Yield Of Nanotubes Research Articles

Buckling and kinking force measurements on individual multiwalled carbon nanotubes

Using an atomic force microscope operated inside a transmission electron microscope, we have studied the forces involved in buckling and kinking an individual multiwalled carbon nanotube while observing its structure. In particular, we have measured an individual nanotube's asymptotic critical buckling load and critical kinking load. The buckling results are well described by classical elastic theory, while the observed kinking behavior requires a more involved analysis. Repeated buckling measurements on the same nanotube indicate an extremely high degree of elasticity and set a lower bound on the nanotube's yield strength of $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, higher than the yield strength of steel. Plastic deformation of the nanotube was eventually observed following kinking.

A Synthesis of High Purity Single-Walled Carbon Nanotubes from Small Diameters of Cobalt Nanoparticles by Using Oxygen-Assisted Chemical Vapor Deposition Process

A successful combination of “oxygen-assisted chemical vapor deposition (CVD) process” and Co catalyst nanoparticles to grow highly pure single walled carbon nanotubes (SWNTs) was demonstrated. Recently, it was reported that addition of small amounts of oxygen during CVD process dramatically increased the purity and yield of carbon nanotubes. However, this strategy could not be applied for discrete Fe nanoparticle catalysts from which appropriate yields of SWNTs could be grown directly on solid substrates, and fabricated into field effect transistors (FETs) quite efficiently. The main reason for this failure is due to the carbothermal reduction which results in SiO2 nanotrench formation. We found that the oxygen-assisted CVD process could be successfully applied for the growth of highly pure SWNTs by switching the catalyst from Fe to Co nanoparticles. The topological morphologies and p-type transistor electrical transport properties of the grown SWNTs were examined by using atomic force microscope (AFM), Raman, and from FET devices fabricated by photolithography.

A Dielectrophoretic Method for High Yield Deposition of Suspended, Individual Carbon Nanotubes with Four-Point Electrode Contact

We have investigated new methods to manufacture four-point contacted suspended, individual multiwalled carbon nanotubes by dielectrophoresis. The necessity of four-point contacted carbon nanotubes is underpinned by the need for more exact measurements of carbon nanotube properties and more complex carbon nanotube based devices. In the experimental study herein, we first evaluated the most important dielectrophoretic parameters of two-point electrode structures. The knowledge gained from this step was applied to the deposition of suspended, individual multiwalled carbon nanotubes over two different four-point electrode designs, a 2-D chip design and a novel 3-D chip design. Our results show that the novel approach using three dimensions for the chip design is superior to the current state of the art 2-D designs. We report a highly improved deposition yield of carbon nanotubes over 3-D chips, which will facilitate the reliable design and manufacturing of future carbon nanotube-integrated devices.

Surface-bound chemical vapour deposition of carbon nanotubes: In situ study of catalyst activation

We combine in situ X-ray photoelectron spectroscopy and ex situ electron and Raman spectroscopy to study chemical interactions of SiO 2-supported Fe catalyst films during C 2H 2 exposure in the 400–600 °C temperature range. Carbon nanotubes nucleate at C 2H 2 pressures below 10 −6 mbar, which allows a time-resolved recording of core level spectra. We find a rapid transition from an initial chemisorbed carbon on metallic Fe catalyst nanoparticles to a sp 2 graphitic carbon network. Pre-treating the Fe catalyst film by H 2 sputtering influences the initial catalyst de-wetting and increases carbon nanotube yield.

Growth and optimization of carbon nanotubes in activated carbon by catalytic chemical vapor deposition

High yield synthesis of carbon nanotubes in activated carbon by Ni-catalyzed chemical vapor deposition is demonstrated and a parametric study on the effect of the most crucial experimental conditions on the growth is performed. Reaction temperature, time and carbon precursor flow rate were systematically varied and their influence on nanotube size, uniformity, density, growth rate, and yield was evaluated. In situ monitoring of growth evolution was achieved by a microbalance system connected on-line to the reaction zone. The porous structure of the support provides high surface area for catalyst particle deposition and improved catalyst particle dispersion properties, while its composition appeared to enhance nanotube yield. The resulting carbon nanotube-loaded activated carbons were characterized by scanning and transmission electron microscopy, Raman spectroscopy, while the pore transformation after nanotube growth was evaluated by mercury and nitrogen porosimetry. A strong effect of the reaction temperature was found on nanotube growth leading to an optimal value of 750 °C, while for a reaction duration longer than 5 min the growth rate was continuously suppressed, the nanotube size distribution was broaden, and the formation of amorphous carbon was favored. Controlled oxidation experiments on the outer surface of the impregnated activated carbon support were performed in order to remove outer layers and observe the catalyst morphology, content, and activity in the inner parts of the material. The composites produced in the present work, combining the exciting characteristics of carbon nanotubes with those of activated carbon, are anticipated to contribute to the development of novel composite materials based on carbon nanotubes with enhanced physical, chemical and electronic characteristics for important applications such as in catalysis, energy, and environment.

