Purification of Carbon Nanotubes
|We have developed numerous techniques for purification as part of our carbon nanotube research. These include:
- Oxidation Tubes are heated at a controlled rate either in wet air or vacuum for an extended time. This removes carbon, although it shortens the tubes and increases the number of structural defects.
- Acid Reflux in HNO3, HCl, or other acid for period of time ranging from 4 to 48 hours. It removes metals and some fullerenes, but HNO3 is the only acid that doesn’t cause degradation to tubes (unless left in HNO3 for extended time frames, usually >16 hrs).
- Centrifugation Centrifuge at 7,000 gs or more for 30 min to 3 hours. Often supernatant is removed after one run and centrifuged again. This removes nanospheres, metal nanoparticles, other carbon particles, but some methods produce a low yield of nanotubes, especially when centrifugation is repeated many times.
- Filtration Soaked into CS2 or distilled water or toluene then filtered through <.2 micron filter, often under vacuum. This removes nanospheres, metal nanoparticles, polyatomic carbons, and fullerenes, without any noted negative effects.
- Ultrasonication Tubes are suspended in distilled water, oDCB, toluene, or acid solution and sonicated with ultrasonic tip for 5-30 minutes. This separates tubes from attached particles, creating dispersion of nanotubes and other particles for better centrifugation, but can induce cutting of tubes. Bundles of nanotubes are often thicker due to random alignment
- Magnetic Tubes suspended in soap or tolulene and nanoparticle powder (ZrO, NHCl4, CaCo3, diamond) is added – slurry is sonicated and then magnetic particles are trapped with permanent magnetic poles, follwed by chemical treatment. This removes catalyst materials and small inorganic particles, few to no known negative effects.
- Functionalization Tubes are organically functionalized and therefore nanotubes become more soluble while metal particles do not. Once this step is done, tubes are purified by other methods then functional groups are removed and nanotubes are annealed. This removesaAmorphous carbon and metal nanoparticles, but purity is not as high as other methods and annealing can cause degradation to nanotubes.
- Microwave Purification Tubes are sonicated, then diluted in HNO3 – HNO3 (or other acid) then microwaved at 100-200 W, and microwave is ramped up to ~200°C over 30 min time period. Microwave is then held at temperature for 30-90 minutes. This removes amorphous carbon, metals, and other nanoparticles, with no known negative effects.
- Annealing Annealing at 900-1500° C in vacuum. This helps reorder tubes, can remove some metals and carbon particles, but can lead to increased defects and shortened tubes. This method has been used less frequently in recent research.
- Intercalation Tubes are immersed in CuCl2-KCl and salt and left at 400° C for a week. Then tubes are placed in He/H2 for 1 hr at 500°C. Nanoparticles and other carbon species can be removed from nanotubes which have closed cage structure, but not all nanoparticles and carbon species are removed, and therefore is not used much due to better methods that have been developed.