|Figure 1. Ball-and-stick diagram of a single-walled carbon nanotube. Image credit: NSF.gov
Carbon nanotubes are molecular tubes formed from rolled-up sheets of graphene. They have incredible physical and electronic properties, which has made them the subject of much research in academia and industry in recent years. Many applications have been predicted, but few have reached a commercial stage due to the high manufacturing costs of high quality nanotubes.
Synthesis Methods for Carbon Nanotubes
Arc-evaporation synthesis, also known as electric arc discharge, has long been known as the best method for synthesizing fullerenes, and it also generates the highest quality carbon nanotubes.
Arc-evaporation apparatus consists of two graphite electrodes under helium. A current of around 50A is passed between the electrodes, causing some of the graphite from the anode to evaporate and condense on the cathode - this deposit contains the carbon nanotubes.
Arc-evaporation with pure graphite electrodes produces mainly multi-wall nanotubes, although single-wall nanotubes can be made by this method by doping the anode with a metal catalyst such as cobalt or nickel.
Whilst the nanotubes produced by arc discharge are of a very high quality, they are mixed with a large amount of amorphous carbon, which makes this technique difficult to scale up.
The laser vaporization (or laser ablation) method was developed in 1995. A pulsed laser is fired at a graphite target in an inert environment, at high temperature and pressure. The target is usually placed at one end of a 50cm quartz tube - the nanotubes are collected at the opposite end.
The shape and structure of the nanotubes produced by laser vaporization are more easily controllable, as they are only affected by a small number of parameters. The yield of carbon nanotubes is also much higher, with very little amorphous carbon produced, but the overall amount generated is very small. This combined with the high operating temperature and pressure make this method highly inefficient for producing large amounts of carbon nanotubes.
Chemical Vapour Deposition (CVD)
Chemical vapour deposition is the method with the most promise for mass production of carbon nanotubes. It operates at much lower temperatures, and produces nanotubes in greater quantities than arc discharge or laser vaporization.
CVD uses a carbon-rich gas feedstock, such as acetylene or ethylene (IUPAC ethyne, ethene). The gas is passed over a metal nanoparticle catalyst (typically iron, nickel, or molybdenum) which has been deposited on a porous substrate (e.g. silica, alumina). Carbon atoms dissociate from the gas molecules as they pass over the catalyst, rearranging on the surface to form nanotubes and fullerenes. This allows nanotubes to be synthesized continuously, making the technique ideal for scaling up to large manufacturing volumes.
The choice of catalyst used for CVD is crucial. Changing the catalyst can entirely alter the quality and yield of the nanotubes produced. The diameter of the carbon nanotubes also depends directly on the catalyst particle size used, as the particles nucleate the growth of the nanotubes.
The properties of the substrate used are also important. The substrate material should be able to retain a high surface area and pore volume at high temperatures - carbon nanotubes grow significantly faster on a porous surface, as carbon atoms can move through the substrate to join growing structures.
Optimizing Chemical Vapour Deposition
Of the well-established methods, CVD shows the most promise for larger-scale synthesis of carbon nanotubes. Much research effort has gone into improving the efficiency of the process.
The main problems which this research is attempting to solve are the poisoning of the catalyst surface by the build-up of amorphous carbon, preventing further nanotube growth, and the number of costly and complicated processes which must take place before and after the synthesis step.
- Water Assisted "Supergrowth" by CVD can produce high purity, vertically aligned single-walled nanotubes, with lengths up to the millimetre range. This is achieved by mixing a small amount of water vapour into the hydrocarbon gas feedstock. It is thought that this cleans the amorphous carbon away from the substrate, keeping the surface clean and allowing unrestricted growth of nanotubes.
- Catalytic Pyrolysis is chemically very similar to CVD, but the preparation and purification steps are significantly simplified. Catalyst particles, or organometallic precursors, are injected into the hydrocarbon (often benzene, toluene, or hexane) stream. The nanotube growth occurs on the reactor wall, or on a specific substrate, which is usually quartz. This generates pure, aligned nanotubes in one step, without the need for separate catalyst/substrate preparation.
