Vertically Aligned Multi-walled Carbon Nanotubes Research Articles

Heat Propagation in Anisotropic Heterogeneous Polymer-CNT Composites

A weak thermal conductivity (TC) of a polymer can be modified by inclusion of nanoparticles with high TC. Here we study the TC enhancement in epoxy resin (ER) based composites by incorporation of carbon nanotubes (CNTs) and demonstrate that the enhancement depends critically on the alignment of CNTs. The highest effect in TC enhancement (18.9) was obtained in ER with vertically aligned multiwall CNTs (VANTs) and in ER with horizontally aligned nanotubes (HANTs) (6.5). We analyze the influence of intrinsic structural factors of CNTs as well as extrinsic factors limiting the enhancement of the composite TC. The dynamics of heat propagation in ER/VANT, a strongly anisotropic and heterogeneous system, was studied experimentally, using laser flash apparatus (LFA), and by computer simulation, applying a coaxial cylinder model. It was found that the thermal resistivity CNT-ER interface to be a key extrinsic factor limiting the dynamics of the heat propagation. We show that these dynamics and the interface resistivity can be efficiently studied using the LFA technique.

Vertically-aligned silicon carbide nanowires as visible-light-driven photocatalysts

Vertically-aligned crystalline silicon carbide nanowires (VASiCs) (1mm long and 50–90nm in diameter) were synthesised in gram scale using SiO2-infiltrated vertically-aligned multi-wall carbon nanotubes (VACNTs) and Si powder. In situ residual gas analysis was employed to study their formation and revealed CO to be the main by-product during synthesis. The in situ studies also showed that the formation of VASiCs begins at 1150°C with the growth rate reaching a maximum at 1350°C. A possible growth mechanism was established based on both, in situ and ex situ characterisation. The VASiCs have an estimated band gap of 2.15eV, are photocatalytically active, and show strong light absorbance of up to 577nm. Under UV–vis light (260–800nm) as grown VASiCs could remove 90% Rhodamine B (RhB) within 30min. Over period of 4h under visible light (400–800nm) more than 95% RhB was removed demonstrating their potential as visible-light-driven photocatalysts.

Simple Potential Bioanalysis Micro reactor Fabricated By Vertically-Aligned Multi-Walled Carbon Nanotubes (MWCNTs)

A simple Microreactor was fabricated by vertically-aligned multi-walled carbon nanotubes (MWCNTs) on the mc-Si substrate. MWCNTs were synthesized by homemade equipment aided with chemical vapor deposition (CVD). The wettability of the surfaces of Microreactor with the synthesized MWCNTs, Au coating and plasma irradiation were investigated, respectively. Results show that the initial surface with the MWCNTs film behaves as a super-hydrophobic surface. However, it changes to a super-hydrophilic surface after coated with Au or irradiated by plasma. It shows that Microreactor with treated MWCNTs film can be used as potential applications for bioanalysis.

On the design and properties of scaffolds based on vertically aligned carbon nanotubes transferred onto electrospun poly (lactic acid) fibers

Herein, we propose the design of a nanoscaffold based on the hot-press transfer of vertically aligned multi-walled carbon nanotubes (VAMWCNT) onto matrices of electrospun poly (lactic acid) (PLA) fibers. To this end, we created a three-dimensional, bioactive and electrically conductive scaffold that combines the potential of PLA as a biomaterial with the physical-chemical and biological properties of VAMWCNT (PLA/VAMWCNT). Given the well-known hydrophobicity of carbon nanotubes, oxygen-plasma functionalization was applied to the scaffolds in order to attach oxygen-containing groups to their surfaces, with the plasma treatment also responsible for the exfoliation of the VAMWCNT's tips. After plasma-functionalization, electrochemical measurements showed that our scaffold presented an increased electroactive area (1.5-fold) with a ko value of 6.87×10−3cms−1, confirming its applicability as an electrode. Furthermore, we have also demonstrated the feasibility of electrodepositing nanohydroxyapatite (nHAp) crystals onto this nanoarchitectured material, improving its biomimetic and bioactive features. Preliminary biological assays revealed the viability of primary human osteoblast cells cultivated onto the PLA/VAMWCNT-nHAp scaffolds. This investigation has shed light on a practical approach to produce 3D–scaffolds by transferring VAMWCNT onto electrospun polymeric matrices. These novel materials may represent promising alternatives for future tissue engineering applications and thus should be studied further.

