Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (-COOH), benzylamine (-Ph-CH₂-NH₂), and perfluorooctylaniline (-Ph-(CF₂)₇-CF₃). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (< 1 m²/g) and roughness factor (R_a = 14 nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70 nm and specific surface areas of 160-180 m²/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.     

Synthesis, characterization and the electrocatalytic behaviour of nickel (II) tetraamino-phthalocyanine chemically linked to single walled carbon nanotubes

In this work we report on the synthesis, characterization and the electrochemical behavior of amide linked nickel (II) tetraamino-phthalocyanine (NiTAPc)-single walled carbon nanotube (SWCNT) nanomaterials (NiTAPc-SWCNT (linked)). UV-vis, XRD, IR and Raman spectroscopies were used in characterization whilst cyclic voltammetry was used to study the electrochemical behavior of NiTAPc-SWCNT (linked)-GCE. Relative to the bare glassy carbon electrode (bare-GCE), SWCNT-GCE, NiTAPc-GCE, and NiTAPc/SWCNT (mixed)-GCE, the NiTAPc-SWCNT (linked)-GCE gave the best current responses for the oxidation of 2-mercaptoethanol (2-ME). The catalytic rate constant is of the magnitude of 10³ M⁻¹ s⁻¹ while the detection limit (LOD) is 0.15 μM using the 3δ notation, with a sensitivity of 2.53 μA μM⁻¹ cm⁻².     

Raman spectroelectrochemistry of a single-wall carbon nanotube bundle

Raman microscopy and spectroelectrochemistry with polymer electrolyte gating is developed to study the effect of charging on Raman spectra of individual single-wall carbon nanotubes (SWCNTs) and bundles. The Raman spectra of a small bundle, consisting of well-separated features from a metallic and a semiconducting SWCNT, have been obtained at different electrochemical charging levels. The broad Fano peak of the metallic SWCNT exhibits an appreciable frequency upshift and simultaneous line narrowing when the charging level, either positive or negative, is increased, in agreement with the presence of a Kohn anomaly in metallic SWCNTs. The radial breathing mode of the metallic tube also shows a similar but much weaker dependence on the charging potential. While the G mode frequencies of the semiconducting SWCNT also increase with the increasing charging level, the magnitude of such change is much smaller than in the metallic SWCNT. At high negative charging potentials the G⁻ peak of the semiconducting SWCNT exhibits a larger upshift than its G⁺ peak, leading to the observation of merging of these two peaks. However, both G⁺ and G⁻ peaks of the semiconducting SWCNT become broader at high charging levels, which are not predicted from previous theoretical studies.     

Self-assembled nano-arrays of single-walled carbon nanotube-octa(hydroxyethylthio)phthalocyaninatoiron(II) on gold surfaces: Impacts of SWCNT and solution pH on electron transfer kinetics

The construction by sequential self-assembly process of reproducible, highly stable and pH-responsive redox-active nanostructured arrays of single-walled carbon nanotubes (SWCNTs) integrated with octa(hydroxyethylthio)phthalocyaninatoiron(II) (FeOHETPc) via ester bonds on a gold surface (Au-Cys-SWCNT-FeOHETPc) is investigated and discussed. The successful construction of this electrode is confirmed using atomic force microscopy and X-ray photoelectron spectroscopy as well as from the distinct cyclic voltammetric and electrochemical impedance spectroscopic profiles. The Au-Cys-SWCNT-FeOHETPc electrode exhibited strong dependence on the reaction of the head groups and the pH of the working electrolytes, the surface pK_a is estimated as 7.3. The high electron transfer capability of the Au-Cys-SWCNT-FeOHETPc electrode over other electrodes (Au-Cys-SWCNT or the Au-Cys-FeOHETPc or the Au-FeOHETPc) suggests that SWCNT greatly improves the electronic communication between FeOHETPc and the bare gold electrode. The electron transfer rate constant (k_app) of Au-Cys-SWCNT-FeOHETPc in pH 4.8 conditions (∼1.7 × 10⁻² cm⁻² s⁻¹) over that of the electrode obtained from SWCNT integrated with tetraaminophthalocyninatocobalt(II) (Au-Cys-SWCNT-CoTAPc) (5.1 × 10⁻³ cm⁻² s⁻¹) is attributed to the possible effect of the central metal on the phthalocyanine core and substituents on the peripheral positions of the phthalocyanine rings. We also prove that aligned SWCNT arrays exhibit much faster electron transfer kinetics to redox-active species in solutions compared to the randomly dispersed (drop-dried) SWCNTs.     

Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.     

Dispersion of pristine single-walled carbon nanotubes using pyrene-capped polystyrene and its application for preparation of polystyrene matrix composites

A pyrene-capped polystyrene (PyPS) is synthesized by an anionic polymerization method and acts as dispersant for dispersion of pristine single-walled carbon nanotubes (SWCNTs). Through a well-known π-stacking interaction confirmed qualitatively by proton nuclear magnetic resonance and fluoroscopic analyses, PyPS is strongly but noncovalently adsorbed onto the nanotube surface, affording highly uniform and stable SWCNT dispersion in chloroform with the nanotube content as high as 250 ± 30 mg L⁻¹. Since no direct chemical reaction takes place on the nanotubes, their intrinsic electronic structure is maintained, thus ensuring them as functional fillers for application in conductive polymer composites. The so-obtained dispersion is subsequently used to prepare polystyrene matrix composites. A solution-based process adopted here preserves the good nanotube dispersing state in dispersion into the composites. Hence, the resultant composites show good optical transmittance and a low electrical percolation threshold of 0.095 wt.% SWCNTs. In comparison, the composites with absence of PyPS prepared by the same process have a relatively high percolation threshold of 0.28 wt.% SWCNTs.     

Synthesis, characterization, and electrochemical testing of carbon nanotubes derivatized with azobenzene and anthraquinone

Multi-walled and single-walled carbon nanotubes were side-wall functionalized with azobenzene and anthraquinone residues, i.e., chemical groups possessing redox activity, for potential utilization in functional catalysis and memory storage devices. Solvent-free synthesis was performed with diazonium salts generated in situ where it was found that it was simple and effective method. Nanotube functionalization was confirmed and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). It is worth noting, that single-walled carbon nanotubes (SWCNTs) functionalized with azobenzene produced Raman modes typical of substituted azobenzenes with spectral peaks at ∼1137, 1412, and 1447 cm⁻¹. The nanotubes containing electroactive substituents were transferred onto electrode substrates using the Langmuir-Blodgett approach and characterized by cyclic voltammetry. The amount of electroactive groups per mg of nanotubes was calculated based on the peak of cathodic current. A highly reproducible voltammetric response was obtained with a single nanotube layer or multiple nanotube/octadecanol layers. It is believed that devices such as these will be invaluable for future high-performance electrodes.     

Electrochemical doping of single-walled carbon nanotubes in double layer capacitors studied by in situ Raman spectroscopy

The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et₄NBF₄ in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was determined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respectively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G⁺ bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G⁺ bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C-C bonds and the corresponding phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical environment of individual SWCNTs upon charging and discharging of the electrochemical double layer within SWCNT bundles.     

Two-dimensional Raman spectroscopic study on the structural changes of a poly(3-chlorothiophene) film during the heating process

Two-dimensional Raman spectroscopy has been applied to provide the information on charge carriers and thermal stability of a doped poly(3-chlorothiophene) (PCTh) film. The strong spectral intensity at 1420 cm⁻¹ shows that positive polarons are the major charge carriers in doped PCTh. On the other hand, peaks in the 2D contour maps separate the overlapped bands around 1386 cm⁻¹, confirming the existence of positive bipolarons in PCTh. The positive asynchronous cross peak located at 1420/1386 cm⁻¹ further indicates that bipolarons have a higher thermal stability compared with polarons in the doped PCTh. The increase of the spectral intensity at 1454 cm⁻¹ and the decrease of the spectral intensity at 1420 cm⁻¹ indicate that during the heating process, a structural change occurs in the PCTh film.     