Flame synthesis of carbon nanotubes with high density on stainless steel mesh

Multi-walled carbon nanotubes grew directly on wires of stainless steel mesh in controllable methane diffusion flames. The formation and morphology of carbon nanotubes were dependent on gas composition of the flames. On pre-etched mesh wires with HCl, high density of carbon nanotubes were synthesized with uniform outer diameters of 60 nm and large inner diameters of 50 nm. With the high yield of carbon nanotubes, less carbon impurities were formed in the process. A mechanistic model was proposed in detail to suggest the formation of catalyst directly on bulk surface and explain the whole process of carbon nanotubes growth in this study.

Synthesis of CNx nanotubes using catalysts prepared from zinc and nickel bimaleates

Nitrogen-containing carbon nanotubes have been prepared via acetonitrile (CH3CN) pyrolysis at 850°C catalyzed by nanoparticles produced by the thermal decomposition of zinc and nickel bimaleates and their solid solutions. The synthesized samples have been characterized by transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. The results demonstrate that increasing the zinc content of catalyst nanoparticles reduces the yield of carbon nanotubes and increases the nitrogen content of the material. The high synthesis temperature gives rise to zinc vaporization, which influences the growth process, increasing the nanotube diameter, reducing the wall thickness, and lowering the structural perfection of the graphite layers.

A new approach towards improving the quality and yield of arc-generated carbon nanotubes

The paper reports the deterministic effects of a focusing electric field in improving the purity and yield of the arc-generated carbon nanotubes (CNTs). The method utilizes a focusing electrostatic field, which was superimposed on the arc symmetrically. The focusing voltage was varied from 0 to 1200 V at steps of 200 V and a number of cathode deposits, thus generated, were collected and thoroughly analysed in their totality with the help of weight balance, Raman spectroscopy, transmission electron microscopy and thermogravimetry. With the optimally configured focusing electric field, the arc generator is found to utilize nearly 85% of the consumed anode material, for converting into cathode deposit consisting of CNTs, as compared with about 35% in the conventional arc plasma method. The sample prepared under optimized conditions exhibited high oxidation temperature (851 °C) in the thermogravimetric analysis, negligible D band intensity along with a reduced G band line-width (14 cm−1) in the Raman spectrum, confirming the presence of high purity CNTs with a high relative yield.

Chemistry of Water-Assisted Carbon Nanotube Growth over Fe−Mo/MgO Catalyst

Introducing a small amount of water vapor in the catalytic chemical vapor deposition (CVD) growth of single- and double-walled carbon nanotubes (SWNTs and DWNTs) extends the catalyst lifetime and increases the nanotube yield. We study the mechanism of this water-assisted nanotube growth over a Fe−Mo/MgO catalyst, based on analysis of the effluent gas, in terms of chemistry of the water-induced oxidation and its effects on catalytic activity. Water vapor was found to etch away carbon precipitate covering the metal catalyst, based primarily on the chemical reaction C + H2O → CO + H2, thus maintaining the catalytic activity. This oxidative etching was strongly dependent on the CVD temperature, and the balance between the etching and carbon precipitation was important for effective nanotube growth. With an optimized water concentration, the etching rate of the carbon precipitate was estimated to be ca. 1/1000 of the formation rate of carbon precipitate consisting mainly of SWNTs and DWNTs.