Soft X-ray absorption spectroscopy study of chemical states, orientation, and oxygen content of ion-irradiated vertically aligned multiwalled carbon nanotubes

Low-energy Ar ions (0.5–2keV) were irradiated onto vertically aligned multiwalled carbon nanotube (VA-MWCNT) films, and the chemical states, orientation, and oxygen content of the irradiated VA-MWCNT films were investigated by soft X-ray absorption spectroscopy (XAS). After irradiation, two additional peaks appeared in the XAS spectra in the C K region, indicating a significant change in the electronic structure. Furthermore, it was found that the VA-MWCNT films were oxidized after irradiation. The results of chemical-state analyses of the XAS spectra in both the C K and O K regions indicated the formation of carboxyl groups (COOH) on the surface of the VA-MWCNT films upon irradiation. The oxygen content of the VA-MWCNT films increased with increasing ion energy and fluence.

Laser-induced nanostructuration of vertically aligned carbon nanotubes coated with nickel oxide nanoparticles

A versatile method is explored to decorate vertically aligned multi-walled carbon nanotubes (VACNTs) with NiO nanostructures. Multi-walled VACNTs are grown by plasma-enhanced chemical vapor deposition and coated with NiO nanoparticles (NPs) by drop casting. After that, the system is submitted to nanosecond pulsed UV laser irradiation in atmospheric environment. Laser irradiation provokes rapid heating–melting–cooling processes which lead to the recrystallization of NiO NPs on the outer walls of VACNTs. In this way, and depending on the laser fluence and the number of accumulated pulses, different nano-architectures such as continuous NiO coatings and spiny features on VACNTs are obtained. High-resolution scanning and transmission electron microscopies and Raman spectroscopy, corroborated with photothermal simulations, suggest that the grown nanostructures are mainly created by the laser-induced high temperatures (photothermal mechanisms). However, the observed reconstruction of the outer graphitic shells of VACNTs point to the catalytic action of NiO NPs, probably induced by the direct action of laser radiation.

Simultaneous Voltammetric Determination of Paracetamol, Codeine and Caffeine on Diamond‐like Carbon Porous Electrodes

AbstractA porous electrode material combining the features of vertically aligned multi‐walled carbon nanotubes (VAMWCNT) and diamond‐like carbon films (DLC) have been developed for a highly sensitive electrochemical sensor. For working electrode preparation, DLC has been grown onto VAMWCNT, forming a porous, conductive and stable composite. The electrochemical performance of this DLC:VAMWCNT electrode has been investigated toward detection and analysis of three well‐known molecules, namely paracetamol, codeine and caffeine. A ternary mixture of these analytes was simultaneously determined under optimum experimental conditions using square‐wave voltammetry. Wide linear concentration ranges and the limits of detection of 3.34×10−7 mol L−1, 1.57×10−7 mol L−1 and 3.67×10−7 mol L−1 were obtained for paracetamol, codeine and caffeine, respectively. We conclude that the proposed voltammetric method and the DLC:VAMWCNT electrode comprise a reliable methodology for simultaneous determination of paracetamol, codeine and caffeine in biological matrix samples.

An electrical bio-chip to transfer and detect electromagnetic stimulation on the cells based on vertically aligned carbon nanotubes

A highly sensitive impedimetric bio-chip based on vertically aligned multiwall carbon nanotubes (VAMWCNTs), was applied in direct interaction with lung cancer cells. Our tool provided both inducing and monitoring the bioelectrical changes in the cells initiated by electromagnetic (EM) wave stimulation. EM wave of 940MHz frequency with different intensities was used. Here, wave ablation might accumulate electrical charge on the tips of nanotubes penetrated into cell's membrane. The charge might induce ionic exchanges into the cell and cause alterations in electrical states of the membrane. Transmembrane electrostatic/dynamic states would be strongly affected due to such exchanges. Our novel modality was that, the cells' vitality changes caused by charge inductions were electrically detected with the same nanotubes in the architecture of electrodes for impedance measurement. The responses of the sensor were confirmed by electron and florescent microscopy images as well as biological assays. In summation, our method provided an effective biochip for enhancing and detecting external EM stimulation on the cells useful for future diagnostic and therapeutic applications, such as wave-guided drug-resistance breakage.

