Metal-free synthesis of carbon nanotubes filled with calcium silicate

A new metal-free catalyst CaSiO₃ was developed to grow carbon nanotubes (CNTs) on a pyrolytic graphite paper tape by an ethanol catalytic chemical vapor deposition method at 1200–1400 °C. The prepared CNTs with a droplet tip had a multi-walled structure and were filled with amorphous CaSiO₃. Temperature, determining the melting of CaSiO₃, was critical for the growth of the CNTs. This new catalyst is suggested to follow the similar roles of transition metals in the growth of CNTs by a molten state to absorb carbon and form CNTs after reaching saturation.     

Synthesis of bamboo-like carbon nanotubes on a copper foil by catalytic chemical vapor deposition from ethanol

Bamboo-like carbon nanotubes (CNTs) were synthesized on a copper foil by catalytic chemical vapor deposition (CVD) from ethanol. The effects of temperature (700–1000 °C) and duration (5–60 min) on the growth of CNTs were investigated. Morphology and structure of the CNTs were characterized by scanning and transmission electron microscopy and Raman spectroscopy. The yield and size of the CNTs increased with temperature. Those prepared at 700 °C had a copper droplet tip and those at 800–900 °C had a copper nanoparticle inside. An amorphous carbon film consisted of a porous and non-porous layer was observed on the surface of the copper substrate, and the CNTs were really grown from this carbon film. The thickness of the carbon film increased from 187 to 900 nm when the duration increased from 5 to 60 min. It was also found that the copper foil became porous after ethanol CVD treatment. The growth mechanism of the CNTs, carbon film and motion of copper catalyst were discussed. It is proposed that a carbon film first deposited on the top surface of the copper foil while the top surface of the copper foil partially melted and migrated across the carbon film, where CNTs formed.     

Low-Temperature Growth of Carbon Nanotubes on Bi- and Tri-metallic Catalyst Templates

Low temperature growth process of carbon nanotubes (CNTs) over bi-metallic (Co–Fe) and tri-metallic (Ni–Co–Fe) catalysts on Si/Al/Al₂O₃ substrates is carried out from acetylene precursor using hydrogen, ammonia or nitrogen as a carrier in a low pressure chemical vapor deposition system. Using the tri-metallic Ni–Co–Fe catalyst template, vertically aligned CNTs of ~700 nm length could be grown already at 450 °C within 10 min using ammonia as a carrier. Within the same period of time, on bi-metallic Co–Fe catalyst templates, ~250 nm long aligned nanotubes emerged already at 400 °C in nitrogen carrier. At low temperatures most of the catalyst materials were elevated from the support by the grown nanotubes indicating tip growth mechanism. The structure of catalyst layers and nanotube films was studied using scanning and transmission electron microscopy and atomic force microscopy.     

Preparation of a HDPE/Carbon Nanotube Composite

This experiment adopted mixed acid (H₂SO₄:HNO₃ = 3:1) to purify multi-walled carbon nanotubes, and used a silane coupling agent n-octyltriethoxysilane (OTES) to modify carbon nanotubes, respectively. Then we mixed OTES-modified carbon nanotubes (CNTs) with high-density polyethylene (HDPE) resin to make a composite. TGA analysis results revealed that as the CNTs'content increased, the Td₁₀ tended to rise. The amount of composite residual at 500°C also increased, as well as the composite electrical conductivity. When a concentration of 5% OTES was used to modify the CNT, the resultant composite exhibited better electrical conductivity.     

Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition

We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH₄–H₂–Ar precursor gas mixture with 1% CH₄, at the chamber pressure and the substrate temperature of 7.5 Torr and 840 °C, respectively. In the H₂-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

CVD growth and field electron emission of aligned carbon nanotubes on oxidized Inconel plates without addition of catalyst

Direct growth of carbon nanotubes (CNTs) on Inconel 600 sheets was investigated using plasma enhanced hot filament chemical vapor deposition in a gas mixture of methane and hydrogen. The Inconel 600 sheets were oxidized at different temperatures (800 °C, 900 °C, 1000 °C, and 1100 °C) before CNT deposition. The structure and surface morphology of the pre-treated substrate sheets and the deposited CNTs were studied by scanning electron microscopy (SEM) and X-ray diffraction. The field electron emission (FEE) properties of the CNTs were also tested. The SEM results show that well aligned CNTs have been grown on the pre-treated Inconel sheets without addition of any catalysts and the higher treatment temperature resulted in CNTs with better uniformity, indicating that the oxidation pre-treatment of the substrate is effective to enhance the CNT growth. FEE testing shows that CNTs with better height uniformity exhibit better FEE characteristics.     

