Field emission characteristics of CNFB-CNT hybrid material grown by one-step MPCVD

A hybrid material consisting of carbon nanotubes (CNTs) and carbon nanoflake balls (CNFBs) was successfully synthesized by microwave-plasma-assisted chemical vapor deposition using a H₂/CH₄/N₂ ratio of 4:1:2 at 80 Torr for 30 min. The precursor used was a sol-gel solution containing ferric nitrate, tetrabutyl titanate, and n-propanol. The carbon hybrid material (CNFB-CNT) exhibited excellent field emission properties, with its turn-on field being 1.77 V/μm. It also showed two field enhancement factors (1536 and 7932) for different electric fields. The emission current density of the hybrid remained higher than 0.65 mA/cm² for more than 50 h and was 0.82 mA/cm² even after 50 h of continuous emission. Further, the field emission properties of the CNFB-CNT hybrid were better than those of other single-structured carbon nanomaterials (CNTs, CNFs, or CNFBs). Therefore, the CNFB-CNT hybrid material should be a promising candidate for use in high-performance field emitters.     

The production of carbon particles of different shapes produced by the chlorination of Cr(C5H5)2

Carbon particles have been obtained by the chlorination of chromocene (Cr(C₅H₅)₂). Changes in their morphology and micro-nanostructure have been monitored at two different temperatures. At 400 °C, filled materials (tubes and spheres) and agglomerated round particles are formed, whereas at 900 °C closed-end tubes, hollow and solid spheres were produced. Transmission electron microscopy shows that these particles are formed of highly disordered graphene-like layers, which is confirmed by the absence of the 2D and 2G bands in the Raman spectrum. The calculated in-plane correlation length of these graphene-like layers is 1.2 ± 0.1 nm. In all the carbon particles, electron energy-loss spectroscopy shows a very similar sp² carbon bonding content (89–98%) and mass density ranging from 1.6 to 1.8 g/cm³, both below standard graphite. Textural studies performed on the sample prepared at 900 °C show Type II adsorption isotherms with a surface area of 694 m²/g.     

Adsorption separation of vinyl chloride and acetylene on activated carbon modified by metal ions

This work mainly involved the adsorption separation of vinyl chloride and acetylene on modified activated carbons. Six metal ions with different hardness were loaded on activated carbon respectively. The effect of metal ions on the adsorption separation performance of vinyl chloride and acetylene was investigated. The experimental results shown that the separation factor of C₂H₃Cl to C₂H₂ over modified activated carbon followed the order: Al(III)/AC > Mg(II)/AC > Fe(III)/AC > AC > Zn(II)/AC ≈ Cu(II)/AC > Ag(I)/AC. The effect of the hardness of metal ions on the adsorption capacity of C₂H₂ was more remarkable than that of C₂H₃Cl, thus the separation factor of C₂H₃Cl to C₂H₂ increased with the rising of absolute hardness of the metal ions.     

Influence of wet oxidation of herringbone carbon nanofibers on the pseudocapacitance effect

Herringbone carbon nanofibers (CNFs) were treated with concentrated HNO₃ and a mixture of diluted HNO₃/H₂SO₄ to obtain a series of oxygen enriched CNF with different oxygen group distribution, but with a similar porous texture. Oxygen functional groups were determined by X-ray photoelectron spectroscopy. CNFs with a very high relative concentration of carbonyl and/or quinone groups and hydroxyl groups were obtained by adjusting the suitable temperature and time of oxidation with HNO₃ and HNO₃/H₂SO₄, respectively. The electrochemical behavior of the samples was studied in three- and two-electrode cells. The performance of oxidized CNFs-based supercapacitors working in 1 mol L⁻¹ H₂SO₄ and 6 mol L⁻¹ KOH was analyzed using cyclic voltammetry and galvanostatic charging/discharging. The specific capacitance of the oxidized CNFs was more than twice enhanced in acidic and alkaline media compared to the pristine CNFs due to the pseudocapacitance effect. It was revealed that not only quinone groups but also hydroxyl groups contribute into the overall capacitance through the pseudocapacitance effect. With increasing surface concentration of the CO and C–OH groups, the capacitance values increase for the capacitors operating in both media.     

