Single-walled Carbon Nanotubes Material Research Articles

Electrochemical properties of double wall carbon nanotube electrodes

Electrochemical properties of double wall carbon nanotubes (DWNT) were assessed and compared to their single wall (SWNT) counterparts. The double and single wall carbon nanotube materials were characterized by Raman spectroscopy, scanning and transmission electron microscopy and electrochemistry. The electrochemical behavior of DWNT film electrodes was characterized by using cyclic voltammetry of ferricyanide and NADH. It is shown that while both DWNT and SWNT were significantly functionalized with oxygen containing groups, double wall carbon nanotube film electrodes show a fast electron transfer and substantial decrease of overpotential of NADH when compared to the same way treated single wall carbon nanotubes.

The purification of HiPco SWCNTs with liquid bromine at room temperature

A new procedure to purify HiPco single-walled carbon nanotubes (SWCNTs) from iron catalyst impurity is introduced. The protocol, which uses liquid bromine at room temperature (RT) as an oxidant, improves nanotube purity from iron by a factor of approximately 10, while maintaining good nanotube integrity as demonstrated by near infrared (NIR) luminescence and absorbance measurements. When HiPco SWCNTs are dissolved in RT Br 2(l) (free of O 2 and H 2O), the metallic iron impurity is quickly oxidized to its bromide salt and easily removed by aqueous washing or by washing with dilute acid. The iron content (by ICP-AE) for the purified SWCNT material was 2.8–3.6% by weight (for three different samples) for a single purification step, but could be lowered to 1.6–1.8% with an additional purification cycle. Characterization of the resulting purified SWCNT material has been achieved by TEM imaging, XPS, ICP-AE analysis, Raman spectroscopy, electronic absorption spectroscopy, and by NIR photoluminescence measurements. Finally, the new Br 2(l) purification procedure has been compared to and contrasted with other established purification procedures for HiPco SWCNTs and found to be a highly desirable alternative.

Nanoscopically Flat Open-Ended Single-Walled Carbon Nanotube Substrates for Continued Growth

Continued growth is a way of growing nanotubes targeted to produce continuous and chirality-controlled single-walled carbon nanotube (SWNT) materials. This growth method strongly depends on efficient preparation of open-ended SWNT substrates. Nanoscopically flat open-ended SWNT substrates have been prepared by cutting the SWNT spun fiber with a focused ion beam cutting technique and followed by etching schemes for cleaning amorphous carbon and opening the ends of the SWNTs. The open ends were effectively characterized through selective etch back of open SWNT ends by carbon dioxide gas at 950 degrees C. High density continued growth was demonstrated from these nanoscopically flat open-ended substrates.

Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes

We present a rational and general method to fabricate a high-densely packed and aligned single-walled carbon-nanotube (SWNT) material by using the zipping effect of liquids to draw tubes together. This bulk carbon-nanotube material retains the intrinsic properties of individual SWNTs, such as high surface area, flexibility and electrical conductivity. By controlling the fabrication process, it is possible to fabricate a wide range of solids in numerous shapes and structures. This dense SWNT material is advantageous for numerous applications, and here we demonstrate its use as flexible heaters as well as supercapacitor electrodes for compact energy-storage devices.

High performance supercapacitor from chromium oxide-nanotubes based electrodes

Single wall carbon nanotubes (SWNTs) filled and doped with chromium oxide have been used as attractive electrodes for supercapacitors. Pseudocapacitance effects related to the presence of nanosized chromium oxide finely dispersed at the nanoscale together with high conducting properties of SWNTs allow building efficient electrodes from this hybrid material. Even if capacitance values are not very high (ca. 60 F g −1), however, extremely quick charge propagation was observed, doubtless due to the overall physical and textural properties of SWNT material. The positive effect – with respect to empty-SWNTs – brought by the presence of chromium oxide in and probably in-between the SWNTs indicates that chromium oxide is accessible to the electrolyte in spite of its encapsulated location, because of the numerous side entries created all along the SWNT walls during the filling step.