The use of aliphatic alcohol chain length to control the nitrogen type and content in nitrogen doped carbon nanotubes

Nitrogen doped carbon nanotubes (N-CNTs) were synthesized using acetonitrile/alcohol mixtures as the nitrogen, carbon and oxygen sources using the chemical vapor deposition (CVD) method. XPS analysis of the CNTs produced from an acetonitrile/ethanol mixture using different CVD temperatures (700–1000 °C), revealed that nitrogen incorporation in N-CNTs decreased with an increase in CVD temperature and that the type of nitrogen species incorporated also varied. Molecular nitrogen and a low content of pyridinic nitrogen was obtained in N-CNTs grown at 700 and 800 °C, while quaternary nitrogen was noted in all N-CNTs grown. Use of 20% acetonitrile/ROH (R = CH₃, C₂H₅, C₄H₉, C₅H₁₁, C₇H₁₅ and C₈H₁₇) mixtures allowed the C/O ratio to be changed whilst the N content in the precursor mixture was kept constant. The N content in the N-CNTs grown at 850 °C increased with the alcohol chain length and also controlled the nitrogen species incorporated, an effect related to the oxygen content of the reactant mixtures.     

Designing of carbon nanofilaments-based composites for innovative applications

The catalytic growth of CNTs and/or CNFs polymer composites has been performed by means of the chemical vapour deposition of a C₂H₄/H₂ gas mixture on polymer supports (i.e. para-aramide powders and fibres) at 500–600 °C. By selecting suitable reaction parameters, the concentration and the growth level of the CNTs and/or CNFs on the polymer surface can be changed, according to the final properties of the obtained composite. Nanofilaments, 20÷120 nm in diameter formed at 500 °C on polymer supported Ni catalysts, give rise, at first, to a carbon-polymer composite. A more dense porous nanotissue, then generated at 600 °C, embeds the polymer substrate with formation of a carbon-carbon composite. In this work, the progressive formation of carbon-polymer and then carbon-carbon composites is investigated by SEM, TEM and AFM microscopies and by XRD analysis.     

Aligned sulfur-coated carbon nanotubes with a polyethylene glycol barrier at one end for use as a high efficiency sulfur cathode

High efficient sulfur cathode materials were constructed by the incorporation of aligned sulfur-coated carbon nanotubes (CNTs) and a polyethylene glycol (PEG) barrier at one end. During the charge and discharge of lithium sulfur batteries, high Li ion storage performance can be achieved on the composite electrode, which was benefited from both the aligned CNT structure and the polymer barrier. Aligned CNT framework afforded high conductivity for electron transportation and ordered pores for lithium ion transportation. Meanwhile, the PEG barrier layer greatly suppressed the shuttle of polysulfides. Therefore, this aligned sulfur-coated CNTs with a PEG barrier showed a high initial discharge capacity of 920 and 1128 mAh g⁻¹ in lithium bis(trifluoromethanesulfonyl)imide/1,3-dioxolane/1,2-dimethoxyethane and electrolyte with LiNO₃ additives, respectively. The PEG coated cathode showed high cycle stability that a low degradation with 0.38% per cycle during the 100 cycles at 0.1 C was achieved in LiNO₃-free electrolytes. These Li storage performance was superior to the aligned sulfur-coated CNT electrode without PEG barrier.     

