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.     

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.     

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.     

One-step fabrication of high quality double-walled carbon nanotube thin films by a chemical vapor deposition process

High-quality double-walled carbon nanotubes (DWCNTs) thin-films have been fabricated in one-step by the catalytic chemical vapor deposition gas-flow reaction process with acetone as a carbon source in an argon flow. The DWCNTs film is formed through the self-assembly of the DWCNTs in the gas flow, which is achieved by controlling the gas rates in the synthesis reaction. The DWCNT film is self-supported and consists of preferentially aligned high-quality DWCNT bundles. Raman spectral analysis shows a low intensity ratio of the D band and the G band with I_D/I_G being 0.025 indicating a high-quality of DWCNTs at a macroscopic scale. Property measurements show that the DWCNT film is mechanically robust and highly electrically conductive. The formation of high-quality DWCNTs can be attributed to the reaction in the argon environment that is inert and does not attack the DWCNTs at the high synthesis temperature (1170 °C). This one-step fabrication process is feasible for large-scale productions of high-quality DWCNTs films with promising structural and functional applications.     

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.     

The effect of phase separation in Fe/Mg/Al/O catalysts on the synthesis of DWCNTs from methane

Double-walled carbon nanotubes (DWCNTs) were prepared by methane decomposition on Fe/Al/Mg/O catalysts with a fixed iron loading of 1.5%. Increasing Al/Mg ratio in the catalyst resulted in the formation of a new MgAl₂O₄ phase, which was characterized by XRD. The size of the MgO crystallites in the support was decreased, due to the phase separation, from 35 nm to 20 nm in the Al/Mg ratio range of 0:1-4:1. At an Al/Mg ratio of 1:200, this effect prevented the sintering of iron on the MgO support and resulted in the synthesis of high-purity DWCNTs in high-yield. Very high-Al/Mg ratio induced the formation of the MgAl₂O₄ phase, which became another catalyst support material. This had a negative effect on the synthesis of DWCNTs due to its acidity and hardness. Simultaneously maintaining MgO as the dominant catalyst support and decreasing its particle size by the phase separation effect are important for good metal dispersion and, consequently, the yield and purity of DWCNTs.     

Synthesis of single-wall carbon nanohorns by arc-discharge in air and their formation mechanism

Single-wall carbon nanohorns (SWCNHs) with different morphologies were generated by direct current arc-discharge between pure graphite rods in different atmospheres, including air, CO₂ and CO. In the arc-discharge process, the O₂ in the air reacted with carbon atoms and transformed into CO so that the formation of SWCNHs was a result of a combination of CO and N₂. The effect of different volume ratios of CO/N₂ on the formation of SWCNHs was examined and a mechanism for the formation of SWCNHs is proposed.     

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.     

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.     

Catalytic activity of cobalt and iron phthalocyanines or porphyrins supported on different carbon nanotubes towards oxygen reduction reaction

Non-noble metal catalysts for O₂ reduction were prepared by dispersing iron(II) phthalocyanine, cobalt(II) tetra-tert-butylphthalocyanine, cobalt(II) 2,3,7,8,12,13,17,18-octaethyl-porphine, and cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrine on carbon nanotubes (CNTs) used as high surface area support. Different types of CNTs (SWCNTs, DWCNTs and MWCNTs) were investigated as an effective substitute for commonly used carbon black in carbon-supported phthalocyanines and porphyrins. The oxygen reduction reaction (ORR) activity of those CNT-supported catalysts in alkaline and acidic solutions was studied. The results show that: (i) all catalytic systems including MWCNTs are more efficient for O₂ reduction than those with SWCNTs and DWCNTs, (ii) the oxidative chemical treatment of the CNTs increases the electrocatalytic performance of the corresponding CNT-supported catalysts, (iii) similarly to Vulcan-supported catalysts, iron(II) phthalocyanine gives the best electroactivity among the investigated CNT-supported materials and (iv) finally, the MWCNT-supported iron(II) phthalocyanine catalyst chemically treated in oxidative conditions shows an ORR catalytic activity comparable to a commonly used Pt/C catalyst with similar current densities and a very low overpotential (60 mV).     

Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C₆₀, C₇₀ and C₆₀H₂₈ molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV-vis-NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C₇₀ molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C₆₀ and C₇₀ peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C₆₀ and C₇₀ caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C₆₀H₂₈@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.     

