How it Works: Ultrasonication of Single-Walled Carbon Nanotubes

Problem: Single-walled carbon nanotubes (SWCNTs) differ from multiwalled carbon nanotubes by their electric properties. The band gap of SWCNTs can vary from zero to 2 eV and their electric conductivity features metallic or semiconducting behavior. As single-walled carbon nanotubes are highly cohesive, one of the major obstacles in processing them is the inherent insolubility of the tubes in organic solvents or water. To use the full potential of SWCNTs, a simple, reliable and scalable deagglomeration process of the tubes is needed—especially since the functionalization of the CNT side walls or open ends necessary to create a suitable interface between the SWCNTs and the organic solvent results in only partial exfoliation of the SWCNTs. Therefore, SWCNTs are mostly dispersed as bundles rather than individual deagglomerated ropes. If the conditions during dispersion are too harsh, the SWCNTs will be shortened to lengths from 80 to 200nm. For the majority of practical applications, i.e. for semiconducting or reinforcing SWCNTs, this is too short.

Hielscher’s ultrasonic processor UIP1000hd with flow cell and pump is a powerful and reliable device for the production of nanomaterials, such as SWCNTs.

Solution: Ultrasonication is a very effective method of dispersion and deagglomeration of carbon nanotubes, as ultrasonic waves of high-intensity ultrasound generate cavitation in liquids. The sound waves propagated in the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is known as cavitation. During the implosion, very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The implosion of the cavitation bubble also results in liquid jets of up to 280 m/s velocity. These liquid jet streams resulting from ultrasonic cavitation overcome the bonding forces between the carbon nanotubes and hence, the nanotubes become deagglomerated. A mild, controlled ultrasonic treatment is an appropriate method to create surfactant-stabilized suspensions of dispersed SWCNTs with high length. For the controlled production of SWCNTs, Hielscher’s ultrasonic processors allow for running at a wide range of ultrasonic parameters sets. The ultrasonic amplitude, liquid pressure and liquid composition can be varied respectively to the specific material and process. This offers variable possibilities of adjustments, such as:

• sonotrode amplitudes of up to 170 micron
• liquid pressures of up to 10 bar
• liquid flow rates of up to 15L/min (depending on the process)
• liquid temperatures of up to 80°C (other temperatures on request)
• material viscosity of up to 100.000cp

Furthermore, ultrasonication, as a polymer-assisted purification method, effectively removes impurities from asgrown SWCNTs. It is difficult to study the chemical modification of SWCNTs at the molecular level, because it is difficult to obtain pure SWNTs. As-grown SWCNTs contain many impurities, such as metal particles and amorphous carbons. Ultrasonication of SWCNTs in a monochlorobenzene (MCB) solution of poly (methyl methacrylate) PMMA followed by filtration is an effective way to purify SWCNTs. This polymer-assisted purification method removes impurities from as-grown SWCNTs. Accurate control of the ultrasonication amplitude allows limiting damages to the SWCNTs.

Hielscher offers high-performance ultrasonic processors for the sonication of every volume. Ultrasonic devices from 50 watts up to 16,000 watts, which can be set up in clusters, are appropriate for various lab applications.

For more information, visit: www.hielscher.com/CNT