Fabrication of free-standing aligned multiwalled carbon nanotube array for Li-ion batteries

We show that a high-temperature CCl4 vapor treatment of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) grown on silicon substrate allows carefully detach the array from the substrate. Moreover, this procedure partially purifies the VA-MWCNTs from the residual iron catalyst. To improve electrical connectivity of free-standing VA-MWCNTs in an electrochemical cell, the array was placed between the layers of Ni foam. Such assembly demonstrated the better performance in Li-battery as compared to the disordered MWCNTs. After 50 cycles, the specific capacity of VA-MWCNT array synthesized from 0.5 wt% ferrocene solution in toluene was 350 mAh g−1 at a current density of 0.1 A g−1, while the battery with the disordered MWCNTs achieved 197 mAh g−1 only. By the results of electrochemical impedance spectroscopy, the higher capacity of VA-MWCNTs was attributed to larger surface area available for electrolyte and Li ions due to the absence of binder coating.

Ultra-stiff large-area carpets of carbon nanotubes.

Herewith, we report the influence of post-synthesis heat treatment (≤2350 °C and plasma temperatures) on the crystal structure, defect density, purity, alignment and dispersibility of free-standing large-area (several cm(2)) carpets of ultra-long (several mm) vertically aligned multi-wall carbon nanotubes (VA-MWCNTs). VA-MWCNTs were produced in large quantities (20-30 g per batch) using a semi-scaled-up aerosol-assisted chemical vapour deposition (AACVD) setup. Electron and X-ray diffraction showed that the heat treatment at 2350 °C under inert atmosphere purifies, removes residual catalyst particles, and partially aligns adjacent single crystals (crystallites) in polycrystalline MWCNTs. The purification and improvement in the crystallites alignment within the MWCNTs resulted in reduced dispersibility of the VA-MWCNTs in liquid media. High-resolution microscopy revealed that the crystallinity is improved in scales of few tens of nanometres while the point defects remain largely unaffected. The heat treatment also had a marked benefit on the mechanical properties of the carpets. For the first time, we report compression moduli as high as 120 MPa for VA-MWCNT carpets, i.e. an order of magnitude higher than previously reported figures. The application of higher temperatures (arc-discharge plasma, ≥4000 °C) resulted in the formation of a novel graphite-matrix composite reinforced with CVD and arc-discharge-like carbon nanotubes.

Carbon Nanotube Surface Regular Topography Improves Cell Response, Depending on Cell Passage

Through mediation of integrin clustering, nanoscale surface structure of carbon nanotubes (CNTs) directly affects cell binding and subsequent behavior. The influence of morphological structures on proliferation, differentiation, and organization of focal adhesions and cytoskeleton of human mesenchymal stem cells (hMSCs) was studied. The following two surface morphologies were fabricated and examined: a random network multiwalled carbon nanotubes film (RNCNT) and a vertically aligned multiwalled carbon nanotube (VACNT) film. hMSCs adhered and spread earlier on both CNT surfaces than on control. A statistically significantly increased number of attached cells were observed on VACNT surfaces. No CNT substrate enhanced differentiation, but both maintained the differentiation property of hMSCs. VACNTs recruited vinculin from the cytoplasm for the formation of focal adhesion complexes earlier in comparison to RNCNT. There is still disparity on how nanostructures regulate the progression toward an osteoblastic phenotype. It is necessary to explore various architectures in order to understand how they initiate osteoinductive or proliferating signals.