Measuring particle size-dependent physicochemical structure in airborne single walled carbon nanotube agglomerates

As-produced single-walled carbon nanotube (SWCNT) material is a complex matrix of carbon nanotubes, bundles of nanotubes (nanoropes), non-tubular carbon and metal catalyst nanoparticles. The pulmonary toxicity of material released during manufacture and handling will depend on the partitioning and arrangement of these components within airborne particles. To probe the physicochemical structure of airborne SWCNT aggregates, a new technique was developed and applied to aerosolized as-produced material. Differential Mobility Analysis-classified aggregates were analyzed using an Aerosol Particle Mass Monitor, and a structural parameter Γ (proportional to the square of particle mobility diameter, divided by APM voltage) derived. Using information on the constituent components of the SWCNT, modal values of Γ were estimated for specific particle compositions and structures, and compared against measured values. Measured modal values of Γ for 150 nm mobility diameter aggregates suggested they were primarily composed of non-tubular carbon from one batch of material, and thin nanoropes from a second batch of material – these findings were confirmed using Transmission Electron Microscopy. Measured modal values of Γ for 31 nm mobility diameter aggregates indicated that they were comprised predominantly of thin carbon nanoropes with associated nanometer-diameter metal catalyst particles; there was no indication that either catalyst particles or non-tubular carbon particles were being preferentially released into the air. These results indicate that the physicochemistry of aerosol particles released while handling as-produced SWCNT may vary significantly by particle size and production batch, and that evaluations of potential health hazards need to account for this.

Nanocomposite Fiber Systems Processed from Fluorinated Single-Walled Carbon Nanotubes and a Polypropylene Matrix

Processing composites of carbon nanotubes into nanotube continuous fibers (NCFs) is an effective way of manipulating the anisotropic properties of the single-walled carbon nanotubes (SWNTs) as it becomes less difficult to transform the SWNTs into an aligned configuration when they are confined in a small diameter fiber. This helps to take fuller advantage of the high mechanical properties of the SWNT materials in the axial direction. However, in creating nanocomposite fiber systems, the issues of dispersion and nanotube−matrix interaction and adhesion become of utmost importance when improved mechanical properties are anticipated. In addressing these issues in this work it has been found that sidewall chemical functionalization can be an effective tool for improving both the dispersion and interaction between the nanotube and the matrix. This work evaluates the effect of sidewall functional groups on fluorinated single-walled carbon nanotubes (F-SWNTs) as a precursor for improved interfacial adhesion in a thermoplastic matrix (polypropylene, PP) via partial defluorination of the F-SWNTs. The partial removal of functional groups from the F-SWNTs during melt processing with PP by shear mixing provides the opportunity for in situ direct covalent bonding between the nanotubes and the matrix during melt processing which ultimately results in better mechanical reinforcement of the composite. The studies conducted herein demonstrate that in comparison with PP composites filled with purified nanotubes (P-SWNTs), improved dispersion, interfacial adhesion, and mechanical properties are achieved for F-SWNT-loaded matrixes due to chemical functionalization.

Novel selective sensors based on carbon nanotube films for hydrogen detection

Novel resistive gas sensors based on single-walled carbon nanotubes (SWNTs) as an active sensing element have been evaluated for hydrogen detection. Sensor films were fabricated by airbrushing SWNT dispersions on alumina substrates. The employed SWNT materials were either chemically functionalized with Pd or sputtered with Pd to enhance their sensitivity to hydrogen. Sensors were characterized by dc electrical measurements in nitrogen and air atmosphere. The response to hydrogen and the cross sensitivity to gases such as ammonia, toluene and octane were studied. Aged Pd-functionalized SWNT films exhibited good sensitivity and selectivity to hydrogen at room temperature. The effect of aging, thermal treatments and the employed carrier gas were also investigated.

