How Carbon Nanotubes Are Made and How Carbon Nanotubes are Used

The term Nanotechnology was originally used to define any work done on the molecular scale, or one billionth of a meter. This term is now used broadly (& loosely) for anything that is really small (hence the nano Ipod). Carbon Nanotubes (CNTs) come in single walled (SWNTs), double walled (DWNTs), and multi walled (MWNTs) varieties. CNTs can best be described as a graphene sheet rolled into a one dimensional structure with axial symmetry. CNTs are one of the primary building blocks which will be critical to the Nanotechnology Revolution. CNTs have many unique and interesting properties, please visit our our applications page to find out more about CNTs.

How are CNTs made?

"There are a number of methods of making CNTs and fullerenes. Fullerenes were first observed after vaporizing graphite with a short-pulse, high-power laser, however this was not a practical method for making large quantities. CNTs have probably been around for a lot longer than was first realized, and may have been made during various carbon combustion and vapor deposition processes, but electron microscopy at that time was not advanced enough to distinguish them from other types of tubes.

The first method for producing CNTs and fullerenes in reasonable quantities – was by applying an electric current across two carbonaceous electrodes in an inert gas (helium or argon) atmosphere. This method is called plasma arcing. It involves the evaporation of one electrode (the anode) as cations, followed by deposition at the other electrode. This plasma-based process is analogous to the more familiar electroplating process in a liquid medium. Fullerenes and CNTs are formed by plasma arcing of carbonaceous materials, particularly graphite. The fullerenes appear in the soot that is formed, while the CNTs are deposited on the opposing electrode, the cathode. Another method of nanotube synthesis involves plasma arcing in the presence of cobalt with a 3% or greater concentration. As noted above, the nanotube product is a compact cathode deposit of rod-like morphology. However when cobalt is added as a catalyst, the nature of the product changes to a web, with strands of 1mm or so thickness that stretch from the cathode to the walls of the reaction vessel. The mechanism by which cobalt changes this process is unclear, however one possibility is that such metals affect the local electric fields and hence the formation of the five-membered rings."

Arc Method

"The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce CNTs, as it is rather simple. However, it is a technique that produces a complex mixture of components, and requires further purification - to separate the CNTs from the soot and the residual catalytic metals present in the crude product.

This method creates CNTs through arc-vaporization of two carbon rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar). Recent investigations have shown that it is also possible to create CNTs with the arc method in liquid nitrogen. A direct current of 50 to 100 A, driven by a potential difference of approximately 20 V, creates a high temperature discharge between the two electrodes. The discharge vaporizes the surface of one of the carbon electrodes, and forms a small rod-shaped deposit on the other electrode. Producing CNTs in high yield depends on the uniformity of the plasma arc, and the temperature of the deposit forming on the carbon electrode."

Laser Methods

"In 1996, a dual-pulsed laser vaporization technique was developed, which produced SWNTs in gram quantities and yields of >70wt% purity. Samples were prepared by laser vaporization of graphite rods with a 50:50 catalyst mixture of Co and Ni (particle size ~1um) at 1200oC in flowing argon, followed by heat treatment in a vacuum at 1000°C to remove the C60 and other fullerenes. The initial laser vaporization pulse was followed by a second pulse, to vaporize the target more uniformly. The use of two successive laser pulses minimizes the amount of carbon deposited as soot. The second laser pulse breaks up the larger particles ablated by the first one, and feeds them into the growing nanotube structure. The material produced by this method appears as a mat of “ropes”, 10-20nm in diameter and up to 100um or more in length. Each rope is found to consist primarily of a bundle of SWNTs, aligned along a common axis. By varying the growth temperature, the catalyst composition, and other process parameters, the average nanotube diameter and size distribution can be varied.

Arc-discharge and laser vaporization are currently the principal methods for obtaining small quantities of high-quality CNTs. However, both methods suffer from drawbacks. The first is that both methods involve evaporating the carbon source, so it has been unclear how to scale up production to the industrial level using these approaches. The second issue relates to the fact that vaporization methods grow CNTs in highly tangled forms, mixed with unwanted forms of carbon and/or metal species. The CNTs thus produced are difficult to purify, manipulate, and assemble for building nanotube-device architectures for practical applications."

