Graphitic Shells Research Articles

High-strength carbon nanotube fibers with near 100% purity acquired via isothermal vacuum annealing

The Fe and carbonaceous particles are the main impurities in a carbon nanotube fiber (CNTF) fabricated by the floating catalyst chemical vapor deposition (FCCVD) method, which have an adverse effect on various applications, leading to strong demand for purification process to obtain high-purity CNTFs. However, the commonly used purification methods combining oxidation and acid refluxing cannot remove the impurities thoroughly, and will introduce undesired damage onto CNT wall at the same time. Herein, we propose an efficient purification strategy involving an isothermal annealing process under high vacuum to achieve mass preparation of CNTFs with high purity and mechanical/electrical properties. The Fe and carbonaceous impurities can be almost completely removed after vacuum annealing at 1800 °C. The purified CNTFs exhibit enhanced oxidation resistance due to the removal of Fe catalyst particles. The diffusion and evaporation of Fe atoms are most likely responsible for the removal of Fe particles enclosed with graphitic shells. The purified CNTF shows a high specific tensile strength of 1.82 ± 0.04 N/tex due to the high alignment of the CNT bundles within fibers, which can retain 80% of the value for pristine CNTF. The purified CNTF exhibits similar trends of electrical conductivity as the specific tensile strength and has a high conductivity of 3.65 ± 0.11 × 105 S/m. The large-scale preparation of high-purity CNTFs with high-performance will promote the real application of CNTFs.

Enhanced dielectric and conductivity properties of carbon-coated SiC nanocomposites in the terahertz frequency range

C-coated SiC nanocomposites (SiC@C NCs) were one-step synthesized under a mixture atmosphere of Ar and CH4 using a DC arc-discharge plasma method. The microstructure of the composites could be controlled by varying the volume ratio of Ar and CH4. A strong response to the terahertz (THz) field was observed due to the existence of a graphite shell. The dielectric properties of SiC@C NCs can be enhanced by altering the thickness of the graphite shell. The thicker graphite shell results in a stronger absorption of THz waves and an enhanced real part of conductivity. Fitting the measured conductivity data using the Drude–Smith model reveals that the carrier transport in the SiC@C NCs and its counterpart, the SiC nanoparticles, is dominated by backscattering. The SiC@C NCs with enhanced conductivity are believed to be fundamental materials for various functionalized optoelectronic devices.

Study on Oxygen Reduction Method of Phosphorus-doped Hydrogenated Nano-silicon

In recent years, hydrogenated nano-silicon has attracted much attention due to its high specific energy density. However, the slow kinetic oxygen reduction reaction (ORR) and oxygen evolution reaction (ORE) reduce the efficiency of hydrogenation of nano-silicon and hinder its practical application. The development of non-precious catalysts with excellent performance and low price is of great significance to the development of hydrogenated nano-silicon. Among many hydrogenation nano-silicon catalysts, cobalt-based compounds have good ORR and ORE catalytic performance, and are rich in Co reserves and low in price. Especially doped compounds have excellent stability. In the thesis, three series of hydrogenated nano-silicon composite materials of Co-SrCO3/NC, Co-Nd/NC and Co-Ce/NC were successfully synthesized through heating reflow method and subsequent high-temperature calcination. XRD, Raman, XPS, TEM, nitrogen isothermal adsorption and desorption and other analytical methods were used to characterize the structure of the above composite materials. Through chemical tests, the composite materials were evaluated for the catalytic performance of ORR and ERR, and their structure-activity relationship was analyzed. The Co_SrC03/NC series composite materials were calcined at different temperatures. The results showed that the generated SrCO3 nanorods were embedded in the graphite shell and contacted with hydrogenated nano-silicon. In an oxygen-saturated 0.1MKOH solution, Co-SrCO3/NC series composites have excellent ORR/ORE catalytic activity, especially Co-SrCO3/NC-600 has more stable activity than precious metal catalysts of 20wt% Pt/C and Ir02. Sex. The air prepared with Co-SrCO3/NC-600 has better performance. During the in-depth study of 10mA_cnT2, Co-SrC03/NC-600 was used as the air to maintain a stable platform of 1.10V, and the platform of 20wt%Pt/C under the same conditions was 1.17V.