Near-infrared photoresponse in single-walled carbon nanotube/polymer composite films

We present a near-infrared photoresponse study of single-walled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (SWCNT/P3HT-b-PS) composite films for different loading ratios of SWCNT in the polymer matrix. Compared to the pure SWCNT film, the photoresponse [(light current − dark current)/dark current] is much larger in the SWCNT/polymer composite films. The photoresponse is up to 157% when SWCNTs are embedded in P3HT-b-PS while for a pure SWCNT film it is only 40%. We also show that the photocurrent strongly depends on the position of the laser spot with maximum photocurrent occurring at the metal-film interface. We explain the photoresponse due to exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode-SWCNT interface.     

Aluminum nitride‐single walled carbon nanotube nanocomposite with superior electrical and thermal conductivities

Development of aluminum nitride (AlN)‐single walled carbon nanotube (SWCNT) ceramic‐matrix composite containing 1‐6 vol% SWCNT by hot pressing has been reported in this article. The composites containing 6 vol% SWCNT are dense (~99% relative density) and show high dc electrical conductivity (200 Sm^?1) and thermal conductivity (62 Wm^?1K^?1) at room temperature. SWCNTs contain mostly metallic variety tubes obtained by controlled processing of the pristine tubes before incorporation into the ceramic matrix. Raman spectroscopy and field emission scanning electron microscopy (FESEM) of the fracture surface of the samples show the excellent survivability of the SWCNTs even after high‐temperature hot pressing. The results indicate the possibility of preparation of AlN nanocomposite for use in plasma devices and electromagnetic shielding.     

Temperature and time dependence study of single-walled carbon nanotube growth by catalytic chemical vapor deposition

The temperature and time dependence of single-walled carbon nanotube (SWCNT) growth by chemical vapor deposition of ethanol on Fe₂O₃/MgO catalyst are compared at both low (∼27 Pa) and atmospheric pressure limits. SWCNTs are synthesized in two reactors with different geometries and operating pressures and are characterized by Raman spectroscopy. Both reactors show SWCNT growth within a relatively narrow temperature window of 700-850 °C, with an optimum growth time of 35 min for the cold wall reactor and 75 min for the quartz tube reactor. A kinetic model comprising of ethanol decomposition, SWCNT formation, and water etching is developed to better understand the growth mechanism. The existence of a temperature window and an optimum growth time in both reactors can be well described by the kinetic model. Simulation results suggest that the temperature and time dependence can be explained by the competition between the growth of SWCNTs and that of amorphous carbon.     

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes

A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10⁻⁴ S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10⁻⁸ S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.     

Protonation and sonication effects on aggregation sensitive Raman features of single wall carbon nanotubes

The G-band and the G′-band (∼2600-2700 cm⁻¹) Raman spectra of single walled carbon nanotubes (SWCNTs) in a sodium dodecyl sulphate aqueous solution were investigated as a function of sonication energy density (ρ). The relative intensity of the G-band feature around ∼1520-1560 cm⁻¹ (G⁻) and the linewidth (Γ) of the G′-band decrease monotonically with ρ where the former dependence is probably due to charge transfer (promoted by sonication induced pH decrease/solubilisation) and consequently not an unambiguous indication of solubilisation. In contrast Γ is shown to be independent of pH, which suggests that the linewidth of the G′-band may be an important indicator of the aggregation state of SWCNTs.     

High yield production of semiconducting p-type single-walled carbon nanotube thin-film transistors on a flexible polyimide substrate by tuning the density of ferritin catalysts

We report on a simple method for fabricating pure p-type single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) on flexible polyimide substrates without selective removal of metallic SWCNTs from the as-grown CNT films. The density of the SWCNTs was controlled by tuning the concentration of ferritin catalyst, resulting in the control of the metallic percolation pathways in the SWCNT TFTs. For a ferritin solution diluted by 1/2000, approximately 60% of the pristine SWCNT TFTs showed p-type behavior with larger on/off current ratios, (I_on/I_off > 10⁴) and a high photosensitivity to the exposure of UV/visible light.     

