In modern materials science, carbon nanotubes (CNTs) are gaining great attention for their excellent mechanical, electrical, and thermal properties. Research on their use in composite materials has been a hot topic. The performance of carbon nanotubes largely depends on how well they are dispersed in the base material. How to disperse carbon nanotubes? In this article, we will focus on the liquid-phase dispersion of CNT powder. We will explain how physical, chemical, and mechanical methods help achieve uniform dispersion.
This area of study addresses a key challenge in real-world CNT applications. It also supports the development of high-performance composite materials. In the long term, it may expand the use of CNTs in electronics, energy, and biomedical fields.
Carbon Nanotubes Dispersion Challenges
Carbon nanotubes (CNTs) are lightweight, strong, tough, and have a high elastic modulus. These features give CNTs better mechanical properties than many traditional materials. CNTs also offer huge potential in various fields. They can be used to make strong materials and fibers, and they show promise in biomedicine, electronics, and energy.
However, CNTs also face some challenges. Because of their large surface area and few surface defects, their hexagonal carbon structure forms many delocalized π-electrons through sp² bonds. This causes nearby CNTs to attract each other through π–π stacking, leading to clumping. This makes them hard to disperse and chemically inactive. As a result, their performance in real applications is limited.
To improve dispersion, CNTs often need strong acid treatment or added dispersants. These methods roughen the surface and add active functional groups, making the CNTs easier to separate and mix.
How to Disperse Carbon Nanotubes: 3 Methods
In recent years, researchers have developed various surface modification methods to improve CNT properties and expand their applications. Currently, there are three main dispersion techniques:
Many researchers have developed various methods to modify the surface of carbon nanotubes
- Mechanical Dispersion
- Covalent Functionalization Dispersion
- Non-Covalent Functionalization Dispersion
1. Mechanical Dispersion
Mechanical dispersion overcomes the van der Waals forces between CNTs by physical means. It mainly includes techniques such as ultrasonic treatment, ball milling, magnetic stirring, and high-speed shearing. They apply mechanical external force to make CNTs disperse evenly in the medium. This avoids the side effects that may be caused by chemical modification while retaining the original characteristics of CNTs.
(1) Ultrasonic Dispersion
- Principle: Ultrasonic waves create mechanical vibration and cavitation. This will produce a strong local impact force in the CNTs dispersion, achieving uniform dispersion of CNTs. The cavitation effect of ultrasound can create tiny bubbles. These bubbles release a huge amount of energy when they collapse, destroying the agglomeration between CNTs.
- Advantages: Easy to operate. It often works as a support method alongside other dispersion techniques to improve results.
- Disadvantages: Long-term ultrasonic treatment may cause a certain degree of structural damage to CNTs. Therefore, it is necessary to optimize the ultrasonic time and power.
(2) Ball Milling
- Principle: Mechanical force is applied to CNTs through the grinding media (such as steel balls or ceramic balls) in the ball mill to achieve dispersion.
- Advantages: CNTs can be effectively dispersed by controlling the ball milling time and conditions.
- Disadvantages: Long-term ball milling may cause structural damage to CNTs. Therefore, it is necessary to balance the dispersion effect and structural integrity.
Note:
Wet ball milling works better than dry milling. It maintains CNT structure and improves dispersion.
(3) Magnetic Stirring and High-speed Shearing
- Principle: Use mechanical force to disperse CNTs in the medium. Magnetic stirring drives the stirring rod to rotate through the magnetic field. High-speed shearing generates strong shear force through the high-speed rotating shear head.
- Advantages: This method can effectively improve the dispersion and stability of CNTs. This is suitable for different types of media and application scenarios.
- Disadvantages: They may damage CNTs. They also require precise control of shear rate, time, and temperature. The operation can be more complex.
2. Covalent Functionalization Dispersion
Covalent functionalization improves the dispersion of carbon nanotubes (CNTs) by adding chemical groups to their surface. These functional groups are bonded directly to the CNTs through chemical reactions. This changes the surface properties and increases their interaction with solvents or base materials.