Single-wall carbon nanotubes synthesized using organic additives to Co–Mo catalystssupported on nanoporous MgO

Carbon nanotubes (CNTs) were synthesized by catalytic decompositionof 20% methane in hydrogen over Co–Mo/MgO catalysts at1000 °C. The catalysts were prepared by a modified sol–gel method, using sorbitol, ethylenediaminetetraacetic acid (EDTA), tartaric acid and citric acid as organic additives. The specific surfacearea (SSA) of the catalysts changes with the organic additives in the order of sorbitol> EDTA> citricacid> tartaric acid. As a result the crystallite size, dispersion of Co–Mo and hence the quality ofthe carbon nanotubes change with the organic additive. Up to 180 wt% yield of highquality single-wall carbon nanotubes (SWNTs) with inner-wall diameter closeto 1.0–1.2 nm are obtained on the catalyst prepared in the presence of sorbitol.

Proteins and Carbon Nanotubes: Close Encounter in Water

The interactions between biological substances and carbon nanotubes (CNTs) and their effect on the nanotubes are of significant importance in this emerging era of nanobiotechnology. Consequently, highly stable dispersions of debundled CNTs in aqueous solution are an important prerequisite for their applications and for the development of nanotube-based molecular electronic and nanobiomedical devices. Here, we report that proteins can work as tools to this end if their primary structure and the pH value of the system are chosen appropriately. Proteins containing a large number of basic residues, for example, histone, are found to be the most promising protein tools for the dispersion of nanotubes. Apart from other interactions, the polarity of the protein seems to play a vital role in obtaining high yields of debundled nanotubes. In addition, an enrichment of metallic nanotubes in the products is observed, which offers a facile approach for separating nanotubes according to their electronic properties in the bulk.

A parametric study of carbon nanotubes production from tetrachloroethylene using a supersonic thermal plasma jet

In this study we produce carbon nanotubes (CNT) using a DC non-transferred plasma torch operated at a power of 30 kW in argon and producing a supersonic jet. Tetrachloroethylene (TCE) is used as the carbon raw material. As an initial demonstration of the supersonic plasma jet approach and in an attempt to simplify the flow/nucleation fields of metal catalyst nanoparticles, the erosion of the torch tungsten electrodes is used as a source of metal vapours nucleating in situ into catalytic nanoparticles within the plasma jet. CNT mass yield values are based on thermogravimetry analysis correlated using electron microscopy and Raman spectroscopy. A parametric study is made to evaluate the influence of the different operating parameters on the yields of carbon nanotubes. The rapid quench generated by the supersonic jet, the high vapour pressures of carbon and a control of the temperature profile within the torch nozzle enable the rapid growth of CNT. Decreasing the reactor pressure from 0.66 to 0.26 atm leads to a CNT yield increase by 22%. High vapour pressure of carbon is obtained by increasing the TCE feed rate. From 0.05 to 0.15 mol/min, the yield of CNT is improved from 38.6 to 53.7 wt%. Beyond 0.15 mol TCE/min, the yield of CNT levels to around 50 wt%. In the start-up phase, the time of operation controls the temperature profile, which in turn drastically increases the yield of CNT from 0 to 8.2 wt% between the 3rd and the 4th minute of operation and to 53.7 wt% after the 5th minute.

Magnetic Brightening of Carbon Nanotube Photoluminescence through Symmetry Breaking

We report that symmetry breaking by a magnetic field can drastically increase the photoluminescence quantum yield of single-walled carbon nanotubes, by as much as a factor of 6, at low temperatures. To explain this we have developed a theoretical model based on field-dependent exciton band structure and the interplay of Coulomb interactions and the Aharonov-Bohm effect. This conclusively explains our data as the first experimental observation of dark excitons 5-10 meV below the bright excitons.

Parameters Affecting the Structure and Yield of Carbon Nanotubes in CVD Method

Carbon nanotubes and nanofibers were synthesized at 400-800°C by a typical CVD method using SiO2, Al2O3, or MgO supported Ni or Co catalyst, and acetylene as feedstock. The effects of temperature, substrates and catalyst on the yield and structure of carbon nanotubes and nanofibers were investigated in detail. The experimental results showed that Ni based catalysts tended to form carbon nanofibers with “herringbone” structure, though for Ni/Al2O3 catalyst, the products at 800°C changed to a structure like carbon nanotube. Co based catalysts tended to form carbon nanotubes.

