Carbon Nanotubes for Solar Energy (1/2)
What are Carbon Nanotubes, how are they made & how are they being used?
Carbon Nanotubes (CNTs) are these cute little structures made out of carbon atoms (except not as big, much stronger and conductive):
I guess they aren’t as cute as we think (they’re fierce). Using these Carbon Nanotubes we can not only unlock much more on a molecular level but also from the products / basic things we use everyday. Like Solar Energy which I’ll be highlighting more in my next article.
But in order to really tap into the potential of CNTs, we need to first understand what they are, how they’re made and their various applications.
What is a Nanometer (nm)? How small is it?
If you’re new to the world of nanotechnology then these are most likely the questions going through your head. In order to really understand what Nanotechnology is we have to get an idea of the units of measure involved.
So here’s the quick rundown (relative to a meter):
- A centimeter is one-hundredth of a meter.
- A millimeter is one-thousandth of a meter.
- A micrometer is one-millionth of a meter.
Yeah, these are all really small measures but all of these are still huge compared to the nanoscale.
A nanometer (nm) is one-billionth of a meter which is smaller than the wavelength of visible light and a hundred-thousandth the width of a human hair.
A human hair is around 75 microns (75μm) or 75,000nm (nanometers) in diameter. The relationship between a nanometer and that hair is similar to the relationship between one mile and an inch (one mile is 63,360 inches).
A Carbon Nanotube
A carbon nanotube is a nano-size cylinder of carbon atoms (also Graphene). Essentially, a sheet of carbon atoms which would look like a sheet of hexagons. If you roll that sheet into a tube, you have a carbon nanotube. Sounds pretty simple but these Carbon Nanotubes can have really interesting properties.
Carbon nanotube properties depend on how you roll the sheet. All carbon nanotubes are made of carbon, they can be very different from one another based on how you align the individual atoms.
- With the right arrangement of atoms, you can create a carbon nanotube that’s hundreds of times stronger than steel, but six times lighter.
- Carbon nanotubes can also be effective semiconductors with the right arrangement of atoms.
They can be single-walled (SWCNT) with a diameter of less than 1 nanometer (nm) or multi-walled (MWCNT), made up of several interlinked nanotubes, with diameters reaching more than 100 nm.
CNTs are chemically bonded with sp2 bonds (which is an extremely strong form of molecular interaction).
This is what makes them unique:
Strong Molecular Interaction + ability to rope together (via van der Waals) = ultra-high strength, low-weight materials → highly conductive electrical and thermal properties.
Armchair nanotubes are called that because of the armchair-like shape of their edges. These are really awesome because of their conductivity. Unlike zigzag nanotubes, which could be semiconductors, they are conductive. Turning a graphene sheet an approx. 30 degrees will change the nanotube from armchair to zigzag or vice versa.
MWCNTs are always conducting and generally receive the same level of conductivity as metals, SWCNTs' conductivity depends on their chiral vector. A vector connecting the centers of the two hexagons is called the chiral vector.
Chirality of single-walled carbon nanotubes
In order to form a tube, you would need to take two of the hexagons in a graphene lattice and overlap them. A vector connecting the centers of the two hexagons is called the chiral vector, and it determines the structure of a single-walled carbon nanotube.
Chiral vector C can be written as C = n a1 + m a2 where a1 and a2 are basis vectors of the graphene lattice. The pair of integers (n,m) is called the chiral index or just chirality. This implies that the structure of a single-walled carbon nanotube is completely determined by chirality.
The most interesting aspect of single walled carbon nanotubes is that their electronic structure can become either semiconducting or metallic depending on the chirality, and it is also interesting that the bandgap energy also depends on chirality.
A band gap is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap is the energy difference (volts) between the top of the valence band and the bottom of the conduction band.
SWCNTs can behave like a metal and be electrically conducting, display the properties of a semi-conductor, or be non-conducting.
Unique thermal and mechanical properties:
Their electrical properties are mostly from graphene but CNTs also have unique thermal and mechanical properties:
- their mechanical strength can be 400 times that of steel.
- they are very light-weight – their density is one-sixth of that of steel.
- their thermal conductivity is better than that of diamond.
- a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor).
- highly chemically stable and resist virtually any chemical impact unless they are exposed to high temperatures and oxygen (makes them extremely resistant to corrosion).
- their hollow interior can be filled with various nanomaterials, separating and shielding them from the surrounding environment (useful for nanomedicine applications like drug delivery).