The Electrochemistry of Carbon Nanotubes: I. Aqueous Electrolyte

The energy storage potentials of many different carbon nanotube (CNT) materials are investigated electrochemically. The electrochemical response of all the investigated materials is rather featureless during charging and discharging and does not show any phase transitions or clear redox responses. The maximum discharge capacity, , is measured for an as-produced single-walled carbon nanotube material. Measurements indicate that the electrochemical activity of the matrix material, used to manufacture composite electrodes, should also not be ignored. Highly pure CNTs yield a substantially lower response, indicating that the CNTs itself can only account for a small part hereof. The measured response of CNT materials is related to a number of processes, including the irreversible oxidation of carbonaceous material, the reversible oxidation/reduction of residual metal catalyst or carbonaceous impurities, and an electrostatic charging component. Steady-state impedance measurements, cross-correlated with cyclic voltammetry, show that (dis)charging of the electrical double layer can be directly linked to this electrostatic charging component. Characterization of highly pure CNT materials shows that more than 90% of the total charge was stored in this way. Only about 25% of the total amount of charge could be explained in this way for the as-produced materials. Based on the overall results it is highly unlikely that a significant amount of hydrogen can be stored in CNT material and that the exact amount of charge that can be reversibly stored in CNT material heavily depends on its morphology and the level of purity.

Small-Angle Neutron Scattering from Labeled Single-Wall Carbon Nanotubes

Small-angle neutron scattering (SANS) is used with the “high-concentration” method to extract single-particle scattering from single-wall carbon nanotubes (SWNTs). The SWNT material was labeled by covalently attaching −C4H9 or −C4D9 groups by use of free radical chemistry. Mixtures of SWNT-C4H9 and SWNT-C4D9 were dispersed in D2O containing 1% sodium lauryl sulfate-d23 (SLS) by use of sonication. SLS matches the neutron contrast of D2O so that all of the SANS is due to the labeled SWNT. Thermogravimetric analysis shows that mass fractions of butyl groups attached to the SWNT-C4H9 and the SWNT-C4D9 were well matched. The scattering had a power law of −2.5, which is characteristic of a “particle” made of clustered SWNTs. The clustering seen in SWNT scattering is due to collections of SWNTs that do not exchange in a dynamic equilibrium with other nanotubes.

Effects of proton irradiation on thermal stability of single-walled carbon nanotubes mat

We report effects of proton irradiation on thermal oxidation of single-walled carbon nanotubes (SWNTs). Following 400 keV proton irradiation to doses of 10 13–10 17 cm −2, thermal oxidation of SWNTs was conducted at temperatures 200–700 °C. The mass of SWNTs during oxidation was monitored with thermogravimetric analysis (TGA) and ion-beam-induced structural modifications in nanotubes were characterized with Raman spectroscopy. Proton irradiation leads to enhanced thermal stability of nanotubes, as indicated by increased values of the temperature ( T max) for maximum oxidation rate, e.g. T max ∼ 495 °C for pristine SWNTs and T max ∼ 525 °C for SWNTs irradiated to the dose of 1 × 10 15 cm −2. The activation energy for oxidation increases gradually from 1.13 ± 0.01 eV for pristine SWNTs, to ∼1.25 ± 0.02 eV for the largest dose of 5 × 10 16 cm −2. Based on the correlation between TGA and Raman data, we discuss possible mechanisms for the observed effects of proton irradiation on SWNT thermal oxidation.

Thermal desorption of deuterium from modified carbon nanotubes and its correlation to the microstructure

The process of deuterium desorption from single-wall carbon nanotubes (SWNTs) modified by atomic (D) and molecular (D 2) deuterium treatment was investigated in an ultrahigh vacuum environment using thermal desorption mass spectroscopy (TDMS). Microstructural and chemical analyses of SWNT material, modified by this deuterium interaction, were performed by means of a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results disclose characteristic features in the TDMS spectra of deuterium evolved from the SWNT material, which can be correlated to the microstructure of nanocarbon material modified by D-treatment. The TDMS spectra of deuterium originating from the large diameter rope type nanotube structures, resulting from a prolonged low-pressure (D + D 2) gas mixture treatment, exhibit three overlapping desorption peaks: a dominant one with a desorption activation energy ( E des) of approx. 2.86 eV and lower intensity peaks at E des of ∼1.50 and 2.46 eV. On the other hand, the TDMS spectra of deuterium taken from the “coral reef”-like carbon nanostructures, obtained after prolonged treatment of SWNTs to a high-pressure (D + D 2) gas mixture produced at high temperature, reveal the coexistence of four superimposed desorption peaks with E des ranging from 1.23 to 4.4 eV. A dominant desorption peak with E des ≈ 4.4 eV, can be attributed to bulk diffusion of D trapped within this nanocapsule bulk structure.

