CNT Growth Research Articles

Chemical Vapor Deposition of Carbon Nanotubes on Monolayer Graphene Substrates: Reduced Etching via Suppressed Catalytic Hydrogenation Using C2H4

In most envisioned applications, the full utilization of a graphene-carbon nanotube (CNT) construct requires maintaining the integrity of the graphene layer during the CNT growth step. In this work, we exhibit an approach toward controlled CNT growth atop graphene substrates where the reaction equilibrium between the source hydrocarbon decomposition and carbon saturation into and precipitation from the catalyst nanoparticles shifts toward CNT growth rather than graphene consumption. By utilizing C2H4 feedstock, we demonstrate that the low-temperature growth permissible with this gas suppresses undesirable catalytic hydrogenation and dramatically reduces the etching of the graphene layer to exhibit graphene-CNT hybrids with continuous, undamaged structures.

Synthesis and Characterization of Carbon Nanotubes Catalyzed by TiO2 Supported Ni, Co and Ni-Co Nanoparticles via CCVD

Monometallic and bimetallic Ni and Co catalytic nanoparticles supported on Titanium dioxide (rutile phase) substrate were prepared by wet impregnation method. These nanoparicles were used as catalysts for synthesis of multiwalled carbon nanotubes (MWCNTs) from acetylene decomposition at 700°C by the catalytic chemical vapor deposition (CCVD) technique. The nanomaterials (catalyst and CNTs) were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman spectroscopy. In this paper, the usage of TiO2 powder as catalyst support was examined and the effect of applied catalyst type on characteristics of grown CNTs was investigated. The results showed that the rutile phase of TiO2 powder can be applied as a suitable catalyst support in CNT growth process. Furthermore, it was observed that the CNTs synthesized on Ni-Co bimetallic catalyst possess smaller average diameters, better quality and less amorphous carbon compared to Ni and Co monometallic catalyst types.

In Situ Observation of the Effect of Nitrogen on Carbon Nanotube Synthesis

The article examines the in situ observation of the effect of nitrogen on carbon nanotube synthesis. Environmental TEM (ETEM) is a relatively new technique ideal for in situ measurements. Researchers apply ETEM to study the effect of nitrogen on CNT growth, elucidating the mechanisms by which nitrogen acts, to guide efforts to optimize synthesis. To the best of our knowledge, this is the first in situ TEM study of the effect of nitrogen on CNT growth. Therefore, in the absence of ammonia, carbon should precipitate first, resulting in an interface with pure iron. Pure iron has a higher surface tension with the nanotube, which explains the separation of the CNT and catalyst immediately following carbon precipitation. A carbide particle will have a lower surface tension with the nanotube due to its higher carbon chemical potential and thus maintain its contact. The pure iron catalyst may become carbide following cap separation, but this will not affect CNT nucleation.

Direct growth of MWCNTs on 316 stainless steel by chemical vapor deposition: Effect of surface nano-features on CNT growth and structure

Multi-walled carbon nanotubes were directly grown by chemical vapor deposition on as-received or pretreated 316 SS without application of an external catalyst. A detailed study of the size distribution of surface features formed by different steps of the synthesis process showed that the heating cycle and any complementary pretreatment may produce significant changes of the surface topography, thus suggesting that the influence of any primary characteristics of the original surface, as well as those caused by a pretreatment, should be assessed in conjunction with the effects of heating. Average lateral size of nano-features less than 60nm (after heating) were shown to favor mainly the carbon nanotube growth while a larger features size was associated predominantly to the carbon nanofiber synthesis. Scanning and transmission electron microscopy observations suggest two different mechanisms for nanotube/nanofiber growth: (1) base growth mode caused by nanosized hills on the surface catalyzing the nanotube/nanofiber synthesis, (2) tip growth mode requiring substrate surface break-up as a preliminary step to form catalytic particles, with similarities to the “metal dusting” mechanisms. While untreated steel showed the best results concerning carbon nanotube coverage and homogeneity, oxidized-reduced samples showed an almost exclusive growth of carbon nanofibers with a full coverage.