Hierarchical Ni-Mo2C/N-doped carbon Mott-Schottky array for water electrolysis

Scalable synthesis of active, durable, low-cost, and self-supported electrodes assembled by electrocatalysts towards hydrogen and oxygen generation reactions for water electrolyzers remains a grand challenge. We develop here a facile fabrication of nickel-molybdenum carbide heterostructures embedded in large-area (100 cm2) hierarchically assembled nitrogen-enriched carbon, forming Mott-Schottky array on nickel foam (Ni-Mo2C/NC@NF). The Ni-Mo2C/NC array is directly applied as the bifunctional catalyst with high activity and durability in alkaline electrolyte. Particularly, an extremely low overpotential of 40 mV is needed to generate hydrogen. Density functional theory calculation revealed that the formation of Ni-Mo2C Mott/NC Schottky interfaces enables favorable electronic structures for electrocatalytic water splitting. Besides, 3D hierarchical structure provides exposed active sites, facilitates mass and charge transfer, graphitic shells enhance stability. A symmetric electrolyzer using Ni-Mo2C/NC@NF generates 10 mA cm−2 at 1.59 V and operates steadily for 150 h, which even outperforms the noble metal couple, Pt/C//RuO2 for water electrolysis. The scalability, activity and durability renders Ni-Mo2C/NC@NF potential industrial application. The assembly-carburization strategy could be further applied for preparing Co-Mo2C/NC@NF and Cu-Mo2C/NC@NF.

Selective Electrocatalytic Reduction of Oxygen to Hydroxyl Radicals via 3-Electron Pathway with FeCo Alloy Encapsulated Carbon Aerogel for Fast and Complete Removing Pollutants.

We reported the selective electrochemical reduction of oxygen (O2 ) to hydroxyl radicals (. OH) via 3-electron pathway with FeCo alloy encapsulated by carbon aerogel (FeCoC). The graphite shell with exposed -COOH is conducive to the 2-electron reduction pathway for H2 O2 generation stepped by 1-electron reduction towards to . OH. The electrocatalytic activity can be regulated by tuning the local electronic environment of carbon shell with the electrons coming from the inner FeCo alloy. The new strategy of . OH generation from electrocatalytic reduction O2 overcomes the rate-limiting step over electron transfer initiated by reduction-/oxidation-state cycle in Fenton process. Fast and complete removal of ciprofloxacin was achieved within 5 min in this proposed system, the apparent rate constant (kobs ) was up to 1.44±0.04 min-1 , which is comparable with the state-of-the-art advanced oxidation processes. The degradation rate almost remains the same after 50 successive runs, suggesting the satisfactory stability for practical applications.

Selective Electrocatalytic Reduction of Oxygen to Hydroxyl Radicals via 3‐Electron Pathway with FeCo Alloy Encapsulated Carbon Aerogel for Fast and Complete Removing Pollutants

AbstractWe reported the selective electrochemical reduction of oxygen (O2) to hydroxyl radicals (.OH) via 3‐electron pathway with FeCo alloy encapsulated by carbon aerogel (FeCoC). The graphite shell with exposed ‐COOH is conducive to the 2‐electron reduction pathway for H2O2 generation stepped by 1‐electron reduction towards to .OH. The electrocatalytic activity can be regulated by tuning the local electronic environment of carbon shell with the electrons coming from the inner FeCo alloy. The new strategy of .OH generation from electrocatalytic reduction O2 overcomes the rate‐limiting step over electron transfer initiated by reduction‐/oxidation‐state cycle in Fenton process. Fast and complete removal of ciprofloxacin was achieved within 5 min in this proposed system, the apparent rate constant (kobs) was up to 1.44±0.04 min−1, which is comparable with the state‐of‐the‐art advanced oxidation processes. The degradation rate almost remains the same after 50 successive runs, suggesting the satisfactory stability for practical applications.

