a current density of 15.0 mA cm−2 and a turnover frequency of 4.1 s−1 at the overpotential of 0.52 V in a near-neutral aqueous solution. Electrochemical reduction of carbon dioxide is a sustainable way of producing carbon-neutral fuels. Here, the authors take a combined nanoscale and molecular approach to develop a highly active and selective cobalt phthalocyanine/carbon nanotube hybrid electrocatalyst for carbon dioxide reduction to carbon monoxide.

Carbon-based supercapacitors for efficient energy storage

Abstract: The advancement of modern electronic devices depends strongly on the highly efficient energy sources possessing high energy density and power density. In this regard, supercapacitors show great promise. Due to the unique hierarchical structure, excellent electrical and mechanical properties, and high specific surface area, carbon nanomaterials (particularly, carbon nanotubes, graphene, mesoporous carbon and their hybrids) have been widely investigated as efficient electrode materials in supercapacitors. This review article summarizes progress in high-performance supercapacitors based on carbon nanomaterials with an emphasis on the design and fabrication of electrode structures and elucidation of charge-storage mechanisms. Recent developments on carbon-based flexible and stretchable supercapacitors for various potential applications, including integrated energy sources, self-powered sensors and wearable electronics, are also discussed.

Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins.

Abstract: Fast water transport through carbon nanotube pores has raised the possibility to use them in the next generation of water treatment technologies. We report that water permeability in 0.8-nanometer-diameter carbon nanotube porins (CNTPs), which confine water down to a single-file chain, exceeds that of biological water transporters and of wider CNT pores by an order of magnitude. Intermolecular hydrogen-bond rearrangement, required for entry into the nanotube, dominates the energy barrier and can be manipulated to enhance water transport rates. CNTPs block anion transport, even at salinities that exceed seawater levels, and their ion selectivity can be tuned to configure them into switchable ionic diodes. These properties make CNTPs a promising material for developing membrane separation technologies.

Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon-Nanotube-Based Electrocatalyst.

Abstract: Ammonia is synthesized directly from water and N2 at room temperature and atmospheric pressure in a flow electrochemical cell operating in gas phase (half-cell for the NH3 synthesis). Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this half-cell. A rate of ammonia formation of 2.2×10−3 gNH3 m−2 h−1 was obtained at room temperature and atmospheric pressure in a flow of N2, with stable behavior for at least 60 h of reaction, under an applied potential of −2.0 V. This value is higher than the rate of ammonia formation obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with a total Faraday efficiency as high as 95.1 % was obtained. Data also indicate that the active sites in NH3 electrocatalytic synthesis may be associated to specific carbon sites formed at the interface between iron particles and CNT and able to activate N2, making it more reactive towards hydrogenation.

Carbon Nanotube-Multilayered Graphene Edge Plane Core-Shell Hybrid Foams for Ultrahigh-Performance Electromagnetic-Interference Shielding.

Abstract: Materials with an ultralow density and ultrahigh electromagnetic-interference (EMI)-shielding performance are highly desirable in fields of aerospace, portable electronics, and so on. Theoretical work predicts that 3D carbon nanotube (CNT)/graphene hybrids are one of the most promising lightweight EMI shielding materials, owing to their unique nanostructures and extraordinary electronic properties. Herein, for the first time, a lightweight, flexible, and conductive CNT-multilayered graphene edge plane (MLGEP) core-shell hybrid foam is fabricated using chemical vapor deposition. MLGEPs are seamlessly grown on the CNTs, and the hybrid foam exhibits excellent EMI shielding effectiveness which exceeds 38.4 or 47.5 dB in X-band at 1.6 mm, while the density is merely 0.0058 or 0.0089 g cm-3 , respectively, which far surpasses the best values of reported carbon-based composite materials. The grafted MLGEPs on CNTs can obviously enhance the penetration losses of microwaves in foams, leading to a greatly improved EMI shielding performance. In addition, the CNT-MLGEP hybrids also exhibit a great potential as nano-reinforcements for fabricating high-strength polymer-based composites. The results provide an alternative approach to fully explore the potentials of CNT and graphene, for developing advanced multifunctional materials.