The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes

Aerosol-assisted catalytic chemical vapor deposition (CCVD) is an efficient single-step process to synthesize vertically aligned carbon nanotube (VACNT) arrays from ferrocene and toluene precursors. While no promoters are needed, adding hydrogen to the carrier gas assists the process. In order to understand the role of hydrogen, we performed syntheses at 850 and 800°C. The product distribution along the length of the reactor was analyzed by electron microscopy to follow the effect of the hydrogen, as well as the characteristics of the CNT and catalyst nanoparticles. Increasing the hydrogen content shifts the CNT growth towards the furnace entrance and strongly reduces the CNT diameters, while the CNT number density increases. Hydrogen demonstrably affects the gas phase phenomena by lowering the decomposition temperature of ferrocene and increasing the nucleation site density of the catalytic nanoparticles. The yield under hydrogen remains reasonable in the aerosol-assisted CCVD process, and is even improved at 800°C. Hydrogen content appears to be an interesting parameter to control the characteristics of vertically aligned CNT.

Effect of Boric Acid on the Nucleation and Growth of Ni Nanoparticles for CNT Growth

The effect of boric acid on the electrochemical deposition of nickel nanoparticles is presented in this paper. We show that lowering the concentration of H3BO3 in the electrolyte increases the particle density. At the same time, it was observed that the deposition of nickel stops at an earlier stage when a low concentration of boric acid was used. Both observations can be explained by the formation of an insoluble Ni(OH)2 which was counteracted by the addition of H3BO3. We were able to reduce the current density needed to obtain similar particle densities by almost an order of magnitude.

3D-Graphitized and Iron Modified Carbon Aerogels for Sustainable Energy Applications

3D-carbon aerogel nanomatrix has been modified with iron nitrate in the amounts of 10 and 20 wt. % by wet incipient method. The crystal structure, morphology, and electrocatalytic activity of the formed nanocomposites have been studied in comparison to the unmodified carbon aerogels after their heat-treatment at 900oC, 1200oC and 1400oC in argon. The mass and specific surface area losses in these materials reduced with iron doping is clear from BET isotherms and BJH pore size analysis. The HRTEM analysis confirms the presence of uniformly distributed ~43.5nm iron nanoparticles surrounded by graphite layers. The XRD analysis show that the level of graphitization increases with the sintering temperature and the concentration of the iron-containing phase. The ORR electrocatalytic activity studies demonstrate the influence of the heat-treatment temperature of samples on the ORR activity. Temperature programed reduction is carried out to understand the reduction behavior of these catalysts to metallic iron, to use them as substrates for direct CNT growth.

Increased CNT growth density with an additional thin Ni layer on the Fe/Al catalyst film

Density of 3×1011/cm2 and diameter of CNTs of 9–12nm were successfully controlled by using the multi-layered catalyst film consisting of an additional Ni layer on Fe/Al catalyst film. EDS analysis for the annealed catalyst films revealed that the increase of the density of Fe catalyst particles corresponded with the decrease of Ni in the films, which strongly suggested that the additional thin Ni layer on the Fe/Al multi-layered catalyst films prevented the fine Fe catalyst particles from agglomeration, resulting in the growth of high-density, and uniform diameter of CNTs.