Synthesis of Fe3C@C core-shell catalysts with controlled shell composition for robust oxygen evolution reaction

In this study, iron carbide (Fe3C) particles were encapsulated in graphitic carbon shells by a facile carbonation process. These core-shell carbides, in which iron carbide is encapsulated in graphene-like layer (Fe3C@C) and nitrogen doped graphene-like layer (Fe3C@C–N) were investigated as oxygen evolution reaction (OER) catalysts to investigate the effect of the carbon shell on catalyst activity and stability. Due to the protective effect of the graphitic carbon shells, the performance of Fe3C@C and Fe3C@C–N were considerably better than the bare Fe3C nanoparticles. The OER activity of Fe3C@C–N is comparable with that of carbon-supported ruthenium oxide (RuO2/C), and this core-shell carbide has a remarkable stability and high turnover frequency under alkaline conditions. To elucidate the real active sites of these core-shell carbides, the role of the Fe3C core, Fe–Nx active sites, and effect of nitrogen doping in the shell were investigated in detail.

Impedance engineered microwave absorption properties of Fe-Ni/C core-shell enabled rubber composites for X-band stealth applications

• A simple, consistent, and economical wet-chemical process for synthesizing large scale Fe-Ni nanocomposites. • A BCC to FCC structural transition with varying Ni concentration in metallic core. • Fe-Ni/C alloy and rubber composites as efficient X-band microwave absorber materials. We report a simple, consistent, and economical wet-chemical method for synthesizing a series of Fe-Ni nanocomposites by varying Fe and Ni fractions. Further, graphite wrapped Fe-Ni alloy nanoparticles in core-shell geometries are prepared by calcination of nanocomposites at 900°C under N 2 gas. XRD studies reveal body-centered and face-centered (bcc/fcc) cubic Fe-Ni alloy core particles and hexagonal graphitic shell formation. More interestingly, a structural transition from bcc to fcc phase is also investigated with varying Ni concentration. The microstructural studies demonstrate ~10 nm graphitic shells together with a nearly spherical ~40 nm diameter Fe-Ni metallic core. These magnetic alloys showed the variation in magnetization and coercive fields with alloy compositions. The Fe-Ni core-shell material with the maximum magnetization is used for fabricating rubber-based composites by varying filler loading in the range 60–80 wt%. The utility of Fe-Ni/C alloy and rubber composite is categorized in terms of electromagnetic impedance matching for the entire X-band (8.2–12.4 GHz) frequency range based on the filler concentration and absorber thickness for microwave absorption applications. These core-shell alloy systems may show potential for stealth of strategic targets because of their very low density (~1.1 g/cc) and lesser thickness than conventional absorbers.

Tailoring of N-doped graphite coated cobalt nanoparticles via arc discharge enables the high microwave absorption

Nitrogen-doped graphite-encapsulated cobalt nanoparticles (Co@G NPs) were fabricated via a one-step arc-discharge method. The core/shell nanostructure and surface functionality were tailored by changing the ammonia content in the working gas mixture. Specifically, the average core diameter changed from 5 to 23 nm, average graphite shell thickness from 1.5 to 3.5 nm as the NH3 ratio increased from 0% to 20%. All these arc products exhibited better microwave absorption properties than the pristine Co micro powders. Co@G NPs prepared with 5% of NH3 presented 1.5 at.% of surface doping nitrogen, showing the optimal reflection loss (RL) value of −37.70 dB at the thickness of 5 mm. For the products prepared with 12.5% of NH3, wide absorption bandwidths of 5.60 GHz (RL < −10 dB) and 1.55 GHz (RL < −20 dB) were achieved. The enhancing electromagnetic loss could be attributed to the proper impedance match of dielectric loss (conductance loss, dielectric relaxation (dipolar polarization) and space-charge polarization) and magnetic loss (the eddy current loss, magnetic hysteresis loss and residual loss). This work unlocked an open platform for the facile fabrication of new generation microwave absorbers with light, thin, strong and wide characteristics.