Metal-Free Carbon Materials for CO2 Electrochemical Reduction.

Abstract: The rapid increase of the CO2 concentration in the Earth's atmosphere has resulted in numerous environmental issues, such as global warming, ocean acidification, melting of the polar ice, rising sea level, and extinction of species. To search for suitable and capable catalytic systems for CO2 conversion, electrochemical reduction of CO2 (CO2 RR) holds great promise. Emerging heterogeneous carbon materials have been considered as promising metal-free electrocatalysts for the CO2 RR, owing to their abundant natural resources, tailorable porous structures, resistance to acids and bases, high-temperature stability, and environmental friendliness. They exhibit remarkable CO2 RR properties, including catalytic activity, long durability, and high selectivity. Here, various carbon materials (e.g., carbon fibers, carbon nanotubes, graphene, diamond, nanoporous carbon, and graphene dots) with heteroatom doping (e.g., N, S, and B) that can be used as metal-free catalysts for the CO2 RR are highlighted. Recent advances regarding the identification of active sites for the CO2 RR and the pathway of reduction of CO2 to the final product are comprehensively reviewed. Additionally, the emerging challenges and some perspectives on the development of heteroatom-doped carbon materials as metal-free electrocatalysts for the CO2 RR are included.

A Review on Design Strategies for Carbon Based Metal Oxides and Sulfides Nanocomposites for High Performance Li and Na Ion Battery Anodes

Abstract: Carbon-oxide and carbon-sulfide nanocomposites have attracted tremendous interest as the anode materials for Li and Na ion batteries. Such composites are fascinating as they often show synergistic effect compared to their singular components. Carbon nanomaterials are often used as the matrix due to their high conductivity, tensile strength, and chemical stability under the battery condition. Metal oxides and sulfides are often used as active material fillers because of their large capacity. Numerous works have shown that by taking one step further into fabricating nanocomposites with rational structure design, much better performance can be achieved. The present review aims to present and discuss the development of carbon-based nanocomposite anodes in both Li ion batteries and Na ion batteries. The authors introduce the individual components in the composites, i.e., carbon matrices (e.g., carbon nanotube, graphene) and metal oxides/sulfides; followed by evaluating how advanced nanostructures benefit from the synergistic effect when put together. Particular attention is placed on strategies employed in fabricating such composites, with examples such as yolk–shell structure, layered-by-layered structure, and composite comprising one or more carbon matrices. Lastly, the authors conclude by highlighting challenges that still persist and their perspective on how to further develop the technologies.

Selective Degradation of Organic Pollutants Using an Efficient Metal-free Catalyst Derived from Carbonized Polypyrrole via Peroxymonosulfate Activation

Abstract: Metal-free carbonaceous materials, including nitrogen-doped graphene and carbon nanotubes, are emerging as alternative catalysts for peroxymonosulfate (PMS) activation to avoid drawbacks of conventional transition metal-containing catalysts, such as the leaching of toxic metal ions. However, these novel carbocatalysts face relatively high cost and complex syntheses, and their activation mechanisms have not been well-understood. Herein, we developed a novel nitrogen-doped carbonaceous nanosphere catalyst by carbonization of polypyrrole, which was prepared through a scalable chemical oxidative polymerization. The defective degree of carbon substrate and amount of nitrogen dopants (i.e., graphitic nitrogen) were modulated by the calcination temperature. The product carbonized at 800 °C (CPPy-F-8) exhibited the best catalytic performance for PMS activation, with 97% phenol degradation efficiency in 120 min. The catalytic system was efficient over a wide pH range (2–9), and the reaction of phenol degradation h...