Simple Potential Bioanalysis Microreactor Fabricated by Vertically-aligned Multi-Walled Carbon Nanotubes (MWCNTs)

Simple Potential Bioanalysis Microreactor Fabricated by Vertically-aligned Multi-Walled Carbon Nanotubes (MWCNTs)

Sensing Properties of Multiwalled Carbon Nanotubes Grown in MW Plasma Torch: Electronic and Electrochemical Behavior, Gas Sensing, Field Emission, IR Absorption

Vertically aligned multi-walled carbon nanotubes (VA-MWCNTs) with an average diameter below 80 nm and a thickness of the uniform VA-MWCNT layer of about 16 μm were grown in microwave plasma torch and tested for selected functional properties. IR absorption important for a construction of bolometers was studied by Fourier transform infrared spectroscopy. Basic electrochemical characterization was performed by cyclic voltammetry. Comparing the obtained results with the standard or MWCNT‐modified screen-printed electrodes, the prepared VA-MWCNT electrodes indicated their high potential for the construction of electrochemical sensors. Resistive CNT gas sensor revealed a good sensitivity to ammonia taking into account room temperature operation. Field emission detected from CNTs was suitable for the pressure sensing application based on the measurement of emission current in the diode structure with bending diaphragm. The advantages of microwave plasma torch growth of CNTs, i.e., fast processing and versatility of the process, can be therefore fully exploited for the integration of surface-bound grown CNTs into various sensing structures.

Atomic Layer Deposition of Titanium Oxide for Pseudocapacitive Functionalization of Vertically-Aligned Carbon Nanotube Supercapacitor Electrodes

Using atomic layer deposition, vertically-aligned multi-walled carbon nanotube (MWCNT) supercapacitors were coated with conformal and uniform layers of titanium oxide to introduce pseudocapacitive charge storage properties to the electrodes. The results suggest that the addition of a controlled thickness of titania to the MWCNT structure improves the performance of the capacitors, with an increase in specific capacitance (more than 300%) as compared to the control supercapacitors, with values up to 73 F/g normalized to electrode mass (MWCNT and titania) and 1,364 F/g normalized to titania mass. Furthermore, the series resistance of the control supercapacitors (1.21 Ω) did not significantly increase with the addition of titania, until 300 atomic layer deposition cycles were performed. Improvements of the specific power and energy were also observed, with the maximum specific power and energy seen being 24.8 ± 4.8 W/g and 14.0 ± 4.1 Ws/g respectively.

Electrochemical performance of porous diamond-like carbon electrodes for sensing hormones, neurotransmitters, and endocrine disruptors.

Porous diamond-like carbon (DLC) electrodes have been prepared, and their electrochemical performance was explored. For electrode preparation, a thin DLC film was deposited onto a densely packed forest of highly porous, vertically aligned multiwalled carbon nanotubes (VACNT). DLC deposition caused the tips of the carbon nanotubes to clump together to form a microstructured surface with an enlarged surface area. DLC:VACNT electrodes show fast charge transfer, which is promising for several electrochemical applications, including electroanalysis. DLC:VACNT electrodes were applied to the determination of targeted molecules such as dopamine (DA) and epinephrine (EP), which are neurotransmitters/hormones, and acetaminophen (AC), an endocrine disruptor. Using simple and low-cost techniques, such as cyclic voltammetry, analytical curves in the concentration range from 10 to 100 μmol L(-1) were obtained and excellent analytical parameters achieved, including high analytical sensitivity, good response stability, and low limits of detection of 2.9, 4.5, and 2.3 μmol L(-1) for DA, EP, and AC, respectively.

Self-assembled plasmonic nanoparticles on vertically aligned carbon nanotube electrodes via thermal evaporation.

This study details the development of a large-area, three-dimensional (3D), plasmonic integrated electrode (PIE) system. Vertically aligned multiwalled carbon nanotube (VA-MWNT) electrodes are grown and populated with self-assembling silver nanoparticles via thermal evaporation. Due to the geometric and surface characteristics of VA-MWNTs, evaporated silver atoms form nanoparticles approximately 15-20 nm in diameter. The nanoparticles are well distributed on VA-MWNTs, with a 5-10 nm gap between particles. The size and gap of the self-assembled plasmonic nanoparticles is dependent upon both the length of the MWNT and the thickness of the evaporated silver. The wetting properties of water of the VA-MWNT electrodes change from hydrophilic (∼70°) to hydrophobic (∼120°) as a result of the evaporated silver. This effect is particularly pronounced on the VA-MWNT electrodes with a length of 1 μm, where the contact angle is altered from an initial 8° to 124°. Based on UV-visible spectroscopic analysis, plasmonic resonance of the PIE systems occurs at a wavelength of approximately 400 nm. The optical behavior was found to vary as a function of MWNT length, with the exception of MWNT with a length of 1 μm. Using our PIE systems, we were able to obtain clear surface-enhanced Raman scattering (SERS) spectra with a detection limit of ∼10 nM and an enhancement factor of ∼10(6). This PIE system shows promise for use as a novel electrode system in next-generation optoelectronics such as photovoltaics, light-emitting diodes, and solar water splitting.