One-step synthesis and performances of Ni@CN nanocapsules with superior dual-function as electrocatalyst and microwave absorbent

Developing high efficient, stable, and earth-abundant materials is still a great challenge both in the electrocatalytic and microwave absorption fields nowadays. Herein, we construct unique core-shell structured N-doped graphite coated Ni (Ni@CN) nanocapsules through a facile one-step arc-discharge method. The as-obtained Ni@CN show superior hydrogen evolution reaction (HER) activities with a low overpotential of 85 mV at 10 mA cm−2, a small Tafel slope of 69 mV dec-1, and strong long-term stability of 10 h. Besides, the Ni@CN nanocapsules exhibit high microwave absorption performance as microwave absorbers. Their optimal value of reflection loss (RL) is -35.8 dB at 11.7 GHz with a thickness of 2.3 mm. The dual-functional behavior of Ni@CN nanocapsules in catalytic and microwave absorption is due to their strong synergistic effect between the N-doped graphite shell and Ni core, the high electrical conductivity of Ni, and the abundant defects caused by the N doping into the graphite shell.

Zeolitic imidazole framework derived N-doped porous carbon/metal cobalt nanoparticles hybrid for oxygen electrocatalysis and rechargeable Zn-air batteries.

Bifunctional electrocatalysts with high catalytic property for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital for high-performance zinc–air batteries (ZnABs). In this study, an efficient bifunctional electrocatalyst with hollow structure (C–N/Co (1/2)) has been successfully prepared through carbonization of ZIF-8@ZIF-67 and evaporation of Zn ions at high temperature. With Co nanoparticles encapsulated by an N-doped porous carbon matrix, the catalyst exhibits excellent stability in aqueous alkaline solution over an extended period and good tolerance to the methanol crossover effect. The integration of an N-doped graphitic carbon outer shell and Co nanoparticles enables high ORR and OER activity, as evidenced by ZnAB using the catalyst C–N/Co (1/2) in an air cathode.

Visible light active Au@g-C3N4 core-shell plasmonic photocatalyst

A core–shell structure of Au nanoparticle and graphitic carbon nitride (g-C3N4) is synthesized by sonication-assisted reduction, using sodium citrate as the reducing agent. Noble metal nanoparticles have low dispersion stability in solution and have tendency to aggregate which reduces their activity. Corrosion of noble metal nanoparticles also restricts practical use of noble metal. Keeping this in mind, a core–shell structure of gold core and semiconductor graphitic carbon nitride shell is developed. Core-shell structure formation is confirmed by structural and morphological characterizations. This nanocomposite Au@g-C3N4 degraded 41% and 94.2% Methylene Blue present in the simulated wastewater in just 60 min with 1 wt% and 5 wt% of Au salt, respectively. This faster degradation of Methylene Blue is due to the large surface area of 2D g-C3N4, surface plasmon resonance of Au nanoparticles, and the interface formed between the two components which increase the lifetime of photogenerated electron-hole pairs.

Obtained Carbon Nano-onions from Underwater Arc Discharge Without the Complex Purification Procedures

Carbon nano-onions (CNOs), which consist of concentric graphitic shells, currently attract much attention because of their unique structural and physical properties, which are different from the properties of the other carbon nanomaterials such as fullerenes, graphene, and carbon nanotubes (CNTs). Due to their small size, the large external surface area and high conductivity, CNOs are used for supercapacitor applications. The arc discharge underwater is an effective and simple method for the synthesis of larger CNOs in reasonable quantities. In this research, we have been obtained carbon nanomaterials using arc discharge in water between two high purity graphite electrodes. The main experimental techniques used to characterize carbon nanostructures have been Transmission Electron Microscopy (TEM) and Raman Spectroscopy. Among them, Raman spectroscopy is the most useful non-destructive technique capable of differentiating between these various structures. Our TEM images showed that the samples collected from the material floating on the water surface consist CNOs with other carbon nanomaterials such as CNTs. We observed for the first time the formation of solid agglomerate on the cathode surface. Raman and TEM results revealed that the agglomerate is made exclusively of CNOs. The defragmentation of such agglomerate allows to obtain CNOs free of other carbon nanomaterials without the complex purification procedures needed for floating nanomaterials.