Chemical Vapor Deposition

"Chemical vapor deposition (CVD) of hydrocarbons over a metal catalyst is a classical method that has been used to produce various carbon materials such as carbon fibers, filaments, etc. for over twenty years. Large amounts of CNTs can be formed by catalytic CVD of acetylene over Co and Fe catalysts supported on silica or zeolite. The carbon deposition activity seems to relate to the cobalt content of the catalyst, whereas the CNTs’ selectivity seems to be a function of the pH in catalyst preparation. Fullerenes and bundles of SWNTs were also found among the MWNTs produced on the carbon/zeolite catalyst.

Some researchers are experimenting with the formation of CNTs from ethylene. Supported catalysts (Fe, Co, Ni), containing either a single metal or a mixture of metals, seem to induce the growth of isolated SWNTs or SWNT bundles in the ethylene atmosphere. The production of SWNTs, as well as double-walled CNTs (DWNTs), on Mo and Mo-Fe alloy catalysts has also been demonstrated. CVD of carbon within the pores of a thin alumina template (called a membrane) with or without a Ni catalyst has been achieved. Ethylene was used with reaction temperatures of 545°C for Ni-catalyzed CVD, and 900°C for an uncatalyzed process. The resultant carbon nanostructures have open ends, with no caps.

Methane has also been used as a carbon source. In particular it has been used to obtain ‘nanotube chips’ containing isolated SWNTs at controlled locations. High yields of SWNTs have been obtained by catalytic decomposition of an H2/CH4 mixture over well-dispersed metal particles (Co, Ni, Fe) on MgO at 1000°C. It has been reported that the synthesis of composite powders containing well-dispersed CNTs can be achieved by selective reduction in an H2/CH4 atmosphere of oxide solid solutions (between a non-reducible oxide such as Al2O3 or MgAl2O4 and one or more transition metal oxides). The reduction produces very small transition metal particles at a temperature of usually >800°C. The decomposition of CH4 over the freshly formed nanoparticles prevents their further growth, and thus results in a very high proportion of SWNTs and fewer MWNTs."

Ball Milling

"Ball milling and subsequent annealing is a simple method for the production of CNTs. Although it is well-established that mechanical attrition of this type can lead to fully nanoporous microstructures, it was not until a few years ago that CNTs of carbon and boron nitride were produced from these powders by thermal annealing. Essentially the method consists of placing graphite powder (99.8% purity) into a stainless steel container along with four hardened steel balls. The container is purged, and argon is introduced. The milling is carried out at room temperature for up to 150 hours. Following milling, the powder is annealed under a nitrogen (or argon) gas flow at temperatures of 1400°C for six hours. The mechanism of this process is not known, but it is thought that the ball milling process forms nanotube nuclei, and the annealing process activates nanotube growth. Research has shown that this method produces more MWNTs and few SWNTs."

Other Methods

"CNTs can also be produced by diffusion flame synthesis, electrolysis, use of solar energy, heat treatment of a polymer, and low-temperature solid pyrolysis. In flame synthesis, combustion of a portion of the hydrocarbon gas provides the elevated temperature required, with the remaining fuel conveniently serving as the required hydrocarbon reagent. Hence the flame constitutes an efficient source of both energy and hydrocarbon raw material. Combustion synthesis has been shown to be scalable for high-volume commercial production."

How are CNTs Purified?

"Purification of CNTs generally refers to the separation of CNTs from other entities, such as carbon nanoparticles, amorphous carbon, residual catalyst, and other unwanted species. The classical chemical techniques for purification (such as filtering, chromatography, and centrifugation) have been tried, but they have not been found to be effective in removing the undesirable impurities. Three basic methods have been used with varying degrees of success, namely gas-phase, liquid-phase, and intercalation methods."