Differentiation of chemical reaction activity of various carbon nanotubes using redox potential: Classification by physical and chemical structures

The present study systematically examined the kinetics of a hydroxyl radical scavenging reaction of various carbon nanotubes (CNTs) including double-walled and multi-walled carbon nanotubes (DWCNTs and MWCNTs), and carbon nano peapods (AuCl₃@DWCNT). The theoretical model that we recently proposed based on the redox potential of CNTs was used to analyze the experimental results. The reaction kinetics for DWCNTs and thin MWCNTs agreed well with the theoretical model and was consistent with each other. On the other hand, thin and thick MWCNTs behaved differently, which was consistent with the theory. Additionally, surface morphology of CNTs substantially influenced the reaction kinetics, while the doped particles in the center hollow parts of CNTs (AuCl₃@DWCNT) shifted the redox potential in a different direction. These findings make it possible to predict the chemical and biological reactivity of CNTs based on the structural and chemical nature and their influence on the redox potential.     

Catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes from α-(Al1−xFex)2O3 powders and self-supported foams

An investigation of the potential interest of α-alumina-hematite foams, as opposed to powders, as starting materials for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapor deposition method was performed. The oxide powders and foams as well as the corresponding CNT-Fe-Al₂O₃ composite powders and foams are studied by X-ray diffraction, specific surface area measurements, electron microscopy, Raman spectroscopy and Mössbauer spectroscopy. The latter technique revealed that four components (corresponding to α-Fe, Fe₃C, γ-Fe-C and Fe³⁺) were present in the Mössbauer spectra of the composite powders, and that an additional sextet, possibly due to an Fe_1−yC_y alloy, is also present in the Mössbauer spectra of the composite foams. Contrary to some expectations, using foams do not lead to an easier reduction and thus to the formation of more α-Fe, Fe₃C and/or γ-Fe-C potentially active particles for the formation of CNTs, and hence to no gain in the quantity of CNTs. However, using foams as starting materials strongly favors the selectivity of the method towards SWCNTs (60% SWCNTs and 40% DWCNTs) compared to what is obtained using powders (5% SWCNTs, 65% DWCNTs and 30% MWCNTs).     

Effect of strain rate on the buckling behavior of single- and double-walled carbon nanotubes

Molecular dynamics simulations are performed on single- (SWCNTs) and double-walled carbon nanotubes (DWCNTs) to investigate the effects of strain rate on their buckling behavior. The Brenner’s second-generation reactive empirical bond order and Lennard-Jones 12-6 potentials are used to describe the short range bonding and long range van der Waals atomic (vdW) interaction within the carbon nanotubes, respectively. The sensitivity of the buckling behavior with respect to the strain rate is investigated by prescribing different axial velocities to the ends of the SWCNTs and DWCNTs in the compression simulations. In addition, the effects of vdW interaction between the walls of the DWCNTs on their buckling behavior are also examined. The simulation results show that higher strain rates lead to higher buckling loads and buckling strains for both SWCNTs and DWCNTs. A distinguishing characteristic between SWCNTs and DWCNTs is that the former experiences an abrupt drop in axial load whereas the axial load in latter decreases over a finite, albeit small, range of strain after buckling initiates. The buckling capability of DWCNT is enhanced in the presence of vdW interaction. DWCNTs can sustain a higher strain before buckling than SWCNTs of similar diameter under otherwise identical conditions.     

Analysis of the vibration characteristics of double-walled carbon nanotubes

The induced electric-field has been applied to measure the elastic modulus of carbon nanotubes. However, the vibrational modes of the multi-walled carbon nanotubes are quite different from those of the single-walled carbon nanotubes. Analysis of the vibration characteristics of double-walled carbon nanotubes (DWCNTs) with simply supported boundary condition are carried out based on Euler–Bernoulli beam theory. The DWCNTs are considered as two nanotube shells coupled through the van der Waals interaction between them. It is found that the vibrational modes of DWCNTs are noncoaxial intertube vibrations, and the deflections of the inner and outer nanotubes can occur in the same or in opposite deflections. In the same vibrational mode, the resonant frequencies of DWCNTs with deflections between the inner and outer nanotubes in the same direction are smaller than those of DWCNTs with the opposite deflections.     