Enhanced Microwave Absorption Performance of Coated Carbon Nanotubes by Optimizing the Fe3O4 Nanocoating Structure

Abstract: It is well accepted that the microwave absorption performance (MAP) of carbon nanotubes (CNTs) can be enhanced via coating magnetic nanoparticles on their surfaces. However, it is still unclear if the magnetic coating structure has a significant influence on the microwave absorption behavior. In this work, nano-Fe3O4 compact-coated CNTs (FCCs) and Fe3O4 loose-coated CNTs (FLCs) are prepared using a simple solvothermal method. The MAP of the Fe3O4-coated CNTs is shown to be adjustable via controlling the Fe3O4 nanocoating structure. The results reveal that the overall MAP of coated CNTs strongly depends on the magnetic coating structure. In addition, the FCCs show a much better MAP than the FLCs. It is shown that the microwave absorption difference between the FLCs and FCCs is due to the disparate complementarities between the dielectric loss and the magnetic loss, which are related to the coverage density of Fe3O4 nanoparticles on the surfaces of CNTs. For FCCs, the mass ratio of CNTs to Fe3+ is then opti...

Foldable interpenetrated metal-organic frameworks/carbon nanotubes thin film for lithium-sulfur batteries.

Abstract: Lithium-sulfur batteries are promising technologies for powering flexible devices due to their high energy density, low cost and environmental friendliness, when the insulating nature, shuttle effect and volume expansion of sulfur electrodes are well addressed. Here, we report a strategy of using foldable interpenetrated metal-organic frameworks/carbon nanotubes thin film for binder-free advanced lithium-sulfur batteries through a facile confinement conversion. The carbon nanotubes interpenetrate through the metal-organic frameworks crystal and interweave the electrode into a stratified structure to provide both conductivity and structural integrity, while the highly porous metal-organic frameworks endow the electrode with strong sulfur confinement to achieve good cyclability. These hierarchical porous interpenetrated three-dimensional conductive networks with well confined S8 lead to high sulfur loading and utilization, as well as high volumetric energy density.

Self-Templated Formation of Interlaced Carbon Nanotubes Threaded Hollow Co3S4 Nanoboxes for High-Rate and Heat-Resistant Lithium-Sulfur Batteries.

Abstract: Lithium–sulfur batteries (Li–S) have attracted soaring attention due to the particularly high energy density for advanced energy storage system. However, the practical application of Li–S batteries still faces multiple challenges, including the shuttle effect of intermediate polysulfides, the low conductivity of sulfur and the large volume variation of sulfur cathode. To overcome these issues, here we reported a self-templated approach to prepare interconnected carbon nanotubes inserted/wired hollow Co3S4 nanoboxes (CNTs/Co3S4–NBs) as an efficient sulfur host material. Originating from the combination of three-dimensional CNT conductive network and polar Co3S4–NBs, the obtained hybrid nanocomposite of CNTs/Co3S4–NBs can offer ultrahigh charge transfer properties, and efficiently restrain polysulfides in hollow Co3S4–NBs via the synergistic effect of structural confinement and chemical bonding. Benefiting from the above advantages, the S@CNTs/Co3S4–NBs cathode shows a significantly improved electrochemical...

A High‐Efficiency Sulfur/Carbon Composite Based on 3D Graphene Nanosheet@Carbon Nanotube Matrix as Cathode for Lithium–Sulfur Battery

Abstract: Carbon materials have attracted extensive attention as the host materials of sulfur for lithium–sulfur battery, especially those with 3D architectural structure. Here, a novel 3D graphene nanosheet–carbon nanotube (GN–CNT) matrix is obtained through a simple one-pot pyrolysis process. The length and density of CNTs can be readily tuned by altering the additive amount of carbon source (urea). Specifically, CNTs are in situ introduced onto the surface of the graphene nanosheets (GN) and show a stable covalent interaction with GN. Besides, in the GN–CNT matrix, cobalt nanoparticles with different diameters exist as being wrapped in the top of CNTs or scattering on the GN surface, and abundant heteroatoms (N, O) are detected, both of which can help in immobilizing sulfur species. Such a rationally designed 3D GN–CNT matrix makes much more sense in enhancing the electrochemical performance of the sulfur cathode for rapid charge transfer and favorable electrolyte infiltration. Moreover, the presence of dispersed cobalt nanoparticles is beneficial for trapping lithium polysulfides by strong chemical interaction, and facilitating the mutual transformation between the high-order polysulfides and low-order ones. As a result, the S/GN–CNT composite presents a high sulfur utilization and large capacity on the basis of the S/GN–CNT composite as active material.

