How are Fullerenes Made?
Today, fullerenes such as C60 are made using three main methods: Huffman-Krätschmer, combustion and microwave. Chemical synthesis techniques, such as laser irradiation and pyrolysis, offer exciting possibilities for producing fullerene derivatives that were previously not accessible. These emerging techniques promise to address current challenges, including low yield and complex procedures. This will pave the way for more efficient and targeted fullerene synthesis in the future.
How were Fullerenes Discovered?
Kroto, Smalley, and Curl used pulsed laser beams to vaporize a graphite disk. Creating a similar environment to that of a star. When the intense laser pulses struck the graphite, they generated a high-temperature plume of carbon vapor. This vapor was then rapidly cooled within a high-density helium atmosphere, a controlled environment that facilitated the condensation of carbon atoms. The rapid cooling process allowed the carbon atoms to bond in unique configurations, resulting in the formation of a series of carbon cluster products. One of which was C60 fullerene.
How are Fullerenes Made?
The three main methods for synthesizing C60 fullerene:
| Method | Advantages | Disadvantages |
|---|---|---|
| Huffman-Krätschmer |
Simple set up High output Low cost Safe Reliable |
Impurities of amorphous carbon and graphite Energy intensive Large amount of solvent required Limited by length of graphite electrode |
| Combustion |
Long-term continuous synthesis Simple Low energy consumption |
Impurities Low yield Difficult to control conditions |
| Microwave |
Energy efficient Rapid heating |
Impurities Equipment costs |
Huffman-Krätschmer Method
The Huffman-Krätschmer method for synthesising fullerene involves the evaporation and recondensation of graphite. It is also known as the arc evaporation or arc discharge method. Graphite is heated in a helium atmosphere under 100-200 Torr pressure. A soot is produced which contains fullerene. The soot is dispersed in benzene and fullerene separated using a Soxhlet extractor. C60 can be produced in gram quantities per day.
The arc process results to be the best technique for production of carbon soot and subsequent extraction of fullerenes. Due to the poor solubility of fullerenes, large quantities of solvents are required during the purification process.
Combustion Method
Another method for producing fullerenes involves the incomplete combustion of hydrocarbons in sooting flames. Fullerenes can be extracted from a laminar sooting flame of a mixture or benzene and oxygen at low pressure.
The key influences over the amount of fullerene produced and its composition are flame conditions such as:
- Pressure
- C:O ratio
- Time the reactants spend in the flame
- Gas velocity
Microwave Method
Fullerene can also be made using microwave irradiation to heat reagents evenly throughout the reaction mixture. One method is to use microwaves to induce naphthalene-nitrogen plasma at atmospheric pressure in a cylindrical coaxial cavity. Toluene is then used to extract fullerene from the soot produced. Plasma conditions can be easily controlled by adjusting the microwave settings, including temperature and power.
Microwave irradiation has also been used to transform graphite powder and fluorinated graphite to fullerenes. The key influences over yield of fullerene is the amount of carbon based starting reagent and microwave power.
Making Fullerenes in the Future
The current main methods for synthesizing fullerene often have uncontrolled reactions and yield unpredictable products. In contrast, chemical synthesis techniques and molecular engineering now make it possible to access fullerenes and their derivatives that were previously inaccessible.
Chemical Synthesis of Fullerene
Laser Irradiation
Laser irradiation at 337 nm induces hydrogen loss for the polycyclic aromatic hydrocarbon (PAH) C60H30 causing it to form C60 fullerene. It is formed by molecular transformation rather than fragmentation and recombination. This is a low yielding process, and the fullerene product cannot be separated.
Pyrolysis
Through molecular engineering, researchers found that chlorine substituted C60H30Cl3 can also form C60 via flash vacuum pyrolysis at 1100°C. The C60 was able to be isolated without generating other fullerenes as by-products. This technique can be used to synthesise other fullerenes, especially those that cannot be produced by graphite vaporization.
The chemical synthesis of fullerenes still has a long way to go to be competitive with the current main methods. There are still issues of low yield and complicated steps. The future is promising and we look forward to seeing what is to come!
C60 Fullerene
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Properties of Fullerene
Fullerene C60 exhibits unique structural, chemical, electronic, and thermal properties. This is due to its highly symmetrical icosahedral structure that contains a mixture of single and double bonds. All fullerenes have partial electron sharing across the molecule and a high affinity for electrons.
Read more... What are Fullerenes?
Fullerenes are an allotrope of carbon and are known for their hollow, cage-like structures. They are composed entirely of carbon atoms that form closed, convex polyhedra. The atoms are bonded together to form hexagonal and pentagonal rings, like the pattern found on a soccer ball or geodesic dome.
Read more...Further Reading
- Fullerenes, Lv, M. et al. Functional Carbon Materials (2022)
- Fullerene: biomedical engineers get to revisit an old friend, Goodarzi, S. et al. materialstoday (2017)