Synthesis of carbon nanotubes, nanofibers and encapsulated nanoparticles by decomposition of acetylene on Ge modified Pd catalysts

Different carbon nanostructures were synthesized by decomposition of acetylene over alumina supported palladium-germanium catalysts. The addition of small amounts of Ge to the Pd(1%)/γ-Al 2 O 3 catalyst increases the yield of carbon nanotubes among other carbon products, in the decomposition of acetylene. The use of a higher Ge concentration (0.3%) in the catalyst induces the formation of carbon nanofibers. For larger Ge concentrations (0.5%) the bimetallic catalyst particles become encapsulated in a graphitic structure.

Soot Activation Energy for a Xylene-Fueled CVD Reactor

The past attempts of Arrhenius plots of carbon nanotube (CNT) yield (or CNT length) to obtain the overall activation energy for CNT formation under changing reaction temperature conditions have created substantial errors. Here we propose an inverse method to accurately account for the effect of temperature on gas-phase reactions and species composition. The overall activation energy of soot formation for a xylene-based CVD reactor was calculated and successfully compared with the measured gas-phase concentration data. Our proposed inverse method is expected to help improve the performance of CVD reactors and optimize its design.

The effect of reduction temperature on Co-Mo/Al2O3 catalysts for carbon nanotubes formation

Abstract Catalyst reduction temperature was observed to affect the yield and diameter uniformity of carbon nanotubes (CNTs) produced on Co-Mo/Al2O3 catalysts. The results show that on the whole, the reduced catalysts gave higher carbon yield. The CNTs obtained from the unreduced catalyst and the catalysts reduced at 400, 550, and 700 °C had diameters of 9.0 ± 1.4, 11.0 ± 1.8, 11.2 ± 1.8, and 12.1 ± 2.7 nm, respectively, showing that an increase in catalyst reduction temperature produced a slightly larger average diameter and wider diameter distribution. Raman spectra further elucidate that better-graphitized CNTs were produced on the catalysts reduced with increasing reduction temperatures.

SWCNT growth on Al/Fe/Mo investigated by in situ mass spectroscopy

The effect of temperature on the growth of single-walledcarbon nanotubes (SWCNTs) was investigated over the range of725–900 °C. A cold-wall reactor consisting of a heated stage (on which the substratefor SWCNT growth (Al/Fe/Mo) was placed) and a showerhead (from whichC2H2 was introduced vertically into the reactor) was used for the growth. The heating was foundto play two roles: (1) it generated complex hydrocarbon radicals during the growth process,as well as (2) promoting catalytic nanoparticles on the substrate during the annealingprocess. The optimum temperature for the highest SWCNT yield was found to be∼860 °C. For the first time, in situ mass spectroscopy was used toidentify the growth precursors generated from thermal pyrolysis ofC2H2 within this temperature range. The peak of the radicals found(C6H9,C5H9 and C6H13) and the highest catalyst support particle density (Fe catalyst supported onAlxOy) wascorrelated to the maximum yield of single walled carbon nanotubes at the optimum growth temperatureof ∼860 °C. Bottom gate SWCNT-FETs (single-walled carbon nanotube based fieldeffect transistors) were fabricated showing a high transconductance of∼0.12 µS andon/off ratioof ∼105 which are both comparable to other state-of-the-art SWCNT-FET.

Preparation of Silica Nanotubes with Cobalt Complex Nanofibers as Templates

Co-silica nanotubes were prepared with nanofibers of [Co(NH3)6](HCO3)(CO3)·2H2O as templates. The yield and morphology of Co-silica nanotubes were influenced by the pH value of the ammonia solution, the concentration of the Co salt solution as well as the rate of ethanol addition. The optimized sample contains 100% nanotubes with 150 – 300 nm in diameter and up to 30 -m in length.

Novel catalytic route to bulk production of high purity carbon nanotube

Carbon nanotubes have been synthesized by catalytic chemical vapour deposition of acetylene diluted with argon using three different catalysts, namely, nickel formate, cobalt formate and ferrocene. The synthesis was carried out at 700°C in a quartz reactor for 30 minutes. Thermal analysis was carried out in order to determine the yield of the nanotube. It was found that the deposit contains 86% nanotube, with nickel-based catalyst, which was the maximum. The yield of nanotube was 71 times that of the nickel loading. The TEM images reveal helical type of nanotubes with iron catalyst while cobalt and nickel catalysts yielded straight nanotubes. This technique can be explored for the bulk production of carbon nanotube in an economic way.