Unraveling the growth of vertically aligned multi-walled carbon nanotubes by chemical vapor deposition

The interaction between the main operational variables during the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) by catalytic chemical vapor deposition is studied. In this contribution, we report the influence of the carbon source (i.e. acetylene, ethylene and propylene), the reaction/activation temperature, the rate of heating, the reaction time, the metal loading, and the metallic nanoparticle size and distribution on the growth and alignment of carbon nanotubes. Fe/Al thin films deposited onto silicon samples by electron-beam evaporation are used as catalyst. A phenomenological growth mechanism is proposed to explain the interaction between these multiple factors. Three different outcomes of the synthesis process are found: i) formation of forests of non-aligned, randomly oriented multi-walled carbon nanotubes, ii) growth of vertically aligned tubes with a thin and homogeneous carbonaceous layer on the top, and iii) formation of vertically aligned carbon nanotubes. This carbonaceous layer (ii) has not been reported before. The main requirements to promote vertically aligned carbon nanotube growth are determined.

High surface area diamond-like carbon electrodes grown on vertically aligned carbon nanotubes

Electrochemically active diamond-like carbon (DLC) electrodes featuring high specific surface area have been prepared by plasma-enhanced chemical vapour deposition (CVD) onto densely packed forests of vertically aligned multiwall carbon nanotubes (VACNTs). The DLC:VACNT composite film exhibits a complex topography with web like features and ridges generated by partial coalescence of the DLC over the CNT arrays. DLC:VACNT electrodes exhibit low background responses over a large potential window, low uncompensated resistance, as well as low charge-transfer impedance in the presence of ferrocyanide as a redox probe. The interfacial capacitance associated with the DLC:VACNT electrode is in the range of 0.6mFcm−2, a value two orders of magnitude larger than in conventional flat carbon electrodes. DLC films grown onto single-crystal Si(100) under identical conditions resulted in essentially insulating layers. Conducting-atomic force microscopy studies reveal that the film electro-activity does not arise from specific topographic features in the highly corrugated film. The ensemble of experimental results suggests that the enhanced electrochemical responses are not connected to areas in which the CNT support is exposed to the electrolyte solution. This is remarkable behaviour considering that no dopants have been included during the DLC film growth.

Covering vertically aligned carbon nanotubes with a multiferroic compound

This work highlights the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO3 (BFO) films were fabricated in-situ deposited on the surface of VA-MWCNTs by RF (radio frequency) magnetron sputtering. For in situ deposition temperature of 400°C and deposition time up to 2h, BFO films cover the MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by piezo force microscopy. G type antiferromagnetic ordering with weak ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700Oe.

A monolithic functional film of nanotubes/cellulose/ionic liquid for high performance supercapacitors

A novel monolithic, pre-fabricated, fully functional film made of a nanostructured free-standing layer is presented for a new and competitive class of easy-to-assemble flexible supercapacitors whose design is in-between the all solid state and the traditional liquid electrolyte. The film is made of two vertically aligned multi-walled carbon nanotube (VANT) electrodes that store ions, embedded-in, and monolithically interspaced by a solution of microcrystalline cellulose in a room temperature ionic liquid (RTIL) electrolyte (1-ethyl-3-methylimidazolium acetate-EMIM Ac). The fine tuning of VANTs length and electrolyte/cellulose amount leads, in a sole and continuous block, to ions storage and physical separation between the electrodes without the need of the additional separator layer that is typically used in supercapacitors. Thus, physical discontinuities that can induce disturbances to ions mobility, are fully eliminated significantly reducing the equivalent series resistance and increasing the knee frequency, hence outclassing the best supercapacitors based on VANTs and non-aqueous electrolytes. The excellent electrochemical response can also be addressed to the chosen electrolyte that, not only has the advantage of leading to a significantly simpler and more affordable fabrication procedure, but has higher ionic conductivity, lower viscosity and higher ions mobility than other electrolytes capable of dissolving cellulose.