Search

Octahedral molecular sieves (OMSs) based on MnO

Two-dimensional (2D) WS2 offers great prospects for assembling next-generation optoelectronic and electronic devices due to its thickness-dependent optical and electronic properties. However, layer-number-controlled growth of WS2 is still a challenge up to now. This work presents controlled growth of bilayer WS2 triangular flakes by carbon-nanoparticle-assisted chemical vapor deposition (CVD) process. The growth mechanism is also proposed. In addition, the field effect transistors (FETs) based on monolayer and bilayer WS2 are also fabricated and investigated. The bilayer FET displays a mobility of 34 cm2·V-1·s-1, much higher than that of the monolayer FET. The high figures of merit make bilayer WS2 a promising candidate in high-performance electronics and optoelectronics.

The reduction in size of field-effect transistors (FETs) comprised of 3D semiconductors is confronted with the issues such as short-channel effects, tunneling effects and thermal dissipation. The emergence of transition metal dichalcogenides (TMDCs) atomic layers has opened up unprecedented opportunities for scaling down of the electronics in view of their unique layered-structure and excellent properties. TMDCs grown directly on high-k dielectric substrates are beneficial for fabricating high-performance FETs. Here, we demonstrate the direct growth of atomically thin MoS2 flakes on high-κ dielectric (HfO2) substrates via a chemical vapor deposition process. The morphology and structure of the as-grown materials were systemically investigated by optical microscope, atomic force microscope, Raman spectroscopy, photoluminescence, transmission electron microscope and X-ray photoelectron spectroscopy. The MoS2 flakes are approximately 5–10 µm in size with polycrystalline monolayer structure. The optical properties of the MoS2 flakes are also found to be substrate-dependent due to optical interference. In addition, back-gate FETs based on the as-grown MoS2 were fabricated and their performance was investigated. The results indicate that the n-type FETs show high on/off current ratio of ~ 106 and a carrier mobility of 9.75 cm2 V−1 s−1.

Elemental sulfur cathodes for lithium/sulfur batteries are receiving intense interest owing to their high theoretical capacity and energy density. However, they still suffer from severe capacity fading and moderate rate capability. Herein, we provide rational design and controllable fabrication of highly uniform flower-like hierarchical carbon nanospheres (FCNS) for sulfur accommodation for lithium/sulfur battery cathodes. The as-prepared three dimension FCNS with a size of around 200 nm seem to be assembled by petal-like carbon nanosheets with a thickness of about 4 nm, forming many mesoporous channels, which lead to their high surface area and large pore volume. With such a tailor-made structure, FCNS/sulfur composite cathodes with high sulfur-loading (81 wt%) deliver high specific capacity, long cycling life and excellent rate capability. Particularly, N-doped flower-like carbon nanospheres (NFCNS) with higher surface area (1223 m2 g−1) and larger pore volume (2.33 cm3 g−1) are also fabricated by treating with NH3 and used to host sulfur in lithium–sulfur battery cathodes, exhibiting more excellent rate capability (829 mA h g−1 at 5C) and cycling stability with a decay of 0.03% per cycle over 200 cycles at 1C. Even though the area density is improved to 2.5 mg sulfur per cm2, the battery still has a decay of 0.056% per cycle over 200 cycles.

Despite diverse carbon materials being intensively applied in energy storage and conversion, efficient optimization of the carbon structure to further improve its performance is still a great challenge. Herein, we design and fabricate a highly uniform 3D hierarchical N-doped carbon nanoflower (NCNF) using low-cost chitosan as the nitrogen and carbon source by a silica template method. The as-prepared NCNF with abundant meso-porous channels displays a high surface area (907 m2 g−1) and large pore volume (1.85 cm3 g−1), thereby demonstrating high performance as a bifunctional material for the electrocatalytic oxygen reduction reaction (ORR) and in lithium–sulfur batteries. As a metal-free ORR electrocatalyst, the NCNF exhibits excellent electrochemical activity comparable to that of commercial Pt/C (20 wt%), and much better methanol tolerance and durability. As sulfur accommodation for a Li–S battery cathode, the NCNF high loading content of sulfur (80 wt%) achieves an extremely high capacity (1633 mA h g−1 at 0.2C), excellent rate capability (916 mA h g−1 at 5C) and good cycling performance with a capacity decay of 0.07% per cycle over 500 cycles at 1C. Even when the area density is improved to 4.5 mgsulfur cm−2, the battery delivers a high areal capacity of ∼5.5 mA h cm−2 (0.37 mA cm−2) and still maintains ∼3 mA h cm−2 after 200 cycles with a smaller capacity decay of 0.07% per cycle at a high area current density of 3.77 mA cm−2. Significantly, the carbon materials recycled from the Li–S cathode after 500 cycles are reused as ORR electrocatalysts, displaying more excellent electrocatalytic activity than Pt/C (20 wt%).