Super growth of vertically-aligned carbon nanofibers and their field emission properties

We demonstrate a very efficient synthesis of vertically-aligned ultra-long carbon nanofibers (CNFs) with sharp tip ends using thermal chemical vapor deposition. Millimeter-scale CNFs with a diameter of less than 50 nm are readily grown on palladium thin film deposited Al₂O₃ substrate, which activate the conical stacking of graphitic platelets. The field emission performance of the as-grown CNFs is better than that of previous CNFs due to their extremely high aspect ratio and sharp tip angle. The CNF array gives the turn-on electric field of 0.9 V/μm, the maximum emission current density of 6.3 mA/cm² at 2 V/μm, and the field enhancement factor of 2585.     

New developments in the growth of 4 Angstrom carbon nanotubes in linear channels of zeolite template

We report new developments on the chemical vapor deposition growth of 0.4 nm single-walled carbon nanotubes (SWCNTs) inside the linear channels of the aluminophosphate zeolite, AlPO₄-5 (AFI), single crystals (0.4 nm-SWCNT@AFI). Ethylene (C₂H₄) and carbon monoxide (CO) were used as the feedstock. Polarized Raman spectroscopy was used to analyze the structure and quality of SWCNTs, both the radial breathing mode and G-band are much clearer and stronger than the samples grown by the old process which used template tripropylamine molecules for growing SWCNT@AFI. From the Raman spectra, it is clearly seen that the RBM is composed of two peaks at 535 and 551 cm⁻¹. By using the pseudopotential module in Material Studio to calculate the Raman lines, the 535 cm⁻¹ peak is attributed to the (5,0) SWCNTs and the 551 cm⁻¹ peak to the (3,3) SWCNTs. The abundance of (4,2) is relatively small. Thermal gravity analysis showed that while the samples grown by CO display less than 1 wt% of carbon, for the samples heated in C₂H₄ atmosphere the weight percentage of SWCNTs is around 10%, which implies ∼30% of the AFI channels are occupied with SWCNTs, a significant increase compared with the previous samples.     

Selective low temperature synthesis of carbon nanospheres via the catalytic decomposition of trichloroethylene

The catalytic growth of structured carbon from a C₂H₄ and C₂HCl₃ feed promoted by Ni/SiO₂ in the presence of H₂ over the temperature range 673 K ≤ T ≤ 1023 K has been examined. The supported Ni phase exhibited an exclusive cubic symmetry (XRD analysis) with a range of Ni particle sizes (TEM analysis) and a net shift in the distribution to larger particles with increasing reduction temperature (from 20 to 36 nm), accompanied by a decrease in H₂ chemisorption. Conversion of C₂H₄ generated hydrogenation (C₂H₆), hydrogenolysis (CH₄) and decomposition (C + H₂) products. Ethane formation was favoured at lower temperatures with C formation increasingly preferred at higher temperatures so that C₂H₄ decomposition was the predominant process at T > 723 K; significant CH₄ production was only observed at T > 900 K. Carbon yield from C₂H₄ passed through a maximum at 773 K and took the form of high aspect ratio graphitic nanofibres with a central hollow core and diameters in the range 5–180 nm. The carbonaceous product has been characterized by a combination of TEM-EDX, SEM, XRD, BET area and temperature programmed oxidation (TPO). Carbon formation from C₂HCl₃ exceeded (by a factor of up to an order of magnitude) that generated via the decomposition of C₂H₄ at the same inlet C:Ni ratio to deliver essentially a carbon yield invariance (9.1 ± 0.3 g_C g_Ni^?1) where 898 K ≤ T ≤ 1023 K, which represents a carbon efficiency (fraction of carbon in the inlet feed that is converted to a solid carbon product) in excess of 96%. Ni/SiO₂ promoted a composite dehydrochlorination/decomposition of C₂HCl₃ to HCl + C. The nature of the carbon product generated from C₂HCl₃ is strongly temperature dependent with a shift from a pseudo-fibrous product at 773 K to a predominant nanosphere formation at 923 K. These nanospheres exhibit a wide diameter range (40–700 nm), a significant Cl content (1.1–2.6%, w/w) and a conglomeration or clustering to give a less ordered carbonaceous product than that generated at the lower temperature (773 K). A tentative carbon growth rationale is presented to account for the observed dependence of carbon structure on carbon-containing precursor and reaction temperature.     