Synthesis of a carbon nanobelt

Abstract: The synthesis of a carbon nanobelt, comprising a closed loop of fully fused edge-sharing benzene rings, has been an elusive goal in organic chemistry for more than 60 years. Here we report the synthesis of one such compound through iterative Wittig reactions followed by a nickel-mediated aryl-aryl coupling reaction. The cylindrical shape of its belt structure was confirmed by x-ray crystallography, and its fundamental optoelectronic properties were elucidated by ultraviolet-visible absorption, fluorescence, and Raman spectroscopic studies, as well as theoretical calculations. This molecule could potentially serve as a seed for the preparation of structurally well-defined carbon nanotubes.

Preparation and Characterization of Highly Aligned Carbon Nanotubes/Polyacrylonitrile Composite Nanofibers

Abstract: In the electrospinning process, a modified parallel electrode method (MPEM), conducted by placing a positively charged ring between the needle and the parallel electrode collector, was used to fabricate highly aligned carbon nanotubes/polyacrylonitrile (CNTs/PAN) composite nanofibers. Characterizations of the samples—such as morphology, the degree of alignment, and mechanical and conductive properties—were investigated by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), universal testing machine, high-resistance meter, and other methods. The results showed the MPEM could improve the alignment and uniformity of electrospun CNTs/PAN composite nanofibers, and enhance their mechanical and conductive properties. This meant the successful preparation of highly aligned CNT-reinforced PAN nanofibers with enhanced physical properties, suggesting their potential application in appliances and communication areas.

CuCo Bimetallic Oxide Quantum Dot Decorated Nitrogen-Doped Carbon Nanotubes: A High-Efficiency Bifunctional Oxygen Electrode for Zn–Air Batteries

Abstract: The large-scale production of metal–air batteries, an appealing solution for next-generation energy storage, requires low-cost, earth-abundant, and efficient oxygen electrode materials, yet insights into active catalyst structures and synergistic reactivity remain largely unknown. Here, a new bifunctional oxygen electrode based on nitrogen-doped carbon nanotubes decorated by spinel CuCo2O4 quantum dots (CuCo2O4/N-CNTs) is reported, outperforming the benchmark of state-of-the-art noble metal catalysts. Combining spectroscopic characterization and electrochemical studies, a prominent synergetic effect between CuCo2O4 and N-doped carbon nanotubes is uncovered: the high conductivity, large active surface area, and increase in the number of catalytic sites induced by Cu doping (i.e., Cu2+ and CuN) can be beneficial to the overall electrocatalytic activities. Remarkably, the native flexibility of CuCo2O4/N-CNTs allows its direct use as reversible oxygen electrodes in Zn–air batteries either with liquid alkaline electrolyte or in the all-solid-state configuration. The prepared devices demonstrate excellent discharging/charging performance, large energy density (83.83 mW cm−2 in liquid state, 1.86 W g−1 in all-solid-state), and long lifetime (48 h in liquid state, 9 h in all-solid-state), holding great promise in the practical application of rechargeable metal–air batteries and other fuel cells.

"Wiring" Fe-Nx -Embedded Porous Carbon Framework onto 1D Nanotubes for Efficient Oxygen Reduction Reaction in Alkaline and Acidic Media.

Abstract: This study presents a novel metal-organic-framework-engaged synthesis route based on porous tellurium nanotubes as a sacrificial template for hierarchically porous 1D carbon nanotubes. Furthermore, an ultrathin Fe-ion-containing polydopamine layer has been introduced to generate highly effective FeNx C active sites into the carbon framework and to induce a high degree of graphitization. The synergistic effects between the hierarchically porous 1D carbon structure and the embedded FeNx C active sites in the carbon framework manifest in superior catalytic activity toward oxygen reduction reaction (ORR) compared to Pt/C catalyst in both alkaline and acidic media. A rechargeable zinc-air battery assembled in a decoupled configuration with the nonprecious pCNT@Fe@GL/CNF ORR electrode and Ni-Fe LDH/NiF oxygen evolution reaction (OER) electrode exhibits charge-discharge overpotentials similar to the counterparts of Pt/C ORR electrode and IrO2 OER electrode.

