Multi-walled Carbon Nanotube Sheets Research Articles

Dry Drawn Multiwall Carbon Nanotube Sheet as a Counter Electrode for Dye-Sensitized Solar Cells: Multilayer Optimization

We demonstrate the fabrication of dye sensitized solar cells using multiwall carbon nanotube (MWNT) sheets as a counter electrode with catalytic activity optimized by layering as an alternative to platinum deposited on fluorinated tin oxide (FTO). The sheets are drawn directly from a highly aligned forest of MWNTs grown on silicon by chemical vapor deposition. We used different number of MWNT layers on the FTO and pure glass. Cell performance was found to vary with the number of MWNT sheets. When using of 10 or more layers we can substitute the FTO with platinum catalytic particles.

Nanoscale spatial resolution probes for scanning thermal microscopy of solid state materials

Scanning thermal microscopy (SThM) uses micromachined thermal sensors integrated in a force sensing cantilever with a nanoscale tip that can be highly useful for exploration of thermal management of nanoscale semiconductor devices as well as mapping of surface and subsurface properties of related materials. Whereas SThM is capable to image externally generated heat with nanoscale resolution, its ability to map and measure thermal conductivity of materials has been mainly limited to polymers or similar materials possessing low thermal conductivity in the range from 0.1 to 1 W m−1 K−1, with lateral resolution on the order of 1 μm. In this paper, we use linked experimental and theoretical approaches to analyse thermal performance and sensitivity of the micromachined SThM probes in order to expand their applicability to a broader range of nanostructures from polymers to semiconductors and metals. We develop physical models of interlinked thermal and electrical phenomena in these probes and their interaction with the sample on the mesoscopic length scale of few tens of nm and then validate these models using experimental measurements of the real probes, which provided the basis for analysing SThM performance in exploration of nanostructures. Our study then highlights critical features of these probes, namely, the geometrical location of the thermal sensor with respect to the probe apex, thermal conductance of the probe to the support base, heat conduction to the surrounding gas, and the thermal conductivity of tip material adjacent to the apex. It furthermore allows us to propose a novel design of the SThM probe that incorporates a multiwall carbon nanotube or similar high thermal conductivity graphene sheet material with longitudinal dimensions on micrometre length scale positioned near the probe apex that can provide contact areas with the sample on the order of few tens of nm. The new sensor is predicted to provide greatly improved spatial resolution to thermal properties of nanostructures as well as to expand the sensitivity of the SThM probe to materials with heat conductivity values up to 100–1000 W m−1 K−1.

The Alignment of Carbon Nanotubes: An Effective Route To Extend Their Excellent Properties to Macroscopic Scale

To improve the practical application of carbon nanotubes, it is critically important to extend their physical properties from the nanoscale to the macroscopic scale. Recently, chemists aligned continuous multiwalled carbon nanotube (MWCNT) sheets and fibers to produce materials with high mechanical strength and electrical conductivity. This provided an important clue to the use of MWCNTs at macroscopic scale. Researchers have made multiple efforts to optimize this aligned structure and improve the properties of MWCNT sheets and fibers. In this Account, we briefly highlight the new synthetic methods and promising applications of aligned MWCNTs for organic optoelectronic materials and devices. We describe several general methods to prepare both horizontally and perpendicularly aligned MWCNT/polymer composite films, through an easy solution or melting process. The composite films exhibit the combined properties of being flexible, transparent, and electrically conductive. These advances may pave the way to new flexible substrates for organic solar cells, sensing devices, and other related applications. Similarly, we discuss the synthesis of aligned MWCNT/polymer composite fibers with interesting mechanical and electrical properties. Through these methods, we can incorporate a wide variety of soluble or fusible polymers for such composite films and fibers. In addition, we can later introduce functional polymers with conjugated backbones or side chains to improve the properties of these composite materials. In particular, cooperative interactions between aligned MWCNTs and polymers can produce novel properties that do not occur individually. Common examples of this are two types of responsive polymers, photodeformable azobenzene-containing liquid crystalline polymer and chromatic polydiacetylene. Aligning the structure of MWCNTs induces the orientation of azobenzene-containing mesogens, and produces photodeformable polymer elastomers. This strategy also solves the long-standing problems from the traditional mechanical rubbing method, which include production of broken debris and structure damage during fabrication and building up electrostatic charge during use. Aligning MWCNTs induces a conformational change in polydiacetylene, which causes the composite fibers to be electrochromatic, a previously unknown reaction in chromatic polymers. Due to their large surface area, flexibility, electrical conductivity, and remarkable electrocatalytic activity, aligned MWCNT films can be used as counter electrodes to produce highly efficient dye-sensitized solar cells. In addition, chemists have developed new electrodes from the aligned MWCNT fibers to make a family of high-performing, wire-shaped dye-sensitized solar cells.