Origin of the 2450 cm Raman bands in HOPG, single-wall and double-wall carbon nanotubes

The second order Raman signals around the G′-band region of graphite and carbon nanotubes have been investigated at more than 15 excitation laser lines. Two distinct Raman bands have been observed around 2700 cm⁻¹; a prominent one is due to the so-called G′-band and the other is a weak band around 2450 cm⁻¹. Both two bands can be from the double resonance process involving two phonons around the K-point in the phonon dispersion of a two-dimensional graphite. The 2450 cm⁻¹-band has exhibited little power dependence, whereas the intensity of G′-band has shown large photon energy dependence as already reported. The 2450 cm⁻¹-band and the G′-band correspond to non-dispersive q = 0 and fully-dispersive q = 2k, respectively. From the phonon dispersion and the corresponding phonon frequency, the 2450 cm⁻¹-band can be assigned as an overtone mode of LO phonon (i.e. 2LO). This is revealed by calculated Raman spectra of graphite with proper electron-phonon matrix elements. The present study is the first report on the origin and assignment of the 2450 cm⁻¹-band, which is based on the double resonance Raman scattering.     

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Low electrical conductivity threshold and crystalline morphology of single-walled carbon nanotubes - high density polyethylene nanocomposites characterized by SEM, Raman spectroscopy and AFM

The electrical conductivities (σ) of nanocomposites of single-walled carbon nanotubes (SWCNTs) and high density polyethylene (HDPE) have been studied for a large number of nanocomposites prepared in a SWCNT concentration range between 0.02 and 8 wt%. The values of σ obey a percolation power law with an SWCNT concentration threshold, p_c = 0.13 wt%, the lowest yet obtained for any kind of carbon-polyethylene nanocomposites. Improved electrical conductivities attest to an effective dispersion of SWCNT in the polyethylene matrix, enabled by the fast quenching crystallization process used in the preparation of these nanocomposites. Characterization by scanning electron microscopy (SEM) and Raman spectroscopy consistently points to a uniform dispersion of separate small SWCNT bundles at concentrations near p_c and increased nanotube clustering at higher concentrations. Near p_c, high activation energies and geometries of long isolated rods suggest that electron transport occurs by activated electron hopping between nanotubes that are close to each other but still geometrically separate. The degree of SWCNT clustering given by Raman spectroscopy and the barrier energy for electrical conductivity are highly correlated. The nanotubes act as nucleants in the crystallization of the polyethylene matrix, and change the type of supermolecular aggregates from spherulites to axialitic-like objects. The size of crystal aggregates decreases with SWCNT loading, however, in reference to the unfilled polyethylene, the three-dimensional growth geometry extracted from the Avrami exponents remains unchanged up to 2 wt%. Consistency between SEM, Raman and electrical transport behavior suggests that the electrical conductivity is dominated by dispersion and the geometry of the SWCNT in the nanocomposites and not by changes or lack thereof in the HDPE semicrystalline structure.     

A comprehensive scenario for commonly used purification procedures of arc-discharge as-produced single-walled carbon nanotubes

The purification of single-walled carbon nanotube (SWCNT) samples was analysed using a multi-technique approach, with structural as well as spectroscopic probes, in order to characterize the samples and to identify important factors for improvement of SWCNT sample quality. The first dry oxidation step (air at 365 °C) is shown to have only a weak selectivity for the removal of the amorphous carbon or weakly organized graphitic species as well as resulting in a partial consumption of the SWCNTs. The functionalization of the SWCNTs is highly specific with formation of carboxyl, hydroxyl and carbonyl groups. On the other hand this oxidation step is highly efficient for the oxidation of the catalytic impurities (Ni, Y) which can be easily removed by subsequent acid treatment. A final high temperature treatment indicates some incomplete restoration of the quality of the SWCNT surface.