Figure 2. A forest of vertically aligned carbon nanotubes, produced by flame-based CVD. Image credit: NSF.gov
Other Synthetic Methods
Several other methods for producing carbon nanotubes have been reported, which are mostly at an early stage of research. Some of these may have the potential to be good mass-production methods, with further development.
- Diffusion flame synthesis, with a variety of metal and metal oxide catalysts and support geometries
- Electrolysis of graphite in molten lithium chloride under an inert atmosphere
- Ball milling and annealing of graphite, catalyzed by iron contamination from the steel milling balls
- Heat treatment of polyesters formed from citric acid and ethylene glycol, at 400C in air
- Hydrothermal treatment of polyethylene with a nickel catalyst under high pressure
- Explosive decomposition of picric acid, in the presence of cobalt acetate and paraffin, produces a high yield of relatively homogeneous, "bamboo-shaped" carbon nanotubes.
Purification of Carbon Nanotubes
Carbon nanotubes are most often produced in combination with a range of fullerenes and amorphous carbon - separating the desirable nanotubes from this other matter can be problematic. Most synthesis methods also produce a range of nanotube sizes and structures. Whilst synthetic methods can be optimized to reduce the degree of purification needed, for demanding applications nanotubes will always need to be cleaned and sorted effectively.
The most common purification methods can recover about 90% of the nanotube yield, and consist of:
- Dispersion - sonification of the crude sample along with a detergent
- Acid reflux - this step uses large quantities of acid over a very long period of time, usually around 10 hours
- Micro-filtration - using a PTFE membrane, usually performed in several steps to achieve the required purity.
Structure-Specific Carbon Nanotube Synthesis
A major problem with most current bulk synthetic methods for carbon nanotube synthesis is the lack of control over their structure. Many high-value applications for nanotubes require a specific type, whether single-walled, chiral, coiled, horn-shaped, etc. It is also extremely useful if the nanotubes can be made with macroscopic structure or arrangement, as this is often how they need to be used, so complicated physical manipulation steps can be avoided.
Figure 3. A single coiled carbon nanotube. Image credit: "Nanoscale Pasta: Toward Nanoscale Electronics" - UCSD Jacobs.
Coiled nanotubes, which are of great interest due to their unique mechanical and electromagnetic properties, have been synthesized using carefully selected catalysts. The most successful catalysts usually consist of iron or nickel, mixed with indium tin oxide (ITO) or alumina, or with a thiophene impurity. Iron/ITO catalysts have been investigated in some detail, and it has been determined that ITO induced the desired helical growth, and the indium/tin ratio controls the yield of nanotubes and nanocoils to some extent.
|Figure 4. Diagram of a single-walled "nanohorn". Image credit: Wikipedia.
Conical or horn-shaped single-walled nanotubes, dubbed nanohorns, are another alternative nanotube structure which is of great interest. Many applications have been proposed which take advantage of their high porosity, excellent current handling, and good catalytic activity.
Whilst synthesizing them in good yield is a great challenge, "flower-like" aggregates of nano-horns have been synthesized by various methods which rapidly quench the carbon vapour from arc evaporation. The most promising method for producing nanohorns on a large-scale is to use an arc torch in a thin graphite tube, submerged in water and under inert gas flow. Single-walled nanohorns are found floating on the surface of the water.
Nanotubes can be grown in an ordered "forest" structure, using modified CVD techniques. By depositing the metal catalyst onto the substrate surface in a regular pattern, the sites at which nanotubes can nucleate are limited, and the tubes grow perfectly parallel, perpendicular to the surface.
Sources and Further Reading
- "Synthesis of Carbon Nanotubes", Awasthi et al, 2005, via arXiv.org.
- "Synthesis of Carbon Nanotubes" - Appropedia.org
- "Carbon nanotubes production by catalytic pyrolysis of benzene", Benito et al, DOI: 10.1016/S0008-6223(98)00039-6
- "Formation of bamboo-shape carbon nanotubes by controlled rapid decomposition of picric acid", Lu et al, DOI: 10.1016/j.carbon.2004.08.003
- "Synthesis of carbon nanohorns by a gas-injected arc-in-water method and application to catalyst-support for polymer electrolyte fuel cell electrodes", Sano et al, DOI: 10.1039/B717302D
- "Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes", Hata et al, DOI: 10.1126/science.1104962