Three-dimensional manganese dioxide-functionalized carbon nanotube electrodes for electrochemical supercapacitors

Three-dimensional manganese dioxide (MnO2)-functionalized multiwalled carbon nanotube (MWCNT) electrodes have been produced by a simple and scalable thermal decomposition process. The electrodes are prepared by treating planar MWCNT sheets with manganese(II) nitrate (Mn(NO3)2) solution and annealing at low temperature (200–300 °C) and ambient pressure. The morphology, chemical composition, and structure of the resulting matrices have been investigated with scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction. Supercapacitors assembled with three-dimensional electrodes exhibit a 14-fold increase in specific capacitance (C sp) in comparison to those containing pristine, two-dimensional MWCNT electrodes. C sp varies linearly with Mn(NO3)2 thermal decomposition temperature (from 100 to 61 F/g at 0.2 A/g), a trend that is discussed in the context of nitrate reaction chemistry and MWCNT structure. This efficient and promising approach allows for simultaneous enhancement of electrode–electrolyte contact area and incorporation of redox-based charge storage within electrochemical capacitors.

Oriented Graphene Nanoribbon Yarn and Sheet from Aligned Multi‐Walled Carbon Nanotube Sheets

Highly oriented graphene nanoribbons sheets and yarns are produced by chemical unzipping of self-standing multiwalled carbon nanotube (MWNT) sheets. The as-produced yarns - after being chemically and thermally reduced - exhibit a good mechanical, electrical, and electrochemical performance.

Terahertz surface plasmon polaritons on freestanding multi-walled carbon nanotube aerogel sheets

We demonstrate that multi-walled carbon nanotubes (MWCNTs) are capable of supporting surface plasmon-polaritons (SPPs) at terahertz (THz) frequencies. To achieve this, we fabricated sub-100 μm-thick freestanding and highly oriented multi-walled carbon nanotube (MWCNT) aerogel sheets. Utilizing terahertz time-domain spectroscopy, we measured the complex index of refraction of the sheets for two orthogonal nanotube orientations. We found that the MWCNT sheets exhibit highly anisotropic THz polarization behavior. Based on the extracted dielectric properties of the medium, which show that it exhibits metallic behavior in the THz spectral range, we investigated the existence and propagation of SPPs by studying the resonantly enhanced transmission through periodic MWCNT hole arrays. We found that carbon nanotubes support SPP excitations that propagate along the tubes, but highly suppress these surface waves in the direction perpendicular to the nanotubes.

Growth of Spin-Capable Multi-Walled Carbon Nanotubes and Flexible Transparent Sheet Films

Iron-catalyzed spin-capable multi-walled carbon nanotubes (MWCNTs) were grown on a SiO2 wafer by chemical vapor deposition, which was carried out at 780 degrees C using C2H2 and H2 gases. We fabricated a flexible transparent film using the spun MWCNTs. The MWCNT sheets were produced by being continuously pulled out from well-aligned MWCNTs grown on a substrate. The MWCNT sheet films were manufactured by simply carrying out direct coating on a flexible film or glass. The thickness of the sheet film decreased remarkably when alcohol was sprayed on the surface of the sheet. The alcohol spraying increased the transmittance and decreased the electrical resistance of the MWCNT sheet films. The sheets obtained after alcohol spraying had a resistance of -699 omega and a transmittance of 81%-85%. The MWCNT sheet films were heated by applying direct current. The transparent heaters showed a rapid thermal response and uniform distribution of temperature. In addition, we tested the field emission of the sheet films. The sheet films showed a turn-on voltage of -1.45 V/microm during field emission.