Effect of Fe catalyst thickness and C2H2/H2 flow rate ratio on the vertical alignment of carbon nanotubes grown by chemical vapour deposition

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst thickness and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs by thermal CVD. In order to investigate these aspects, four Fe metal films with thickness of 2.5, 3.5, 7.5 and 16 nm were deposited on Si/SiO₂ substrate and CNTs were grown with different C₂H₂/H₂ flow rate ratios, from 5/95 to 30/70 by thermal CVD at 750 °C. Results showed that CNTs were not vertically aligned with 16 nm catalyst thickness for all flow rate ratios, while CNTs were always vertically aligned for iron thickness less than 3.5 nm and vertically aligned only for a C₂H₂/H₂ flow rate ratio greater than 20/70 for the 7.5 nm catalyst thickness. Morphology and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Our results also indicate that for each flow rate ratio exists a critical thickness of iron catalyst under which vertically aligned CNTs are obtained.     

A layer-by-layer deposition mechanism for producing a pyrolytic carbon coating on carbon nanotubes

Pyrolytic carbon (PyC) was deposited on carbon nanotubes (CNTs) in order to modify them by introducing defects to their surface. The deposition of PyC was carried out at temperature between 800 and 1000 °C using propane as carbon source with or without a hydrogen carrier gas at low pressure of 20 kPa. The structure of PyC coatings was examined using transmission electron microscopy. The PyC coating could be distinguished from the original CNT walls due to the difference of the structure, with the coating showing a less orderly layer structure. When H₂ was introduced during deposition, PyC coating started to form at 900 °C, and the deposition rate increased rapidly with increasing temperature. Without H₂, PyC coating with a thickness of a few layers could be formed at temperatures between 800 and 900 °C in 10 min. The outmost layer of the PyC coating showed a structure of rough and curved carbon fragment. A layer-by-layer mechanism is proposed for the deposition consisting of alternating fragment formation (nucleation) and lateral growth to layer.     

Synthesize carbon nanotubes by a novel method using chemical vapor deposition-fluidized bed reactor from solid-stated polymers

Carbon nanotubes (CNTs) were synthesized from solid-stated polymers-polycarbosilane (PCS) and polyethylene (PE), by using a chemical vapor deposition-fluidized bed reactor (CVD-FBR). MgO and Fe(NO₃)₃ acted as the catalysts. The experimental results indicate that diameters of CNTs, using PCS as carbon source, are from 15 to 90nm and lengths are several μm when reaction temperatures are at 850–950 °C. Using PE as carbon source, the diameter of CNTs is from 25 to 90 nm and the length can reach 1 μm since reaction temperatures are at 750–850 °C. Comparing these two different carbon sources, the CNTs synthesized from PE have better quality and a higher degree of graphitization.     

Influences of NH3 pre-treatment and radicals on the growth of carbon nanotubes

A triple-layered catalyst (Al/Fe/Mo) undergoes considerable restructuring of surface morphology during NH₃ annealing prior to carbon nanotube (CNT) growth. The diameter (or density) of Al_xO_y–Fe clusters formed during the annealing is found to be dependent on the concentration ratio of NH₃ to H₂O present inside the chamber, which is confirmed by in-situ mass spectroscopy. The different diameter clusters then affect the types of CNTs (i.e. single or multi-walled CNTs) during the growth. Here, a growth model is also presented, where hydrocarbon radicals (C₅H₉, C₆H₉, and C₆H₁₃) generated from C₂H₂ pyrolysis (~ 800 °C) can be used as effective precursors to synthesize CNTs.     

Purity-controllable growth of bamboo-like multi-walled carbon nanotubes over copper-based catalysts

The growth of bamboo-like multi-walled carbon nanotubes (CNTs) without the formation of amorphous carbons was performed using copper-based catalysts by catalytic chemical vapour deposition (CVD) with diluted ethylene at 700–900 °C. The as-grown CNT soot was characterised by transmission electron microscopy, thermogravimetric analysis and Raman spectroscopy. The weak metal–support interaction of a sulphate-assisted copper catalyst (CuSO₄/SiO₂) can provide high-purity growth with remarkable yields of CNTs (2.24–6.10 CNT/g Cu·h) at 850–900 °C. Additionally, hydrogen-assisted CVD can activate inert copper catalysts, e.g., Cu(NO₃)₂/SiO₂ or Cu(CH₃COO)_2/SiO₂, for the growth of CNTs.     