The ongoing search for cheap and efficient hydrogen evolution reaction (HER) electrocatalysts to replace currently used catalysts based on Pt or its alloys has been considered as an prevalent strategy to produce renewable and clean hydrogen energy. Herein, inspired by the neuron structure in biological systems, we demonstrate a novel fabrication strategy via a simple two-step method for the synthesis of a neuronlike interpenetrative nanocomposite network of Co-P embedded in porous carbon nanotubes (NIN-Co-P/PCNTs). It is found that the interpenetrative network provides a natural transport path to accelerate the hydrogen production process. The embedded-type structure improves the utilization ratio of Co-P and the hollow, tubelike, and porous structure of PCNTs further promote charge and reactant transport. These factors allow the as-prepared NIN-Co-P/PCNTs to achieve a onset potential low to 43 mV, a Tafel slope as small as 40 mV/decade, an excellent stability, and a high turnover frequency value of 3.2 s(-1) at η = 0.2 V in acidic conditions. These encouraging properties derived from the neuronlike interpenetrative network structure might offer new inspiration for the preparation of more nanocomposites for applications in other catalytic and optoelectronic field.

The formidable sustainability challenges in advancing energy storage technologies call for game-changing research in battery designs. The previous pursuing of novel cathode materials with high redox potentials impedes the vast applications due to the simultaneous electrolyte decomposition at high potentials, though they are expected to deliver high specific capacities. Eventually, people start thinking in an opposite way, desirable anode materials with low redox potentials can also own high specific capacities. Among all the promising candidates, phosphorus-based anodes in sodium ion batteries (SIBs) have received considerable attention owing to the low cost and relatively high natural abundance of phosphorus. More importantly, phosphorus can store three sodium atoms and enable a high theoretical capacity of 2596 mAh g−1, which overwhelms any other SIB anode currently available. However, the poor electronic conductivity and large volume change of phosphorus during cycling severely deteriorate battery performance. The most widely used strategy is to confine phosphorus within well-designed carbon hosts. We thereby introduce a new type of porous hollow carbon with conductive-network interior as phosphorus host, which not only improves the electrical conductivity, but also creates enough interior surface for maximizing phosphorus utilization and shortening the ion's diffusion distance, compared to those conventional hollow carbon hosts. Therefore, the as-prepared red phosphorus-carbon spheres composites (RP/CS) exhibit superior rate performance (~1083 mAh g−1 at 4 A g−1, ~837 mAh g−1 even at 8 A g−1) and excellent cycle life (1027 mAh g−1 at 4 A g−1 more than 2000 cycles).

Developing highly active heterogeneous catalyst with strong basicity and porous structure is a highly attractive strategy for the base-catalyzed organic chemistry. Herein, we directly synthesized the mesoporous zeolite ETS-10 (METS-10) from using a water glass contained cationic copolymer with quaternary ammonium groups as a template. Furthermore, when the nickel nitrate solution was added into the starting synthesis gel, the nickel species facilitated the mesopore formation, and the Ni-containing mesoporous zeolite ETS-10 (Ni-METS-10) was obtained. Catalytic test results showed that the conversions of the benzaldehyde and citral over Ni-METS-10 (88.8 and 63.2 %) and METS-10 (85.5 and 60.1 %) catalysts are higher than those over both mesopore-free ETS-10 (77.3 and 47.7 %) and mesoporous NaX (MNaX, 55.0 and 35.2 %) catalysts in the condensation reactions with ethyl cyanoacetate. The higher activity of METS-10 and Ni-METS-10 than ETS-10 and MNaX is assigned to the fact that the strong basic sites on the catalysts can activate the reaction substrate and the mesoporous in the catalyst benefit the mass transfer and enhance the catalytic activity.