Carbon Nanotube vs Nanofibers
Carbon nanotubes are different than carbon nanofibers (CNFs). Carbon fibers have been used for decades to strengthen compound but they do not have the same lattice structure as CNTs. They consist of a combination of several forms of carbon and/or several layers of graphite, which are stacked at various angles on an amorphous carbon (where atoms do not arrange themselves in ordered structures). CNFs have similar properties as CNTs, but their strength is lower and they are not hollow inside.
How are carbon nanotubes made?
Three main methods are currently available for the production of CNTs: arc discharge, laser ablation of graphite, and chemical vapor deposition (CVD).
Arc Discharge & Laser Ablation:
In these two processes, graphite is combusted electrically by a laser, and the CNTs developing in the gaseous phase are separated. All three methods require the use of metals (e.g. iron, cobalt, nickel) as catalysts.
Here’s a more detailed diagram if you’re curious in learning more:
The CVD process is the most common, since it allows the production of larger quantities of CNTs under more easily controllable conditions and at lower cost.
In the CVD process, you combine a metal catalyst (such as iron) with carbon-containing reaction gases (such as hydrogen or carbon monoxide) to form carbon nanotubes on the catalyst inside a high-temperature furnace.
Small catalyst particles of the size of a CNT diameter develop, on which the nanotubes start growing. The catalyst particle is either at the top or at the bottom of the emerging nanotube. Growth will stop if the catalyst particle is deactivated through the development of a carbon envelope.
The CVD process can be purely catalytic or plasma-supported. The latter requires slightly lower temperatures than the catalytic process and has more of a 'lawn-like' CNT growth.
Even though these techniques have been improved to obtain high-purity carbon nanotubes, there is generally also the formation of foreign particles.
This is why carbon nanotubes need to be purified with the help of various methods such as acid treatment or ultrasound at the end of the production process.
Carbon nanotube uses and applications
CNTs are well-suited for virtually any application requiring high strength, durability, electrical conductivity, thermal conductivity and lightweight properties compared to conventional materials.
But these are some of the most common uses:
Carbon nanotube enabled nanocomposites have received much attention as a highly attractive alternative to conventional composite materials.
It has been estimated that advanced CNT composites could reduce the weight of aircraft and spacecraft by up to 30%.
Here are some of the other use cases:
- sporting goods (bicycle frames, tennis rackets, hockey sticks etc.)
- textiles (smart textiles, bullet-proof vests, water-resistant and flame-retardant textiles)
- automotive, aeronautics and space (light-weight, high-strength structural composites)
- industrial engineering (e.g. coating of wind-turbine rotor blades, industrial robot arms)
Carbon nanotubes are so attractive for catalysis because of their high surface area combined with the ability to attach anything to their sidewalls. Already, CNTs have been used as catalysts in many relevant chemical processes but controlling their activity is the hard part.
Usually, carbon nanotubes have been combined with molecules (via very strong covalent bonds) that lead to very stable compounds. But this also leads to a change in the structure of the nanotube + ts properties.
Weak non-covalent forces have also been used, which keep the structure of the nanotubes intact, but typically yield kinetically unstable compounds.
BUT good for us, researchers already are developing methods for the chemical modification of carbon nanotubes by mechanical bonding. This type of compounds is as stable as covalent compounds, but still respect the structure.
Semiconducting single-walled carbon nanotubes are still being used as strong, thin-film transistors as well as for opto-electronic devices to replace silicon electronics (quick plug, you can read more about that here: "20 years of nanotube transistors").
Many studies have shown that although CNTs are robust, their electrical properties are extremely sensitive to the effects of charge transfer and chemical change by various molecules.
The functionalization of CNTs is important for making them selective to the target analyte. Different types of sensors are based based on molecular recognition interactions.
For instance, researchers have developed flexible hydrogen sensors using single-walled carbon nanotubes:
+ Many More … including Solar Energy!
But that’s for next time. The applications of Carbon Nanotubes in Solar Energy are out of this world (also literally)!!! I’ll be explaining more of that in the next (2/2) part of this article.
We just love renewable energy:
See you in my next article in this 2 part series!
Hi. I’m Alishba.
I’m so excited to be working in the space of clean energy + sustainability using Nanotechnology, for the next few months. I’d love to connect with you and chat more about the tech + research I’m doing. Feel free to reach out!