Excellent Field Emission Properties of Short Conical Carbon Nanotubes Prepared by Microwave Plasma Enhanced CVD Process

Randomly oriented short and low density conical carbon nanotubes (CNTs) were prepared on Si substrates by tubular microwave plasma enhanced chemical vapor deposition process at relatively low temperature (350–550 °C) by judiciously controlling the microwave power and growth time in C₂H₂ + NH₃ gas composition and Fe catalyst. Both length as well as density of the CNTs increased with increasing microwave power. CNTs consisted of regular conical compartments stacked in such a way that their outer diameter remained constant. Majority of the nanotubes had a sharp conical tip (5–20 nm) while its other side was either open or had a cone/pear-shaped catalyst particle. The CNTs were highly crystalline and had many open edges on the outer surface, particularly near the joints of the two compartments. These films showed excellent field emission characteristics. The best emission was observed for a medium density film with the lowest turn-on and threshold fields of 1.0 and 2.10 V/μm, respectively. It is suggested that not only CNT tip but open edges on the body also act as active emission sites in the randomly oriented geometry of such periodic structures.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Growth of carbon nanotubes on carbon fibers using the combustion flame oxy-acetylene method

Multi-walled carbon nanotubes (MWCNTs) were directly grown on carbon fibers (CFs) using the combustion flame oxy-acetylene method. Ferrocene deposited on the fiber surface acts as a catalyst for carbon nanotubes (CNTs) growth. The effects of ferrocene concentration on the morphology of the CNTs coating were investigated. Growth temperature ranges from 500 to 650 °C at atmospheric pressure, while growth surface is a continuous 10 × 1000 mm² tape. CNTs are produced with a dense entanglement, covering the CFs uniformly. Tube outer diameters are in the range of 20–40 nm. Tube length is quite long (about 4–5 μm) and uniform. Particularly, growth times are very short: about 0.3–0.6 s. Growth morphology and other characteristics of the as-grown tubes were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy dispersive X-ray (EDX) and by Raman spectroscopy.     

Hot filament enhanced CVD synthesis of carbon nanotubes by using a carbon filament

The low temperature synthesis of single-walled carbon nanotubes (CNTs) on a Si substrate has been reported. Single-walled CNTs were grown from a C₂H₂ and H₂ mixture by a hot-filament enhanced CVD method using a carbon filament. The catalyst was silica-supported iron–cobalt prepared by sol–gel method. We observed the influence of the catalyst material, C₂H₂ concentration, growth pressure and substrate temperature on the formation of the CNTs. By optimizing the catalyst, both single-walled CNTs and multiwalled CNTs could be synthesized, depending on the reaction conditions. The formation of single-walled CNTs occurred when the carbon supply was kept low, i.e. low C₂H₂ concentration and low reaction pressure. The diameter of tubes decreased with increasing the substrate temperature. By optimizing the growth conditions, a small diameter of 0.65 nm single-walled CNTs, estimated from Raman scattering spectrum, was achieved even the low substrate temperature as 660 °C.     

Growth mechanisms of carbon nanofilaments on Ni-loaded diamond catalyst

Nickel-loaded oxidized powdered diamond (Ni/O-Dia) was used for the synthesis of carbon nanofibers (CNFs).CNFs grown at 400 °C, from both CH₄ and C₂H₆ over Ni/O-Dia, had herring-bone type graphene sheets and those grown at 600 °C had tube type graphene sheets. The amount of CNFs was greatly affected by the growth temperature in both CH₄ and C₂H₆. In all the cases, Ni particle was found on the tip end of the grown CNFs.No carbon formation was observed at 650 °C or higher temperature from both CH₄ and C₂H₆, because Ni particles on O-Dia were covered with graphene sheets.Termination of the growth of CNFs was ascribed to the rapid decomposition of hydrocarbons on active Ni surface as compared to the dissolution and diffusion of the carbon on Ni surface into bulk Ni particle to give CNFs on the opposite side of active Ni surface.