Well-defined gold embedded maghemite hybrid nanowires are synthesized, and their structures are fully characterized. They are composed of porous γ-Fe2O3 shells and embedded gold nanoparticles (3-10 nm), which is novel and very different from the conventional "surface decoration" configuration. These hybrid nanowires are produced by the de-alloying of Au-Fe alloy nanowires and subsequent heat treatment. The reaction mechanism is proposed and validated. The results of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and thermogravimetry techniques prove consistently that the Fe composition of Au-Fe alloy nanowires change to γ-FeOOH first and then to γ-Fe2O3. The embedded gold particles are help to enhance the gas response properties of the hybrid nanowires, which is attributed to the nano open-circuit Schottky junctions between γ-Fe2O3 and the Au nanoparticles. The gas sensing experiment data with high repeatability demonstrate that these hybrid nanowires are excellent sensing materials, especially for ethanol, and have shown both high selectivity and high sensitivity.

We demonstrate that alkali metal salts, including KCl, NaCl, K2SO4, Na2SO4, K2CO3, and Na2CO3, can act as catalysts for carbon nanotube (CNT) growth in chemical vapor deposition (CVD). The solution of alkali metal salt, water and ethanol was nebulized and was introduced into the CVD reactor, producing CNT with a multi-walled structure. Individual CNT are terminated with an onion-shaped carbon tip even when different alkali metal salt catalysts are used. Through observation and analysis of the catalyst particles and the resulting product, we elucidate the mechanism by which the alkali metal salt nanoparticles are served as “seeds” and provide nucleation sites for CNT growth. The ethanol decomposes to release carbon atoms into the catalyst particles, and the carbon nucleates and then begins to assemble on the surface of the catalyst particles, resulting in the CNT growth. By altering growth conditions, branched CNT and single-walled CNT also can be grown on alkali metal salt nanoparticles.

The current investigation of lithium–sulfur (Li–S) batteries faces three practical problems: (1) the poor conductivity of sulfur; (2) the notorious shuttle mechanism; and (3) the volume variation of the sulfur cathode. In principle, carbon nanotubes (CNTs) have a strong potential for improving sulfur usage because of their high electrical conductivity. Furthermore, opening holes in CNTs or creating pores on the walls is also a useful approach to not only enhance the diffusion of Li ions, but also enable more sulfur to fill the interior of the CNTs, which would be beneficial in retaining the soluble poly-sulfide intermediates and accommodate volume variations. Herein, we designed a mild one-step oxidation approach to create porous CNTs (PCNTs) through a chemical reaction between CNTs and rare oxygen sourced from a nebulized water stream at high temperatures. The higher specific surface area and pore volume values confirmed that PCNTs had significant porosity, compared with raw CNTs. When the PCNTs–S composites were tested as cathode materials in Li–S batteries, the cathode with 78 wt% S content exhibited an initial reversible capacity of 1382 mA h g−1 at 0.2 C. Furthermore, a reversible capacity of 150 mA h g−1 can be preserved, even at a very high current rate of 15 C. More importantly, it is also confirmed that a cathode with 89 wt% S content unexpectedly delivered a reversible capacity as high as 1165 mA h g−1sulfur/830 mA h g−1electrode at the initial cycle, and 792 mA h g−1sulfur/564 mA h g−1electrode after 200 cycles at a current rate of 0.2 C. To the best of our knowledge, such a high rate performance (15 C) and S loading (89 wt%) in cathodes of advanced Li–S batteries have been infrequently reported in previous research.

GeSe2nanobelts were synthesized via a simple thermal-evaporation process by using gold particles as catalyst and GeSe2flakes as starting materials. The morphology, crystal structure, and composition were characterized with scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS). SEM micrographs show that most of GeSe2nanobelts have distinct segmented structures (wide belt, zigzag belt, and narrow belt). A possible mechanism was proposed for the growth of segmented nanobelts. It is possible that the growth of the segmented nanobelts is dominated by both vapor-liquid-solid and vapor-solid mechanisms. Devices made of single GeSe2nanobelt have been fabricated and their photoelectrical property has been investigated. Results indicate that these nanobelt devices are potential building blocks for optoelectronic applications.