Fabrication of carbon fiber reinforced ceramic matrix composites with improved oxidation resistance using boron as active filler

Boron was introduced into C_f/SiC composites as active filler to shorten the processing time of PIP process and improve the oxidation resistance of composites. When heat-treated at 1800 °C in N₂ for 1 h, the density of composites with boron (C_f/SiC-BN) increased from 1.71 to 1.78 g/cm³, while that of composites without boron (C_f/SiC) decreased from 1.92 to 1.77 g/cm³. So when boron was used, two cycles of polymer impregnation and pyrolysis (PIP) could be reduced. Meanwhile, the oxidation resistance of composites was greatly improved with the incorporation of boron-bearing species. Most carbon fiber reinforcements in C_f/SiC composite were burnt off when they were oxidized at 800 °C for 10 h. By contrast, only a small amount of carbon fibers in C_f/SiC-BN composite were burnt off. Weight losses for C_f/SiC composite and C_f/SiC-BN composite were about 36 and 16 wt%, respectively.     

Kinetically controlled synthesis of carbon nanofibers with different morphologies by catalytic CO disproportionation over iron catalyst

Carbon nanofibers (CNFs) were synthesized by CO disproportionation on iron catalyst at CO concentration between 58.3% and 75.0%, H₂ concentration between 8.3% and 25.0% and reaction temperature between 833 and 913 K. The time-depending rate of CNFs growth as a function of time was determined by an on-line mass spectrometer and the morphologies of all CNFs products were observed by electronic microscopy. Not only the CNFs growth rate but also the morphology of the grown CNFs were shown to vary with the three operating variables. SEM and TEM images showed that the three-dimensional morphologies of the CNFs were twist, helical or straight and an interesting relationship between the maximal growth rate and the morphology was observed. When the growth rate was between 0.8 and 0.9 mmol/(s gcat), the CNFs were twist. As the growth rate increased to 1.0 mmol/(s gcat), more helical nanofibers appeared. Straight nanofibers were produced when the growth rate reached the level of 1.2 mmol/(s gcat). Finally when the rate of CNFs growth was high at 1.8 mmol/(s gcat), the absolute majority of the solid products was amorphous carbon coexisting with some short and thick nanofibers. Under different operating conditions, the crystal faces of the catalysts had different anisotropy properties for carbon deposition, thus producing CNFs with different morphologies.     

Robust growth of herringbone carbon nanofibers on layered double hydroxide derived catalysts and their applications as anodes for Li-ion batteries

Herringbone carbon nanofibers (CNFs) were efficiently produced by chemical vapor deposition on Ni nanoparticles derived from layered double hydroxide (LDH) precursors. The as-obtained CNFs with a diameter ranging from 40 to 60 nm demonstrated herringbone morphologies when they grew on Ni/Al LDH derived catalysts both in the fixed-bed and fluidized-bed reactor. The Ni/Mg/Al, Ni/Cu/Al, as well as Ni/Mo/Mg/Al catalysts were also effective to grow herringbone CNFs. The diameter and specific surface area of the as-obtained CNFs highly depended on the catalyst composition and the growth temperature. When CNFs were grown at 550 °C on Ni/Al catalyst, the as-obtained products had an outer diameter of ca. 50 nm and a specific surface area of 242 m² g⁻¹, possessed a discharge capacity of 330 mAh g⁻¹ as the electrode in a two-electrode coin-type cell. With the increase of the surface area, the discharge capacity increased at a rate of 0.90 mAh cm⁻², while the initial coulombic efficiency decreased gradually on nanocarbon anodes. This is attributed to the fact that CNFs with higher surface area afford smaller sp² carbon layer that facilitated more Li ions to extract from the anodes.     

Single-walled carbon nanotubes functionalized with polydiphenylamine as active materials for applications in the supercapacitors field