Effect of the Tube Diameter Distribution on the High-Temperature Structural Modification of Bundled Single-Walled Carbon Nanotubes

We present results of a systematic high-resolution transmission electron microscopy study of the thermal evolution of bundled single-walled carbon nanotubes (SWNTs) subjected to approximately 4-h high-temperature heat treatment (HTT) in a vacuum at successively higher temperatures up to 2200 degrees C. We have examined purified SWNT material derived from the HiPCO and ARC processes. These samples were found to thermally evolve along very different pathways that we propose depend on three factors: (1) initial diameter distribution, (2) concomitant tightness of the packing of the tubes in a bundle, and (3) the bundle size. Graphitic nanoribbons (GNR) were found to be the dominant high-temperature filament in ARC material after HTT = 2000 degrees C; they were not observed in any heat-treated HiPCO material. The first two major steps in the thermal evolution of HiPCO and ARC material agree with the literature, i.e., coalescence followed by the formation of multiwall carbon nanotubes (MWNTs). However, ARC material evolves to bundled MWNTs, while HiPCO evolves to isolated MWNTs. In ARC material, we find that the MWNTs collapse into multishell GNRs. The thermal evolution of these carbon systems is discussed in terms of the diameter distribution, nanotube coalescence pathways, C-C bond rearrangement, diffusion of carbon and subsequent island formation, as well as the nanotube collapse driven by van der Waals forces.

Raman and IR Spectroscopy of Chemically Processed Single-Walled Carbon Nanotubes

IR and Raman spectroscopy has been used to study the evolution of the vibrational spectrum of bundled single-walled carbon nanotubes (SWNTs) during the purification process needed to remove metal catalyst and amorphous carbon present in arc-derived SWNT soot. We have carried out a systematic study to define the different outcomes stemming from the purification protocol (e.g., DO, DO/HCl, DO/HNO(3), H(2)O(2), H(2)O(2)/HCl), where dry oxidation (DO) or refluxing in H(2)O(2) was used in a first purification step to remove amorphous carbon. The second step involves acid reflux (HCl or HNO(3)) to remove the residual growth catalyst (Ni-Y). During strong chemical processing, it appears possible to create additional defects where carbon atoms are eliminated, the ring structure is now open, localized C=C bonds are created, and O-containing groups can be added to this defect to stabilize the structure. Evolution of SWNT skeletal disorder obtained via chemical processing was studied by Raman scattering. Higher intensity ratios of R- and G-band (I(R)/I(G)) are more typically found in SWNT materials with low D-band intensity and narrow G-band components. Using IR transmission through thin films of nanotubes, we can resolve the structure due to functional groups that were present in the starting material or added through chemical processing. After high-temperature vacuum annealing of the purified material at 1100 degrees C, IR spectroscopy shows that most of the added functional groups can be removed and that the structure that remains is assigned to the one- and two-phonon modes of SWNTs.

Electrochemical tuning and investigations on actuator mechanism of single-wall carbon nanotubes

Electrochemical tuning of single-wall carbon nanotubes has been investigated using in situ Raman spectroscopy. We built a linear actuator from single-wall carbon nanotube mat and studied in several alkali metal (Li, Na, and K) and alkaline earth (Ca) halide solutions. The variation of bonding with electrochemical biasing was monitored using in situ Raman. This is since Raman can detect changes in C–C bond length: the radial breathing mode (RBM) at ∼190 cm − 1 varies inversely with the nanotube diameter and the G band at ∼1590 cm − 1 varies with the axial bond length. In addition, the intensities of both the modes vary significantly in a non-monotonic manner pointing at the emptying/depleting or filling of the bonding and anti-bonding states—electrochemical charge injection. We discuss the variation of spectroscopic observables (intensity/frequency) of these modes providing valuable information on the charge transfer dynamics on the single-wall carbon nanotubes mat surface. We found the in-plane compressive strain (∼− 0.25%) and the charge transfer per carbon atom ( f c∼− 0.005) as an upper bound for the electrolytes used i.e. CaCl 2. These results can be quantitatively understood in terms of the changes in the energy gaps between the one-dimensional van Hove singularities in the electron density of states arising possibly due to the alterations in the overlap integral of π bonds between the p orbitals of the adjacent carbon atoms. Moreover, the extent of variation of the absolute potential of the Fermi level or alternatively modification of band gap is estimated from modeling Raman intensity to be around 0.1 eV as an upper bound for CaCl 2.