Despite the good progress in developing carbon catalysts for oxygen reduction reaction (ORR), the current metal-free carbon catalysts are still far from satisfactory for large-scale applications of fuel cell. Developing hollow graphene balls with a self-supporting structure is considered to be an ideal method to inhibit graphene stacking and improve their catalytic performance. Herein, we fabricated metal-free hollow graphene balls with a self-supporting structure, through using a new strategy that involves direct metal-free catalytic growth from assembly of SiO2 spheres. To our knowledge, although much researches involving the synthesis of graphene balls have been reported, investigations into the direct metal-free catalytic growth of hollow graphene balls are rare. Furthermore, the electrocatalytic performance shows that the resulting hollow graphene balls have significantly high catalytic activity. More importantly, such catalysts also possess much improved stability and better methanol tolerance in alkaline media during the ORR compared with commercial Pt/C catalysts. The outstanding performances coupled with an easy and inexpensive preparing method indicated the great potential of the hollow graphene balls with a self-supporting structure in large-scale applications of fuel cell. Hollow graphene balls with a self-supporting structure have been successfully fabricated, through using a new strategy that involves direct metal-free catalytic growth from 3D assembly of SiO2 spheres. The hollow graphene balls can exhibit a high catalytic activity, long-term stability, and an excellent methanol tolerance for the oxygen reduction reaction

Electric-arc-synthesized single-walled carbon nanotube (SWNT) bundles were sonicated in boric acid, dried and sintered under vacuum at 1200°C using a spark plasma sintering process. The nominal boric acid concentrations were 0%, 2.5%, 5%, 7.5%, 12.5% and 15%. This resulted in a nonmonotonic variation of carrier concentration in the resulting samples. The 7.5% boric acid treated sample showed the least magnitude and weakest temperature-dependence for the thermopower with a distinct phonon-drag peak that was absent in the other samples. These results are discussed within the framework of the doping-induced shift of the Fermi level and changes in the electron–phonon coupling.

Millimeter long, vertically oriented, multi-walled carbon nanotube (MWCNT) arrays were pre-aligned and densified using spark plasma sintering (SPS) technique to form aligned MWCNT bulk samples. The combined results of X-ray powder diffraction, Raman spectroscopy, scanning electron microscopy and high-resolution transmission electron microscopy show that the MWCNTs largely retain their orientation and individual tubular morphology in the aligned MWCNT bulk samples, and that the SPS process induces inter- and intra-tubular bonding as well as graphene formation. In view of the one-dimensional nature of individual MWCNTs, it is particularly noteworthy that the transverse electrical resistivity ρT is slightly lower than the longitudinal resistivity ρL, whereas the transverse thermal conductivity κT is ∼50% of κL. The room temperature κL is ∼31 W/(m K), one of the highest reported in MWCNT bulk samples. In addition, the thermopower measurements show anisotropy and features of phonon drag.

Helically coiled carbon nanotubes (HCNTs) and nanowires (HCNWs) represent novel nanostructural morphology and have technological and scientific significance. Their potential applications span high frequency electronics, tactile and magnetic sensors, and structural foams for cushioning and energy dissipation. It is also interesting to make a connection between the CVD synthesized helical carbon nanostructures to organic forms found in nature, such as DNA and proteins, and indeed, these structures can be used for templates in collagen growth. It was also suggested that a coil could correspond to a sequence of alternating metallic/semiconducting junctions, which is very interesting from the point of view of application to nanoelectronic systems. While several groups have previously reported on the synthesis of coiled nanostructures using chemical vapor deposition (CVD), patterned substrates were always used. While mostly HCNWs are seen, there have also been a few reports on the synthesis of HCNTs. Patterned growth of coiled nanotubes based on using previously aligned straight CNTs as templates was also demonstrated. It is typically found that such methods, in addition to limiting the amount of material due to the catalyst distribution, is often accompanied by the formation of linear multiwalled nanotubes. It would be desirable to develop a process that is independent of the underlying substrate, utilizing gas-phase synthesis alone, with controllable coiling characteristics (i.e., length, pitch etc.). In this paper, we report on a liquid-precursor-based synthesis method which has the additional advantage that either coiled nanotubes or nanowires, with differing electrical and mechanical properties, can be fabricated. We provide a rational explanation for the distinct growth modes based on an analysis of the binary equilibrium phase diagrams, where the mutual affinity of secondary catalysts (In/Sn), with the primary catalyst (Fe) in the ferrocene–xylene mixture, promotes nanotube/-wire formation. Coiled carbon nanostructures have been predicted to be energetically stable, and various mechanisms have been posited for their formation. While the curvature, in the case of helically coiled single-walled nanotubes could possibly be due to the regular insertion of pentagon–heptagon pairs at the junctions, it is unclear if a similar mechanism could hold for multiwalled nanotubes and HCNWs. Other formation mechanisms invoke localized stresses and anisotropic rates of carbon deposition on faceted catalyst particles. However, there is no experimental evidence for the above, as it is seen that helical structure is induced even though catalyst particles are not obviously present in the structure. It is also noted that the above mechanisms cannot be invoked for amorphous carbon nanocoils and compound (e.g., boron carbide) nanowires. To provide a comprehensive explanation, we have proposed a thermodynamic model, where helix/coil formation is explained on the basis of the interactions between specific catalyst particles and the growing nanostructure. We use the degree of wettability of the nanostructure surface by the catalysts as a criterion for coiling. It is seen, through the Young equation, that the wetting angle h cos 1 csv csl clv of