Composites based on polydiphenylamine (PDPA) doped with heteropolyanions of H₃PW₁₂O₄₀ and single-walled carbon nanotubes (SWNTs) were prepared by electrochemical polymerization of diphenylamine (DPA) on carbon nanotube films deposited onto Pt electrodes. HRTEM studies reveal that the electrochemical polymerization leads to the filling the spaces between tubes which compose the bundles, creating a monolithic film on the Pt electrode. The resulting composites were tested as active materials in supercapacitors. Resonant Raman scattering studies showed that the electropolymerization of DPA in the presence of H₃PW₁₂O₄₀ and SWNTs leads to the covalent functionalization of SWNTs with doped PDPA. The covalent functionalization of SWNTs with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions was revealed by FTIR spectroscopy, based on the changes in the vibrational features of PDPA and H₃PW₁₂O₄₀. These changes included i) a down-shift of the PDPA IR bands, which was attributed to the C–H bending vibrational mode of benzene (B), C_aromatic–N, C–C stretching (B) + C–H bending (B) and C–C stretching vibrations of the B ring, from 1174, 1321, 1495 and 1603 cm^− 1 to 1165, 1313, 1487 and 1599 cm^− 1, respectively; ii) a change in the peak positions of IR bands associated with the W = O and P-O-W vibration modes of H₃PW₁₂O₄₀; and iii) a down-shift of the IR band situated in the spectral range 650–725 cm^− 1, which was assigned to the inter-ring deformation vibration mode.The characterization of symmetric solid-state supercapacitors was performed for electrodes prepared from i) SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions, ii) SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions, and iii) PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. Preliminary results indicate high discharge capacitance values of up to 157.2 mF/cm² for SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. The discharge capacitance of this material is superior to those recorded for SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions (~ 18.2 mF/cm²) and PDPA doped with H₃PW₁₂O₄₀ heteropolyanions (~ 62.1 mF/cm²).     

Synthesis and characterization of nickel phosphonopropionate hybrid materials

Hydrothermal reaction of nickel acetate with 3-phosphonopropionic acid and 4,4′-bipyridine resulted in two novel phosphonate compounds Ni(HO₃PC₂H₄COO)(4,4′-bpy)(H₂O) · 0.5(4,4′-bpy) (bpy = bipyridine) (I) and Ni(O₃PC₂H₄COOH)(4,4′-bpy)(H₂O)₃ · H₂O (II). Single-crystal X-ray studies reveal that self-assemblies between the ligands and metal ion result in layer (I) and chain (II) structures. Magnetic measurement of I indicate there are ferromagnetic couplings between adjacent Ni²⁺ ions (C = 1.29 cm³ mol⁻¹ K and θ = 2.25 K).     

Tunable and selective formation of micropores and mesopores in carbide-derived carbon

Pore structure of carbide-derived carbon (CDC) was tunable by chlorination of Ti(C_1−xA_x) solid–solution carbides (A = O or N). High-energy ball milling method was used to synthesize various nanocrystalline Ti(C_1−xA_x) phases. We were able to obtain specific dimension of pore volumes in the range of micropore (<2 nm) or mesopore size (2–50 nm), depending on the compositions of the precursors. The substitutional atoms and their contents effectively modify the characteristics of pores i.e., pore size, volume and their distributions. The micropore volume density, total pore volume density and specific surface area (SSA) of Ti(C_0.7O_0.3) CDCs were found 1.55 cm³/g, 1.72 cm³/g and 3100 m²/g, respectively. In contrast, Ti(C_0.5N_0.5) CDCs showed enhancement of mesopore formation with 3.34 cm³/g, 3.45 cm³/g and 522 m²/g for mesopore volume density, total pore volume density and SSA, respectively.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.     

Synthesis,structure and magnetic properties of a copper molybdate hybrid inorganic/organic solid with a novel 10-connected three-dimensional network topology

Hydrothermal reaction of CuCl₂, MoO₃, and 4,4′-dipyridylketone (4,4′-dpk) afforded green crystals of the mixed metal oxide phase {[Cu₂(MoO₄)₂(4,4′-dpk)(H₂O)]·H₂O}_n (1). According to single-crystal X-ray diffraction, {Cu₂O₂} dimers link into 1-D {Cu₂O₂(μ-H₂O)}_n chains via bridging aqua ligands. These chains form [Cu₂(MoO₄)₂(H₂O)]_n slabs via linkage through tetrahedral molybdate anions. In turn, the copper molybdate slabs are pillared through tethering 4,4′-dpk ligands into a 10-connected three-dimensional lattice with an unprecedented 3¹²4³⁰5²6 topology. Variable temperature magnetic data above 140 K were fit to the Curie–Weiss law, with C = 0.17 cm³ K/mol Cu and Θ = 70 K, indicating likely ferromagnetic coupling within the dinuclear kernels of 1; low temperature data points towards the possibility of interdimer antiferromagnetic interactions.     