Improved field emission properties of double-walled carbon nanotubes decorated with Ru nanoparticles

The field emission properties of double-walled carbon nanotubes (DWCNTs) were remarkably improved by decorating their surface with ruthenium (Ru) metal nanoparticles. The Ru nanoparticles were attached effectively on the surface of DWCNTs via a chemical procedure. The Ru-decorated DWCNTs showed lower turn-on voltage, higher emission current density, and improved emission uniformity compared with pristine DWCNTs. The effect of Ru nanoparticles on the work function and density of states was evaluated by the first-principles calculation. The enhanced field emission properties of Ru-DWCNTs were mainly attributed to the Ru nanoparticles which increased the field enhancement factor and the density of emission sites. Our results indicate that the Ru-decorated DWCNTs can be used as an effective field emitter for various field emission devices.     

Transport properties of double-walled carbon nanotubes and carbon boronitride heteronanotubes

Using the density functional theory combined with the nonequilibrium Green's function, the transport properties of double-walled carbon nanotubes (DWCNTs) and carbon boronitride (CBN) heteronanotubes were investigated. As the hopping length increases, the conductance of DWCNTs shows a dramatic variation that is independent of the intertube space. The transport of the CBN heterojunctions also displays abnormal behavior when the hopping length is changed, which is very different from the behavior of DWCNTs. The currents of the forward in the CBN heterojunctions are about 3–15 times as large as those of the back under lower bias voltages. The negative differential resistance (NDR) effect occurs in the CBN heterojunctions, and the peak-to-valley ratio in the additional NDR regions is about 2–4 for the current–voltage relationship. The hopping length and BN parts have a great influence on the transport of the double-walled nanodevices.     

Comparison of double-walled with single-walled carbon nanotube electrodes by electrochemistry

Double-walled carbon nanotubes (DWCNTs) were selectively functionalised by treatment with concentrated nitric and sulphuric acid, resulting in carboxylated outer and pristine inner tube constituents. The functionalised DWCNTs were then incorporated into two types of pre-existing carbon nanotube (CNT) electrode platforms, and the performance of each was compared to single-walled carbon nanotubes (SWCNTs). To make the CNT electrode platforms DWCNTs were covalently bound to fluorinated tin oxide glass (FTO) or electrografted aminophenyl tether layers on silicon. The performance of single- compared to double-walled CNTs on FTO or silicon supported electrodes was then determined through electrochemical methods, using the redox probes, ferrocene and ruthenium hexaamine, respectively. The DWCNTs showed an improved heterogeneous rate constant. This improvement was attributed to the protection of the electronic properties of the inner wall of the DWCNT during the chemical modification and suggests that DWCNTs may offer a useful alternative to SWCNTs in future electronic devices.     

Adsorption of perchlorate onto raw and oxidized carbon nanotubes in aqueous solution

Perchlorate (ClO₄^?) is an emerging trace contaminant. The adsorption of ClO₄^? on raw and oxidized carbon nanotubes (CNTs) was investigated to elucidate the affinity mechanism of CNTs with anion pollutants. The adsorption of ClO₄^? into different CNTs increased in the order multi-walled CNTs < single-walled CNTs < double-walled CNTs (DWCNTs). Co-existing anions (SO₄^2?, NO₃^?, Cl^?) significantly weakened ClO₄^? adsorption, while the co-existence of Fe³⁺ and cetyltrimethylammonium cations increased ClO₄^? adsorption 2- to 3-fold. ClO₄^? adsorption was promoted by oxidized DWCNTs due to the introduction of more oxygen-containing functional groups, which served as additional adsorption sites. The pH values significantly affected the zeta potential of raw and oxidized DWCNTs and thus ClO₄^? adsorption. The pH-dependent curves of ClO₄^? adsorption on CNTs were distinct from those of conventional sorbents (e.g., activated carbon and resin). Maximum ClO₄^? adsorption occurred at pH = the isoelectric point (pH_IEP) + 0.85 rather than at pH < pH_IEP, which cannot be explained by electrostatic interactions alone. Hydrogen bonding is proposed to be a dominant mechanism at neutral pH for the interaction of ClO₄^? with CNTs, and variations of ClO₄^? affinity with CNTs in different pH ranges are illustrated.     

Double‐wall carbon nanotube‐reinforced polyester nanocomposites: Improved dispersion and mechanical properties

Double‐walled carbon nanotube (DWCNT)‐reinforced polyester nanocomposites were prepared and tested to characterize their mechanical properties. The DWCNTs were functionalized to improve their dispersion within the polyester matrix. The improvement in the mechanical properties shows that the functionalized DWCNTs have better distribution within, and good adhesion with, the polyester matrix. A comparison of the mechanical properties of nanocomposites reinforced by functionalized and nonfunctionalized DWCNTs confirms that the functionalization leads to substantially improved composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers