Arc synthesis of double-walled carbon nanotubes in low pressure air and their superior field emission properties

Double-walled carbon nanotubes (DWCNTs) have been effectively synthesized by direct current (DC) arc discharge in low pressure air using a mixture of Fe catalyst and FeS promoter. Compared with conventional arc methods, this method is easier to implement without using expensive high purity gas sources. A tip structural DWCNT film has been successfully fabricated by a mixing process of electrophoresis, electroplating and electrocorrosion. The field emission properties of tip structural nanotube film are significantly increased compared with DWCNT film fabricated by electrophoresis. The turn-on electric field E_to decreases from 1.25 to 0.92 V/μm, the low threshold electric field E_th decreases from 1.45 to 1.13 V/μm, and the field enhancement factor β increases from about 2210 to 4450. Meanwhile, this tip structural CNT film shows remarkably stable within 2% fluctuations for several hours. The high-performance emitter material and preparation technologies are both easy to scale up to large areas.     

Toughening and hardening in double-walled carbon nanotube/nanostructured magnesia composites

Dense double-walled carbon nanotube (DWCNT)/nanostructured MgO composites were prepared using an in situ route obviating any milling step for the synthesis of powders and consolidation by spark-plasma-sintering. An unambiguous increase in both toughness and microhardness is reported. The mechanisms of crack-bridging on an unprecedented scale, crack-deflection and DWCNT pullout have been evidenced. The very long DWCNTs, which appear to be mostly undamaged, are very homogeneously dispersed at the grain boundaries of the matrix, greatly inhibiting the grain growth during sintering. These results arise because the unique microstructure (low content of long DWCNTs, nanometric matrix grains and grain boundary cohesion) provides the appropriate scale of the reinforcement to make the material tough.     

Optimisation of reaction conditions for the synthesis of single‐walled carbon nanotubes using response surface methodology

The effects of three preparation variables, i.e. reaction temperature, reaction time and reaction gas (methane/nitrogen) flow rate, on the ratio of the intensity of the Raman D band to the intensity of the G band (I_D/I_G), carbon mass and the presence of radial breathing mode (RBM) peaks were investigated by using a central composite design to develop two linear models. The most influential factor in each experimental design‐response was identified using the analysis of variance. The predicted I_D/I_G ratio, carbon mass and presence of RBM peaks determined during the process optimisation were found to agree satisfactorily with the experimental values. The optimum conditions for synthesising single‐walled carbon nanotubes were determined to be a reaction temperature of 900°C, a reaction time of 59 min and a reaction gas flow rate of 54 mL/min. © 2011 Canadian Society for Chemical Engineering     

Growth of double-walled carbon nanotubes from silicon oxide nanoparticles

Double-walled carbon nanotubes (DWCNTs) were synthesized by a metal-catalyst-free chemical vapor deposition method using silicon oxide nanoparticles as a catalyst. The diameters and lengths of the DWCNTs are in the ranges of 3–5 nm and 1–5 μm, respectively. The amount of DWCNTs produced is about 70%, while the remainder is single-walled carbon nanotubes. A heat treatment of the SiO₂/Si substrate used was found to be crucial for controlling the size of the catalyst nanoparticles, and hence for the growth of the DWCNTs. Flat or cone-shaped caps were observed for the DWCNTs, indicating that the growth of the DWCNTs from the non-metal catalyst follows a vapor–solid–solid mechanism. Energy dispersive spectroscopy and X-ray photoelectron spectroscopy characterizations confirmed that no metal impurity exists in the obtained DWCNT samples.     

Nitrogen doping effects on the structure behavior and the field emission performance of double-walled carbon nanotubes

The nitrogen (N) doping effect and field emission properties of double-walled carbon nanotubes (DWCNTs) were investigated. Diameter transformation and defect generation in the N-doped DWCNTs mainly depend on the amount of nitrogen employed. By applying N-doping into DWCNTs (1.5 N at.%), the average diameters of the DWCNTs were increased from 1.7 to 2.4 nm, and the crystallinity (I_G/I_D) was decreased from 13.5 to 5. Field emission properties were enhanced by the N doping into DWCNTs. The turn-on field, corresponding to a current density of 0.1 μA/cm², was about 0.9 V/μm for the N-doped DWCNTs (1.5 N at.%). The field enhancement factor of the N-doped DWCNTs was higher than that of the undoped DWCNTs. It was found that the field emission properties were controlled by pyridine-like N in the graphite due to N-doping.     

Effect of H2O adsorption on the electrical transport properties of double-walled carbon nanotubes

Water molecules adsorbed on a double-walled carbon nanotube (DWCNT) serve as charge trapping centers when present in low density and as electron donors when present in high density. There is a discontinuous change between the low- and high-density regions. H₂O molecules are apt to be adsorbed on the outer surface of DWCNTs, and in this case the electrical transport properties are extremely sensitive to environment, which suggests that DWCNTs are hole doped and act as an electric dipole with the inner tube.     

Synthesis of High‐Quality,Double‐Walled Carbon Nanotubes in a Fluidized Bed Reactor

Double‐walled carbon nanotubes (DWCNTs) were synthesized in a packed bed reactor (PBR) and a fluidized bed reactor (FBR) by cracking CH₄ on a Fe/MgO catalyst. It is observed that the dominant carbon product changes drastically from DWCNTs to multi‐walled CNTs along the axial direction of PBR. The studies indicated that the high concentration of H₂ from the high conversion of CH₄ causes the quick reduction and sintering of the iron catalyst and inhibits the nucleation of DWCNTs. Based on these results, the batch or continuous feeding mode of small amounts of catalyst was adopted in a FBR to maintain a high space velocity of CH₄ and to inhibit the negative effect of excess H₂. Finally, a DWCNT product with a specific surface area of 950 m²/g and a purity of 98 %, was obtained.     

The confined growth of double-walled carbon nanotubes in porous catalysts by chemical vapor deposition

Double-walled carbon nanotubes (DWCNTs) were prepared from methane using a Fe/MgO porous catalyst. A series of catalyst powders with different pore size distributions were obtained by compression at pressures of 0-233 MPa. These were used to decompose methane and synthesize DWCNTs which differed in activity, purity, yield and degree of perfection. Characterization by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, thermo-gravimetric analysis, N₂ adsorption measurement (Brunauer-Emmett-Teller (BET)) and Hg penetration provided direct evidence that a compact catalyst structure is not good for the nucleation and growth of DWCNTs, e.g., a catalyst with a compact structure that did not have pores larger than 30-50 nm mostly produced multi-walled carbon nanotubes. The confined growth and buckling model of DWCNTs inside the porous catalysts are proposed to explain the growth behavior. These results suggest that a porous catalyst for DWCNT synthesis should have a large pore size distribution or loose stacked structure, which provides new guidelines for catalyst design.     

Partially graphitized carbon filaments from as-synthesized silica/surfactant composite

The carbonization behavior of surfactants templated within mesoporous silica is studied in detail. Cetyltrimethylammonium bromide and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) are used as the structure-directing agents for MCM-41 and SBA-15 synthesis, respectively. Thermal treating the as made silica/surfactant composites under argon flow at 900 °C produces partially graphitized carbon filaments as a result of the carbonization of the surfactants within the mesopores. Furthermore, the carbon materials derived from P123 in SBA-15 yield a more developed graphite structure than the carbon obtained from CTAB in MCM-41, as evidenced by the narrower X-ray Bragg reflections in the powder XRD and larger I_G/I_D ratio in the Raman spectra.     

Electrochemistry of double-wall carbon nanotubes encapsulating C60 and their spectral characterization

Electrochemistry of double-wall carbon nanotubes (DWCNTs) encapsulating C₆₀ (C₆₀@DWCNT) have been studied by preparing a C₆₀@DWCNT modified electrode, and three pairs of reversible electro-reduction waves corresponding to electron transfer reactions of C₆₀ inside DWCNTs have been obtained in a mixed solvent of toluene and acetonitrile (4:1, v:v) containing tetrabutylammonium cation as supporting electrolyte, which indicates that DWCNTs act as molecular wires to allow electrical communication between the underlying electrode and the redox-active guest C₆₀. The influencing factors on the electrochemistry of C₆₀@DWCNT modified electrodes have been investigated. The results suggest that the voltammetric behavior of C₆₀@DWCNT is dependent on the nature of the supporting electrolyte and the solvent system. In addition, spectral characterization of the C₆₀@DWCNT modified electrodes before and after electrochemical scanning reveals interaction between C₆₀ and DWCNT and verifies the reduction of C₆₀ encapsulated in DWCNTs. C₆₀ molecules inside DWCNTs retains their redox activity, and can also act as an electron-transfer mediator to electrocatalyze the reduction of halohydrocarbon.     

Stability comparison of simulated double-walled carbon nanotube structures

The focus of this research is to systematically study and classify electronic energy trends in different double-walled carbon nanotube (DWCNT) structures through ab initio simulations. Simulations comparing the stability of DWCNTs with different interwall spacings, tube types (armchair or zigzag), lengths, diameters, and endcaps were performed at a variety of computational levels. These simulations showed that DWCNTs nucleate from end caps and become energetically more stable as length and diameter increase. Another finding of this research was that the interwall spacing is dependent on which type of tube is in the outer position of the DWCNT. High stability configurations occurred when the interwall spacing was approximately 3.3 Å and a zigzag tube was in the outer position or when the interwall spacing was approximately 3.5 Å and an armchair tube was in the outer position. It was also seen that endcaps affected which tube combinations were more stable; the armchair@armchair DWCNT was the most energetically stable combination for capped tubes, while the armchair@zigzag DWCNT had the highest stability of uncapped tubes. Understanding if there is a preferred structural motif for DWCNTs and clarifying which nucleation and growth paths are favored by nanotubes will elucidate if controlled fabrication can be achieved.     

Behavior of the high frequency Raman modes of double-wall carbon nanotubes after doping with bromine or iodine vapors

The effects of two different halogen dopants (bromine and iodine) at different concentrations on the higher frequency modes (the so-called G and G′ bands) of the Raman spectra of double-wall carbon nanotube (DWCNT) “buckypaper” are investigated. The effects of dopants on different DWCNT configurations (metallic inner/semiconducting outer and vice versa) are studied by changing the laser excitation energy. The doping causes the loss of the Breit–Wigner–Fano line shape and the appearance of less metallic behavior. An increase of the relative intensity of the G⁺ band, which is more sensitive for the outer metallic tubes, is clearly observed with increasing Br₂ concentration in the sample. By analysis of the G⁺ band and the G′ band it is possible to measure the changes in the electron–phonon coupling, due to the charge-transfer between the dopant (Br₂ or I₂) and the tubes in the DWCNT. The doping effect causes an upshift of the G⁺ band and a suppression of the contribution of the inner tubes to the G′ band signal and as a consequence, the observed G′ band is dominated by the contribution from the outer tubes.     

Carbon nanowalls deposited by inductively coupled plasma enhanced chemical vapor deposition using aluminum acetylacetonate as precursor

Well aligned carbon nanowalls, a few nanometers thick, were fabricated by continuous flow of aluminum acetylacetonate (Al(acac)₃) without a catalyst, and independent of substrate material. The nanowalls were grown on Si, and steel substrates using inductively coupled plasma-enhanced chemical vapor deposition. Deposition parameters like flow of argon gas and substrate temperature were correlated with the growth of carbon nanowalls. For a high flow of argon carrier gas, an increased amount of aluminum in the film and a reduced lateral size of the carbon walls were found. The aluminum is present inside the carbon nanowall matrix in the form of well crystallized nanosized Al₄C₃ precipitates.     

Preparation of graphene sheets by the reduction of carbon monoxide

Graphene sheets were prepared by a synthesis method in which aluminum sulfide (Al₂S₃) was calcined under a mixed gas flow of carbon monoxide (CO) and argon. The reaction of CO with Al₂S₃ produced α-Al₂O₃ and graphene sheets. The graphene sheets were characterized by X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy. A reaction mechanism is proposed in which CO is reduced to gaseous carbon by the reaction with Al₂S₃ and the gaseous carbon crystallizes in the graphene sheets.     

Synthesis of double-walled carbon nanotube films and their field emission properties

Continuous double-walled carbon nanotube (DWCNT) films were synthesized using an Fe-Mo catalyst by the arc discharge method. This new catalyst has dramatically improved the purity and selectivity of DWCNT product. High-resolution transmission electron microscopy indicates that the outer and inner diameter of DWCNT are 1.9-4.7 nm and 1.2-3.8 nm, respectively. The field emission properties of DWCNT films have been studied. The directly grown film was transferred onto quartz substrates and used as emission cathodes, and has demonstrated a quite good emission performance. Moreover, the emissions of DWCNT films have been further improved by heat treatment. The film after 400 °C oxidation shows excellent field emission property with a low turn-on (E_to = 0.6 V/μm) and threshold field (E_th = 0.9 V/μm) corresponding to the emission current density of 1 μA/cm² and 1 mA/cm², respectively.     

The influence of doping on the Raman intensity of the D band in single walled carbon nanotubes

The D band in the Raman spectra of single walled carbon nanotubes is considered as an indicator of defects in carbon nanotubes. However, its dependence on charge-transfer doping is generally ignored, despite the studied samples are often naturally doped. We studied the intensity of the D band, the ratio of the intensities of the D band and TG band (I_D/I_TG) and the ratio of the intensities of the D and G′ band (I_D/I_G′) in the Raman spectra of the single walled carbon nanotubes in dependence on a doping level. We tested two laser excitation energies viz 2.41 and 1.92 eV, which are in resonance with semiconducting and metallic tubes, respectively in our sample. It is shown that the D band intensity is significantly attenuated in doped carbon nanotubes sample for both semiconducting and metallic tubes. The I_D/I_TG ratio is weakly dependent on doping for semiconducting tubes but for metallic tubes the I_D/I_TG ratio exhibits strong dependence on doping. The I_D/I_G′ ratio is suggested for evaluation of the defects in carbon nanotubes samples since it is less sensitive to doping both for semiconducting and metallic tubes. Nevertheless, for highly doped samples even the I_D/I_G′ ratio exhibits significant dependence on doping level.     

Spectroscopic study of double-walled carbon nanotube functionalization for preparation of carbon nanotube / epoxy composites

A spectroscopic study of the amino functionalization of double-walled carbon nanotubes (DWCNT) is performed. Original experimental investigations by near edge X-ray absorption fine structure spectroscopy at the C and O K-edges allow one to follow the efficiency of the chemistry during the different steps of covalent functionalization. Combined with Raman spectroscopy, the characterization gives direct evidence of the grafting of amino-terminated molecules on the structural defects of the DWCNT external wall, whereas the internal wall does not undergo any change. Structural and mechanical investigations of the amino functionalized DWCNT/epoxy composites show coupling between epoxy molecules and the DWCNTs. Functionalization improves the interface between amino-functionalized DWCNT and the epoxy molecules. Electrical transport measurements indicate a percolating network formed only by the inner metallic tubes of the DWCNTs. The activation energy of the barrier between connected metallic tubes is determined to be around 20 meV.     

Atomically resolved Moiré-type superstructures in double-walled carbon nanotubes

Double-wall carbon nanotubes (DWCNTs) are investigated with high resolution by scanning tunneling electron microscopy at 78 and 4.5 K. Besides the atomic structure of the DWCNT surface, an additional honeycomb-type superstructure is revealed at these low temperatures. This periodic structure can be interpreted in terms of a Moiré pattern resulting from a rotation between the rolled up graphene sheets that constitute the DWCNT. Deviations of the observed Moiré patterns from the perfect hexagonal Moiré patterns that are commonly resolved for multi-graphene layer surfaces are found to be related to the intrinsic different curvature of the inner and outer tube of the DWCNT. Investigation of such Moiré patterns in the surface of DWCNTs allows one to gain more insight into the structure and chirality of the otherwise inaccessible inner shell of the DWCNTs.     

Orientation of graphitic planes during annealing of “dip deposited” amorphous carbon film: A carbon K-edge X-ray absorption near-edge study

Annealing effect of amorphous carbon thin films on Si(1 0 0) substrates is studied by normal incidence and angle dependent carbon K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The angle dependence of the XANES signal shows that the graphitic basal planes are oriented perpendicular to the surface when the film is annealed at 1000 °C. Micro-Raman spectroscopy reveals two well-separated bands the D band at 1355 cm⁻¹ and G band at ∼1600 cm⁻¹, and their I_D/I_G intensity ratio indicates the formation of more graphitic film at higher annealing temperatures. X-ray diffraction pattern of 1000 °C temperature annealed film confirms the formation of graphite structure.     

Deposition of HMDSO-based coatings on PET substrates using an atmospheric pressure dielectric barrier discharge

This paper presents results on the formation of coatings in an atmospheric pressure dielectric barrier discharge using hexamethyldisiloxane (HMDSO) as gaseous precursor. Plasma-polymerized films are deposited onto polyethylene terephthalate (PET) films using argon and argon/air mixtures as carrier gases. The chemical and physical properties of the obtained coatings are discussed in detail using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). FTIR and XPS results show that the composition of the gas phase and the chemical structure of the obtained coatings are clearly correlated. When pure argon is used as working gas, the film is polymeric with a structure close to [(CH₃)₂–Si–O]_n, which means that the deposited films resemble PDMS. However, if plasma-polymerization occurs in argon/air mixtures, the deposited film is silica-like containing only few carbon atoms. These dense SiO_x coatings generally exhibit high barrier properties, while pure HMDSO-derived films might be of importance for selective permeation. From this point of view, the capability of controlling the film composition by varying the operation conditions opens interesting perspectives.