The use of XAFS to determine the nature of interaction of iron and molybdenum metal salts within PS-b-P2VP micelles

The poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) micelle route is a well established method for the preparation of bimetallic nanoparticles used for the catalysis of carbon nanotubes and other applications like ultrahigh density storage devices, yet to date no information is available concerning the internal structure of the P2VP-metal salt complex. For the first time, XAFS measurements were performed on micelles loaded with either iron(III) chloride or molybdenum(V) chloride and a combination of both. Analysis of the data revealed that iron is tetrahedrally coordinated within the core, whereas molybdenum is octahedrally coordinated in the pure loaded micelles and trigonally coordinated in the mixed micelles. For the bimetallic samples, analysis of the Fe and Mo K-edge data revealed the existence of an interaction between iron and molybdenum. This approach to obtain detailed structural information during the preparation of these catalyst samples will allow for a deeper understanding of the effects of structure on the function of catalysts used for CNT growth i.e. to explain differences in yield as well as potentially providing a deeper understanding of the CNT growth mechanism itself.

The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth

We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron–carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes.

Screening and optimisation of the single- and double-walled carbon nanotube parameter space using a high-throughput methodology

A high-throughput methodology comprising of an automated micro-chemical vapour deposition (CVD) reactor and statistical design of experiments were used for screening and the first pass optimisation of the single- (SWCNT) and double-walled carbon nanotube (DWCNT) parameter space using an alumina supported iron-molybdenum catalyst.The parameters of reaction temperature, metal loading, bi-metallic ratio, total gas flow, and gas flow ratio were initially investigated using an L18 experimental design to identify the most significant reaction parameters. Product characterisation metrics included carbon yield (determined from thermogravimetric analysis), IG/ID (ratio of G band to D band from Raman spectroscopy), and the presence of SWCNTs/DWCNTs from radial breathing modes (Raman spectroscopy) and transmission electron microscopy (TEM).A good model fit for Reaction Carbon Yield and IG/ID was obtained for the L18 experimental design (R2–Q2<0.2–0.3, model validity>0.25, reproducibility>0.5), and temperature and metal loading were found to be the most influential variables. Temperature and metal loading were consequently optimised using a D-Optimal experimental matrix to maximise both output metrics. The optimisation generated model predicted a metal loading of 4.9wt% and a synthesis temperature of 900°C to result in a carbon yield of 11.4wt% and an IG/ID of 8.0, in excellent agreement with experimental data of 11.8wt% and an IG/ID of 8.1–8.7.This study confirms that the efficient small-scale optimisation protocol is a valid alternative to resource intensive large-scale experiential screening and offers a route towards completely mapping the CNT growth space. However, results from the high-throughput CVD reactor are yet to be transferred with high accuracy to a larger scale fluidized bed apparatus (52mm ID), and further research is required to scale-up the process.

Effect of Boric Acid on the Nucleation and Growth of Ni Nanoparticles for CNT Growth

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Charge storage in carbon nanotube–TiO2 hybrid nanoparticles

Multi-walled carbon nanotubes (MWNTs) were directly deposited onto Ni modified TiO2 particles using chemical vapour deposition (CVD). These novel hybrid nanostrucured MWNT/TiO2 composites were characterised by scanning electron microscopy (SEM), revealing a novel hybrid structure wherein the CNT growth sprouted from the TiO2 particles and thermogravimetric analysis (TGA) that revealed a carbon content of 8%. Electrochemical methods were used to determine capacitance values as high as 146Fg−1 using acetonitrile containing 0.1M tetrabutylammonium perchlorate as electrolyte. An energy density and power density of 21Whkg−1 and 10kWkg−1 respectively were obtained.

Cold-gas chemical vapor deposition to identify the key precursor for rapidly growing vertically-aligned single-wall and few-wall carbon nanotubes from pyrolyzed ethanol

Vertically-aligned carbon nanotubes (VA-CNTs) were rapidly grown from ethanol and their chemistry has been studied using a “cold-gas” chemical vapor deposition (CVD) method. Ethanol vapor was preheated in a furnace, cooled down and then flowed over cobalt catalysts upon ribbon-shaped substrates at 800°C, while keeping the gas unheated. CNTs were obtained from ethanol on a sub-micrometer scale without preheating, but on a millimeter scale with preheating at 1000°C. Acetylene was predicted to be the direct precursor by gas chromatography and gas-phase kinetic simulation, and actually led to millimeter-tall VA-CNTs without preheating when fed with hydrogen and water. There was, however a difference in CNT structure, i.e. mainly few-wall tubes from pyrolyzed ethanol and mainly single-wall tubes for unheated acetylene, and the by-products from ethanol pyrolysis possibly caused this difference. The “cold-gas” CVD, in which the gas-phase and catalytic reactions are separately controlled, allowed us to further understand CNT growth.