A new gas-phase method has been developed at the NASA Glenn Research Center to purify gram-scale quantities of single-wall CNTs. This method, a modification of a gas-phase purification technique previously reported by Smalley and others, uses a combination of high-temperature oxidations and repeated extractions with nitric and hydrochloric acid. This improved procedure significantly reduces the amount of impurities (residual catalyst, as well as non-nanotube forms of carbon) within the CNTs, increasing their stability significantly.

The current liquid-phase purification procedure follows certain essential steps:

  • preliminary filtration to get rid of large graphite particles;
  • dissolution to remove fullerenes (in organic solvents) and catalyst particles (in concentrated acids);
  • centrifugal separation
  • microfiltration; and
  • chromatography to either separate MWNTs and unwanted nanoparticles or SWNTs and the amorphous carbon impurities.

"It is important to keep the CNTs well-separated in solution, so the CNTs are typically dispersed (using a surfactant) prior to the last stage of separation.

Generally, a centrifugal separation is necessary to concentrate the SWNTs in low-yield soot before the micro-filtration operation, since the nanoparticles easily contaminate membrane filters. The advantage of this method is that unwanted nanoparticles and amorphous carbon are removed simultaneously and CNTs are not chemically modified. However 2-3 M nitric acid is useful for chemically removing impurities.

It is now possible to cut CNTs into smaller segments, by extended sonication in concentrated acid mixtures. The resulting CNTs form a colloidal suspension in solvents. They can be deposited on substrates or further manipulated in solution, and can have many different functional groups attached to the ends and sides of the CNTs."

How do I Disperse the CNTs Once I have Received Them From Cheap Tubes?

Cheap Tubes Inc exclusively uses the SONICS VCX750 ultrasonic equipment. We routinely disperse CNTs using the following process. Cheap Tubes Inc's customers receive a 5% discount on the Sonics & Materials Line of ultrasonic systems when the orders are placed through Cheap Tubes Inc.

Dispersion of Carbon Nanotubes

The solution is composed of MWNTs, PVP, and water, in the proportions of 10 parts CNTs: ~1-2 parts PVP: 2,000 parts water or other solvents. The required dispersion (sonication) time is ~2 to 8 minutes with an interruption of 10 seconds every 30 seconds at full or high amplitude. If the power of your ultrasonic equipment is less than that of the SONICS VCX750 unit then you must prolong the sonication time accordingly. For Dispersing SWNTs, we recommend a constant sonication for 20 minutes at 40% amplitude. We have found that to effectively disperse SWNTs that we need to set the amplitude at 40% for much longer times to break apart the Van der Waals physical bonds which make the SWNTs agglomerate into bundles. Since SWNTs are such a fine particle, the agglomerated bundles are harder to disperse. We also recommend using a magnetic stirrer for mechanical agitation to assist the dispersion process.

Although both probe style and bath style ultrasonic systems can be used for dispersing CNTs, it is widely believed that the probe style ultrasonic systems work better for dispersing CNTs. It is also widely known that adding a dispersing reagent (surfactant) into the solution will accelerate the dispersion effect and help to keep the CNTs well separated. The reagent Polyvinyl Pyrrolidone (PVP) is a good dispersion agent for our CNTs. Some people like other surfactants such as Sodium Dodecyl Benzene Sulfonate (SDBS),Poly Vinyl Alcohol (PVA), or Triton X100. We have found that the dispersing reagent and proportions listed above do change when using different solvents. When trying to disperse CNTs please note that your chemistry makes a difference. Typically we disperse our COOH functionalized CNTs in IPA or Acetone, our OH into ethanol, & our standard purified into Di Water and use the ultrasonic process detailed above. In our experience, much less reagent is used for dispersing in DI water than other solvents. We believe this is due to the high polarity of water compared to other solvents. Typically, it is a question of chemistry, proper ratios, & sonication time to achieve a stable dispersion. A stable dispersion will last for days, weeks, or months with little to no settling.

In some applications, achieving a stable dispersion can require other agents in the solution to prevent the CNTs from falling out of solution over time. Emulsifier T-60 (also known as Tween 60) is commonly used with Di water or Isopropyl Alcohol. Organic titanates can be used with Acetone and Xylene. The specific application determines whether these agents remain in the solution when further processing, or if they need to be removed. Some organic titanates can be removed by heating the solution above 2500°C. The addition of the OH and COOH functional groups assist the CNTs dispersion as well as the chemical bonding to other materials during further processing.