Carbon nanofibres produced from electrospun cellulose nanofibres

Cellulose nanofibres have been fabricated by electrospinning of a cellulose acetate solution followed by deacetylation. The cellulose nanofibres were then carbonized using temperatures in the range 800–2200 °C and the resulting carbon nanofibres (CNFs) were characterized using transmission electron microscopy, X-ray diffraction and Raman spectroscopy. A graphitic structure was observed for CNFs treated at a relatively low temperature of 1500 °C, with no obvious skin-core heterogeneity observed for fibres treated up to 2200 °C, suggesting a possible advantage of using nano-scale precursors. The effective Young’s modulus of the CNFs was assessed using an in situ Raman spectroscopic technique following the shift in the position of the G′ (2D) band located at ∼2660 cm⁻¹ and relating this to a calibration established for a range of other carbon fibres. Using this approach the moduli of the CNFs were found to be ∼60 and ∼100 GPa for samples carbonized at 1500 and 2200 °C, respectively.     

Solvothermal in situ synthesis of (Schiff-base)-containing dinuclear cobalt compound

A new dinuclear cobalt compound, namely Co₂(L)(H₂O)Cl₂ (1, H₂L = N,N′-o-phenylenebis(salicylide-neimine) was obtained by one-pot solvothermal self-assembly of CoCl₂, 1,2-phenylenediamine, and salicylaldehyde in C₂H₅OH. The magnetic studies suggest weak antiferromagnetic behavior and the magnetic data were interpreted by means of a dinuclear cobalt model with the parameters of g = 2.12, J = ?1.25 cm^?1, θ = ?3.12 K.     

Densification and properties of transition metal borides-based cermets via spark plasma sintering

Engineering borides like TiB₂ and ZrB₂ are difficult to sinter materials due to strong covalent bonding, low self-diffusion coefficient and the presence of oxide layer on the powder particles. The present investigation reports the processing of hard, tough and electrically conductive transition metal borides (TiB₂ and ZrB₂) based cermets sintered with 6 wt.% Cu using spark plasma sintering (SPS) route. SPS experiments were carried out with a heating rate of 500 K/min in the temperature range of 1200–1500 °C for a varying holding time of 10–15 min and the optimization of the SPS conditions is established. A maximum density of ∼95% ρ_th in ZrB₂/Cu and ∼99% ρ_th in TiB₂/Cu is obtained after SPS processing at 1500 °C for 15 min. While the optimized TiB₂/Cu cermet exhibits hardness and fracture toughness of ∼17 GPa and ∼11 MPa m^1/2, respectively, the optimized ZrB₂/Cu cermet has higher hardness of ∼19 GPa and fracture toughness of ∼7.5 MPa m^1/2, respectively. High electrical conductivity of ∼0.20 MΩ⁻¹ cm⁻¹ (TiB₂/Cu) and ∼0.15 MΩ⁻¹ cm⁻¹ (ZrB₂/Cu) are also measured with the optimally sintered cermets.     

Gas phase oxidation as a tool to introduce oxygen containing groups on metal-loaded carbon nanofibers

Oxygen containing groups were introduced, onto carbon nanofibers (CNFs) that were previously loaded with palladium, using HNO₃ vapor. Using traditional liquid-phase oxidations this is not possible due to severe metal leaching. For the samples oxidized using HNO₃ vapor temperature programmed desorption and X-ray photoelectron spectroscopy revealed the presence of two major classes of oxygen containing groups, i.e. carboxylic acid groups which are thermally stable up to 300 °C and less acidic (e.g. phenol) and basic groups which were stable up to 700 °C. The amount of acidic oxygen containing groups introduced by this gas-phase treatment ranged from 0.1 to 0.3 mmol/g, as determined by titration. The latter amount is comparable to that introduced by traditional liquid-phase treatment in 65% HNO₃ on bare CNFs. Transmission electron microscopy and H₂-chemisorption measurements show a gradual increase of the average metal particle size from 2.1 nm for the starting Pd/CNF to 4.5 nm for Pd/CNF treated for 75 h in HNO₃ vapor indicating that the extent of sintering with gas-phase treatment is limited. Elemental analysis showed that no leaching occurred upon gas-phase oxidation, whereas 90% of the metal was lost with a liquid-phase reflux HNO₃ treatment.