Tailoring structural and electrochemical properties of vertical aligned carbon nanotubes on metal foil using scalable wet-chemical catalyst deposition

A scalable process for the synthesis of vertical aligned carbon nanotubes (VA-CNT) with precise control over structural properties is described. Direct growth on metal foils is achieved using a dip-coating step for the wet-chemical catalyst layer deposition and a subsequent chemical vapor deposition step for CNT growth. Two optimized Fe/Co and Fe/Mo 2-ethylhexanoate/2-propanol solutions are applied as precursors for the catalyst layer deposition and the influence of catalyst film thickness on resulting CNT film properties is investigated. The catalyst layer thickness can be controlled by the precursor salt concentration in the dip coating solution. By using a Fe and Co (2:3 ratio) catalyst, the CNT film homogeneity dramatically drops with lowered catalyst concentration due to a strong Ostwald ripening behavior of the catalyst layer. By using the FeMo (47:3 ratio) catalyst system, homogeneous VA-CNT films are obtained and the average CNT diameter can be adjusted in a range of 5–20nm by the catalyst layer thickness. The electrochemical properties of the CNT electrodes are investigated in a symmetric supercapacitor test cell. The lowest CNT diameters of 5nm results in a specific double layer capacitance up to 60Fg−1, while the density of the films directly correlates with its pore resistance.

Carbon nanotube growth from metallic nanoparticles deposited by pulsed-laser deposition on different substrates

Carbon nanotubes carpets were grown by RF plasma enhanced chemical vapor deposition on various substrates coated by Fe and Ni transition metals that act as catalyst. C2H2 gas was used for the carbon source. The results show that carbon nanotubes CNT can be grown on Si3N4/Si and SiO2/Si substrates only with an Fe catalyst. They are typically formed by multi-walled graphene layers, and can be obtained for a temperature as low as 550°C. Nanotubes grown on TiN/SiO2/Si substrate from Fe or Ni catalysts present bamboo-like nanostructures and are obtained for particular experimental conditions. This study demonstrates substrate-to-catalyst effect on the CNT growth and their microstructures indicating that the adhesion force of nanoparticles on substrates is a main parameter. Catalyst particles are spherical and several tens of nm in diameter (weak adhesion strength) when deposited onto SiO2/Si or Si3N4/Si, the tip growth mode of nanotube is favored. On TiN/SiO2/Si substrate, particles are larger (large adhesion strength) and CNT growth is no more in tip mode, bamboo-like structures are obtained. When an Fe-Ni catalyst multilayer has been deposited onto the different substrates, carbon nanotube microstructures show multi-walled graphene parallel layers on Si3N4/Si and SiO2/Si insulating substrates, and bamboo-like microstructures on TiN/SiO2/Si conductor substrate.

Role of Gas-phase Reactions and Thermal Gradient Control in Carbon Nanotube Synthesis