What are Functionalized CNTs?

Pristine nanotubes are unfortunately insoluble in many liquids such as water, polymer resins, and most solvents. Thus they are difficult to evenly disperse in a liquid matrix (for example, epoxies and other polymers). This complicates efforts to utilize the nanotubes’ outstanding physical properties in the manufacture of composite materials, as well as in other practical applications (biological, optical, magnetic, etc.) which require preparation of uniform mixtures of CNTs with many different organic, inorganic, and polymeric materials.

To make nanotubes more easily dispersible in liquids, it is necessary to physically or chemically attach certain molecules (functional groups) to their smooth sidewalls without significantly changing the nanotubes’ desirable properties. This process is called functionalization.

The production of robust composite materials requires strong covalent chemical bonding between the filler particles (CNTs) and the polymer matrix, rather than the much weaker van der Waals physical bonds which occur if the CNTs are not properly functionalized.

Functionalization methods such as chopping, oxidation, and “wrapping” of the CNTs in certain polymers can create more active bonding sites on the surface of the nanotubes.

For biological uses, CNTs can be functionalized by attaching biological molecules, such as lipids, proteins, biotins, etc. to them. Then they can usefully mimic certain biological functions, such as protein adsorption, and bind to DNA and drug molecules. This would enable medially and commercially significant applications such as gene therapy and drug delivery.

In biochemical and chemical applications (for example, development of very specific biosensors), molecules such as carboxylic acid (COOH), poly m-aminobenzoic sulfonic acid (PABS), polyimide, and polyvinyl alcohol (PVA) have been used to functionalize CNTs, as have amino acid derivatives, halogens, and compounds such as potassium permanganate. Poly (acrylic acid)-functionalized CNTs are soluble in water and other highly polar, aqueous solvents such as DMSO

What is the Thermal Conductivity of CNTs?

The thermal conductivity of mwnts is approximately 2000W/m·k, and thermal conductivity of swnts is approximately 4000W/m·k.

What is the PH of OH and COOH Functionalized CNTs Dispersed in DI Water?

Using app 150 mg of OH and COOH functionalized CNTs in app 150-200mls of DI water. After stabilizing, the PH of the OH was 3.8 and the COOH was 4.2.

Why is the Specific Surface Area (SSA) Spec Lower Than Theoretically Possible?

The theoretical Specific Surface area of swnts is about 1300m2/g. We test the SSA of our CNTs through the nitrogen adsorbing method. As many of you know, the SWNTs are clumps of tubes. So there are many areas can't adsorb the nitrogen. So the testing data is often lower than expected and we spec the SSA of our SWNTs at 407m2/g.

What is the Aspect Ratio of CNTs and Why is it Important?

Aspect Ratio is the measurement of the length times diameter. The aspect ratio of CNTs is very high, about 1000:1 which enables CNTs to impart electrical conductivity at lower loadings, compared to conventional additive materials such as carbon black, chopped carbon fiber, or stainless steel fiber.

Can Cheap Tubes Sell Dispersed CNTs?

Yes, we can sell small quantities of dispersed CNTs in DI Water and Isopropyl Alcohol. Please call or email us for more information. Other dispersions will be offered soon.

What is the Typical Loading Ratio People use When Adding CNTs to Products?

One of the many benefits of CNTs over other conductive or strengthening materials is the low loading percentage needed to achieve typical results. Most companies/researchers start by using 1-3% loading ratios and adjust up or down depending on desired results vs. actual achieved results.

Can Cheap Tubes Supply Fullerenes?

Cheap Tubes is currently finalizing our agreement with a well known, well respected fullerene manufacturer. We can currently supply fullerenes however there is a lead time and they do not ship from stock.

This information has been sourced, reviewed and adapted from materials provided by Cheap Tubes Inc.

For more information on this source, please visit Cheap Tubes Inc.


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