Hydrogen-fuel-powered bell segments of biomimetic jellyfish

Artificial muscles powered by a renewable energy source are desired for joint articulation in bio-inspired autonomous systems. In this study, a robotic underwater vehicle, inspired by jellyfish, was designed to be actuated by a chemical fuel source. The fuel-powered muscles presented in this work comprise nano-platinum catalyst-coated multi-wall carbon nanotube (MWCNT) sheets, wrapped on the surface of nickel–titanium (NiTi) shape memory alloy (SMA). As a mixture of oxygen and hydrogen gases makes contact with the platinum, the resulting exothermic reaction activates the nickel–titanium (NiTi)-based SMA. The MWCNT sheets serve as a support for the platinum particles and enhance the heat transfer due to the high thermal conductivity between the composite and the SMA. A hydrogen and oxygen fuel source could potentially provide higher power density than electrical sources. Several vehicle designs were considered and a peripheral SMA configuration under the robotic bell was chosen as the best arrangement. Constitutive equations combined with thermodynamic modeling were developed to understand the influence of system parameters that affect the overall actuation behavior of the fuel-powered SMA. The model is based on the changes in entropy of the hydrogen and oxygen fuel on the composite actuator within a channel. The specific heat capacity is the dominant factor controlling the width of the strain for various pulse widths of fuel delivery. Both theoretical and experimental strains for different diameter (100 and 150 µm) SMA/MWCNT/Pt fuel-powered muscles with dead weight attached at the end exhibited the highest magnitude under 450 ms of fuel delivery within 1.6 mm diameter conduit size. Fuel-powered bell deformation of 13.5% was found to be comparable to that of electrically powered (29%) and natural jellyfish (42%).

Field Emission Device Made from Multiwalled Carbon Nanotube Sheet

ABSTRACTA new type of cathode for electron field emission (FE) was fabricated. The cathode was made from ultra-thin multiwalled carbon nanotube (CNT) sheets. These sheets were drawn directly from a CNT forest, stacked layer-by-layer together and densified by isopropyl alcohol. CNT emitters were formed by utilizing laser beam to cut the sheet. The FE performance of the proposed devices has been enhanced dramatically. The threshold field for electron emission (at which the emission current is 10 mA/cm2) was 0.88 V/μm. The current density of 36 A/cm2 was achieved at the electric field of 2 V/μm. The enhanced performance is the result of the thin, uniformly distributed and aligned array of the CNT emitters.

Regulation of morphogenesis and neural differentiation of human mesenchymal stem cells using carbon nanotube sheets

In order to successfully utilize stem cells for therapeutic applications in regenerative medicine, efficient differentiation into a specific cell lineage and guidance of axons in a desired direction is crucial. Here, we used aligned multi-walled carbon nanotube (MWCNT) sheets to differentiate human mesenchymal stem cells (hMSCs) into neural cells. Human MSCs present a preferential adhesion to aligned CNT sheets with longitudinal stretch parallel to the CNT orientation direction. Cell elongation was 2-fold higher than the control and most of the cells were aligned on CNT sheets within 5° from the CNT orientation direction. Furthermore, a significant, synergistic enhancement of neural differentiation was observed in hMSCs cultured on the CNT sheets. Axon outgrowth was also controlled using nanoscale patterning of CNTs. This CNT sheet provides a new cellular scaffold platform that can regulate morphogenesis and differentiation of stem cells, which could open up a new approach for tissue and stem cell regeneration.

Potassium perruthenate-treated carbon nanotube sheets for flexible supercapacitors

High-throughput production of flexible, all-solid-state supercapacitors can be accomplished by integrating redox-functionalized commercial carbon nanotube sheets with H2SO4/PVA/H2O polymer hydrogel electrolyte. The chemical composition, elemental oxidation states, and crystallinity of the pristine and 0.02M potassium perruthenate-treated multi-walled carbon nanotube (MWCNT) sheets are examined using X-ray photoelectron spectroscopy and X-ray diffraction. In assembled hydrogel-based supercapacitor cells, ruthenium oxide-decorated MWCNT sheet electrodes exhibit a specific capacitance of 96F/g at 1.0A/g and good cycle stability.

Catalytic Twist‐Spun Yarns of Nitrogen‐Doped Carbon Nanotubes

AbstractThe treatment of free‐standing sheets of multiwalled carbon nanotubes (MWNTs) with a NH3/He plasma results in self‐supporting sheets of aligned N‐doped MWNTs (CNx). These CNx sheets can be easily twist spun in the solid state to provide strong CNx yarns that are knottable, weavable, and sewable. The CNx yarns exhibit tunable catalytic activity for electrochemically driven oxygen reduction reactions (ORR), as well as specific capacitances (up to 39 F·g−1) that are 2.6 times higher than for the parent MWNTs. Due to a high degree of nanotube alignment, the CNx yarns exhibit specific strengths (451 ± 61 MPa·cm3·g−1) that are three times larger than observed for hybrid CNx/MWNT biscrolled yarns containing 70 wt.% CNx in the form of a powder. This difference in mechanical strength arises from substantial differences in yarn morphology, revealed by electron microscopy imaging of yarn cross‐ sections, as well as the absence of a significant strength contribution from CNx nanotubes in the biscrolled yarns. Finally, the chemical nature and abundance of the incorporated nitrogen within the CNx nanotubes is studied as function of plasma exposure and annealing processes using X‐ray photoelectron spectroscopy and correlated with catalytic activity.