Rapid synthesis of FeS nanoparticles encapsulated with nanoporous graphitic shells for high performance sodium- and lithium-ion batteries

FeS encapsulated with nanoporous graphitic shells could avoid the large volume change caused by the insertion/extraction of Na+.

θ-Fe3C dominated Fe@C core–shell catalysts for Fischer-Tropsch synthesis: Roles of θ-Fe3C and carbon shell

θ-Fe3C formation is usually considered as a deactivation factor in Fischer-Tropsch synthesis (FTS). Herein, θ-Fe3C dominated Fe@C core–shell catalysts were fabricated by pyrolysis of MIL-101(Fe) in ethyne for studying the roles of θ-Fe3C and carbon shell in FTS. In the evolution of MIL-101(Fe) into Fe@C catalysts, ethyne decomposition promotes the formation of θ-Fe3C and carbon shells around nanoparticles. By modulating the pyrolysis temperature, θ-Fe3C and/or α-Fe nanoparticles coated with different carbon shells (amorphous carbon, graphene or graphite shells) are acquired. By exploring the relationship between phase composition, structure of catalysts and FTS performance, it is found that θ-Fe3C is confirmed to be FTS active, which exhibits superior CC chain growth ability, i.e., higher C5+ selectivity and lower CH4 selectivity. Meanwhile, graphene shell inhibits iron nanoparticles aggregation and stabilizes the catalysts.

Proof-of-concept fabrication of carbon structure in Cu–N–C catalysts of both high ORR activity and stability

N-doped hierarchical carbon (NHC) composed of hierarchically arranged highly graphitic graphene shells with high surface/bulk ratio is demonstrated as a proof of concept, and utilized as carbon source in Cu–N–C catalysts. The special structure is contributed to Fe nanoparticles induced to PANI nanospheres, which serving as hard templates greatly alter the thermal decomposition/carbonization process of PANI. When employed as carbon source in Cu–N–C ORR catalyst, not only is the catalytic activity boosted as a result of highly loaded Cu centers anchored at the rather accessible sites to oxygen, the stability is greatly enhanced as well.

Spatial distribution of metallic heteroatoms in soot nanostructure mapped by aberration-corrected STEM-EELS

Soot from the thermochemical conversion of solid and liquid fuels can be infused with metallic heteroatoms originating from the fuel – these heteroatoms alter the nanostructure and the reactivity of the soot. Here, we investigate the spatial distribution of metallic heteroatoms in soot generated by biomass gasification, using aberration-corrected Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). The technique allowed for the mapping of heteroatom distribution in soot at the nanoscale, and thereby for the direct correlation of heteroatom concentration with the graphitic nanostructure. Spherical soot particles were coated with a thin layer of silicon, possibly in the form of quartz that may be linked to minor distortions of the nanostructure of the graphitic shell of the particles. Further results on non-spherical soot and inorganic-carbon fused aggregates suggest that the chemistry of formation was affected by the presence of gaseous ash-forming elements, especially calcium, with carbon-oxygen functional groups forming as intermediates in the graphite-inorganic reaction; i.e., prior to the formation of the thermodynamically stable carbonate bonds. The analytical approach demonstrated here can potentially help select fuel additives or aid in the design of fuel blends that minimize the formation of similar, hybrid carbon nanoparticles in combustion or gasification systems.