ABSTRACTWe investigate the role of precursor thermal rearrangement and surface catalytic reactions in the synthesis of vertically aligned carbon nanotubes (VA-CNTs) by acetylene-based, chemical vapor deposition (CVD) and demonstrate a millimeter-long growth of single-walled CNT (SWNT) without water assistance. A substrate heater was used to create an ascending temperature gradient from gas injection to catalyst substrate. Whereas temperature of catalyst substrates primarily determines their catalytic activity, it is a thermal condition of a gaseous mixture in the CVD chamber that also influence growth yield and structural features of as-grown CNTs. Employing Egloff’s characterization, [1] we discuss the importance of various gas thermal zones in producing high-quality nanotubes with augmented growth efficiency. We continue to report production of millimeter-long, VA-SWNT having a mean diameter of 1.7 ± 0.7 nm, catalyzed by iron on an alumina support. Important finding is that a million of aspect ratio of SWNT arrays can be produced, without water assistance, via combined action of an ascending temperature gradient toward catalyst substrate and low partial pressures of acetylene carbon feedstock. Our results do not only emphasize the role of precursor thermal rearrangement in CNT synthesis, but also offer a practical route to the modulation of such complex phenomena for an ultrahigh-yield growth of narrow VA-SWNT.

Integration of Bulk Nanoporous Elements in Microfluidic Devices With Application to Biomedical Diagnostics

We demonstrate integration of ultraporous (99% porous) elements composed of nanoporous forests of vertically aligned carbon nanotubes (VA-CNTs, or VACNTs) in microelectromechanical systems (MEMS), illustrating their use in fluidic applications for biomedical diagnostics. Our method enables definition of high-aspect-ratio (~104, heights ~100 μm, and pore spacings ~10 nm) nanoporous elements and preserves the VACNT elements' shapes even under flowthrough conditions. The fluid permeability of the VACNT elements is measured experimentally and shown to be comparable to that of much larger micro- and macroscopic porous materials. Permeability tailoring of VACNT elements through manipulation of both material and CNT growth process parameters is also experimentally demonstrated. Distinct from solid designs (e.g., Si and polydimethylsiloxane), our approach enables fluid flow both around and through the nanoporous microfluidic elements, hence enhancing isolation efficiency by increased physical interaction between the particles in the flow and the functional elements. Simultaneous multiphysics (chemical and mechanical) and multiscale (several orders of magnitude in size) isolation of bioparticles using microfluidic devices with nanoporous elements is demonstrated as well.

Synthesis and characterization of iron-filled multi-walled nanotubes

The growth of iron filled multiwalled carbon nanotubes (Fe-MWCNT) using chemical vapour deposition (CVD) has been widely studied. Considering the remarkable magnetic and structural properties of Fe-MWCNT, these materials have been applied in numerous areas. In particular their biomedical application has been explored, where Fe-MWCNT can be used in hyperthermia, acting as a local nano-heater at cellular level. Regarding this aim, the reproducible and highly purified ferromagnetically filled samples of carbon nanotubes are still required. There are several parameters during the synthesis process that influence the properties of the nanotubes. The most favourable temperature of the CNT growth is probably one of the most important issues and its optimisation is crucial. In the current study, the Fe-MWCNT were grown at different temperatures ranging from 650 °C to 1050 °C. Additionally, a comparison between two different CVD systems and two carbon sources are also here presented. The Fe-MWCNT were characterised using diverse techniques regarding the evaluation of their morphology, filling ratio, and purity. Observations showed a strong influence of the growth temperature on the morphology and properties of the Fe-MWCNT. The samples characterisation was performed using Raman spectroscopy, thermogravimetric analysis (TGA), X ray diffraction (XRD), and transmission electron microscopy analysis (TEM).

Composite films prepared by immersion deposition of manganese oxide in carbon nanotubes grown on graphite for supercapacitors

Manganese oxide/carbon nanotube (CNT) composite films on graphite were prepared by growing CNTs on the substrate using chemical vapor deposition (CVD), followed by immersion in an aqueous solution of potassium permanganate. The CVD growth created favorable conditions for deposition of the oxide on the electrode, and an aligned porous structure of the composite films, which originated from the CNT growth, could be managed. Electrochemical behaviors of the CNT and the composite films for supercapacitors were studied in 1 M Na2SO4 solution. While the oxide deposition in the CNT films was identified as contributing to capacitance enhancement, it was also found that a mild heat treatment could improve performance of the composite films.