Semi-transparent small molecule organic solar cells with laminated free-standing carbon nanotube top electrodes

We have developed semi-transparent n–i–p organic solar cells (OSC) with free-standing multi-wall carbon nanotube (f-CNT) sheets as transparent top electrodes. By a simple and damage-free room temperature orthogonal liquid solution assisted self-laminating process, f-CNT top electrodes are successfully deposited on top of ZnPc:C 60 bulk heterojunction small molecule OSCs. The cells show high fill factors above 58% as well as efficiencies up to 1.5% and greater long-term stability compared to the device having a metal electrode. For the given cell structure with f-CNT semi-transparent electrodes, the influence of an optical spacer on light absorption is studied by a systematic variation of the hole transport layer thickness, supported by optical simulations. The results strongly indicate that OSCs with f-CNT top electrodes and optimized thin film stack are highly promising for semi-transparent OSCs, which can be used in tandem devices, in tinted smart windows, and similar applications by a simple and damage-free process in roll-to-roll configuration that can be scaled to large area manufacturing.

EMI shielding effectiveness of silver nanoparticle-decorated multi-walled carbon nanotube sheets

With the aim of exploring the excellent properties of multi-walled carbon nanotubes (MWNTs) in modern composite technologies, various macrostructures of nanotubes have been developed from one to three dimensions, e.g. fibers, networks, sheets (buckypapers) and pellets. The MWNT sheets discussed here were fabricated by a vacuum filtration procedure, a process that has potential for large-scale manufacturing. In order to further enhance the transport properties of MWNT sheets by reducing the contact resistance between nanotubes, highly conductive silver nanoparticles were introduced by an in situ photochemical reduction method. TEM analysis showed that highly acid-treated MWNTs dispersed in the presence of Triton X-100 (TX-100) under UV light was a controllable processing method for preparation of a narrow size distribution of silver nanoparticles that were anchored onto the nanotubes. The free-standing MWNT/Ag nanohybrid sheets possessed a sharp increase in electrical conductivity from 27.7 to 40.0 S/cm, which consequently led to a much improved electromagnetic interference shielding effectiveness (SE). In principle, the SE could reach 3500 dB/cm with a thickness of 110 μm, which matched the experimental results well. In addition, the nanohybrid sheets are robust and can be folded with a thickness of 30 μm, which opens a promising way to integrate MWNT sheets into conventional composite laminates.

The manufacture of a transparent film heater by spinning multi-walled carbon nanotubes

A flexible, transparent heater was manufactured by spinning multi-walled carbon nanotubes (MWCNTs). MWCNT sheets were produced by continuously pulling on well-aligned MWCNTs, and films were produced by directly coating a flexible film. Single and double sheets were produced with sheet resistances of ∼699 Ω/sq and ∼349 Ω/sq and transmittances of 81% to 85% and 67% to 72%, respectively. Both films were heated by applying a direct current, and showed rapid thermal responses and uniform temperature distribution. The rate of temperature change was found to depend on the power applied. Frost on the surface of these films was rapidly removed so that they may have applications as automatic defrosters in vehicles of goggles.

Sound of carbon nanotube assemblies

Strong thermo- and photoacoustic responses have been detected for aligned arrays of multiwalled carbon nanotube (MWNT) forests and solid drawn MWNT sheets. When heated using alternating current or a near-IR laser modulated in 100–20000Hz range, the nanotube assemblies generated loud, audible sound, with higher sound pressure being detected from the MWNT sheets. An evaluation of nonlinear distortions of the thermoacoustic signal revealed a highly peculiar behavior of the third and fourth harmonics produced from forests grown on silicon wafers. The peculiarities were especially pronounced for short forests and can be associated with the heat transfer from the MWNT layer to the substrate. For both types of nanotube assemblies, the acoustic signal’s amplitude varied with frequency approximately by the power low fp. The power factor p was found to be unexpectedly high for short forests probably due to heat loss to the substrate. The observed peculiarities can be used for the characterization of the prepared MWNT assemblies. The dependencies can also be helpful for evaluating the properties of thermal interfaces, in particular, those based on carbon nanotubes.

Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses andquenching of phonon modes

The extremely high thermal conductivity of individual carbon nanotubes, predictedtheoretically and observed experimentally, has not yet been achieved for large nanotubeassemblies. Resistances at tube–tube interconnections and tube–electrode interfaces havebeen considered the main obstacles for effective electronic and heat transport. Here weshow that, even for infinitely long and perfect nanotubes with well-designed tube–electrodeinterfaces, excessive radial heat radiation from nanotube surfaces and quenching of phononmodes in large bundles are additional processes that substantially reduce thermal transportalong nanotubes. Equivalent circuit simulations and an experimental self-heating3ω technique were used to determine the peculiarities of anisotropic heat flow andthermal conductivity of single MWNTs, bundled MWNTs and aligned, free-standingMWNT sheets. The thermal conductivity of individual MWNTs grown by chemicalvapor deposition and normalized to the density of graphite is much lower (κMWNT = 600 ± 100 W m−1 K−1)than theoretically predicted. Coupling within MWNT bundles decreases this thermal conductivity to150 W m−1 K−1. Further decrease of the effective thermal conductivity in MWNT sheets to 50 W m−1 K−1 comes from tube–tube interconnections and sheet imperfections like dangling fiber ends,loops and misalignment of nanotubes. Optimal structures for enhancing thermalconductivity are discussed.

Tuning of Fe catalysts for growth of spin-capable carbon nanotubes

Growing spin-capable multi-walled carbon nanotube (MWCNT) forests in a repeatable fashion will become possible through understanding the critical factors affecting the forest growth. Here we show that the spinning capability depends on the alignment of adjacent MWCNTs in the forest which in turn results from the synergistic combination of a high areal density of MWCNTs and short distance between the MWCNTs. This can be realized by starting with both the proper Fe nanoparticle size and density which strongly depend on the sheet resistance of the catalyst film. We prove that a simple measurement of the sheet resistance can allow one to reliably predict the growth of spin-capable forests. Further investigation into the properties of pulled MWCNTs sheets demonstrated the relationship between their electrical resistance and optical transmittance. Overlaying either 3, 5, or 10 sheets pulled out from a single forest produces much more repeatable characteristics.

Semitransparent organic photovoltaic cell with carbon nanotube-sheet anodes and Ga-doped ZnO cathodes

We demonstrate a semitransparent organic photovoltaic (OPV) cell with two transparent electrodes: a multi-wall carbon nanotube (MWCNT) sheet and a Ga-doped ZnO (GZO) are used as a transparent anode and cathode, respectively. As an active layer, we used a bulk heterojunction structure with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The photovoltaic cell has a GZO/P3HT:PCBM/poly(3,4-ethylene-dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/MWCNT/PEDOT:PSS structure. For comparison, an OPV cell with a MWCNT-sheet anode and an indium tin oxide (ITO) cathode were also fabricated. The OPV cell with the GZO cathode showed better performance than that with the ITO cathode, owing to the lower work function of the former. The semitransparent OPV cell with GZO cathode demonstrated a short-circuit current of 3.4 mA/cm 2, open-circuit voltage of 0.51 V, and efficiency of 0.45%.

Electron field emission from transparent multiwalled carbon nanotube sheets for inverted field emission displays

Strong, conducting, transparent carbon nanotube sheets were prepared by solid-state draw from well-ordered, aligned multiwalled carbon nanotube (MWCNT) forests [Zhang et al., 2005] [1]. Study of electron field emission from such transparent MWCNT sheets shows threshold fields of less than 0.5 V/μm with current densities high enough for display applications. Step-like field emission current increase and hysteresis behavior in I– V curves has been observed. The origin of such behavior is discussed in terms of mechanical rearrangement of the nanotube network in high electric field. Studied MWCNT transparent sheet field emission cathodes have several advantages when used as multi-functional electrodes. They are high current, high stability, transparent, and flexible field emission sources and can be used in an inverted geometry, with cathode being in front of the light emitting plate. At the same time transparent CNT sheets may serve as a transparent conducting electrode for electrical connection and pixel addressing in field emission displays (FEDs). Also, these sheets can be used as an optical polarizer in FEDs.