Electrochemically splitting water for hydrogen evolution reaction (HER) has been viewed as a promising approach to produce renewable and clean hydrogen energy. However, searching for cheap and efficient HER electrocatalysts to replace the currently used Pt-based catalysts remains an urgent task. Herein, we develop a one-step carbon nanotube (CNT) assisted synthesis strategy with CNTs' strong adsorbability to mediate the growth of subnanometer-sized MoS(x) on CNTs. The subnanometer MoS(x)-CNT hybrids achieve a low overpotential of 106 mV at 10 mA cm(-2), a small Tafel slope of 37 mV per decade, and an unprecedentedly high turnover frequency value of 18.84 s(-1) at η = 200 mV among all reported non-Pt catalysts in acidic conditions. The superior performance of the hybrid catalysts benefits from the presence of a higher number of active sites and the abundant exposure of unsaturated S atoms rooted in the subnanometer structure, demonstrating a new class of subnanometer-scale catalysts.

Two series of Cd-Ln clusters: nano-drum [Ln₈Cd₂₄L₁₂(OAc)₄₈] and nano-double-drum [Ln₁₂Cd₄₄L₂₀Cl₃₀(OAc)₅₄] (Ln = Nd and Yb) were prepared using a flexible Schiff base ligand bearing two aryl-Br groups. Chloride (Cl(-)) ions, together with the interactions of Br with other electronegative atoms, play a key role in the formation of the nano-double-drums. The structures were studied by TEM and photophysical properties were determined.

In this report, we describe a method for modifying electrical and optoelectrical properties of CdS nanobelts using low-energy (lower than 10 keV) e-beam irradiation in a scanning electron microscope. The electrical conductivity of the nanobelts was dramatically improved via the irradiation of e-beams. The modified conductivity of the nanobelts depends on the energy of the e-beam; it exhibits a larger photocurrent and higher external quantum efficiency but slower time-response than that before the modification. A possible mechanism about the modification is the increase of electron accumulation (injected electrons) in the nanobelts due to e-beam irradiation. In addition, the optoelectrical modification could be caused by the trapped electrons in the nanobelts and the decrease of contact resistance between the nanobelts and metal electrodes induced by e-beam irradiation. The results of this work are significant for the in situ study of semiconductor nanostructures in the electron microscope. Besides, the method of electrical and optoelectrical modification presented here has potential application in electronics and optoelectronics.

We find that the electrical and thermal connectivity in multiwalled carbon nanotube buckypaper can be tuned using a spark plasma sintering (SPS) technique. Elevated SPS temperatures promote the formation of inter-tube connections and consequently impact the electrical resistivity, thermoelectric power and thermal conductivity of the buckypaper. In particular, the electrical resistivity as a function of SPS temperature exhibits a percolation-type behavior while the low temperature lattice thermal conductivity shows a crossover behavior in the sample dimensionality. The results are discussed in terms of the quasi-one-dimensional metallic nature of multiwalled carbon nanotubes, the packing density and the electron-phonon coupling.