(1) Oxidation Treatment
Oxidation treatment is one of the most commonly used methods for covalent modification. By refluxing CNTs in a strongly acidic environment (such as H₂SO₄, HNO₃, H₂O₂, etc.), oxygen-containing functional groups such as carboxyl (-COOH), hydroxyl (-OH), and carbonyl (-C=O) can be introduced on their surface. These functional groups increase the surface polarity of CNTs and improve their dispersibility in polar solvents (such as water, ethanol, dimethylformamide, etc.). For example, oxidized CNTs disperse better in solutions, making them useful for preparing composite materials.
(2) Further Functionalization
In addition to oxidation, other chemical reactions can add more functional groups to the CNT surface. For example, esterification introduces ester groups, while amidation adds amide groups.
These functionalization treatments improve the dispersibility of CNTs and can also impart specific chemical properties. For instance, hydroxyl groups help in silane bonding in polymer nanocomposites. Carboxyl groups can be converted with SOCl₂ and react with other groups for use in biomedical or sensor applications.
(3) Pros and Cons of Covalent Functionalization
- Pros: This method greatly improves CNT dispersion. Treated CNTs can be evenly distributed in various solvents and base materials.
- Cons: Chemical reactions may damage the CNTs’ sp² structure, reducing their electrical and mechanical properties. The process is complex and requires careful control of conditions like temperature, acid strength, and time. Also, added functional groups may increase surface roughness and affect performance in some applications.
3. Non-Covalent Functionalization Dispersion
The non-covalent modification method improves the dispersibility of CNTs by adsorbing specific molecules or polymers on the surface of CNTs. This method does not destroy the original chemical structure of CNTs. It uses intermolecular interactions (such as electrostatic effects, π-π stacking, hydrogen bonds, etc.) to achieve stable dispersion.
(1) Surfactant Adsorption
Surfactant is one of the most commonly used dispersants in non-covalent modification. It adsorbs surfactant molecules on the surface of CNTs, effectively reducing the van der Waals forces between CNTs and achieving dispersion.
The hydrophilic and hydrophobic ends of the surfactant interact with the solvent and the surface of CNTs, respectively. This can form a stable dispersion system. For example, surfactants such as sodium dodecyl sulfate (SDS) and sodium cholate have been widely used to disperse CNTs.
(2) Polymer Wrapping
The polymer wrapping is another non-covalent modification method. It adsorbs polymer molecules on the surface of CNTs to form a protective film. This layer prevents the CNTs from aggregating.
This method improves the dispersibility of CNTs and retains their original electrical and mechanical properties. For example, polymers such as polyvinylpyrrolidone (PVP) and polystyrene (PS) have been successfully used to disperse CNTs. Additionally, adjusting the molecular weight and side chain structure of the polymer can further enhance the dispersion effect.
(3) Other Non-Covalent Interactions
In addition to surfactants and polymer wrapping, other non-covalent interactions can also be used to achieve the dispersion of CNTs. This includes electrostatic dipole interactions and metal-organic coordination.
For example, metal-polyphenol coordination reactions can adsorb metal ions and polyphenol molecules on the surface of CNTs. This will form a stable dispersion system.
(4) Pros and Cons of Non-Covalent Functionalization
- Pros: This method keeps the original structure and properties of CNTs. It provides excellent dispersion. It is relatively simple to operate and does not require complex chemical reactions. This method is also suitable for large-scale applications.
- Cons: The dispersion effect may be affected by factors such as solvent properties and temperature. In some cases, the dispersion stability is not as good as the covalent modification method. In addition, non-covalent modifiers may introduce additional impurities or affect the surface properties of CNTs in some applications.
Conclusion
Proper carbon nanotube dispersion is key to use effectively. Mechanical, covalent, and non-covalent methods each have their pros and cons. The best method depends on the application. With better dispersion, CNTs can reach their full potential in advanced materials.