FeN@N-doped graphitic biochars derived from hydrothermal-microwave pyrolysis of cellulose biomass for fuel cell catalysts

Iron-nitrogen moieties encapsulated in the N-doped graphitic and hierarchical biochars are derived from the hydrothermal pyrolysis and microwave NH3 activation of cellulose and lignin. The physicochemical properties of these biochars are identified by using various analytical instruments and their application as electrocatalysts in oxygen reduction reaction (ORR) is also evaluated in alkaline solution. As compared to the lignin-derived biochars (named as L-y), the cellulose-derived biochars (named as C-x) have superior activities toward ORR due to the formation of highly graphitic carbon shells (as proofed by using X-ray diffraction and high-resolution transmission electron microscope) in the biochars. Moreover, the C-1% (i.e., 1% of iron in the cellulose-derived biochars) exhibits the best electrocatalytic performance (onset potential = −0.05 V vs. Ag/AgCl) and follows a near four-electron ORR route, which could be ascribed to the more active iron-nitrogen sites (as evidenced by X-ray photoelectron and X-ray absorption spectroscopy) covered with hierarchical carbon shells (as identified by N2 adsorption-desorption isotherms). The cellulose-derived biochars with distinctive iron-nitrogen retained in the shell of N-doped graphitic and hierarchical carbons by employing a time-saving and energy-efficient microwave activation procedure may render a potential application in the ORR.

Studies on the thermal stability of nanosized powder of WC1-x-based product prepared by plasma dynamic method, compaction feasibility of the powder and preparation of composite with aluminium

Obtaining ceramics and composites based on cubic tungsten carbide WC1-x requires knowledge of its thermal behavior, which is poorly studied now. The paper presents the results of a thermal analysis of the nanosized WC1-x-based product with the stoichiometry of WC0.72-0.74 synthesized by a plasma dynamic method. Taking into account the known literature data on the structure of WC1-x products, an investigation of synthesized nanoparticles allows us to conclude that WC1-x cannot exist without a crystalline graphite shell and/or an amorphous carbon matrix. Similarly, the destruction of both carbon shells and a matrix leads to either quick oxidation of WC1-x to WO3 when heated in air or the phase transition WC1-x → WC when heated in vacuum. The thermal analysis shows WC1-x is stable up to 550 °C in air and up to 1200 °C in vacuum and argon. The obtained data made it possible to formulate recommendations for preserving WC1-x material in bulk specimens when compacting from the dispersed powder. The preparation of binderless ceramics from the WC1-x-based product by the SPS method seems to be impossible due to carbon diffusion during the sintering process, while the use of WC1-x-based products as a reinforcing additive to a metal (Al) matrix seems to be the only way to preserve this crystalline phase in a bulk form at the present time.

Hydrothermal Activation of Porous Nitrogen-Doped Carbon Materials for Electrochemical Capacitors and Sodium-Ion Batteries.

Highly porous nitrogen-doped carbon nanomaterials have distinct advantages in energy storage and conversion technologies. In the present work, hydrothermal treatments in water or ammonia solution were used for modification of mesoporous nitrogen-doped graphitic carbon, synthesized by deposition of acetonitrile vapors on the pyrolysis products of calcium tartrate. Morphology, composition, and textural characteristics of the original and activated materials were studied by transmission electron microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared spectroscopy, and nitrogen gas adsorption method. Both treatments resulted in a slight increase in specific surface area and volume of micropores and small mesopores due to the etching of carbon surface. Compared to the solely aqueous medium, activation with ammonia led to stronger destruction of the graphitic shells, the formation of larger micropores (1.4 nm vs. 0.6 nm), a higher concentration of carbonyl groups, and the addition of nitrogen-containing groups. The tests of nitrogen-doped carbon materials as electrodes in 1M H2SO4 electrolyte and sodium-ion batteries showed improvement of electrochemical performance after hydrothermal treatments especially when ammonia was used. The activation method developed in this work is hopeful to open up a new route of designing porous nitrogen-doped carbon materials for electrochemical applications.