Polydopamine-Inspired, Dual Heteroatom-Doped Carbon Nanotubes for Highly Efficient Overall Water Splitting

Abstract: Overall water splitting involved hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are critical for renewable energy conversion and storage. Heteroatom-doped carbon materials have been extensively employed as efficient electrocatalysts for independent HER or OER processes, while those as the bifunctional catalysts for simultaneously generating H-2 and O-2 by splitting water have been seldom reported. Inspired by the unparalleled virtues of polydopamine, the authors devise the facile synthesis of nitrogen and sulfur dual-doped carbon nanotubes with in situ, homogenous and high concentration sulfur doping. The newly developed dual-doped electrocatalysts display superb bifunctional catalytic activities for both HER and OER in alkaline solutions, outperforming all other reported carbon counterparts. Experimental characterizations confirm that the excellent performance is attributed to the multiple doping together with efficient mass and charge transfer, while theoretical computations reveal the promotion effect of secondary sulfur dopant to enhance the spin density of dual-doped samples and consequently to form highly electroactive sites for both HER and OER.

Ti3C2 MXenes modified with in situ grown carbon nanotubes for enhanced electromagnetic wave absorption properties

Abstract: Ti3C2Tx MXenes modified with in situ grown carbon nanotubes (CNTs) are fabricated via a simple catalytic chemical vapor deposition (CVD) process. The as-prepared Ti3C2Tx/CNT nanocomposites show that one-dimensional (1D) carbon nanotubes are uniformly distributed in the interlayers of two-dimensional (2D) Ti3C2Tx MXene flakes. Compared with the pristine Ti3C2Tx MXenes, the hierarchical sandwich microstructure makes a contribution to the excellent electromagnetic wave absorption performance in the frequency range of 2–18 GHz, including higher absorption intensity (the minimum reflection coefficient reaches −52.9 dB, ∼99.999% absorption), broader effective absorption bandwidth (4.46 GHz), lower filler loading (35 wt%) and thinner thickness (only 1.55 mm). In addition, with the adjustment of thickness from 1.55 to 5 mm, the effective absorption bandwidth can reach up to 14.54 GHz (3.46–18 GHz). Different absorption mechanisms mainly based on polarization behaviors and conductivity loss are discussed. This work not only proposes the design of a novel electromagnetic wave absorber, but also provides an effective route for extending further the applications of 2D MXene materials in the field of electromagnetic wave absorption.

Arrays of horizontal carbon nanotubes of controlled chirality grown using designed catalysts

Abstract: The semiconductor industry is increasingly of the view that Moore's law-which predicts the biennial doubling of the number of transistors per microprocessor chip-is nearing its end. Consequently, the pursuit of alternative semiconducting materials for nanoelectronic devices, including single-walled carbon nanotubes (SWNTs), continues. Arrays of horizontal nanotubes are particularly appealing for technological applications because they optimize current output. However, the direct growth of horizontal SWNT arrays with controlled chirality, that would enable the arrays to be adapted for a wider range of applications and ensure the uniformity of the fabricated devices, has not yet been achieved. Here we show that horizontal SWNT arrays with predicted chirality can be grown from the surfaces of solid carbide catalysts by controlling the symmetries of the active catalyst surface. We obtained horizontally aligned metallic SWNT arrays with an average density of more than 20 tubes per micrometre in which 90 per cent of the tubes had chiral indices of (12, 6), and semiconducting SWNT arrays with an average density of more than 10 tubes per micrometre in which 80 per cent of the nanotubes had chiral indices of (8, 4). The nanotubes were grown using uniform size Mo2C and WC solid catalysts. Thermodynamically, the SWNT was selectively nucleated by matching its structural symmetry and diameter with those of the catalyst. We grew nanotubes with chiral indices of (2m, m) (where m is a positive integer), the yield of which could be increased by raising the concentration of carbon to maximize the kinetic growth rate in the chemical vapour deposition process. Compared to previously reported methods, such as cloning, seeding and specific-structure-matching growth, our strategy of controlling the thermodynamics and kinetics offers more degrees of freedom, enabling the chirality of as-grown SWNTs in an array to be tuned, and can also be used to predict the growth conditions required to achieve the desired chiralities.