Electronic devices based on purified carbon nanotubes grown by high-pressure decomposition of carbon monoxide

The excellent properties of transistors, wires and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems. Gas-phase growth procedures such as the high-pressure decomposition of carbon monoxide (HiPCO) method yield large quantities of small-diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. As-grown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade the properties of SWNT devices. Here we demonstrate a purification, deposition and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source-drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field-effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material.

Alignment of Liquid-Crystalline Polymers by Field-Oriented, Carbon Nanotube Directors

Bulk liquid-crystalline polymer (LCP) material can be macroscopically aligned by seeding the growth of LCP domains with preoriented, single-wall carbon nanotubes (SWNTs). SWNT seeds dispersed in a columnar polyoxazoline melt were first oriented by applying an ac electric field across the molten material. Then, cooling the material below the liquid-crystalline transition temperature yielded domains that were oriented in the direction of the applied field. The orientation of these domains was characterized by temperature-controlled polarized light microscopy, static birefringence, and wide-angle X-ray scattering. These experiments demonstrate that domains can be oriented at field strengths that are orders of magnitude lower than those used to pole anisotropic polymers alone, and that the seeded domains can grow together to form homogeneously oriented bulk material. Because the nanotubes act only as nucleants, very little SWNT material is required. We anticipate that this general concept could aid the fabrication of monolithic objects with tailored anisotropic properties from SWNT−polymer composites.

Raman Spectroscopy of Individual Single-Walled Carbon Nanotubes from Various Sources

Resonance Raman spectroscopy/microscopy was used to study individualized single-walled carbon nanotubes (SWNTs) both in aqueous suspensions as well as after spin-coating onto Si/SiO2 surfaces. Four different SWNT materials containing nanotubes with diameters ranging from 0.7 to 1.6 nm were used. Comparison with Raman data obtained for suspensions shows that the surface does not dramatically affect the electronic properties of the deposited tubes. Raman features observed for deposited SWNTs are similar to what was measured for nanotubes directly fabricated on surfaces using chemical vapor deposition (CVD) methods. In particular, individual semiconducting tubes could be distinguished from metallic tubes by their different G-mode line shapes. It could also be shown that the high-power, short-time sonication used to generate individualized SWNT suspensions does not induce defects in great quantities. However, (additional) defects can be generated by laser irradiation of deposited SWNTs in air, thus giving rise to an increase of the D-mode intensity for even quite low power densities (approximately 10(4) W/cm2).

Functionalization and Extraction of Large Fullerenes and Carbon-Coated Metal Formed during the Synthesis of Single Wall Carbon Nanotubes by Laser Oven, Direct Current Arc, and High-Pressure Carbon Monoxide Production Methods

Large fullerenes and carbon-coated metal nanoparticles that are formed during the synthesis of carbon nanotubes have been functionalized by the addition of alkyl radicals and isolated by extraction into chloroform. The soluble, functionalized fullerenes have been isolated from raw single-wall carbon nanotube (SWNT) material prepared by laser oven, direct current arc, and high-pressure carbon monoxide production methods. Analyses of the extracted large fullerenes were carried out by thermogravimetric analysis, UV-vis-near-IR, laser desorption ionization mass spectrometry, and high-resolution transmission electron microscopy.

Influence of catalyst metal particles on the hydrogen sorption of single-walled carbonnanotube materials

The hydrogen sorption capacity of arc-synthesized single-walled carbon nanotubes(SWNTs) was studied using a specially built high-pressure rig coupled to a Toepler pumpcapable of directly measuring the desorbed volumes of gas. The samples studied wereprepared using the arc-evaporation method and were as-produced SWNT material formedat the cathode (collar), as-produced SWNTs deposited in the soot and a purified sample ofSWNTs. The three samples had similar diameter ranges, the major difference betweenthem being the concentration of remaining metal particles from the Ni/Y/C catalyst usedin the arc-synthesis. The effect of the presence of these residual catalyst metal particles hasbeen analysed and seen to strongly influence the hydrogen storage capacity of the samples.